**Figure 16.** a)The map of wavelength distribution over image (a) and its horizontal (b) and vertical (c) cross sections for a transient process of holographic layer shrinking and b) for a stationary state of sensor AA-AMPh-bis – 87-12-1 mol. %.(JPEG-format) The lines on the maps show the cross-section directions.

**Figure 17.** Isometric presentation of the wavelength distribution map (a) in the case of transient shrinkage of the holographic layer of sensor AA-AMPh-bis – 87-12-1 mol.% and for the hologram in a stationary state (b).

By the digital image of the mercury spectrum for the yellow doublet lines and for underly‐ ing continuous spectrum we determined the standard deviation of the recovered wave‐ length in the limits of a narrow window oriented along the central (with respect to the spectrum) part of the mercury line. The window width was 4 pixels, which was less than the line width. The window height was 350 pixels and covered almost all the image of the mer‐ cury line and the whole corresponding part of the continuous spectrum. For the mercury doublet lines the wavelengths of 577 nm (the standard deviation is 0.16) and 579 nm (the standard deviation is 0.19) were obtained.

The developed method was employed for studying holographic sensors. The results are shown in Figs 16, 17. The spectrum of radiation reflected from a holographic sensor is wider and the problem of possible inaccuracy in determining the wavelength requires particular investigations. One should keep in mind that in using holographic sensors it is important to know the shift of the wavelength under the action of solution surrounding the sensor rather than the absolute value of the wavelength itself. Data presented in Figs 16a and 17a refer to a transient process to the hologram initially reflecting in the red range. Data for this holo‐ gram in ending stationary state is presented in Figs.16b. and 17b. One can see that the spread of wavelengths is noticeably reduced as compared to the transient process, and is less than 2 nm over the whole sensor surface. The local spread also strongly reduced and was less than 1 nm. The map comprises approximately 500 000 points.

The hologram quality and uniformity of the processes occurring during swelling can be esti‐ mated from their noise characteristics. In Fig. 18, the standard deviation of the calculated wavelengths is shown versus the width of the averaging window. The standard deviation of wavelength from mean *λ*¯is:

$$A\_{\mathbb{k}} = \sqrt{\frac{\sum\_{i}^{\text{in}} (\lambda\_i - \overline{\lambda})^2}{m}}$$

**Figure 16.** a)The map of wavelength distribution over image (a) and its horizontal (b) and vertical (c) cross sections for a transient process of holographic layer shrinking and b) for a stationary state of sensor AA-AMPh-bis – 87-12-1 mol.

**Figure 17.** Isometric presentation of the wavelength distribution map (a) in the case of transient shrinkage of the

holographic layer of sensor AA-AMPh-bis – 87-12-1 mol.% and for the hologram in a stationary state (b).

%.(JPEG-format) The lines on the maps show the cross-section directions.

120 Holography – Basic Principles and Contemporary Applications

where *λi* is the calculated wavelength in the *i-*th pixel of the image and *m* is the number of points fitting the window, was averaged over *N* image points covering the whole studied domain of the sensor. The parameter *S* = 1 *N* ∑ *k N Ak* we will term the noise value. Data present‐ ed in Fig. 18 correspond to variation of *m* from 4 to 2500 pixels. The standard deviation was averaged over the image domain of 500x500 points. With increasing *m* in the transient proc‐ ess the standard deviation varied from 0.5 to 1.8 nm. A steady increase in noise with the in‐ creasing window is related to a large-scale hologram inhomogeneity. In the steady state it was 0.16 – 0.32 nm at the same values of *m*. An increase in noise at the initial part of the dependence is explained by small-scale inhomogeneity, and the saturation at large *m* is re‐ lated to the absence of large-scale inhomogeneity. The ratio of nonstationary noise to sta‐ tionary level in this range of *m* increases from 3.4 to 5.6. These facts bear witness that, first, the hologram in the steady state is highly uniform and, second, variations of its swelling over the surface in the nonstationary state are noticeably inhomogeneous.

studied these ranges are 470 – 540 nm and 570 – 600 nm). The accuracy of the deter‐ mined wavelength is not worse than 1 nm. The method was tested on the yellow dou‐ blet of the mercury spectrum and on a continuous spectrum of the incandescent lamp covering the working interval 570 – 600 nm. By using this method the uniformity of holographic sensor swelling was studied both in a stationary state and in dynamics.

Holographic Sensors for Detection of Components in Water Solutions

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123

**Figure 20.** a) The photograph of the hologram. b) The distribution of samples of 0.5% acetic acid on the sensors sur‐

The design of holographic sensors allows their use in multi-channel mode, when one sensor can simultaneously analyze multiple samples of the same type or define a few parameters of a sample. The holographic sensor is actually a thick hologram plane mirror about the size of a square centimeter. By placing such plate in a special cuvette, containing 24 cells of 2 mm diameter, each contains 50 μL of fluid to be analyzed; it can be used in a multi-channel mode for the simultaneous determination of all samples. The response of the sensor - the wavelength of reflected light - from the cell is easily determined by means of small-sized fi‐ ber-optic spectrometer (Fig. 20, 21) or in combination with the developed colorimetric meth‐ od; it can greatly simplify and speed up the analysis (Fig. 19). As can be seen from Fig. 20, the wavelength of the reflected light at different points on the surface is almost the same (the

kinetics of the holographic sensor, to conduct research of the spatial distribution of process‐ es, with the help of special devices to conduct simultaneous analysis of different compo‐ nents or different samples. Fig. 21 presents the results of the determination of glucose in the blood serum of diabetic patients with glucose sensor (see Fig.) based on aminophenylboron‐ ic acid. Despite a significant decrease in the sensitivity of the sensor to the glucose concen‐ tration in blood plasma, determined by glucose clearly distinguished spectrometrically.

face and wavelength of reflected light in cells (fiber-optic spectrometer).

standard deviation is 1.7 nm). This way you can see the spatial pattern of

**Figure 18.** The standard deviation of the calculated wavelength versus the number of points in the averaging window for the transient process (1) and stationary sensor state (2).

Further development of the colorimetric method with the use of RAW-format can signifi‐ cantly increase the working range of wavelengths (440-620 nm), which can be seen from Fig. 19.

**Figure 19.** a) The photograph of the hologram in the steady state. b) In the upper right-hand column shows the hori‐ zontal and vertical cross-section of the distribution of responses. (RAW-format)

Thus, here is described the method for measuring the distribution of the average wave‐ length for narrow-band radiation over the source surface by means of a commercial digi‐ tal camera. There are the following limitations in using the method (without RAWformat): the radiation spectrum should be narrow (the average wavelength is determined); measurements are performed in the spectral range in which at least two sensor types of detecting array are simultaneously sensitive (for the most of cameras studied these ranges are 470 – 540 nm and 570 – 600 nm). The accuracy of the deter‐ mined wavelength is not worse than 1 nm. The method was tested on the yellow dou‐ blet of the mercury spectrum and on a continuous spectrum of the incandescent lamp covering the working interval 570 – 600 nm. By using this method the uniformity of holographic sensor swelling was studied both in a stationary state and in dynamics.

**Figure 18.** The standard deviation of the calculated wavelength versus the number of points in the averaging window

Further development of the colorimetric method with the use of RAW-format can signifi‐ cantly increase the working range of wavelengths (440-620 nm), which can be seen from

**Figure 19.** a) The photograph of the hologram in the steady state. b) In the upper right-hand column shows the hori‐

Thus, here is described the method for measuring the distribution of the average wave‐ length for narrow-band radiation over the source surface by means of a commercial digi‐ tal camera. There are the following limitations in using the method (without RAWformat): the radiation spectrum should be narrow (the average wavelength is determined); measurements are performed in the spectral range in which at least two sensor types of detecting array are simultaneously sensitive (for the most of cameras

zontal and vertical cross-section of the distribution of responses. (RAW-format)

for the transient process (1) and stationary sensor state (2).

122 Holography – Basic Principles and Contemporary Applications

Fig. 19.

**Figure 20.** a) The photograph of the hologram. b) The distribution of samples of 0.5% acetic acid on the sensors sur‐ face and wavelength of reflected light in cells (fiber-optic spectrometer).

The design of holographic sensors allows their use in multi-channel mode, when one sensor can simultaneously analyze multiple samples of the same type or define a few parameters of a sample. The holographic sensor is actually a thick hologram plane mirror about the size of a square centimeter. By placing such plate in a special cuvette, containing 24 cells of 2 mm diameter, each contains 50 μL of fluid to be analyzed; it can be used in a multi-channel mode for the simultaneous determination of all samples. The response of the sensor - the wavelength of reflected light - from the cell is easily determined by means of small-sized fi‐ ber-optic spectrometer (Fig. 20, 21) or in combination with the developed colorimetric meth‐ od; it can greatly simplify and speed up the analysis (Fig. 19). As can be seen from Fig. 20, the wavelength of the reflected light at different points on the surface is almost the same (the standard deviation is 1.7 nm). This way you can see the spatial pattern of

kinetics of the holographic sensor, to conduct research of the spatial distribution of process‐ es, with the help of special devices to conduct simultaneous analysis of different compo‐ nents or different samples. Fig. 21 presents the results of the determination of glucose in the blood serum of diabetic patients with glucose sensor (see Fig.) based on aminophenylboron‐ ic acid. Despite a significant decrease in the sensitivity of the sensor to the glucose concen‐ tration in blood plasma, determined by glucose clearly distinguished spectrometrically.

**Acknowledgements**

**Author details**

Medicine, Moscow, Russia

Moscow, Russia

**References**

13-20

my of Sciences, Moscow, Russia

Sciences for Medicine" of the Presidium of RAS.

Vladimir A. Postnikov1\*, Aleksandr V. Kraiskii2

Anal. Chem. 1995; 67,4229-4233

alcohol sensor. Anal. Chem. 1999; 71, 3390-3396

sensing materials. Nature. 1997; 389, 829–32.

\*Address all correspondence to<EMAIL_ADDRESS>
This work was supported by grant of the Program of Fundamental Studies "Fundamental

1 Laboratory of Medical Nanotechnology, Scientific Research Institute of Physical-Chemical

2 G.S. Landsberg Optical Department, P.N.Lebedev Physical Institute of the Russian Acade‐

3 Department of Biophysics, Scientific Research Institute of Physical-Chemical Medicine,

[1] Millington R, Mayes A, Blyth J, and Lowe C. A Holographic sensor for Proteases.

[2] Mayes A, Blyth J, Kyrollo1inen-Reay M, Millington R. and Lowe C. A holographic

[3] Holtz J, Asher S. Polymerized colloidal crystal hydrogel films as intelligent chemical

[4] Reese E, Baltusavich M, Keim J, Asher S. Development of an intelligent polymerized crystalline colloidal array colorimetric reagent. Anal Chem. 2001; 73, 5038–42.

[5] Marshall A, Young D, Kabilan S, Hussain A, Blyth J. and Lowe C. Holographic sen‐ sors for the determination of ionic strength. Analytica Chimica Acta. 2004; 527(1),

[6] González B, Christie G, Davidson C, Blyth J. and Lowe C. Divalent metal ion-sensi‐

[7] Alexeev V, Sharma A, Goponenko A, Das S, Lednev I, Wilcox C, et al. High ionic

tive holographic sensors. Analytica Chimica Acta. 2005; 528(2), 219-228

strength glucose-sensing photonic crystal. Anal Chem 2003; 75, 2316–23.

and Valerii I. Sergienko3

Holographic Sensors for Detection of Components in Water Solutions

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125

**Figure 21.** a). The distribution of blood serum (\* ) [glucose], mM) samples with different glucose concentrations and 0.5% acetic acid on the sensors surface. b). Response (λnm) of glucose sensor (AA-GA- AMPh - bis: 87,5 - 6 – 6 - 0.5 mol. %) (fiber-optic spectrometer).

## **3. Conclusion**

Thus, we have developed a colorimetric method of determination of the wavelength with the digital camera to study processes in inhomogeneous systems and to check the quality of the holographic layer. On this basis we revealed the multichannel simultaneous methods of analysis of spatially inhomogeneous objects and processes. We have found that the change in the ionic composition of solution is accompanied by the change in the distance between silver nanograin layers and in the diffraction efficiency of holograms. Based on this, we are formulated conditions for optimization of the operating mode of the holographic layer. Transition processes revealed variations in the reflection line shape, caused by the inhomo‐ geneity of the sensitive layer, and non-monotonic changes in the emulsion thickness and dif‐ fraction efficiency. In this relation it was developed the computer model of propagation in a layered medium for one-dimensional case 33.

We have developed the method for manufacturing holographic sensors of different types and selected the composition of components of the hydrogel medium for the systems that can be used as bases for glucose sensors. The maximum mean holographic response in the mmol. L-1 region concentrations of glucose (1-20 mmol. L-1) per 1 mmol. L-1 of glucose in model solutions achieves ~40 nm/( mmol. L-1). It has been shown that holographic sensors can be used to determine the quality of water, in particular, for drinking, the acidity of media, etha‐ nol concentration, ionic strength, metal ions and glucose in blood serum.

## **Acknowledgements**

This work was supported by grant of the Program of Fundamental Studies "Fundamental Sciences for Medicine" of the Presidium of RAS.

## **Author details**

Vladimir A. Postnikov1\*, Aleksandr V. Kraiskii2 and Valerii I. Sergienko3

\*Address all correspondence to<EMAIL_ADDRESS>
1 Laboratory of Medical Nanotechnology, Scientific Research Institute of Physical-Chemical Medicine, Moscow, Russia

2 G.S. Landsberg Optical Department, P.N.Lebedev Physical Institute of the Russian Acade‐ my of Sciences, Moscow, Russia

3 Department of Biophysics, Scientific Research Institute of Physical-Chemical Medicine, Moscow, Russia

## **References**

**Figure 21.** a). The distribution of blood serum (\*

124 Holography – Basic Principles and Contemporary Applications

layered medium for one-dimensional case 33.

solutions achieves ~40 nm/( mmol.

L-1 region concentrations of glucose (1-20 mmol.

nol concentration, ionic strength, metal ions and glucose in blood serum.

%) (fiber-optic spectrometer).

**3. Conclusion**

mmol.

) [glucose], mM) samples with different glucose concentrations and

L-1) per 1 mmol.

L-1). It has been shown that holographic sensors can be

L-1 of glucose in model

0.5% acetic acid on the sensors surface. b). Response (λnm) of glucose sensor (AA-GA- AMPh - bis: 87,5 - 6 – 6 - 0.5 mol.

Thus, we have developed a colorimetric method of determination of the wavelength with the digital camera to study processes in inhomogeneous systems and to check the quality of the holographic layer. On this basis we revealed the multichannel simultaneous methods of analysis of spatially inhomogeneous objects and processes. We have found that the change in the ionic composition of solution is accompanied by the change in the distance between silver nanograin layers and in the diffraction efficiency of holograms. Based on this, we are formulated conditions for optimization of the operating mode of the holographic layer. Transition processes revealed variations in the reflection line shape, caused by the inhomo‐ geneity of the sensitive layer, and non-monotonic changes in the emulsion thickness and dif‐ fraction efficiency. In this relation it was developed the computer model of propagation in a

We have developed the method for manufacturing holographic sensors of different types and selected the composition of components of the hydrogel medium for the systems that can be used as bases for glucose sensors. The maximum mean holographic response in the

used to determine the quality of water, in particular, for drinking, the acidity of media, etha‐

[8] Asher S, Alexeev V, Goponenko A, Sharma A, Lednev I, Wilcox C, et al. Photonic crystal carbohydrate sensors: low ionic strength sugar sensing. J Am Chem Soc 2003; 125, 3322–9.

[22] Sartain FK, Yang X, Lowe CR. Holographic lactate sensor. Anal Chem. 2006;78(16),

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[24] Postnikov VA, Kraiskii AV, Tikhonov VE, Sultanov TT, Khamidulin AV. Hydrogel holographic sensors for detection of components in biological fluids. In Proceedings of CAOL 2008: 4th International Conference on Advanced Optoelectronics and La‐ sers ,2008, ), Alushta, Crimea South Coast, Ukraine, September 29 – October 4, art.

[25] Postnikov VA, Kraiskii AV, Tikhonov VE, Sultanov TT, Khamidulin AV. Hydrogel holographic sensors for detection of components in biological fluids. In Proceedings of CAOL 2008: 4th International Conference on Advanced Optoelectronics and La‐ sers ,2008, ), Alushta, Crimea South Coast, Ukraine, September 29 – October 4, art.

[26] Postnikov VA, Kraiskii AV, Sultanov TT, Deniskin VV. Holographic sensors of glu‐ cose in model solution and serum. In Conference Proceedings - 5th International Conference on Advanced Optoelectronics and Lasers, CAOL' 2010, September 10 -14,

[27] Postnikov VA, Kraiskii AV, Sultanov TT. Holographic sensors. In Conference Pro‐ ceedings - 11th International Conference on Laser and Fiber-Optical Networks Mod‐ eling, LFNM 2011, Kharkov, Ukraine, September 5 – 8, 2011, art. no. 6145033, 369-371

[28] Kraiskii AV, Postnikov VA, Sultanov TT, Khamidulin AV. Holographic sensors diag‐ nostics of solution components. Quantum Electronics. 2010; 40 (2), 178 -182

[29] Hansen J, Christensen J, Petersen J, Hoeg-Jensen T, Norrild J. Arylboronic acids: A diabetic eye on glucose sensing. Sensors and Actuators. 2012; B 161, 45– 79

[30] Steiner M-S, Duerkop A, Wolfbeis O. Optical methods for sensing glucose. Chem.

[31] Christopher R. Cooper and Tony D. James. Synthesis and evaluation of D-glucosa‐ mine-selective fluorescent sensors. J. Chem. Soc., Perkin Trans. 2000; 1 ,963–969

[32] Egawa Y, Seki T, Takahashi S, Anzai J-i. Electrochemical and optical sugar sensors based on phenylboronic acid and its derivatives. Materials Science and Engineering

[33] Kraiskii AV, Postnikov VA, Sultanov TT, Mironova TV, Kraiskii AA. On optical properties of holographic sensors based on silver emulsions. In Conference Proceed‐ ings - 11th International Conference on Laser and Fiber-Optical Networks Modeling, LFNM 2011, Kharkov, Ukraine, September 5 – 8, 2011 , art. no. 5634214, 191-192

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[9] Kabilan S, Marshall A, Sartain F, Lee M.-C, Hussain A, Yang X, Blyth J, Karangu N, James K, Zeng J, Smith D, Domschke A. and Lowe C. Holographic glucose sensors.

[10] Yang X, Lee M.-C, Sartain F, Pan X, Lowe C.R. Designed Boronate Ligands for Glu‐

[11] Horgan A, Marshall A, Kew S, Dean K, Creasey C. and Kabilan S. Crosslinking of phenylboronic acid receptors as a means of glucose selective holographic detection.

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[13] Yang X, Pan X, Blyth J, Lowe C.R. Towards the real-time monitoring of glucose in tear fluid: Holographic glucose sensors with reduced interference from lactate and

[14] Alexeev VL, Das S, Finegold DN, Asher SA. Photonic crystal glucose-sensing materi‐ al for noninvasive monitoring of glucose in tear fluid. Clin Chem. 2004; 50, 2362–9. [15] Bhatta D, Christie G, Madrigal-González B, Blyth J. and Lowe C.R. Holographic sen‐ sors for the detection of bacterial spores. Biosensors and Bioelectronics. 2007; 23(4),

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**Chapter 6**

**Photopolymer Holographic Optical Elements for**

Making use of the sun's radiation as an alternate energy resource has become increasingly worthwhile in recent years both on a domestic and large industrial scale. Rooftop solar collectors for domestic water heating are now common even in regions where direct sunlight is somewhat limited, and large installations for commercial electricity generation are increas‐ ing in sunnier climates. In 2010 in the US there was a 45% increase in the number of grid connected photovoltaic systems installed compared to the preceding year, raising the cumu‐ lative grid-connected capacity to 2.15 GWdc. In the same year, the largest solar concentrating

There are three key technologies for the conversion of solar energy; thermal heating, photo‐

In thermal heating systems, water is heated directly or indirectly by the sun, typically in insulated tubing on the premises roof, and used for domestic or sometimes for commercial water heating. Photovoltaic cells generate electricity directly and are widely used in domestic

Thermal to electric involves mechanical heat engines and requires higher temperatures, so to date it has been mostly used in large scale commercial generation plants where concentrating

> © 2013 Naydenova et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

> © 2013 Naydenova et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Application in Solar Energy Concentrators**

Izabela Naydenova, Hoda Akbari, Colin Dalton,

Additional information is available at the end of the chapter

plant since the 1980s (75 MWa) was completed in Florida [1].

Mohamed Yahya so Mohamed Ilyas, Clinton Pang Tee Wei, Vincent Toal and

Suzanne Martin

**1. Introduction**

http://dx.doi.org/10.5772/55109

voltaic and thermal to electric.

and commercial applications.

collectors can be used.

## **Photopolymer Holographic Optical Elements for Application in Solar Energy Concentrators**

Izabela Naydenova, Hoda Akbari, Colin Dalton, Mohamed Yahya so Mohamed Ilyas, Clinton Pang Tee Wei, Vincent Toal and Suzanne Martin

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/55109

**1. Introduction**

[34] Kraiskii AV, Mironova TV, Sultanov TT, Postnikov VA, Sergienko VI, Tikhonov VE.

[35] Kraiskii AV, Mironova TV, Sultanov TT. Measurement of the surface wavelength dis‐ tribution of narrow-band radiation by a colorimetric method. Quantum Electronics.

[36] Kraiskii AV, Mironova TV, Sultanov TT, Postnikov VA. Measuring surface distribu‐ tion of narrowband radiation wavelength by colorimetric method. In Conference Proceedings - 5th International Conference on Advanced Optoelectronics and Lasers, CAOL' 2010, September 10 -14, 2010, Sevastopol, Crimea, Ukraine, art. no. 5634214,

[37] Judd D.B., Wyszecki G. Color in Business, Science and Industry (New York: Wiley,

Sposob izmereniya dliny volny…, Pat. RF No. 2390738, prior. 21.05.2008.

2010; 40 (7), 652 – 658 36.

128 Holography – Basic Principles and Contemporary Applications

1975; Moscow: Mir, 1978).

191-192

Making use of the sun's radiation as an alternate energy resource has become increasingly worthwhile in recent years both on a domestic and large industrial scale. Rooftop solar collectors for domestic water heating are now common even in regions where direct sunlight is somewhat limited, and large installations for commercial electricity generation are increas‐ ing in sunnier climates. In 2010 in the US there was a 45% increase in the number of grid connected photovoltaic systems installed compared to the preceding year, raising the cumu‐ lative grid-connected capacity to 2.15 GWdc. In the same year, the largest solar concentrating plant since the 1980s (75 MWa) was completed in Florida [1].

There are three key technologies for the conversion of solar energy; thermal heating, photo‐ voltaic and thermal to electric.

In thermal heating systems, water is heated directly or indirectly by the sun, typically in insulated tubing on the premises roof, and used for domestic or sometimes for commercial water heating. Photovoltaic cells generate electricity directly and are widely used in domestic and commercial applications.

Thermal to electric involves mechanical heat engines and requires higher temperatures, so to date it has been mostly used in large scale commercial generation plants where concentrating collectors can be used.

© 2013 Naydenova et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Naydenova et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

However availability of new materials and technologies currently under development may well change the applicability of each technology. For example, a recent paper in Nature[2] describes the use of nanostructured thermoelectric materials and spectrally selective solar absorbers in a solar thermal to electric power conversion system. This has efficiency 7–8 times greater than the previously reported best value for a flat-panel solar thermal to electric (STEG) system and could lead to a much wider use of STEG systems.

**3. Advantages of HOE solar energy concentrators**

for tracking or mechanical movement.

light can be focussed in one spot [6, 7].

energy collection can be classified as:

the wavelength of the recording light.

wavelength of recording light.

orientations and locations.

**4. Types of HOEs**

The collection of light from a moving source (such as sun) which exhibits a broad spectral range of wavelengths is a complex process. HOEs have the capability to perform a range of functions in one element thus providing a potential solution to this problem without the need

Photopolymer Holographic Optical Elements for Application in Solar Energy Concentrators

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131

Holographic solar concentrators can use flat optics for the collection of sunlight because they can be designed to have a very wide field of view which would make them attractive for improving power conversion efficiencies in energy conversion devices which have fixed

HOEs can be designed to redirect, concentrate or block the incident light, such as that from the sun. They may also be designed for wavelength selectivity, so a range of wavelengths can be directed to one position while other wavelengths go to another position and the diffracted

Depending on their effect on the incident light holographic optical elements for use in solar

**•** Focussing elements: optical elements that produce a converging wavefront, having the same effect as spherical or cylindrical lenses. The focal length can vary depending on the devices; they can have a dual role in solar collectors by focusing the light and redirecting the beam.

**•** Reflection HOEs: The incident beam and the diffracted beam propagate on the same side of the hologram; they allow diffuse light to be transmitted whilst the direct beam is diffracted.

In this type of HOE the fringes due to interference between the recording light beams, form planes that are usually parallel to the recording material surface. The spacing between fringes depends on the angular separation between the reference beam and the object beam and on

**•** Transmission HOEs: In a transmission HOE the incident and the diffracted beams are both transmitted through the optical element. The fringes due to interference between the recording light beams can be perpendicular to the layer surface (unslanted gratings) or at an angle (slanted gratings). As in the case of the reflection gratings the spacing between the interference fringes depends on the angle between the two recording beams and the

**•** Non focussing elements: optical elements that are used simply to redirect light;

Depending on the geometry of the recording, HOEs can be classified as:

Solar collector technologies in current use can be divided into concentrating and non-concen‐ trating types. Concentrators have obvious advantages where the solar conversion surface is expensive (photovoltaics) or requires high temperatures to work efficiently (thermal to electric systems). The most common concentrators currently used in commercial systems are either cylindrical or hemispherical reflectors. They concentrate the light by reflectance off a curved surface either to a line or spot where the solar energy is converted.

The holographic equivalent is a concentrating diffractive optical element, or holographic lens, which will focus the collected light in a similar fashion.

## **2. Solar application for holographic optical elements**

Holographic Optical Elements (HOE) have also been studied for controlling and directing the radiation of the sun with high potential for energy saving.

Photovoltaic energy conversion is very suitable for solar energy generation but the main disadvantage of photovoltaic electrical energy generation is the cost.

In order to solve this problem a significant amount of expensive photovoltaic material can be replaced by an optical concentrator. By providing complex optical functions in thin, low cost layers which can be used with other PV components, benefits could be expected.

HOEs are very good examples of optical concentrators and have been suggested for use as solar concentrators [3]. HOEs are produced by dividing coherent light into monochromic waves with the same polarization and equal intensities. An optical lens is placed in one of the beams and focuses the incident beam; the HOE can be recorded where two beams overlap with each other. HOEs have several unique features such as ability to diffract light through a large angle, and Bragg selectivity. They also have the potential for multifunctionality by multiplex‐ ing a number of optical components in the same layer. They are thin, flat and lightweight, making HOEs attractive for solar collector/ concentrator devices.

Another type of HOE has been used for window shading in buildings with a defined orienta‐ tion [4, 5]; the holograms have been designed and produced to shade the windows of a building with a facade facing 56º east of south. HOEs were recorded at 45º and 60º and tested in a solar simulator for an entire year. The test revealed that the maximum illumination took place at 11 a.m. Comparison of the spectral characteristics proved that the HOES recorded at 45 º are more suitable for window shading. Due to the ammonium dichromate in the HOEs they showed some absorption in the blue spectral range and due to iron ions (which can be found in standard green glass) they showed some absorption in the red.

## **3. Advantages of HOE solar energy concentrators**

The collection of light from a moving source (such as sun) which exhibits a broad spectral range of wavelengths is a complex process. HOEs have the capability to perform a range of functions in one element thus providing a potential solution to this problem without the need for tracking or mechanical movement.

Holographic solar concentrators can use flat optics for the collection of sunlight because they can be designed to have a very wide field of view which would make them attractive for improving power conversion efficiencies in energy conversion devices which have fixed orientations and locations.

HOEs can be designed to redirect, concentrate or block the incident light, such as that from the sun. They may also be designed for wavelength selectivity, so a range of wavelengths can be directed to one position while other wavelengths go to another position and the diffracted light can be focussed in one spot [6, 7].

## **4. Types of HOEs**

However availability of new materials and technologies currently under development may well change the applicability of each technology. For example, a recent paper in Nature[2] describes the use of nanostructured thermoelectric materials and spectrally selective solar absorbers in a solar thermal to electric power conversion system. This has efficiency 7–8 times greater than the previously reported best value for a flat-panel solar thermal to electric (STEG)

Solar collector technologies in current use can be divided into concentrating and non-concen‐ trating types. Concentrators have obvious advantages where the solar conversion surface is expensive (photovoltaics) or requires high temperatures to work efficiently (thermal to electric systems). The most common concentrators currently used in commercial systems are either cylindrical or hemispherical reflectors. They concentrate the light by reflectance off a curved

The holographic equivalent is a concentrating diffractive optical element, or holographic lens,

Holographic Optical Elements (HOE) have also been studied for controlling and directing the

Photovoltaic energy conversion is very suitable for solar energy generation but the main

In order to solve this problem a significant amount of expensive photovoltaic material can be replaced by an optical concentrator. By providing complex optical functions in thin, low cost

HOEs are very good examples of optical concentrators and have been suggested for use as solar concentrators [3]. HOEs are produced by dividing coherent light into monochromic waves with the same polarization and equal intensities. An optical lens is placed in one of the beams and focuses the incident beam; the HOE can be recorded where two beams overlap with each other. HOEs have several unique features such as ability to diffract light through a large angle, and Bragg selectivity. They also have the potential for multifunctionality by multiplex‐ ing a number of optical components in the same layer. They are thin, flat and lightweight,

Another type of HOE has been used for window shading in buildings with a defined orienta‐ tion [4, 5]; the holograms have been designed and produced to shade the windows of a building with a facade facing 56º east of south. HOEs were recorded at 45º and 60º and tested in a solar simulator for an entire year. The test revealed that the maximum illumination took place at 11 a.m. Comparison of the spectral characteristics proved that the HOES recorded at 45 º are more suitable for window shading. Due to the ammonium dichromate in the HOEs they showed some absorption in the blue spectral range and due to iron ions (which can be found in standard

layers which can be used with other PV components, benefits could be expected.

system and could lead to a much wider use of STEG systems.

130 Holography – Basic Principles and Contemporary Applications

surface either to a line or spot where the solar energy is converted.

**2. Solar application for holographic optical elements**

disadvantage of photovoltaic electrical energy generation is the cost.

making HOEs attractive for solar collector/ concentrator devices.

green glass) they showed some absorption in the red.

which will focus the collected light in a similar fashion.

radiation of the sun with high potential for energy saving.

Depending on their effect on the incident light holographic optical elements for use in solar energy collection can be classified as:

Depending on the geometry of the recording, HOEs can be classified as:

**•** Reflection HOEs: The incident beam and the diffracted beam propagate on the same side of the hologram; they allow diffuse light to be transmitted whilst the direct beam is diffracted.

In this type of HOE the fringes due to interference between the recording light beams, form planes that are usually parallel to the recording material surface. The spacing between fringes depends on the angular separation between the reference beam and the object beam and on the wavelength of the recording light.

**•** Transmission HOEs: In a transmission HOE the incident and the diffracted beams are both transmitted through the optical element. The fringes due to interference between the recording light beams can be perpendicular to the layer surface (unslanted gratings) or at an angle (slanted gratings). As in the case of the reflection gratings the spacing between the interference fringes depends on the angle between the two recording beams and the wavelength of recording light.

## **5. HOEs used in solar concentrators**

Prism Solar Technologies manufacture a solar cell concentrator which at present has a limited bandwidth and low conversion efficiency [8]. The sunlight is reflected and concentrated onto the photovoltaic cell (PV) with all components supported by a substrate. The HOE reflector is placed in a waveguide. The waveguide has been used in this application to receive the sunlight and redirect it to the PV cell. The concentrators produce uncompensated aberration such as dispersion and wavelength shift produced by the reflector, so that the spectral bandwidth of reflected band may not be precisely matched to the energy band gaps of the PV solar cells.

efficiency achieved was 42% since the concentration ratio for diffracted wavelengths was about

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Holographic solar concentrators have been theoretically modelled [11] and several useful aspects of holographic gratings have been investigated for use in solar concentrator applica‐ tions. The basic relationships for designing holographic elements have also been presented.

A solar radiation receiver is described in [12]. This combined system uses a holographic film to concentrate the solar radiation and to optimize the efficiency of the sensor. A mathematical model is used to calculate the Volt-Ampere behaviour and the thermal and photovoltaic

The design and optimization of photopolymer based holographic solar concentrators was recently reported in [13]. The authors demonstrated the recording of broad band spectrally splitting holographic solar concentrator in HoloMer photopolymer material with an efficiency of 70% and an average efficiency of 56.6% for a wavelength range from 633nm to 442 nm. The recorded elements showed a narrow angular selectivity hence tracking would be required for

A simple technique to realize a compact and nearly all-angle solar energy concentrator using a volume holographic element is presented in [14]. The theoretical modelling of the HOE predicts up to a fivefold concentration of energy per unit area of photovoltaic material.

In the following section we present experimental results from the recording of simple focusing holographic optical elements in a photopolymer layer, namely a spherical lens and a cylindrical lens. Furthermore we have explored the possibility of multiplexing a number of elements in

The photosensitive layer was prepared as previously described [15]. Briefly, 2ml of trietha‐ nolamine was added to 17.5 ml stock solution of polyvinyl alcohol (PVA) (10% w/w). Then the monomers, 0.6g acrylamide and 0.2 g of N,N Methylene bisacrylamide and 2ml of initiator, TEA, were added. Finally, 4ml of Erythrosin B dye was added (stock solution concentration - 1.1mM) to sensitise at 532 nm. The solution was made up to 25ml by adding distilled water.

Different amounts of photopolymer solution were spread evenly on a 50x50 mm2 glass plate placed on a levelled surface and allowed to dry. This resulted in layers of thickness varying

Methylene blue sensitised samples of thickness 50 μm were used to record at 633nm.

between 50 and 120 μm. The drying time was usually 18-24 hours.

efficiencies to demonstrate the advantages of the suggested system.

an effective photovoltaic concentrator system.

**6.1. Photopolymer solution preparation**

c= 100.

the same layer.

**6. Experimental**

**6.2. Layer preparation**

Another application of HOEs in light harvesting that could be useful in solar energy collection is reported in [9]. Holographic diffractive optical elements were used in order to increase the light collection from fluorescence-based biochips. The HOEs increased the transmitted fluorescence intensity and also served to filter out the undesired wavelengths. This was possible due to their high angular selectivity. The diffracted intensity of the HOE was meas‐ ured to be about 50% of that of the incident beam. The diffraction efficiency was relatively low due to a complexity of the recording process that covers a large spatial frequency range (0-2800 lines/mm). It was found that the HOE can collect fluorescent light coming from a spot with the same size as that of the HOE.

Another example of holographic solar application described in [10] uses a sensor and feedback system to maintain 0.5 degree tracking accuracy with one-axis tracking holographic planar concentrators (HPCs). It was found that in the polar one-axis tracking HPC system the efficiency increases by 43.8% compared to non tracking HPC systems due to high overall module optical efficiency and higher levels of irradiance.

Dispersive concentrating systems based on transmission phase HOEs for solar applications are reported in [2]. The authors demonstrate that volume based transmission HOEs can be used advantageously in solar concentrators due to their high diffraction efficiency, low absorption and adjustable dispersion. The ratio of diffracted intensity to incident beam intensity is defined as diffraction efficiency. In solar applications the measurement of the diffraction efficiency as a function of wavelength is essential. The transmissivity of HOEs as a function of wavelength was measured when white light illuminated the phase holograms and due to diffraction the light was split spatially and spectrally. It was determined that a minimum angle of 20° is required between the recording beams for achieving high diffraction efficiency in one diffraction order so that only an off-axis zone plate was suitable. A zone plate with diameter of 8 cm and a focal length of 25 cm was recorded at 488 nm wavelength. The diffraction of efficiency of the recorded zone plate was about 70% for monochromatic light. The shrinkage of the gelatin layer caused a change in the Bragg angle depending on the shape of recorded interference patterns and the intensities of the recording beams.

Volume HOES are suitable for multiplexing; a range of HOES with various angles between the recording beams can be recorded in one photosensitive layer and this allows spatial separation of the red and the blue spectral ranges of sunlight into different areas. Three solar cell systems with various band gaps and multiplexed HOEs were tested [3]. The maximum efficiency achieved was 42% since the concentration ratio for diffracted wavelengths was about c= 100.

Holographic solar concentrators have been theoretically modelled [11] and several useful aspects of holographic gratings have been investigated for use in solar concentrator applica‐ tions. The basic relationships for designing holographic elements have also been presented.

A solar radiation receiver is described in [12]. This combined system uses a holographic film to concentrate the solar radiation and to optimize the efficiency of the sensor. A mathematical model is used to calculate the Volt-Ampere behaviour and the thermal and photovoltaic efficiencies to demonstrate the advantages of the suggested system.

The design and optimization of photopolymer based holographic solar concentrators was recently reported in [13]. The authors demonstrated the recording of broad band spectrally splitting holographic solar concentrator in HoloMer photopolymer material with an efficiency of 70% and an average efficiency of 56.6% for a wavelength range from 633nm to 442 nm. The recorded elements showed a narrow angular selectivity hence tracking would be required for an effective photovoltaic concentrator system.

A simple technique to realize a compact and nearly all-angle solar energy concentrator using a volume holographic element is presented in [14]. The theoretical modelling of the HOE predicts up to a fivefold concentration of energy per unit area of photovoltaic material.

In the following section we present experimental results from the recording of simple focusing holographic optical elements in a photopolymer layer, namely a spherical lens and a cylindrical lens. Furthermore we have explored the possibility of multiplexing a number of elements in the same layer.

## **6. Experimental**

**5. HOEs used in solar concentrators**

132 Holography – Basic Principles and Contemporary Applications

same size as that of the HOE.

module optical efficiency and higher levels of irradiance.

Prism Solar Technologies manufacture a solar cell concentrator which at present has a limited bandwidth and low conversion efficiency [8]. The sunlight is reflected and concentrated onto the photovoltaic cell (PV) with all components supported by a substrate. The HOE reflector is placed in a waveguide. The waveguide has been used in this application to receive the sunlight and redirect it to the PV cell. The concentrators produce uncompensated aberration such as dispersion and wavelength shift produced by the reflector, so that the spectral bandwidth of reflected band may not be precisely matched to the energy band gaps of the PV solar cells.

Another application of HOEs in light harvesting that could be useful in solar energy collection is reported in [9]. Holographic diffractive optical elements were used in order to increase the light collection from fluorescence-based biochips. The HOEs increased the transmitted fluorescence intensity and also served to filter out the undesired wavelengths. This was possible due to their high angular selectivity. The diffracted intensity of the HOE was meas‐ ured to be about 50% of that of the incident beam. The diffraction efficiency was relatively low due to a complexity of the recording process that covers a large spatial frequency range (0-2800 lines/mm). It was found that the HOE can collect fluorescent light coming from a spot with the

Another example of holographic solar application described in [10] uses a sensor and feedback system to maintain 0.5 degree tracking accuracy with one-axis tracking holographic planar concentrators (HPCs). It was found that in the polar one-axis tracking HPC system the efficiency increases by 43.8% compared to non tracking HPC systems due to high overall

Dispersive concentrating systems based on transmission phase HOEs for solar applications are reported in [2]. The authors demonstrate that volume based transmission HOEs can be used advantageously in solar concentrators due to their high diffraction efficiency, low absorption and adjustable dispersion. The ratio of diffracted intensity to incident beam intensity is defined as diffraction efficiency. In solar applications the measurement of the diffraction efficiency as a function of wavelength is essential. The transmissivity of HOEs as a function of wavelength was measured when white light illuminated the phase holograms and due to diffraction the light was split spatially and spectrally. It was determined that a minimum angle of 20° is required between the recording beams for achieving high diffraction efficiency in one diffraction order so that only an off-axis zone plate was suitable. A zone plate with diameter of 8 cm and a focal length of 25 cm was recorded at 488 nm wavelength. The diffraction of efficiency of the recorded zone plate was about 70% for monochromatic light. The shrinkage of the gelatin layer caused a change in the Bragg angle depending on the shape

Volume HOES are suitable for multiplexing; a range of HOES with various angles between the recording beams can be recorded in one photosensitive layer and this allows spatial separation of the red and the blue spectral ranges of sunlight into different areas. Three solar cell systems with various band gaps and multiplexed HOEs were tested [3]. The maximum

of recorded interference patterns and the intensities of the recording beams.

#### **6.1. Photopolymer solution preparation**

The photosensitive layer was prepared as previously described [15]. Briefly, 2ml of trietha‐ nolamine was added to 17.5 ml stock solution of polyvinyl alcohol (PVA) (10% w/w). Then the monomers, 0.6g acrylamide and 0.2 g of N,N Methylene bisacrylamide and 2ml of initiator, TEA, were added. Finally, 4ml of Erythrosin B dye was added (stock solution concentration - 1.1mM) to sensitise at 532 nm. The solution was made up to 25ml by adding distilled water. Methylene blue sensitised samples of thickness 50 μm were used to record at 633nm.

#### **6.2. Layer preparation**

Different amounts of photopolymer solution were spread evenly on a 50x50 mm2 glass plate placed on a levelled surface and allowed to dry. This resulted in layers of thickness varying between 50 and 120 μm. The drying time was usually 18-24 hours.

#### **6.3. Recording of HOE consisting of a single optical component**

A standard holographic optical setup (Fig.1) was used to record transmission gratings and lenses using a 532nm Nd:YVO4 laser. The recording intensity was controlled by a variable neutral density filter. The inter-beam angle was adjusted to be 9 degrees in order to obtain a spatial frequency of recording of 300 lines/mm. At the end of the holographic recording, the focusing beam was blocked and the collimated beam was used to probe the recorded HOE. The intensity of the diffracted beam was measured using an optical power meter (Newport 1830-C) to determine the diffraction efficiency of the recorded grating or lens respectively.

**Figure 1.** Optical set-up for recording of a single lens HOE.

The recording set up at 633 nm was similar to that shown in Fig. 1. A single off-axis HOE of focal length 5 cm was recorded. The recording intensity of the beams was 1 mW/cm2 and the average spatial frequency of recording was 650 l/mm.

**Figure 2.** Optical set-up for recording multiplexed HOEs in this case - diffraction gratings of different spatial frequen‐

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Figure 2 shows the experimental set up for the recording of multiplexed transmission gratings. The photopolymer sample was kept at a fixed distance from the beam splitter and the reference beam was varied in direction by using five mirrors fixed at different distances from the beam splitter to reflect the light onto the photopolymer layer. The photopolymer sample was adjusted so that the object beam and the reference beam from mirror 5 overlapped in the plane of the photosensitive medium with the sample normal bisecting the interbeam angle. This ensured the grating was unslanted when recorded by the beam reflected by mirror 5 and the beam transmitted by the beam splitter. The gratings were recorded in the same photopolymer layer starting with the lowest spatial frequency (mirror1). Mirror 1 was then removed and the next grating was recorded in the same area by the light reflected onto the photopolymer layer from mirror 2 and the light transmitted by the beam splitter. This procedure was repeated for the other three mirrors, with mirror 5 corresponding to the largest spatial frequency. Cylin‐

cies. B S (Beam Splitter), C L (Collimating Lens), S F (Spatial Filter), P S (Photopolymer Sample)

#### **6.4. Recording of HOEs by multiplexing**

The aim of this experiment was to record a holographic optical element which would direct the light in a fixed direction independently of the direction of incoming light.

**6.3. Recording of HOE consisting of a single optical component**

134 Holography – Basic Principles and Contemporary Applications

**Figure 1.** Optical set-up for recording of a single lens HOE.

**6.4. Recording of HOEs by multiplexing**

average spatial frequency of recording was 650 l/mm.

A standard holographic optical setup (Fig.1) was used to record transmission gratings and lenses using a 532nm Nd:YVO4 laser. The recording intensity was controlled by a variable neutral density filter. The inter-beam angle was adjusted to be 9 degrees in order to obtain a spatial frequency of recording of 300 lines/mm. At the end of the holographic recording, the focusing beam was blocked and the collimated beam was used to probe the recorded HOE. The intensity of the diffracted beam was measured using an optical power meter (Newport 1830-C) to determine the diffraction efficiency of the recorded grating or lens respectively.

The recording set up at 633 nm was similar to that shown in Fig. 1. A single off-axis HOE of focal length 5 cm was recorded. The recording intensity of the beams was 1 mW/cm2 and the

The aim of this experiment was to record a holographic optical element which would direct

the light in a fixed direction independently of the direction of incoming light.

**Figure 2.** Optical set-up for recording multiplexed HOEs in this case - diffraction gratings of different spatial frequen‐ cies. B S (Beam Splitter), C L (Collimating Lens), S F (Spatial Filter), P S (Photopolymer Sample)

Figure 2 shows the experimental set up for the recording of multiplexed transmission gratings. The photopolymer sample was kept at a fixed distance from the beam splitter and the reference beam was varied in direction by using five mirrors fixed at different distances from the beam splitter to reflect the light onto the photopolymer layer. The photopolymer sample was adjusted so that the object beam and the reference beam from mirror 5 overlapped in the plane of the photosensitive medium with the sample normal bisecting the interbeam angle. This ensured the grating was unslanted when recorded by the beam reflected by mirror 5 and the beam transmitted by the beam splitter. The gratings were recorded in the same photopolymer layer starting with the lowest spatial frequency (mirror1). Mirror 1 was then removed and the next grating was recorded in the same area by the light reflected onto the photopolymer layer from mirror 2 and the light transmitted by the beam splitter. This procedure was repeated for the other three mirrors, with mirror 5 corresponding to the largest spatial frequency. Cylin‐ drical lenses were also recorded in the same volume of the recording medium using a cylindrical lens of 15 cm focal length placed in the path of the beam transmitted by the beam splitter so that this light was focused into a thin line just behind the photopolymer sample. In order to find the optimum recording conditions for HOEs with equal diffraction efficiencies the transmission gratings were recorded in two ways. First, the intensity was kept constant and the exposure time varied from one recording to the next. The second approach was to keep the exposure time constant and vary the intensity. The recorded HOEs initially consisted of three gratings utilising mirrors M1, M2 and M5 and then five gratings utilising all five mirrors (M1, M2, M3, M4 and M5).

The spatial frequencies of the recorded gratings using the different mirrors in the recording set up were respectively: M1- 450 lines/mm; M2 - 1065 lines/mm; M3 -1295 lines/mm; M4 -1470 lines/mm and M5 -1700 lines/mm.

### **6.5. Characterisation of the recorded HOEs**

Two procedures were used in order to measure the maximum diffraction efficiencies (*η*) of the HOEs at different spatial frequencies.

Probing the recorded holograms using Nd:YVO4 laser beam (532nm)

After the recording of the transmission gratings with the photopolymer sample fixed in the same position, one of the recording beams was stopped and the HOE was illuminated only with the other recording beam, but with intensity much less than that used to record the grating, in order to avoid further polymerization. A photo detector was used to measure the intensities of the diffracted beam, the incident beam and the beam reflected from the photo‐ polymer surface.

The percentage diffraction efficiency (*η*) was calculated from the equation

$$
\eta = \frac{I\_d}{I\_1 - I\_r} \times 100.\tag{1}
$$

**Figure 3.** Characterisation of the HOE at 633nm. *I*1 is the intensity of the probe beam,

efficiency is nearly 45 % and it is reached after 100 s exposure time.

from the photopolymer surface.

**7. Results and discussion**

**7.1. Recording of focusing HOEs**

each was obtained at the appropriate Bragg angle.

*Id* the intensity of the reconstructed or diffracted beam and *Ir* the intensity of the beam reflected

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The intensity *Id* was measured using a photo detector, and equation (1) was used to calculate the diffraction efficiency. In the case of multiplexed gratings the value of *Id* for each grating was measured in turn by further rotation of sample until the diffracted intensity maximum for

The diffraction efficiency of a single lens recorded in a red sensitive layer of thickness of 50 μm as a function of recording time is presented in Fig. 4. It is seen that the maximum diffraction

where *I*1 is the intensity of the incident beam, *Id* is the intensity of the diffracted beam and *Ir* is the intensity of the beam reflected from the front photopolymer surface.

Probing the recorded holograms using a Helium-Neon laser (633nm)

Figure 3 shows the experimental set up for measuring the diffraction efficiencies using a Helium-Neon laser beam that has a much smaller diameter than that of the HOE. The Helium-Neon laser was positioned so that it probed the centre of the HOE that was recorded on the photopolymer layer. The photopolymer is not sensitive to light of wavelength 633 nm, therefore further polymerisation does not occur. The photopolymer sample was rotated until the maximum diffracted intensity of the laser beam was observed on a screen behind the photopolymer. The angle of incidence of the probe beam at which the maximum diffraction efficiency is obtained is known as the Bragg angle.

**Figure 3.** Characterisation of the HOE at 633nm. *I*1 is the intensity of the probe beam,

*Id* the intensity of the reconstructed or diffracted beam and *Ir* the intensity of the beam reflected from the photopolymer surface.

The intensity *Id* was measured using a photo detector, and equation (1) was used to calculate the diffraction efficiency. In the case of multiplexed gratings the value of *Id* for each grating was measured in turn by further rotation of sample until the diffracted intensity maximum for each was obtained at the appropriate Bragg angle.

## **7. Results and discussion**

drical lenses were also recorded in the same volume of the recording medium using a cylindrical lens of 15 cm focal length placed in the path of the beam transmitted by the beam splitter so that this light was focused into a thin line just behind the photopolymer sample. In order to find the optimum recording conditions for HOEs with equal diffraction efficiencies the transmission gratings were recorded in two ways. First, the intensity was kept constant and the exposure time varied from one recording to the next. The second approach was to keep the exposure time constant and vary the intensity. The recorded HOEs initially consisted of three gratings utilising mirrors M1, M2 and M5 and then five gratings utilising all five mirrors

The spatial frequencies of the recorded gratings using the different mirrors in the recording set up were respectively: M1- 450 lines/mm; M2 - 1065 lines/mm; M3 -1295 lines/mm; M4 -1470

Two procedures were used in order to measure the maximum diffraction efficiencies (*η*) of the

After the recording of the transmission gratings with the photopolymer sample fixed in the same position, one of the recording beams was stopped and the HOE was illuminated only with the other recording beam, but with intensity much less than that used to record the grating, in order to avoid further polymerization. A photo detector was used to measure the intensities of the diffracted beam, the incident beam and the beam reflected from the photo‐

Probing the recorded holograms using Nd:YVO4 laser beam (532nm)

The percentage diffraction efficiency (*η*) was calculated from the equation

is the intensity of the beam reflected from the front photopolymer surface.

Probing the recorded holograms using a Helium-Neon laser (633nm)

efficiency is obtained is known as the Bragg angle.

1

100. *d r I I I* h

where *I*1 is the intensity of the incident beam, *Id* is the intensity of the diffracted beam and *Ir*

Figure 3 shows the experimental set up for measuring the diffraction efficiencies using a Helium-Neon laser beam that has a much smaller diameter than that of the HOE. The Helium-Neon laser was positioned so that it probed the centre of the HOE that was recorded on the photopolymer layer. The photopolymer is not sensitive to light of wavelength 633 nm, therefore further polymerisation does not occur. The photopolymer sample was rotated until the maximum diffracted intensity of the laser beam was observed on a screen behind the photopolymer. The angle of incidence of the probe beam at which the maximum diffraction

= ´ - (1)

(M1, M2, M3, M4 and M5).

polymer surface.

lines/mm and M5 -1700 lines/mm.

HOEs at different spatial frequencies.

**6.5. Characterisation of the recorded HOEs**

136 Holography – Basic Principles and Contemporary Applications

#### **7.1. Recording of focusing HOEs**

The diffraction efficiency of a single lens recorded in a red sensitive layer of thickness of 50 μm as a function of recording time is presented in Fig. 4. It is seen that the maximum diffraction efficiency is nearly 45 % and it is reached after 100 s exposure time.

**7.2. Recording of three multiplexed holographic gratings**

20

30

Diffraction

of the diffraction efficiencies is developed.

100 μm.

efficiency,%

40

50

60

70

Initial experiments were carried out in layers of thickness of 100 μm. In order to find the optimum exposure times required to obtain gratings with diffraction efficiency above 50 %, gratings were separately recorded at 1000, 1500 and 2000 l/mm. At the next stage of the experiment, three gratings of spatial frequencies 2000, 1500 and 1000 l/mm were recorded in the same volume of the recording medium, first by using the measured recording times required to achieve 50% diffraction efficiency at each of the spatial frequencies. This produced gratings with unequal diffraction efficiencies. In order to equalize the diffraction efficiency of the gratings the recording time was varied. The diffraction efficiencies of previously recorded gratings were measured after each exposure to observe how the recording of the gratings at

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1 2 3

Exposure number

**Figure 6.** Diffraction efficiency of three gratings multiplexed in the same region of a photopolymer layer of thickness

It is seen from Fig. 6 that the recording of the grating at 1500 lines/mm had a large effect on the diffraction efficiency of the grating previously recorded at 2000 lines/mm. The diffraction efficiency at 2000 lines/mm increased by 23.8% after the recording of the second grating at 1500 lines/mm. It increased a further 14% due to the recording of the third grating at 1000 lines/mm. This shows that the recording of a grating affects the diffraction efficiency of a previously recorded grating. This must be taken into account when the exposure schedule for equalization

This procedure was repeated several times by varying the exposure times, but it was not possible to equalise the diffraction efficiencies of the three gratings and to achieve diffraction efficiency above 50% for all three of them.. From Fig. 6 is seen that the maximum diffraction

1500lines/mm and 1000 lines/mm affected the grating recorded at 2000 lines/mm.

 2000 l/mm 1500 l/mm 1000 l/mm

**Figure 4.** Diffraction efficiencies of single lenses recorded in 50 micrometer layers. Recording wavelength was 633 nm. Recording frequency was 650 l/mm and recording intensity was 1 mW/cm2.

Much higher diffraction efficiency was achieved in green sensitised layers of thickness 50 μm (Fig.5). The total recording intensity of the beams was 1 mW/cm2 . The spatial frequency of recording was 300 l/mm.

**Figure 5.** Diffraction efficiency of gratings recorded in 50 μm layers. Recording wavelength was 532 nm.

#### **7.2. Recording of three multiplexed holographic gratings**

0 50 100 150 200

**Figure 4.** Diffraction efficiencies of single lenses recorded in 50 micrometer layers. Recording wavelength was 633

Much higher diffraction efficiency was achieved in green sensitised layers of thickness 50 μm

0 50 100 150 200

**Figure 5.** Diffraction efficiency of gratings recorded in 50 μm layers. Recording wavelength was 532 nm.

Exposure time, s

. The spatial frequency of

Exposure time, s

0

Diffraction

efficiency,%

nm. Recording frequency was 650 l/mm and recording intensity was 1 mW/cm2.

(Fig.5). The total recording intensity of the beams was 1 mW/cm2

10

Diffraction

recording was 300 l/mm.

efficiency,%

138 Holography – Basic Principles and Contemporary Applications

20

30

40

50

Initial experiments were carried out in layers of thickness of 100 μm. In order to find the optimum exposure times required to obtain gratings with diffraction efficiency above 50 %, gratings were separately recorded at 1000, 1500 and 2000 l/mm. At the next stage of the experiment, three gratings of spatial frequencies 2000, 1500 and 1000 l/mm were recorded in the same volume of the recording medium, first by using the measured recording times required to achieve 50% diffraction efficiency at each of the spatial frequencies. This produced gratings with unequal diffraction efficiencies. In order to equalize the diffraction efficiency of the gratings the recording time was varied. The diffraction efficiencies of previously recorded gratings were measured after each exposure to observe how the recording of the gratings at 1500lines/mm and 1000 lines/mm affected the grating recorded at 2000 lines/mm.

**Figure 6.** Diffraction efficiency of three gratings multiplexed in the same region of a photopolymer layer of thickness 100 μm.

It is seen from Fig. 6 that the recording of the grating at 1500 lines/mm had a large effect on the diffraction efficiency of the grating previously recorded at 2000 lines/mm. The diffraction efficiency at 2000 lines/mm increased by 23.8% after the recording of the second grating at 1500 lines/mm. It increased a further 14% due to the recording of the third grating at 1000 lines/mm. This shows that the recording of a grating affects the diffraction efficiency of a previously recorded grating. This must be taken into account when the exposure schedule for equalization of the diffraction efficiencies is developed.

This procedure was repeated several times by varying the exposure times, but it was not possible to equalise the diffraction efficiencies of the three gratings and to achieve diffraction efficiency above 50% for all three of them.. From Fig. 6 is seen that the maximum diffraction efficiencies measured at spatial frequencies of 1500 lines/mm and 2000 lines/mm were greater than the set target of 50%. The lower diffraction efficiency at 1000lines/mm was assumed to be due to the dynamic range of the photopolymer layer being consumed.

The results of these experiments are represented in Fig. 8.

450 l/mm

1065 l/mm

**Exposure time, s**

1 450 1 10 12 3

2 1065 3 30 30 7.5 3 1295 5 50 60 15

4 1470 15 150 150 37.5

5 1700 19 190 188 47

layer is insufficient to produce all five gratings with high diffraction efficiency..

From Fig.8 it is seen that recording the gratings with constant intensity produced less variation in the diffraction efficiencies of the recorded gratings. In both cases the dynamic range of the

**Table 1.** Exposure time and intensity for recording of the five multiplexed gratings.

1295 l/mm

**Frequency l/mm**

Diffraction

**Grating No. Spatial**

efficiency,%

0 50 100 150 200

Exposure, mJ/cm2

**Exposure energy mJ/cm2**

**Figure 8.** Diffraction efficiencies of five multiplexed gratings recorded by using two exposure schedules- constant ex‐ posure intensity of 10 mW/cm2 (black symbols) with variable exposure time and constant exposure time of 4s (red symbols) with variable intensity. The order of recording and the exposure times and intensities are given in Table 1.

constant recording intensity 10mW/cm2

Photopolymer Holographic Optical Elements for Application in Solar Energy Concentrators

1470 l/mm

**Constant Intensity of 10mW/cm2 Constant Exposure time of 4 s**

**Exposure energy mJ/cm2** **Exposure Intensity, mW/cm2**

http://dx.doi.org/10.5772/55109

141

1700 l/mm

constant recording time 4s

In order to achieve gratings with diffraction efficiency higher than 50% layers with greater thickness and dynamic range were prepared. The next set of gratings was recorded in layers of 120 μm thickness.

**Figure 7.** Diffraction efficiency of three gratings multiplexed in the same region of a photopolymer layer of thickness 120 μm.

It can be observed in Fig.7 that the three gratings of different spatial frequencies were suc‐ cessfully recorded with nearly equal diffraction efficiencies around 50%. This confirmed the assumption that in the thinner layers the main obstacle to equalising the gratings with diffraction efficiencies above 50% was insufficient dynamic range of the photopolymer layer. The order of recording and corresponding exposure times for the three gratings were 450, 1065 and 1700 l/mm and 2, 4 and 18 seconds.

#### **7.3. Recording of five multiplexed holographic gratings**

After successfully multiplexing three gratings in the same layer the next step was to try to increase the number of holograms and in this way to increase the number of solar light incidence angles that can be exploited. The aim of this experiment was to obtain optimum diffraction efficiencies from five gratings recorded in the same volume of the recording medium. This was achieved by using two different recording conditions, constant intensity with variable expsosure time and constant exposure time with variable intensity. The experimental set-up used was as shown in Fig. 2. The diffraction efficiencies were measured using a He-Ne laser.

The results of these experiments are represented in Fig. 8.

efficiencies measured at spatial frequencies of 1500 lines/mm and 2000 lines/mm were greater than the set target of 50%. The lower diffraction efficiency at 1000lines/mm was assumed to be

In order to achieve gratings with diffraction efficiency higher than 50% layers with greater thickness and dynamic range were prepared. The next set of gratings was recorded in layers

400<PHONE_NUMBER> 1200 1400 1600 1800

Spatial frequency of the grating, l/mm

**Figure 7.** Diffraction efficiency of three gratings multiplexed in the same region of a photopolymer layer of thickness

It can be observed in Fig.7 that the three gratings of different spatial frequencies were suc‐ cessfully recorded with nearly equal diffraction efficiencies around 50%. This confirmed the assumption that in the thinner layers the main obstacle to equalising the gratings with diffraction efficiencies above 50% was insufficient dynamic range of the photopolymer layer. The order of recording and corresponding exposure times for the three gratings were 450, 1065

Aftersuccessfullymultiplexingthreegratingsinthesamelayerthenextstepwastotrytoincrease the number of holograms and in this way to increase the number of solar light incidence angles that can be exploited. The aim of this experiment was to obtain optimum diffraction efficiencies from five gratings recorded in the same volume of the recording medium. This was achieved by using two different recording conditions, constant intensity with variable expsosure time and constant exposure time with variable intensity. The experimental set-up used was as shown in

due to the dynamic range of the photopolymer layer being consumed.

0

10

20

Diffraction

and 1700 l/mm and 2, 4 and 18 seconds.

**7.3. Recording of five multiplexed holographic gratings**

Fig. 2. The diffraction efficiencies were measured using a He-Ne laser.

efficiency,%

30

40

50

60

70

140 Holography – Basic Principles and Contemporary Applications

of 120 μm thickness.

120 μm.

**Figure 8.** Diffraction efficiencies of five multiplexed gratings recorded by using two exposure schedules- constant ex‐ posure intensity of 10 mW/cm2 (black symbols) with variable exposure time and constant exposure time of 4s (red symbols) with variable intensity. The order of recording and the exposure times and intensities are given in Table 1.

**Table 1.** Exposure time and intensity for recording of the five multiplexed gratings.

From Fig.8 it is seen that recording the gratings with constant intensity produced less variation in the diffraction efficiencies of the recorded gratings. In both cases the dynamic range of the layer is insufficient to produce all five gratings with high diffraction efficiency..

#### **7.4. Recording of five multiplexed holographic lenses**

The method of varying the exposure time for the recording of the gratings was repeated with a cylindrical lens in the path of the beam transmitted by the beamsplitter focussing the beam into a long narrow line. The diffraction efficiencies were measured using the He-Ne laser and compared with the diffraction efficiencies measured for gratings recorded under the same conditions with two collimated beams (Fig. 9).

400<PHONE_NUMBER> 1200 1400 1600 1800

Photopolymer Holographic Optical Elements for Application in Solar Energy Concentrators

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143

Spatial frequency of the grating, l/mm

It was demonstrated that high diffraction efficiency HOE consisting of a single spherical lens can be recorded in a relatively thin photopolymer layer of 50 μm thickness. The advantage of using thin layers and lower spatial frequency of recording in this application is the larger

It was possible to equalise the diffraction efficiencies of three multiplexed gratings at 51.9±3.5 %. A study of the influence of the exposure schedule – keeping the intensity constant and changing the time or keeping the exposure time constant and varying the intensity, revealed that the

Three HOEs - containing five gratings with a range of spatial frequencies from 450 to 1700 l/mm, five cylindrical lenses and four cylindrical lenses, were successfully recorded in the same photopolymer layer. These can be considered as successful first steps in the design and fabrication of holographic solar concentrators fabricated in acrylamide based photopolymer.

10

**Figure 10.** Diffraction efficiencies of multiplexed holographic cylindrical lenses.

first schedule delivered better equalisation of the diffraction efficiency.

20

30

Diffraction

acceptance angle of the optical component.

**8. Conclusions**

efficiency,%

40

50

60

70

80

**Figure 9.** Comparison of the diffraction efficiencies of HOEs of five multiplexed gratings (squares) and five multi‐ plexed lenses (circles).

From Fig. 9 it is seen that the diffraction efficiencies of the first three gratings (recorded at low spatial frequencies) were around the same value above 50% and that the diffraction efficiency is lower for the last two gratings recorded at larger spatial frequencies.

In a further experiment just four lenses instead of five were recorded to reduce the risk of the dynamic range being consumed. The recording conditions were optimised again (Table 2) and are presented in Fig. 10.

**Table 2.** Recording conditions for multiplexing of four cylindrical lenses in a 120 μm thick photopolymer layer.

**Figure 10.** Diffraction efficiencies of multiplexed holographic cylindrical lenses.

## **8. Conclusions**

**7.4. Recording of five multiplexed holographic lenses**

conditions with two collimated beams (Fig. 9).

142 Holography – Basic Principles and Contemporary Applications

0

10

20

Diffraction

plexed lenses (circles).

are presented in Fig. 10.

efficiency,%

30

40

50

60

450 l/mm

70

The method of varying the exposure time for the recording of the gratings was repeated with a cylindrical lens in the path of the beam transmitted by the beamsplitter focussing the beam into a long narrow line. The diffraction efficiencies were measured using the He-Ne laser and compared with the diffraction efficiencies measured for gratings recorded under the same

1295 l/mm

1065 l/mm

is lower for the last two gratings recorded at larger spatial frequencies.

0 50 100 150 200

Exposure, mJ/cm2

**Figure 9.** Comparison of the diffraction efficiencies of HOEs of five multiplexed gratings (squares) and five multi‐

From Fig. 9 it is seen that the diffraction efficiencies of the first three gratings (recorded at low spatial frequencies) were around the same value above 50% and that the diffraction efficiency

In a further experiment just four lenses instead of five were recorded to reduce the risk of the dynamic range being consumed. The recording conditions were optimised again (Table 2) and

**Lenses No. Spatial Frequency l/mm Exposure energy mJ/cm2 Exposure Intensity,**

 450 6 1.5 1065 13 3.2 1295 60 15 1700 160 40

**Table 2.** Recording conditions for multiplexing of four cylindrical lenses in a 120 μm thick photopolymer layer.

1470 l/mm

1700 l/mm

**Constant Exposure time of 4 s**

**mW/cm2**

It was demonstrated that high diffraction efficiency HOE consisting of a single spherical lens can be recorded in a relatively thin photopolymer layer of 50 μm thickness. The advantage of using thin layers and lower spatial frequency of recording in this application is the larger acceptance angle of the optical component.

It was possible to equalise the diffraction efficiencies of three multiplexed gratings at 51.9±3.5 %.

A study of the influence of the exposure schedule – keeping the intensity constant and changing the time or keeping the exposure time constant and varying the intensity, revealed that the first schedule delivered better equalisation of the diffraction efficiency.

Three HOEs - containing five gratings with a range of spatial frequencies from 450 to 1700 l/mm, five cylindrical lenses and four cylindrical lenses, were successfully recorded in the same photopolymer layer. These can be considered as successful first steps in the design and fabrication of holographic solar concentrators fabricated in acrylamide based photopolymer.

## **Author details**

Izabela Naydenova1 , Hoda Akbari1 , Colin Dalton1 , Mohamed Yahya so Mohamed Ilyas2 , Clinton Pang Tee Wei2 , Vincent Toal1 and Suzanne Martin1

1 Centre for Industrial and Engineering Optics, School of Physics, College of Sciences and Health, Dublin Institute of Technology, Kevin Street, Dublin, Ireland

[10] D. Zhang, J.M. Castro and R.K. Kostuk, "One-axis tracking holographic planar con‐

Photopolymer Holographic Optical Elements for Application in Solar Energy Concentrators

http://dx.doi.org/10.5772/55109

145

[11] A. Kumar, N. Deo, H.L. Yadav, "Analysis of design parameters for wavelength selec‐ tive holographic solar concentrators", IEEE conference paper 978-1-4244-1641 (2008)

[12] O. Iurevych, S. Gubin, M. Dudeck, "Combined receiver of solar radiation with holo‐ graphic planar concentrator", IOP Conference paper, Materials Science and Engi‐

[13] S. Sam Tl, A. Kumar PT, "Design and Optimization of Photopolymer Based Holo‐ graphic Solar Concentrators", American Institute of Physics conference *Optics: Phe‐ nomena, Materials, Devices, and Characterization,* Conf. Proc. 1391, 248-250 (2011); doi:

[14] M. Hsieh, S. Lin, K. Y. Hsu, J. Burr, and S. Lin, "An Efficient Solar Concentrator using Volume hologram", in CLEO:2011 - Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB8, http://

[15] S. Martin, C.A. Feely and V. Toal, "Holographic recording characteristics of an acryl‐

www.opticsinfobase.org/abstract.cfm?URI=CLEO: A and T-2011-PDPB8

amide-based photopolymer", Appl. Optics 36, 5757-5768, (1997)

centrator system", Journal of Photonics for Energy, 1947-7988 (2011)

neering 29 (2012) 012016, doi:10.1088/1757-899X/29/1/012016

10.1063/1.3646844

2 School of Chemical & Life Sciences, Singapore Polytechnic, Singapore

## **References**

[10] D. Zhang, J.M. Castro and R.K. Kostuk, "One-axis tracking holographic planar con‐ centrator system", Journal of Photonics for Energy, 1947-7988 (2011)

**Author details**

Izabela Naydenova1

**References**

Council ) June 2011

538, (2011)

3739-3742 (1982)

649, 64901 (2007)

33, 22, 2614 (2008)

Holography, 13.2, p.5 (2002)

Clinton Pang Tee Wei2

, Hoda Akbari1

144 Holography – Basic Principles and Contemporary Applications

, Vincent Toal1

Health, Dublin Institute of Technology, Kevin Street, Dublin, Ireland

2 School of Chemical & Life Sciences, Singapore Polytechnic, Singapore

, Colin Dalton1

and Suzanne Martin1

[1] U.S. Solar market Trends 2010, Larry Sherwood, IREC (Interstate Renewable Energy

[2] Daniel Kraemer, Bed Poudel, Hsien-Ping Feng, J. Christopher Caylor, Bo Yu, Xiao Yan, Yi Ma, Xiaowei Wang, Dezhi Wang, Andrew Muto, Kenneth McEnaney, Matteo Chiesa, Zhifeng Ren & Gang Chen , "High-performance flat-panel solar thermoelec‐ tric generators with high thermal concentration", Nature Materials, Volume 10, 532 –

[3] Bloss, W. H., Griesinger, M. and Reinhardt, E. R., "Dispersive concentrating systems based on transmission phase holograms for solar applications", Appl. Opt., 21,

[4] C.G. Stojanoff, "Holographic optical elements for window shading in buildings",

[5] P.A.B. James, A.S. Bahaj, "Holographic optical elements: various principles for solar

[6] Ludman, J.E., Riccobono, J., Semenova, I.V., Reinhand, N.O., Tai, W., Li, X., Syphers, G., Rallis, E., Sliker, G., Martin, J., "The optimization of a holographic system for so‐

[7] R. K. Kostuk, and G. Rosenberg, "Analysis and design of holographic solar concen‐

[8] R.K. Kostuk, J. Castillo, and J.M. Russo and G. Rosenberg "Spectral-shifting and holo‐ graphic planar concentrators for use with photovoltaic solar cells", Proc. of SPIE vol

[9] P. Macko and M.P. Whelan, "Fabrication of holographic diffractive optical elements for enhancing light collection from fluorescence-based biochips", Optics Letters, vol.

control of conservatories and sunrooms", Solar Energy 78, 441- 454 (2005)

lar power generation", Solar Energy 60(1), 1-9 (1996)

trators", Proc. SPIE 7043, 70430I (2008)

1 Centre for Industrial and Engineering Optics, School of Physics, College of Sciences and

, Mohamed Yahya so Mohamed Ilyas2

,

**Chapter 7**

**Optically Accelerated Formation of One- and**

**Two-Dimensional Holographic Surface Relief**

Polymer-based periodic structures nowadays are very useful for many applications in optoelectronics and photonics, such as optical filters, distributed-feedback laser, wave‐ guide coupling, photonic crystals, etc. [1-3]. Different methods based on photoinduced transformation of photosensitive materials have been used to fabricate the desired period‐ ic structures. A common point of these fabrication technologies is the use of a so-called photoresist to record the light pattern following by a development process. The later one allows to wash out the remaining unpolymerized photoresists, resulting in a polymerized structure, which is a duplication of the light pattern. However, development techniques based on solvent dissolutions affect the fabrication process, leading often to a deforma‐ tion of the fabricated structures. Recently, new organic materials named azobenzenecontaining polymers (azopolymers) have attracted much attention for the fabrication of submicrometer structures, thanks to a particular mass transport effect under a modulated light irradiation. This phenomenon allows to fabricate a so-called surface relief grating (SRG) by a simple one-step process, without development process [4-5]. Figure 1 shows the chemi‐ cal structure of one typical commercial azopolymer named Disperse Red 1-poly-methylmethacrylate (DR1/PMMA). Here, the DR1 molecule is attached to the polymer backbone MMA monomers through covalent bonds with a 30/70 molar ratio between them. This copolymer presents very good thermal and temporal stability with high glass transition temperature (Tg = 125°C). Experimental results have been shown that the SRG formation

> © 2013 Wu et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Wu et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

**Gratings on DR1/PMMA**

Xiao Wu, Thi Thanh Ngan Nguyen,

Ngoc Diep Lai

**1. Introduction**

http://dx.doi.org/10.5772/53788

Isabelle Ledoux-Rak, Chi Thanh Nguyen and

Additional information is available at the end of the chapter

## **Optically Accelerated Formation of One- and Two-Dimensional Holographic Surface Relief Gratings on DR1/PMMA**

Xiao Wu, Thi Thanh Ngan Nguyen, Isabelle Ledoux-Rak, Chi Thanh Nguyen and Ngoc Diep Lai

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/53788

## **1. Introduction**

Polymer-based periodic structures nowadays are very useful for many applications in optoelectronics and photonics, such as optical filters, distributed-feedback laser, wave‐ guide coupling, photonic crystals, etc. [1-3]. Different methods based on photoinduced transformation of photosensitive materials have been used to fabricate the desired period‐ ic structures. A common point of these fabrication technologies is the use of a so-called photoresist to record the light pattern following by a development process. The later one allows to wash out the remaining unpolymerized photoresists, resulting in a polymerized structure, which is a duplication of the light pattern. However, development techniques based on solvent dissolutions affect the fabrication process, leading often to a deforma‐ tion of the fabricated structures. Recently, new organic materials named azobenzenecontaining polymers (azopolymers) have attracted much attention for the fabrication of submicrometer structures, thanks to a particular mass transport effect under a modulated light irradiation. This phenomenon allows to fabricate a so-called surface relief grating (SRG) by a simple one-step process, without development process [4-5]. Figure 1 shows the chemi‐ cal structure of one typical commercial azopolymer named Disperse Red 1-poly-methylmethacrylate (DR1/PMMA). Here, the DR1 molecule is attached to the polymer backbone MMA monomers through covalent bonds with a 30/70 molar ratio between them. This copolymer presents very good thermal and temporal stability with high glass transition temperature (Tg = 125°C). Experimental results have been shown that the SRG formation

© 2013 Wu et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Wu et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

on an azopolymer film depends on different parameters: sample temperature, light polarization, intensity of the interference beams, etc. Many theoretical models have been proposed to explain the mechanism of SRG formation [6-10] on DR1/PMMA azopolymer. Despite these efforts, the mechanism is still not fully explained. The most plausible and widely used model is the gradient force model [7]. According to this model, the pressure and resultant force can be achieved thanks to the light irradiation, which induces a *trans*form ↔ *cis*-form photoisomerization effect in azobenzene molecules. Inset of the Fig. 1 illustrates the photoisomerization process between the *trans*-form and *cis*-form of a DR1 chromophore molecule. At room temperature, the *trans*-form is fundamentally stable as compared with the *cis*-form. When the DR1 molecule is subjected to a visible illumina‐ tion, the *trans*-form will be transformed to a *cis*-form, which later relaxes back to the *trans*-form by thermal relaxation (slow process) or by UV irradiation (fast process). Under a spatially modulated irradiation, the DR1 molecules, after many *trans*-form ↔ *cis*-form cycles, will move from the position of highest intensity to the position at which the light intensity is lowest. Therefore, the necessary condition to form the SRG is a photoisomeriza‐ tion process under the irradiation of a modulated light intensity.

a VIS beam (532 nm-wavelength), of the same wavelength as the interference beams, is used to accelerate the photoisomerization from the *trans*-form to the *cis*-form. Both techniques allowed to enhance the SRG formation, resulted in a better DE and a larger depth of the SRGs. The mechanisms of these two methods are different but both of them are polarization de‐

Optically Accelerated Formation of One- and Two-Dimensional Holographic Surface Relief Gratings on DR1/PMMA

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149

**Figure 1.** Chemical structure of the DR1/PMMA copolymer. Insert shows the *trans*-form↔*cis*-form photoisomeriza‐

Laser interference technologies have been widely used to fabricate polymer-based periodic and quasi-periodic micro- and nano-structures with large areas. We propose to use the twobeam interference technique to fabricate 1D and 2D SRG structures by one- and two-exposure. Indeed, two-beam interference is the simplest configuration (Fig. 2(a)), which allows to create

Considering two-beam interference as illustrated in Fig. 2(a). Two coherent laser beams of the same profiles and same intensities and coming from the same laser source propagate toward the same sample area. These laser beams are symmetrically oriented around the sample normal direction and make an angle θ with respect to this axis. The electric fields, **E**1(**r**, t) and **E**2(**r**, t),

of the two plane waves corresponding to these two laser beams are given by

tion process induced by different optical excitations and thermal relaxation.

a spatial modulation of light intensity in one dimension.

**2. Two-beam interference**

**2.1. Theory of two-beam interference**

Finally, we will make some conclusions of our work and also release some prospects.

pendent.

This chapter presents the fabrication of one- and two-dimensional (1D and 2D) holographic SRGs based on one- or multi-exposure of the azobenzene copolymer (DR1/PMMA) to twoand/or three-beam interference patterns. A single exposure of the two-beam interference pattern is used to form 1D SRG. 2D periodic structure then can be fabricated either by two exposures of the sample to a two-beam interference pattern or by only a single exposure to a three-beam interference pattern. Also, the formation of SRGs is optically accelerated by assisting it with an independent UV or VIS laser.

Section 2 shows the fabrication of SRG by using the two-beam interference technique. Because of the important role of the intensity and polarization modulations during the formation of SRG, the theory of two-beam interference will be presented in detail. The diffraction efficiency (DE) and the depth of SRGs fabricated with different polarization configurations have been characterized and discussed. The relationship between the depth and period of SRG was also studied and reported. The 2D holographic SRG is realized by using two exposures to the twobeam interference pattern. This fabrication method is simple and easy to control the SRG periodicity, but it is time consuming and the 2D SRG structures are not symmetric.

In order to rapidly fabricate uniform and symmetric 2D SRG structures, the three-beam interference technique has been used as shown in Section 3. The theory of three-beam interference is first studied, analysing in particular the influence of polarizations of the three laser beams on the intensity and polarization modulations. Then, 2D hexagonal structures have been fabricated by this fabrication technique with a particular polariza‐ tion configuration (circular-circular-circular). Comparison with the results obtained by the multiple-exposure two-beam interference technique and discussions of the advantages of this technique will also be presented.

In Section 4, we investigate experimentally the optimization of 1D SRG formation by using an additional laser to assist the photoisomerization process. First, an UV beam (355 nm-wave‐ length) is used to accelerate the photoisomerization from the *cis*-form to the *trans*-form. Second, a VIS beam (532 nm-wavelength), of the same wavelength as the interference beams, is used to accelerate the photoisomerization from the *trans*-form to the *cis*-form. Both techniques allowed to enhance the SRG formation, resulted in a better DE and a larger depth of the SRGs. The mechanisms of these two methods are different but both of them are polarization de‐ pendent.

Finally, we will make some conclusions of our work and also release some prospects.

**Figure 1.** Chemical structure of the DR1/PMMA copolymer. Insert shows the *trans*-form↔*cis*-form photoisomeriza‐ tion process induced by different optical excitations and thermal relaxation.

## **2. Two-beam interference**

on an azopolymer film depends on different parameters: sample temperature, light polarization, intensity of the interference beams, etc. Many theoretical models have been proposed to explain the mechanism of SRG formation [6-10] on DR1/PMMA azopolymer. Despite these efforts, the mechanism is still not fully explained. The most plausible and widely used model is the gradient force model [7]. According to this model, the pressure and resultant force can be achieved thanks to the light irradiation, which induces a *trans*form ↔ *cis*-form photoisomerization effect in azobenzene molecules. Inset of the Fig. 1 illustrates the photoisomerization process between the *trans*-form and *cis*-form of a DR1 chromophore molecule. At room temperature, the *trans*-form is fundamentally stable as compared with the *cis*-form. When the DR1 molecule is subjected to a visible illumina‐ tion, the *trans*-form will be transformed to a *cis*-form, which later relaxes back to the *trans*-form by thermal relaxation (slow process) or by UV irradiation (fast process). Under a spatially modulated irradiation, the DR1 molecules, after many *trans*-form ↔ *cis*-form cycles, will move from the position of highest intensity to the position at which the light intensity is lowest. Therefore, the necessary condition to form the SRG is a photoisomeriza‐

This chapter presents the fabrication of one- and two-dimensional (1D and 2D) holographic SRGs based on one- or multi-exposure of the azobenzene copolymer (DR1/PMMA) to twoand/or three-beam interference patterns. A single exposure of the two-beam interference pattern is used to form 1D SRG. 2D periodic structure then can be fabricated either by two exposures of the sample to a two-beam interference pattern or by only a single exposure to a three-beam interference pattern. Also, the formation of SRGs is optically accelerated by

Section 2 shows the fabrication of SRG by using the two-beam interference technique. Because of the important role of the intensity and polarization modulations during the formation of SRG, the theory of two-beam interference will be presented in detail. The diffraction efficiency (DE) and the depth of SRGs fabricated with different polarization configurations have been characterized and discussed. The relationship between the depth and period of SRG was also studied and reported. The 2D holographic SRG is realized by using two exposures to the twobeam interference pattern. This fabrication method is simple and easy to control the SRG

In order to rapidly fabricate uniform and symmetric 2D SRG structures, the three-beam interference technique has been used as shown in Section 3. The theory of three-beam interference is first studied, analysing in particular the influence of polarizations of the three laser beams on the intensity and polarization modulations. Then, 2D hexagonal structures have been fabricated by this fabrication technique with a particular polariza‐ tion configuration (circular-circular-circular). Comparison with the results obtained by the multiple-exposure two-beam interference technique and discussions of the advantages of

In Section 4, we investigate experimentally the optimization of 1D SRG formation by using an additional laser to assist the photoisomerization process. First, an UV beam (355 nm-wave‐ length) is used to accelerate the photoisomerization from the *cis*-form to the *trans*-form. Second,

periodicity, but it is time consuming and the 2D SRG structures are not symmetric.

tion process under the irradiation of a modulated light intensity.

assisting it with an independent UV or VIS laser.

148 Holography – Basic Principles and Contemporary Applications

this technique will also be presented.

Laser interference technologies have been widely used to fabricate polymer-based periodic and quasi-periodic micro- and nano-structures with large areas. We propose to use the twobeam interference technique to fabricate 1D and 2D SRG structures by one- and two-exposure. Indeed, two-beam interference is the simplest configuration (Fig. 2(a)), which allows to create a spatial modulation of light intensity in one dimension.

#### **2.1. Theory of two-beam interference**

Considering two-beam interference as illustrated in Fig. 2(a). Two coherent laser beams of the same profiles and same intensities and coming from the same laser source propagate toward the same sample area. These laser beams are symmetrically oriented around the sample normal direction and make an angle θ with respect to this axis. The electric fields, **E**1(**r**, t) and **E**2(**r**, t), of the two plane waves corresponding to these two laser beams are given by

**Figure 2.** Optical arrangement of two-beam (a) and three-beam interference (b) techniques. Each laser beam is sym‐ metrically oriented around the vertical axis by an angle θ. **k**i (i = 1, 2, 3) represents the wave vector of each laser beam.

$$\mathbf{E}\_1(\mathbf{r}, t) = \text{Re}\left[E\_{01} \exp\left(i\left(\mathbf{k}\_1 \cdot \mathbf{r} - \alpha t\right)\right) \mathbf{e}\_1\right],\tag{1}$$

In this work, all numerical calculations were realized by using Matlab software with personal codes, which allowed to investigate all properties of the interference pattern as a function of

Optically Accelerated Formation of One- and Two-Dimensional Holographic Surface Relief Gratings on DR1/PMMA

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151

Figure 3 shows the simulation results of the intensity modulation of the two-beam interference pattern, obtained with different polarization configurations and at different incident angles θ. We have considered four particular polarization configurations of the two laser beams: S-S, P-P, S-P, and RC-LC (right circular and left circular). Since the interference cannot be realized with S-P polarization configuration, this particular case is not shown here. For the three other polarization configurations, we can see that intensity modulation depends not only on the polarization configuration, but also on the incident angle θ. Namely, for a S-S polarization configuration, the polarizations are the same for both laser beams at any angle θ, resulting in a maximal amplitude modulation (100%) of the intensity interference pattern. However, the amplitude modulation increases from 0 to 100% when θ increases in the case of RC-LC polarization configuration. Also, for the P-P configuration, the amplitude modulation firstly decreases then increases when θ varies from 0° to 90°. We note that for the last case, there is a position shifting the interference pattern by a distance of Λ/2 along *x*-axis for θ = 45°, but this

In order to evaluate the influence of polarizations configurations on the intensity modulation,

min are the maximum and minimum of the intensity, respectively.

Figure 4 shows the interference contrast as a function of θ (from 0° to 85°) for different polarization configurations. It is clear that the interference contrast is constant for S-S polari‐ zation configuration and keeps increasing with θ for the RC-LC polarization configuration. On the other hand, for the P-P case, the interference contrast decreases from 100% to 0 when the incident angle is varied from 0° to 45°, then increases again from 0 to 100% when θ increases from 45° to 90°. The reason is that in the case of P-P polarization configuration, two polariza‐ tions tend to be orthogonal when θ approaches 45°. At this particular angle, two polarizations are perpendicular and there is no interference. Therefore, for a standard optical lithography, a two-beam interference pattern with S-S polarization configuration is used in order to ensure

However, when the two-beam interference technique is applied to SRG formation, the modulation condition of the interference intensity is not enough, since the photoisomerization of DR1 molecules depends also on the illuminated light polarization. The polarization

( ) ( ) max min max min C , *TT TT* =- + *II II* (6)

we introduce a well-known parameter called visibility (interference contrast), C

different polarizations of the laser beams.

does not affect the fabrication of SRGs.

the best intensity modulation contrast.

*2.1.2. Polarization modulation*

where *IT*

max and *IT*

*2.1.1. Intensity distribution*

$$\mathbf{E}\_2\left(\mathbf{r}, t\right) = \text{Re}\left[E\_{02} \exp\left(i\left(\mathbf{k}\_2 \cdot \mathbf{r} - at\right)\right)\mathbf{e}\_2\right],\tag{2}$$

where **k** 1 and **k** 2 are the corresponding wave vectors, **r** is the position vector in the overlapping space, *E*01 and *E*02 are the real electric field amplitudes, and **e**1 and **e**2 are the unit vectors of the polarizations. At the overlapping area of the two beams, the interference electric field is the sum of the electric fields of individual plane waves

$$\mathbf{E}\_{\rm T}(\mathbf{r},t) = \mathbf{E}\_{\rm 1}(\mathbf{r},t) + \mathbf{E}\_{\rm 2}(\mathbf{r},t). \tag{3}$$

The interference intensity distribution of the resultant wave is given by

$$I\_T\left(\mathbf{r}, t\right) = \left<\mathbf{E}\_T^\*\left(\mathbf{r}, t\right) \bullet \mathbf{E}\_T\left(\mathbf{r}, t\right)\right>\_{t'} \tag{4}$$

where ... *t* represents the time average of the resultant electric field.

Using equations (3) and (4), we can easily calculate both the intensity distribution and the polarization distribution of the interference pattern. The periodicity of the interference pattern depends on the angle θ and the laser wavelength (λ), and it is determined by

$$
\Lambda = \frac{\lambda}{2\sin\theta}.\tag{5}
$$

In this work, all numerical calculations were realized by using Matlab software with personal codes, which allowed to investigate all properties of the interference pattern as a function of different polarizations of the laser beams.

### *2.1.1. Intensity distribution*

1 ( ) 01 1 ( ( )) 1 , Re exp *tEi t* = é ù -

**Figure 2.** Optical arrangement of two-beam (a) and three-beam interference (b) techniques. Each laser beam is sym‐

2 ( ) 02 ( ( 2 )) 2 , Re exp *tEi t* = é ù -

the polarizations. At the overlapping area of the two beams, the interference electric field is

Using equations (3) and (4), we can easily calculate both the intensity distribution and the polarization distribution of the interference pattern. The periodicity of the interference pattern

> . 2sin l

q

space, *E*01 and *E*02 are the real electric field amplitudes, and **e**1

The interference intensity distribution of the resultant wave is given by

where ... *t* represents the time average of the resultant electric field.

depends on the angle θ and the laser wavelength (λ), and it is determined by

the sum of the electric fields of individual plane waves

metrically oriented around the vertical axis by an angle θ. **k**i

150 Holography – Basic Principles and Contemporary Applications

where **k** 1 and **k** 2 w

w

are the corresponding wave vectors, **r** is the position vector in the overlapping

, ë û **E r kr e** g (1)

, ë û **E r kr e** g (2)

and **e**2

(i = 1, 2, 3) represents the wave vector of each laser beam.

( ) ( ) ( ) 1 2 , , ,. *T* **E r Er Er** *tt t* = + (3)

*T TT* ( , , ,, ) ( ) ( ) *t It t t* \* **r Er Er** = (4)

L = (5)

are the unit vectors of

Figure 3 shows the simulation results of the intensity modulation of the two-beam interference pattern, obtained with different polarization configurations and at different incident angles θ. We have considered four particular polarization configurations of the two laser beams: S-S, P-P, S-P, and RC-LC (right circular and left circular). Since the interference cannot be realized with S-P polarization configuration, this particular case is not shown here. For the three other polarization configurations, we can see that intensity modulation depends not only on the polarization configuration, but also on the incident angle θ. Namely, for a S-S polarization configuration, the polarizations are the same for both laser beams at any angle θ, resulting in a maximal amplitude modulation (100%) of the intensity interference pattern. However, the amplitude modulation increases from 0 to 100% when θ increases in the case of RC-LC polarization configuration. Also, for the P-P configuration, the amplitude modulation firstly decreases then increases when θ varies from 0° to 90°. We note that for the last case, there is a position shifting the interference pattern by a distance of Λ/2 along *x*-axis for θ = 45°, but this does not affect the fabrication of SRGs.

In order to evaluate the influence of polarizations configurations on the intensity modulation, we introduce a well-known parameter called visibility (interference contrast), C

$$\mathbf{C} = \left| \left( I\_T^{\max} - I\_T^{\min} \right) \left\langle \left( I\_T^{\max} + I\_T^{\min} \right) \right\rangle \right| \tag{6}$$

where *IT* max and *IT* min are the maximum and minimum of the intensity, respectively.

Figure 4 shows the interference contrast as a function of θ (from 0° to 85°) for different polarization configurations. It is clear that the interference contrast is constant for S-S polari‐ zation configuration and keeps increasing with θ for the RC-LC polarization configuration. On the other hand, for the P-P case, the interference contrast decreases from 100% to 0 when the incident angle is varied from 0° to 45°, then increases again from 0 to 100% when θ increases from 45° to 90°. The reason is that in the case of P-P polarization configuration, two polariza‐ tions tend to be orthogonal when θ approaches 45°. At this particular angle, two polarizations are perpendicular and there is no interference. Therefore, for a standard optical lithography, a two-beam interference pattern with S-S polarization configuration is used in order to ensure the best intensity modulation contrast.

#### *2.1.2. Polarization modulation*

However, when the two-beam interference technique is applied to SRG formation, the modulation condition of the interference intensity is not enough, since the photoisomerization of DR1 molecules depends also on the illuminated light polarization. The polarization

**Figure 3.** Interference intensity distributions along the *x*-axis, calculated for different polarizations configurations (S-S, P-P, RC-LC) at θ = 5º, 50º and 85°, respectively. S and P represent two orthogonal linear polarizations; RC and LC are right and left circular polarizations, respectively. Note that: i) the intensity modulation keeps the same shape (except periodicity) for any θ−value in the case of S-S polarization configuration; ii) there is a Λ/2 switching position of the interference pattern for θ = 45° in the case of P-P polarization configuration, and iii) there is no intensity modulation at any θ−value in the case of S-P polarization configuration (not shown).

elliptic and circular forms, as seen in Fig. 5. In particular, for the S-P polarization configuration, a periodical modulation of the resultant polarization is obtained, while the resultant intensity modulation cannot be achieved, as discussed above. That will explain why SRG can be realized with such polarization configuration, thanks to the interference polarization modulation. Finally, for the case of RC-LC polarization configuration, the resultant polarization becomes a linear polarization for any θ-value, but the polarization direction changes periodically in the *xy*-plane. We note that, similar to the case of P-P polarization configuration, the polarization amplitude in the *xy*-plane decreases when θ increases. Besides, the interference polarization pattern is shifted by a distance of Λ/4 along the *x*-axis with respect to the interference intensity

**Figure 4.** Interference contrast or visibility of the two-beam interference pattern as a function of the angle θ. Three curves are obtained by using different polarizations configurations (S-S: blue line, P-P: red line, RC-LC: black line). There is a Λ/2 switching position of the interference pattern for θ = 45° in the case of P-P polarization configuration,

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As discussed before, the formation of SRG on azo-copolymer depends on the modulation of the irradiating light pattern and also on its resultant polarization. Therefore, polarization modulation and intensity distribution of the interference pattern are two necessary conditions, complementary and also competitive, for creating SRG. In this work, different SRGs were experimentally realized and the polarization dependence in the formation of SRG was also

SRGs were fabricated by the following procedure: i) preparation of a thin film sample by spincoating DR1/PMMA on cleaned glass substrate and baking in the oven at a temperature of 120°C for 2 hours to remove the solvent; ii) exposure by a two-beam interference pattern.

In order to build-up the fabrication technique, we have first recorded the UV–Visible absorp‐ tion spectrum of the DR1/PMMA thin film. The absorption band ranges from 400 nm to 570

pattern, as shown in Fig. 5.

corresponding to a minimum contrast at this angle.

**2.2. Experimental demonstration**

investigated.

distribution of the resultant field of the two laser beams has been then evaluated. Figure 5 represents the polarization distributions of the two-beam interference pattern in the *xy*-plane for different polarization configurations and for different incident angles. The background illustrates the intensity pattern where the deep colour corresponds to high intensity and the light colour corresponds to low intensity. The polarization of the resultant field is calculated for several particular positions, corresponding to *x* = ±Λ/2; ±Λ/4; 0. For the S-S polarization configuration, the interference polarization keeps the same direction, as those of the two laser beams, for any position and for all incident angles. In the case of the P-P polarization config‐ uration, the interference polarization direction is also the same for different θ-values, but the polarization amplitude decreases with increasing θ. In fact, when θ increases, the polarizations of both laser beams become parallel to the *z*-axis, i. e. perpendicular to the *xy*-plane, resulting in a diminution of the interference polarization amplitude in this plane. For the two other polarization configurations, the interference polarization varies periodically between linear,

**Figure 4.** Interference contrast or visibility of the two-beam interference pattern as a function of the angle θ. Three curves are obtained by using different polarizations configurations (S-S: blue line, P-P: red line, RC-LC: black line). There is a Λ/2 switching position of the interference pattern for θ = 45° in the case of P-P polarization configuration, corresponding to a minimum contrast at this angle.

elliptic and circular forms, as seen in Fig. 5. In particular, for the S-P polarization configuration, a periodical modulation of the resultant polarization is obtained, while the resultant intensity modulation cannot be achieved, as discussed above. That will explain why SRG can be realized with such polarization configuration, thanks to the interference polarization modulation. Finally, for the case of RC-LC polarization configuration, the resultant polarization becomes a linear polarization for any θ-value, but the polarization direction changes periodically in the *xy*-plane. We note that, similar to the case of P-P polarization configuration, the polarization amplitude in the *xy*-plane decreases when θ increases. Besides, the interference polarization pattern is shifted by a distance of Λ/4 along the *x*-axis with respect to the interference intensity pattern, as shown in Fig. 5.

As discussed before, the formation of SRG on azo-copolymer depends on the modulation of the irradiating light pattern and also on its resultant polarization. Therefore, polarization modulation and intensity distribution of the interference pattern are two necessary conditions, complementary and also competitive, for creating SRG. In this work, different SRGs were experimentally realized and the polarization dependence in the formation of SRG was also investigated.

#### **2.2. Experimental demonstration**

distribution of the resultant field of the two laser beams has been then evaluated. Figure 5 represents the polarization distributions of the two-beam interference pattern in the *xy*-plane for different polarization configurations and for different incident angles. The background illustrates the intensity pattern where the deep colour corresponds to high intensity and the light colour corresponds to low intensity. The polarization of the resultant field is calculated for several particular positions, corresponding to *x* = ±Λ/2; ±Λ/4; 0. For the S-S polarization configuration, the interference polarization keeps the same direction, as those of the two laser beams, for any position and for all incident angles. In the case of the P-P polarization config‐ uration, the interference polarization direction is also the same for different θ-values, but the polarization amplitude decreases with increasing θ. In fact, when θ increases, the polarizations of both laser beams become parallel to the *z*-axis, i. e. perpendicular to the *xy*-plane, resulting in a diminution of the interference polarization amplitude in this plane. For the two other polarization configurations, the interference polarization varies periodically between linear,

any θ−value in the case of S-P polarization configuration (not shown).

152 Holography – Basic Principles and Contemporary Applications

**Figure 3.** Interference intensity distributions along the *x*-axis, calculated for different polarizations configurations (S-S, P-P, RC-LC) at θ = 5º, 50º and 85°, respectively. S and P represent two orthogonal linear polarizations; RC and LC are right and left circular polarizations, respectively. Note that: i) the intensity modulation keeps the same shape (except periodicity) for any θ−value in the case of S-S polarization configuration; ii) there is a Λ/2 switching position of the interference pattern for θ = 45° in the case of P-P polarization configuration, and iii) there is no intensity modulation at

> SRGs were fabricated by the following procedure: i) preparation of a thin film sample by spincoating DR1/PMMA on cleaned glass substrate and baking in the oven at a temperature of 120°C for 2 hours to remove the solvent; ii) exposure by a two-beam interference pattern.

> In order to build-up the fabrication technique, we have first recorded the UV–Visible absorp‐ tion spectrum of the DR1/PMMA thin film. The absorption band ranges from 400 nm to 570

*2.2.1. Formation of SRG with different polarization configurations*

molecule from high intensity to low intensity area.

polarization dependence of the SRG formation.

Figure 6 shows experimental results of the SRGs formation. The incident angle θ is 9°, corresponding to a grating period of 1.7 μm. Figure 6(b) shows the AFM image of a 1D SRG obtained by the RC-LC polarization configuration, which is perfectly consistent with the simulation result of the interference pattern illustrated in Fig. 6(a). Actually, the shape of the interference intensity pattern and that of SRG are similar, with a sinusoidal form of the same period. However, we note that the highest intensity of the interference pattern corresponds to the valley of SRG, due to the mass transport effect, which induces the transfer of the DR1

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Figures 6(c) and (d) show the first-order diffraction efficiency (DE) and the amplitude of the SRGs fabricated using different polarization configurations. Note that the probing beam (red laser) applied to the film surface during the whole formation process has no influence on the sample structure because its wavelength is out of the absorption band of the DR1 molecules [11]. Figure 6(c) shows the time dependence of the DE, which is obviously polarization dependence. The diffraction signal indicates the creation of the SRG and depends on the exposure time. For all the polarization configurations, DE increases as a function of time, except for the RC-LC polarization configuration for which DE saturates and decreases after 10 min of exposure. However, we note that during the light irradiation process, the DE results from the formation of three different gratings for the azobenzene copolymer: two gratings were caused by refractive index change in the bulk copolymer material because of the different refractive index values between *trans*-form and *cis*-form of DR1, and only the third one is related to SRG [12]. An investigation of the SRG amplitude is necessary to fully analyse the

Figure 6(d) shows the amplitude of the SRGs measured by AFM. The time-dependence of the SRG depth is similar to that of the first-order diffraction intensity, except in the case of RC-LC polarization configuration, for which SRG depth doesn't decrease after saturation. This explains the behaviour observed in Fig. 6(c) for RC-LC polarization configuration, in which there is an exchange of the DE between the three different gratings, i. e. while the DE of two other refractive gratings increases then decreases, the DE of the SRG just continues to increase.

Besides, we also found that the RC-LC polarization configuration is the best case, which allows to achieve SRGs with the largest amplitude, as seen in Fig. 6 (d). On the other hand, the S-S polarization configuration, theoretically providing 100% of intensity modulation, allows obtaining SRGs with a depth of only several tens nanometers. Surprisingly, even in the case of S-P polarization configuration, which does not create any intensity modulation, allows to obtain SRGs with large depth, similar to those obtained by using a P-P polarization configu‐ ration. The formation of SRGs therefore depends strongly on the polarization distribution, or more precisely, it depends on the compromise between the intensity and the polarization modulations. Furthermore, we observed that the SRG depth reaches to a saturation level and remains unchanged after. This saturation level depends mostly on the thickness of the sample, on the light irradiation intensity, and also on the periodicity of the interference pattern. The dependence of the SRG formation with respect to the two first parameters has been studied

We conclude that the SRG formation plays a dominant role in DE evolution.

**Figure 5.** Polarization distributions in *xy*-plane of the two-beam interference pattern, obtained with different polari‐ zation configurations. The interference pattern is calculated for one period (from -Λ/2 to +Λ/2). The polarization mod‐ ulation varies as a function of θ-values (5°, 50° and 85°). The white-green colour of the background represents intensity interference pattern.

nm, with an absorption peak at 480 nm. A green laser is therefore suitable for the SRG formation.

The experimental setup for two-beam interference can be realized easily according to Fig. 2 (a). A green laser (λ = 532 nm) beam is split into two identical secondary beams by a 50/50 nonpolarized beam splitter. The two beams were then combined, by using two mirrors, and interfered on the azo-copolymer film. The polarizations of the two laser beams were controlled independently by using different wave plates (quarter-wave plate and half-wave plate). The incident angle θ was adjusted by tuning the two mirrors symmetrically, and the SRG perio‐ dicity varied from several micrometers to hundreds nanometers. The intensity of each interference beam was fixed to about 84 mW/cm2 at the sample position. In order to monitor the dynamics of SRG formation, a red laser beam (λ = 633 nm, power = 1 mW) was sent into the interference area, and the first-order diffraction intensity was measured as a function of time, indicating the formation of SRG. The fabricated SRGs were then examined by using an atomic force microscope (AFM) and/or a scanning electron microscope (SEM).

### *2.2.1. Formation of SRG with different polarization configurations*

Figure 6 shows experimental results of the SRGs formation. The incident angle θ is 9°, corresponding to a grating period of 1.7 μm. Figure 6(b) shows the AFM image of a 1D SRG obtained by the RC-LC polarization configuration, which is perfectly consistent with the simulation result of the interference pattern illustrated in Fig. 6(a). Actually, the shape of the interference intensity pattern and that of SRG are similar, with a sinusoidal form of the same period. However, we note that the highest intensity of the interference pattern corresponds to the valley of SRG, due to the mass transport effect, which induces the transfer of the DR1 molecule from high intensity to low intensity area.