$$\mathcal{A}\_m = \frac{2d\sqrt{n\_{\rm cld}^2 - n\_{\rm cor}^2}}{m}, \ m = 1, 2, 3 \dots \tag{10}$$

To realize a highly sensitive detection of vaporized analyte in the hollow core, the resonant waves in the pipe wall must be sufficiently evanescent toward the hollow core because the sufficient interaction is required. Based on the transverse power distributions of resonant waves in a pipe waveguide [24], the higher THz transmission power in the hollow core is observed in the higher-order resonant-dip, i.e., the dips in the high frequency range. **Figure 8(a)** shows the vapor-sensing example based on a 30 cm-long glass pipe, possessing the inner core diameter and the pipe-wall thickness are, respectively, 5.57 and 1.17 mm. The vapor inducing dip-frequency-shift only occurs in a resonance dip of 0.452 THz and other resonance dips in the low-frequency range do not exhibit any spectral shift. It does not follow the simulated results in **Figure 8(b)** because the leaky power of the low-order resonance-wave in the hollow core is insufficient to sense the vapor molecules. It also practically presents the evanescent field strength of a pipe resonant modes within the inner core is critical for the vapor sensing.

**Figure 8.** (a) Spectral dip shift of the glass-pipe resonance wave at 0.452 THz for sensing different volatile liquids. (b) Theoretical estimation of the inner core refractive indices to match the related spectral dip positions for the vapor-sensing results. (c) The spectral relation of the inner core refractive indices for different vapors. (d) Relation between the spectral dip shift and the related molecular densities (Reprinted from Ref. [24] © 2012 OSA).

The vapors are prepared by dropping 0.05 cm3 liquids individually of water, hydrochloric acid (HCl), acetone and ammonia inside the hollow core of glass pipe with vapor pressure saturation. The sensing result shows the spectral dip at 0.452 THz can be shifted with different levels (**Figure 8(a)**), locating at 0.461, 0.465 and 0.477 THz, respectively, for the hydrochloric acid, acetone and ammonia vapors. Trying to match the spectral dip position, the effective inner core refractive indices can be calculated by FDTD method (**Figure 8(b)**), which are 1.016, 1.035 and 1.102, respectively, for the vapors of hydrochloric acid, acetone and ammonia. The relation is then summarized in **Figure 8(c)** between the spectral dip frequencies and the effective coreindices of the glass-pipe fiber, which is filled with different vapors. Those inner core refractive indices are distinct due to the different vapors even though the same liquid volume is inserted inside the pipe hollow core. The relation between the effective inner core refractive index and resonance frequency in **Figure 8(c)** can be fit with one polynomial to evaluate any slight index variation inside the pipe hollow core. For the example of water sample, the relating spectral dip is consistent with and without inserting the vapor because of the considerably low density, comparing to those volatile liquids. It means the effective inner core index of water case approximates to 1.0.

Different volatile liquids have individual vapor pressures at normal atmospheric pressure and temperature. There are different quantities of vaporized molecules in the same hollow core volume. Vapor pressures of volatile liquids are proportional to the quantities of vaporized molecules in the same chamber. High vapor molecule density in the hollow core eventually results in the higher effective core-index than the air core, performing apparent blue shift at the resonant spectral dip at 0.452 THz. When we consider the vapor pressures in literatures for water, hydrochloric acid, acetone and ammonia at 1 atmosphere and 20°C, their saturated vapor pressures are, respectively, around 17 mm-Hg [25], 38 mm-Hg [26], 202 mm-Hg [26] and 308 mm-Hg [25]. Thereby, the effective core-index is proportional to the increase of the sample vapor pressure. When the vaporized molecular density is estimated on the basis of the ideal gases, the corresponding densities are illustrated in **Figure 8(d)**. The relation is fit with one nonlinear curve, representing the blue shift level would cease approaching the value of 10 nano-mole/mm3 . The minimum detectable molecular quantity would be approximately 7.8 micromole in the glass pipe, corresponding to a molecular density of 1.6 nano-mole/mm3 when the system spectral resolution is about 4 GHz.

## **5. Conclusion**

In this chapter, THz fiber sensors are successfully demonstrated in a PS-plastic wire, a PEplastic ribbon integrated with a metal grating, a PP- and glass pipes. The presented fiber sensors are highly sensitive to the existence and variation of minute molecules and enable sensing analytes with different formations, including the liquids, powder grains, vapor gases, thin films and overlayers. A subwavelength plastic wire fiber has large percentages of evanescent field surrounding the fiber cores and there is negative waveguide dispersion sensitive to the refractive index variation in the air cladding. Fluidic channel is successfully integrated to detect minute concentration of melamine grains dissolved in the ethanol solutions ranged from 20 to 80 ppm. A plastic ribbon fiber also has the extended evanescent waveguide modes, but it is further advantageous to integrate a metal grating becoming a hybrid plasmonic waveguide based on the optical properties of a THz ribbon fiber, including the TM polarized wave guidance and the broadband transmission. When the optimal overlapping between the metal grating and ribbon waveguide modes, subwavelength confined surface waves can be excited and used for sensing powder grains or thin-film analytes. When the spectral resolution of THz system is reduced down to 1 GHz, the nano-scaled OPD detection could be realized by the plastic ribbon fiber sensor. The dielectric pipe fiber with a hollow air core is also considered as one important THz fiber sensor because of the built-in resonator along the pipe wall. To achieve highly sensitive detection, the applicable resonant modes for thin-film and vapor detections are absolutely different because of the different cross locations of the target analytes. Using the pipe fiber to adsorb liquids with high viscosities as one thin film layer on the inner pipe wall, the low frequency resonance modes or small pipe wall thicknesses are required to achieve the best sensitivity due to the much extended evanescent resonance field. For sensing the inserted vapors at the pipe hollow core, the sensitive THz waves contribute at the high order resonant modes because their resonant field well confined at the inner core efficiently interacting the vapors. In the sensing experiments, the detectable overlayer thickness and vaporized molecular density via THz pipe fiber sensors can be reduced to 2.9μm (~λ/ 225)and 1.6 nano-mole/mm3 , respectively.

## **Author details**

The vapors are prepared by dropping 0.05 cm3

78 Terahertz Spectroscopy - A Cutting Edge Technology

approximates to 1.0.

nano-mole/mm3

**5. Conclusion**

the system spectral resolution is about 4 GHz.

liquids individually of water, hydrochloric acid

(HCl), acetone and ammonia inside the hollow core of glass pipe with vapor pressure saturation. The sensing result shows the spectral dip at 0.452 THz can be shifted with different levels (**Figure 8(a)**), locating at 0.461, 0.465 and 0.477 THz, respectively, for the hydrochloric acid, acetone and ammonia vapors. Trying to match the spectral dip position, the effective inner core refractive indices can be calculated by FDTD method (**Figure 8(b)**), which are 1.016, 1.035 and 1.102, respectively, for the vapors of hydrochloric acid, acetone and ammonia. The relation is then summarized in **Figure 8(c)** between the spectral dip frequencies and the effective coreindices of the glass-pipe fiber, which is filled with different vapors. Those inner core refractive indices are distinct due to the different vapors even though the same liquid volume is inserted inside the pipe hollow core. The relation between the effective inner core refractive index and resonance frequency in **Figure 8(c)** can be fit with one polynomial to evaluate any slight index variation inside the pipe hollow core. For the example of water sample, the relating spectral dip is consistent with and without inserting the vapor because of the considerably low density, comparing to those volatile liquids. It means the effective inner core index of water case

Different volatile liquids have individual vapor pressures at normal atmospheric pressure and temperature. There are different quantities of vaporized molecules in the same hollow core volume. Vapor pressures of volatile liquids are proportional to the quantities of vaporized molecules in the same chamber. High vapor molecule density in the hollow core eventually results in the higher effective core-index than the air core, performing apparent blue shift at the resonant spectral dip at 0.452 THz. When we consider the vapor pressures in literatures for water, hydrochloric acid, acetone and ammonia at 1 atmosphere and 20°C, their saturated vapor pressures are, respectively, around 17 mm-Hg [25], 38 mm-Hg [26], 202 mm-Hg [26] and 308 mm-Hg [25]. Thereby, the effective core-index is proportional to the increase of the sample vapor pressure. When the vaporized molecular density is estimated on the basis of the ideal gases, the corresponding densities are illustrated in **Figure 8(d)**. The relation is fit with one nonlinear curve, representing the blue shift level would cease approaching the value of 10

micromole in the glass pipe, corresponding to a molecular density of 1.6 nano-mole/mm3 when

In this chapter, THz fiber sensors are successfully demonstrated in a PS-plastic wire, a PEplastic ribbon integrated with a metal grating, a PP- and glass pipes. The presented fiber sensors are highly sensitive to the existence and variation of minute molecules and enable sensing analytes with different formations, including the liquids, powder grains, vapor gases, thin films and overlayers. A subwavelength plastic wire fiber has large percentages of evanescent field surrounding the fiber cores and there is negative waveguide dispersion sensitive to the refractive index variation in the air cladding. Fluidic channel is successfully integrated to detect minute concentration of melamine grains dissolved in the ethanol solutions

. The minimum detectable molecular quantity would be approximately 7.8

Borwen You1 and Ja-Yu Lu2\*

\*Address all correspondence to<EMAIL_ADDRESS>
1 Division of Applied Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan

2 Department of Photonics, National Cheng-Kung University, Tainan, Taiwan

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[4] L. Tong, J. Lou and E. Mazur. Single-mode guiding properties of subwavelengthdiameter silica and silicon wire waveguides. Opt. Express. 2004;12:1025–1035.

[5] H. W. Chen, Y. T. Li and C. L. Pan, J. L. Kuo, J. Y. Lu, L. J. Chen and C. K. Sun. Investigation on spectral loss characteristics of subwavelength terahertz fibers. Opt. Lett.

[6] B. Ferguson and X. C. Zhang. Materials for terahertz science and technology. Nat. Mater.

[7] B. You, T. A. Liu, J. L. Peng, C. L. Pan and J. Y. Lu. A terahertz plastic wire based evanescent field sensor for high sensitivity liquid detection. Opt. Express.

[8] R. E. N. Baozeng, L. I. Chen, Y. U. A. N. Xiaoliang and W. A. N. G. Fu'an. Determination and correlation of melamine solubility. Chin. J. Chem. Eng. 2003;54:1001–1003.

[9] C. Yeh, F. Shimabukuro and P. H. Siegel. Low-loss terahertz ribbon waveguides. Appl.

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[11] L. Shen, X. Chen and T. J. Yang. Terahertz surface plasmon polaritons on periodically

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[13] B. You, J. Y. Lu, W. L. Chang, C. P. Yu, T. A. Liu and J. L. Peng. Subwavelength confined terahertz waves on planar waveguides using metallic gratings. Opt. Express.

[14] C. R. Williams, S. R. Andrews, S. A. Maier, A. I. F. Dominguez, L. M. Moreno and F. J. G. Vidal. Highly confined guiding of terahertz surface plasmon polaritons on struc-

[15] J. F. O'Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A. J. Taylor and W. Zhang. Thinfilm sensing with planar terahertz metamaterials: sensitivity and limitations. Opt.

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80 Terahertz Spectroscopy - A Cutting Edge Technology

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**Provisional chapter**

## **Terahertz Detectors (THzDs): Bridging the Gap for Energy Harvesting Energy Harvesting**

**Terahertz Detectors (THzDs): Bridging the Gap for**

Kapil Bhatt, Shilpi Shriwastava, Sandeep Kumar, Sandeep and Chandra Charu Tripathi Sandeep Kumar, Sandeep and Chandra Charu Tripathi

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66347

Kapil Bhatt, Shilpi Shriwastava,

#### **Abstract**

It is indispensable to integrate electronics with environment for better lives. Huge amount of solar energy, dark energy, and unused microwave energy is untapped till now due to insufficient availability of high frequency THz detectors. The difference between THz wave detection and THz electric field detection must be clear. THz wave detection connects the detection of explosives, drugs, astronomy, metals, and imaging applications, etc. On the other hand, THz electric field detection involves the conversion of electromagnetic (EM) radiations to usable DC power. The optimum choice of detectors for energy harvesting is a highly diverse area. The latter part is concentrated on the non‐ linear behavior of the incoming radiations and has been highlighted also. In this chapter, metal**‐**insulator‐metal (MIM) diode detectors have been explored to become a best choice for high frequency detectors.

**Keywords:** metal‐insulator‐metal (MIM) diodes, terahertz detectors (THzD), rectenna, tunneling

## **1. Introduction**

In order to fulfill the world energy demands increasing day by day, it is perquisite to have some alternate for the conventional energy resources and energy harvesting methods. Since last few decades, advancement in the field of the THz rectification reveal another way to develop highly sensitive uncooled THz detector. Undoubtedly, a terahertz measurement requires a highly sensitive detector to obtain distinct spectra. One problem faced by the detection of THz waves is low photon energy of milli‐electron volts (meV), making the development of a

© 2016 The Author(s). Licensee InTech. This chapter is 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. © 2017 The Author(s). Licensee InTech. This chapter is 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.

high‐performance terahertz detector a difficult task. By serendipity, accurate modeling and applications of the nanoscale materials and devices have conquered these difficulties.

There is a fine line between the egoistical requirement and legitimate demand of energy around the world. By 2030, the demand for energy is projected to increase by 55%, while requirement of oil is increased from 11.4 to 17.7 billion tons [1]. Between 2005 and 2030, energy consumption is expected to increase by 50%, with the bulk of the demand coming from devel‐ oping countries. The energy provided by the Sun at the Earth's surface on 1 day is more than enough to fulfill the needs of the whole Earth for more than 25 years, if utilizes efficiently. But due to low conversion efficiency, a prevalent commercial usage of solar energy is hardly seen. A portion of the increasing developments, utilizing the enhancement in nanotechnology may be able to change this situation. During the clear climate conditions, the Earth gets 1000 W/m2 solar irradiation from the Sun [2]. This enormous energy received from the Sun has opened the ways to look for novel systems to convert unused energy radiated continuously from the Sun into usable electricity. The continuous research results in environmentally friendly and clean energy sources and will replace our conventional sources of energy such as coal, gas, oil, hydro, combustible, petroleum energy, nuclear energy, etc. By‐products of these sources lead to greenhouse effects and the solar energy is considered to be the most excellent substitute for it as compared to other energy resources. There is a good scope for renewable/green energy corner as its share is less than 1% as compared to present energy resources.

The requirements of efficient energy utilization could be estimated from the fact that dur‐ ing the 2012 Olympics held in London, organizer had managed new facilities with energy efficient, sustainable, and recyclable designs and they cut down the watts to maintain the games clean, green, and energy efficient. The London games had left a benchmark as the most energy‐efficient Olympics to till date. A similar kind of legacy is being followed by Rio Olympics 2016. This scenario has motivated the researchers toward exploring renewable energy and unused energy resources (dark energy, unused heat and radiations).

Rectenna is one of the energy harvesting devices, which is a combination of rectifying high frequency diode and an antenna. Rectenna (Rectifier antenna) is a system that is capable for harvesting the infrared and visible wavelengths. The heart of the rectenna system is a metal**‐** insulator‐metal (MIM) device that can convert such an energy collected by nanoantenna into usable electric energy.

## **2. Why THz region?**

A lower electromagnetic (EM) region has been occupied for mobile communication systems, and the higher EM region has been occupied for optical communication. However, the THz region is still untapped and has not been completely in use, and this regime has become of colossal importance in extensive disciplines. This region embraces numerous aspects of space research, medical studies, atmospheric studies, natural science, plasma physics, defense and security, process control, etc. The close correlation of electrons and THz radiation is also reflected in the use of THz spectroscopy, which is a contemporary field for the researcher to attract toward it. THz spectroscopy is publicized to provide insight into the electron mobility in semiconductive and conductive nanostructures (like in MIM nanostructures and tunnel‐ ing spectroscopy) that are suitable for fast electronics systems, which are used to convert solar power into electrons. MIM diodes are able to work in semiclassical and quantum region (**Figure 1**) further broader this window.

It is also applicable for nonlinear optics and image spectroscopy. THz imaging has become a significant function in this region. The initial development of the premature terahertz time domain spectroscopy was the innovation that a range of materials, which are opaque in the visible and near‐IR region together with clothing and packaging materials, are reasonably transparent over a great extent of the THz range [3]. Room temperature operation of the infra‐ red and far infrared frequencies detectors too increased the interest toward this region.

The popularity of the study of this region can be estimated by the fact that about 50% of the luminosity and most of the photons energy emitted since Big Bang of the space are contained by the THz radiations [4]. Every technology has dark side as well as brighter one; research in THz range has been obstructed by the absorption of the Earth's atmosphere amongst various main factors is the water vapors present there.

**Figure 1.** Schematic of the EM spectrum shows the working region for MIM devices.

## **3. Rectenna made easy**

high‐performance terahertz detector a difficult task. By serendipity, accurate modeling and

There is a fine line between the egoistical requirement and legitimate demand of energy around the world. By 2030, the demand for energy is projected to increase by 55%, while requirement of oil is increased from 11.4 to 17.7 billion tons [1]. Between 2005 and 2030, energy consumption is expected to increase by 50%, with the bulk of the demand coming from devel‐ oping countries. The energy provided by the Sun at the Earth's surface on 1 day is more than enough to fulfill the needs of the whole Earth for more than 25 years, if utilizes efficiently. But due to low conversion efficiency, a prevalent commercial usage of solar energy is hardly seen. A portion of the increasing developments, utilizing the enhancement in nanotechnology may be able to change this situation. During the clear climate conditions, the Earth gets 1000 W/m2 solar irradiation from the Sun [2]. This enormous energy received from the Sun has opened the ways to look for novel systems to convert unused energy radiated continuously from the Sun into usable electricity. The continuous research results in environmentally friendly and clean energy sources and will replace our conventional sources of energy such as coal, gas, oil, hydro, combustible, petroleum energy, nuclear energy, etc. By‐products of these sources lead to greenhouse effects and the solar energy is considered to be the most excellent substitute for it as compared to other energy resources. There is a good scope for renewable/green energy

applications of the nanoscale materials and devices have conquered these difficulties.

84 Terahertz Spectroscopy - A Cutting Edge Technology

corner as its share is less than 1% as compared to present energy resources.

energy and unused energy resources (dark energy, unused heat and radiations).

usable electric energy.

**2. Why THz region?**

The requirements of efficient energy utilization could be estimated from the fact that dur‐ ing the 2012 Olympics held in London, organizer had managed new facilities with energy efficient, sustainable, and recyclable designs and they cut down the watts to maintain the games clean, green, and energy efficient. The London games had left a benchmark as the most energy‐efficient Olympics to till date. A similar kind of legacy is being followed by Rio Olympics 2016. This scenario has motivated the researchers toward exploring renewable

Rectenna is one of the energy harvesting devices, which is a combination of rectifying high frequency diode and an antenna. Rectenna (Rectifier antenna) is a system that is capable for harvesting the infrared and visible wavelengths. The heart of the rectenna system is a metal**‐** insulator‐metal (MIM) device that can convert such an energy collected by nanoantenna into

A lower electromagnetic (EM) region has been occupied for mobile communication systems, and the higher EM region has been occupied for optical communication. However, the THz region is still untapped and has not been completely in use, and this regime has become of colossal importance in extensive disciplines. This region embraces numerous aspects of space research, medical studies, atmospheric studies, natural science, plasma physics, defense and security, process control, etc. The close correlation of electrons and THz radiation is also reflected in the use of THz spectroscopy, which is a contemporary field for the researcher to

We are more than 7 billion today and multiplying every second around the globe. Specialists believe that further 50% extra energy will be required by 2050 to prolong humanity. Nevertheless, it does not mean that population is the only stumbling block, but the ever inflating ego and diminishing humanity is also responsible for more energy needs. There is a fine line between the egoistical requirement and legitimate demand of energy around the world. Among various developed countries, Americans build up no more than 4.5% of the world's population and yet consume almost 20% of its energy. Its energy consumption is truly extraordinary [5]. In order to cope up with these problems, appropriate measures intended at reducing the dependence on the present fuels are needed, and the exploration for clean and renewable unconventional energy resources is one of the most urgent confront to the sustain‐ able development of creature evolution.

There are several unconventional sources of energy like thermal energy, nuclear, vibration, wind, water, solar energy, etc. Solar energy is estimated to provide a significant contribution for the solution of energy problem today. Just a small portion of the Sun's energy that hits the Earth is sufficient to meet all our power needs. A little amount of the solar energy reaching the Earth is utilized for producing electricity by using present solar systems. Still the energy demand met by using solar energy is very less. Solar cells based on photovoltaic technology have many applications in solar ponds, vehicles based on solar energy, water pumps, lights, satellites calculators, solar telephones, and many more. But there are several shortcomings of this technology in terms of bulky solar panels, costly installation, low conversion efficiency, and dependencies on the weather conditions. The photovoltaic technology used for energy harvest‐ ing has limitation of maximum conversion efficiency by the Shockley‐Queisser limit. However, in comparison the rectenna system is able to provide monospectral radio frequency‐DC (RF‐ DC) with a conversion efficiency of 100% [6]. Moreover, photovoltaic technology is expensive and some poisonous by‐products are produced during the manufacturing process. Therefore, in this perspective to obtain high efficiency, a green source of renewable energy, rectenna, has emerged as one of the best solutions for meeting the growing energy demand. A system that converts unused EM radiations and radiations from the Sun into usable DC is a rectenna sys‐ tem. It consists of a high frequency antenna to capture the incoming signal and provides the signal to prerectification block. The band‐limited signal from prerectification block has been converted to DC which after sufficient filtration applied to output load, as shown in **Figure 2**.

**Figure 2.** Block diagram of rectenna system.

The term rectenna, i.e., rectifying antenna, was coined by William C. Brown, he used an array of diodes for conversion of incoming microwave power into DC [7]. However, the rectenna system in contrast also suffered from limited multispectral efficiency. The progress in the computer‐aided design structure has currently cut down the design processes, in which com‐ plex electronic circuits such as high‐frequency filters, antennas, and rectenna can be designed with a high level of buoyancy and reliability at very low cost and in small period of time.

### **3.1. Nanoscale antennas**

Nevertheless, it does not mean that population is the only stumbling block, but the ever inflating ego and diminishing humanity is also responsible for more energy needs. There is a fine line between the egoistical requirement and legitimate demand of energy around the world. Among various developed countries, Americans build up no more than 4.5% of the world's population and yet consume almost 20% of its energy. Its energy consumption is truly extraordinary [5]. In order to cope up with these problems, appropriate measures intended at reducing the dependence on the present fuels are needed, and the exploration for clean and renewable unconventional energy resources is one of the most urgent confront to the sustain‐

There are several unconventional sources of energy like thermal energy, nuclear, vibration, wind, water, solar energy, etc. Solar energy is estimated to provide a significant contribution for the solution of energy problem today. Just a small portion of the Sun's energy that hits the Earth is sufficient to meet all our power needs. A little amount of the solar energy reaching the Earth is utilized for producing electricity by using present solar systems. Still the energy demand met by using solar energy is very less. Solar cells based on photovoltaic technology have many applications in solar ponds, vehicles based on solar energy, water pumps, lights, satellites calculators, solar telephones, and many more. But there are several shortcomings of this technology in terms of bulky solar panels, costly installation, low conversion efficiency, and dependencies on the weather conditions. The photovoltaic technology used for energy harvest‐ ing has limitation of maximum conversion efficiency by the Shockley‐Queisser limit. However, in comparison the rectenna system is able to provide monospectral radio frequency‐DC (RF‐ DC) with a conversion efficiency of 100% [6]. Moreover, photovoltaic technology is expensive and some poisonous by‐products are produced during the manufacturing process. Therefore, in this perspective to obtain high efficiency, a green source of renewable energy, rectenna, has emerged as one of the best solutions for meeting the growing energy demand. A system that converts unused EM radiations and radiations from the Sun into usable DC is a rectenna sys‐ tem. It consists of a high frequency antenna to capture the incoming signal and provides the signal to prerectification block. The band‐limited signal from prerectification block has been converted to DC which after sufficient filtration applied to output load, as shown in **Figure 2**.

The term rectenna, i.e., rectifying antenna, was coined by William C. Brown, he used an array of diodes for conversion of incoming microwave power into DC [7]. However, the rectenna

able development of creature evolution.

86 Terahertz Spectroscopy - A Cutting Edge Technology

**Figure 2.** Block diagram of rectenna system.

For the collection of the EM field which is a combination of electric and magnetic fields, opti‐ mized antenna design and analysis must be taken care. Maxwell's field equations describe the full apparatus of EM fields. Antenna's geometric structure to associated wavelength and its polarization play a very important role. The receiving antenna must be located in the plane of polarization antenna for highest absorption of the electromagnetic fields. Several THz detec‐ tors and emitters establish the polarization with associated spectral response, e.g., dipole antennas are mainly based on linear polarization, whereas systems in incoming fields can either respond circularly, elliptically, or linearly polarized light [8].

Several nano‐sized antennas, available [9–11] to capture the small wavelength signals, are half‐wave dipole, spiral‐type, Bow‐Tie, Yagi‐Uda, etc. The shape and size of the antenna is very important for THz energy collection. The μm‐shaped antenna is required to collect the small wavelength signals. Latest nanotechnology for terahertz detection has now demanded for diode‐integrated antennas or antenna‐coupled diode detectors. The schematic diagram of integrated antenna with diode is shown in **Figure 3**.

Baily gave the concept of capturing the solar power with antennas for the first time employing the wave nature of incoming power [12]. Antenna‐coupled detectors with micro‐bolometer, metal‐insulator‐metal diode, and Schottky diodes have been used for detection in the sub‐ mm‐wave and mm‐wave regions with various antenna configurations [13, 14]. The surface plasmons are generated on the antenna due to incident EM radiations. Due to the appropriate impedance matching between the antenna geometry and the coupled MIM diode, these plas‐ mons transfer to diode and it generates output DC.

**Figure 3.** A typical schematic of antenna coupled diode detector.

#### **3.2. Potential detectors available**

Commonly available detectors for high‐frequency (HF) range are Schottky diode, geometric diode, and MIM/MIIM. High challenges are involved in designing of these detectors due to complex geometries involved, HF challenges and increased fabrication cost. Nevertheless, due to the advancement in technology, highly optimized diode detectors are available nowa‐ days which perform well up to several hundred THz.

#### *3.2.1. Schottky diode detector*

For THz detection, a long time battle has been continued between Schottky and MIM diodes [15]. The *n*‐doped GaAs semiconductor is the key contender for THz applications, used in the metal‐semiconductor structure. Schottky diode's series resistance is high‐frequency‐depen‐ dent parameter and essentially becomes a complex function of device geometry and material conductivity [16]. The diode conversion efficiency *Ƞ*d [17] is the key to determine the system's performance given by <sup>ƞ</sup>diode <sup>=</sup> DC Output power \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ RF Power incident on diode (1)

$$\eta\_{\text{dolo}} = \frac{\text{DC Output power}}{\overline{\text{RF Power incident on diode}}} \tag{1}$$

Let

$$
\eta\_{\text{doloo}} = \frac{1}{A+B} \tag{2}
$$

The low‐frequency term is given by

$$\begin{aligned} \boldsymbol{\eta}\_{\text{dolo}} &= \frac{1}{A+B} \\ \text{term is given by} \\ A &= 1 + \frac{R\_{\text{L}}}{\pi R\_{\text{S}}} \left( 1 + \frac{V\_{\text{bi}}}{V\_{\text{D}}} \right)^{2} \left[ \theta\_{\text{ON}} \left( 1 + \frac{1}{2 \cdot \cos^{2} \theta\_{\text{ON}}} \right) - \frac{3}{2} \tan \theta\_{\text{ON}} \right] \\ &+ \frac{R\_{\text{L}}}{\pi R\_{\text{S}}} \left( 1 + \frac{V\_{\text{bi}}}{V\_{\text{D}}} \right) \frac{V\_{\text{bi}}}{V\_{\text{D}}} \left( \tan \theta\_{\text{ON}} - \theta\_{\text{ON}} \right) \end{aligned} \tag{3}$$

And high‐frequency term is given by

$$
\pi R\_{\rm s} \left( \begin{array}{c} V\_{\rm D} \end{array} \right) \left( \begin{array}{c} V\_{\rm D} \end{array} \right) \left( \begin{array}{c} \cdots \longrightarrow \stackrel{\sim}{\longrightarrow} \end{array} \right)
$$

and high-frequency term is given by

$$
B = \frac{R\_{\rm s} R\_{\rm L} \, \mathrm{c}^2 \, \mathrm{o}^2}{2 \pi} \left( 1 + \frac{V\_{\rm h}}{V\_{\rm D}} \right) \left( \frac{\pi - \theta\_{\rm ON}}{\cos^2 \theta\_{\rm ON}} + \tan \theta\_{\rm ON} \right) \tag{4}
$$

where *c*<sup>j</sup> is the diode's junction capacitance given by

$$\begin{array}{ccccc}\hline \text{minimize } c\_{\text{j}} & \text{minimize } c\_{\text{j}} & \text{minimize } \mathbf{g} \times \mathbf{h}\_{\text{j}} \\\\ c\_{\text{j}} & = & c\_{\text{j}} \sqrt{\frac{V\_{\text{bi}}}{V\_{\text{bi}} + V\_{\text{D}}}} \\\\ \end{array} \tag{5}$$

Also

$$
\tan\Theta\_{\text{ON}} - \Theta\_{\text{ON}} = \frac{\pi R\_{\text{S}}}{R\_{\text{L}} \left(1 + \frac{V\_{\text{in}}}{V\_{\text{B}}}\right)}\tag{6}
$$

The cut‐off frequency of Schottky is given by

$$f\mathbf{c} = \begin{array}{c} \frac{1}{2\pi |\mathcal{R}\_\* c\_j|} \end{array} \tag{7}$$

where *R*<sup>L</sup> is the DC load resistance and *R*<sup>S</sup> is the diode's series resistance, *c*<sup>j</sup> is the junction capacitance, *V*bi is the diode's built‐in voltage in the forward bias region, *V*D is the self‐bias voltage due to rectification across the terminals of the diode, *θ*ON is forward‐bias turn‐on angle depends on diode input power, *ω* is the angular frequency. For higher frequency operation, the diode series resistance and junction capacitance must be lower. Planar Schottky diodes are able to operate at higher frequency with additional cooling mechanism required. Moreover, Schottky diodes cannot operate at zero bias; hence, MIM diodes are the best choice.

#### *3.2.2. MIM diodes*

**3.2. Potential detectors available**

88 Terahertz Spectroscopy - A Cutting Edge Technology

*3.2.1. Schottky diode detector*

performance given by

Let

where *c*<sup>j</sup>

Also

where *R*<sup>L</sup>

days which perform well up to several hundred THz.

<sup>ƞ</sup>diode <sup>=</sup> \_\_\_\_ <sup>1</sup>

+ *R*\_\_\_\_*<sup>L</sup> <sup>π</sup> <sup>R</sup>*S(<sup>1</sup> <sup>+</sup>

*R*\_\_\_\_<sup>L</sup> *<sup>π</sup> <sup>R</sup>*<sup>S</sup> (<sup>1</sup> <sup>+</sup>

is the diode's junction capacitance given by

tanθON <sup>−</sup> *<sup>θ</sup>*ON <sup>=</sup> πR \_\_\_\_\_\_\_ <sup>S</sup>

*V*\_\_\_bi *<sup>V</sup>*D) 2

*V*\_bi *<sup>V</sup>*D) *V*\_\_\_bi *V*D

<sup>2</sup> *ω*<sup>2</sup> \_\_\_\_\_\_\_\_ <sup>2</sup>*<sup>π</sup>* (<sup>1</sup> <sup>+</sup>

*A* = 1 +

*<sup>c</sup>*<sup>j</sup> <sup>=</sup> *<sup>c</sup>*j0 <sup>√</sup>

The cut‐off frequency of Schottky is given by

*<sup>f</sup>*<sup>c</sup> <sup>=</sup> \_\_\_\_\_ <sup>1</sup>

is the DC load resistance and *R*<sup>S</sup>

The low‐frequency term is given by

And high‐frequency term is given by

*<sup>B</sup>* <sup>=</sup> *<sup>R</sup>*<sup>S</sup> *<sup>R</sup>*<sup>L</sup> *<sup>c</sup>*<sup>j</sup>

Commonly available detectors for high‐frequency (HF) range are Schottky diode, geometric diode, and MIM/MIIM. High challenges are involved in designing of these detectors due to complex geometries involved, HF challenges and increased fabrication cost. Nevertheless, due to the advancement in technology, highly optimized diode detectors are available nowa‐

For THz detection, a long time battle has been continued between Schottky and MIM diodes [15]. The *n*‐doped GaAs semiconductor is the key contender for THz applications, used in the metal‐semiconductor structure. Schottky diode's series resistance is high‐frequency‐depen‐ dent parameter and essentially becomes a complex function of device geometry and material conductivity [16]. The diode conversion efficiency *Ƞ*d [17] is the key to determine the system's

<sup>ƞ</sup>diode <sup>=</sup> DC Output power \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ RF Power incident on diode (1)

[*θ*ON(<sup>1</sup> <sup>+</sup> \_<sup>1</sup>

(tan *θ*ON − *θ*ON)

*π* − *θ* \_ON

cos<sup>2</sup> *θ*ON

\_\_\_\_\_\_ *<sup>V</sup>* \_\_\_\_\_\_ bi *V*bi + *V*<sup>D</sup>

> *R*L(1 + *V*\_bi *<sup>V</sup>*D)

is the diode's series resistance, *c*<sup>j</sup>

2*π R*<sup>s</sup> *c*<sup>j</sup>

capacitance, *V*bi is the diode's built‐in voltage in the forward bias region, *V*D is the self‐bias voltage due to rectification across the terminals of the diode, *θ*ON is forward‐bias turn‐on angle depends on diode input power, *ω* is the angular frequency. For higher frequency operation, the diode series resistance and junction capacitance must be lower. Planar Schottky diodes are

*V*\_bi *<sup>V</sup>*D)( <sup>2</sup> cos<sup>2</sup> *<sup>θ</sup>*ON) <sup>−</sup> \_<sup>3</sup>

*<sup>A</sup>* <sup>+</sup> *<sup>B</sup>* (2)

+ tan *θ*ON) (4)

(3)

(5)

(6)

(7)

is the junction

<sup>2</sup> tan *θ*ON]

The MIM diode is a combination of two metal plates separated by a very thin insulating layer of the order of few nanometers. The contact area between upper and lower layer determines the active device area. The area may vary from few μm2 to several hundred μm2 depends upon the operating frequency. There are various parameters needed to be optimized for high conversion efficiency like metal work function, insulator electron affinity, and its thickness, which are responsible for the best detector. The dependencies of these parameters can be estimated from the fact that the small deviation (in the range of Å) of an insulating layer and diode area can cause a significant change in the tunneling characteristics, and *I*‐*V* responses [18]. Depending upon the application involved, the type of detector will be used. Comparison of different parameters has been shown in **Table 1**. Also, the fabrication of geometric diodes and other geometric field enhancement diodes is much more complex and expensive in com‐ parison with MIM diodes. Detailed comparative analysis of these diodes has been discussed in our previous publication [19].

#### *3.2.3. Geometric diode*

It is the geometry of the device, which allows a preferential motion of charge carriers in a direction defined by its geometry. The asymmetric structure of the device forces to flow the charge carriers in one direction only, and it rectifies an alternating current as a result and hence gives the diode like actions. The diode acts as a funnel for the flow of carriers moving from left to right or right to left (depends on the design of geometry), with restricted flow in one direction and ease flow in other direction as shown in **Figure 4**. These diodes are able to convert from few THz to 1000 THz into usable DC [20].

**Table 1.** Comparison of High high‐fFrequency detectors.

**Figure 4.** Schematic diagram of geometric diode.

## **4. Metal‐insulator‐metal (MIM) diodes**

MIM diodes are the successor of Schottky diodes. The Schottky diodes have been rou‐ tinely used for high‐frequency applications like rectification and mixing. MIM diodes are not influenced by parasitic capacitors as compared to Schottky diodes. Extremely broader bandwidth, minute size, room temperature operation, effortlessly integrability with CMOS innovation [28], and zero bias voltage requirements are different encouraging factors for MIM diodes to be investigated with increased interest from last two decades. It is an excep‐ tionally encouraging innovation for terahertz regime. The femtosecond quick exchanging time of these diodes makes them valuable in numerous low power and ultra‐fast appli‐ cations going from rectenna sunlight‐based cells as an energy harvesting device [29] to imaging, sensing, and hot electron transistors, also in numerous refined microelectronic products such as switching memories, display devices (LCD backplanes), field emission cathodes [30], etc. MIM diode is a thin‐film device in which the electrons tunnel through the insulator layer from the first metal layer to the second metal. **Figure 5** shows the schematic representation of the MIM diode.

Charge transport across the insulator occurs due to quantum‐mechanical tunneling of elec‐ trons. A similar concept applies to MIM diodes, the process in which a particle penetrates or tunnel via the energy barrier in place of jump over it in contrary to classical mechanics. When the electron reaches the barrier, the insulator in between the metal electrodes absorbs some part of the energy passes through the barrier reaching to the second electrode, but another part reflects back due to some irregularities in thee insulator. How much the energy absorbs and reflected originally depends upon the different characteristics of material used such as work‐function difference of metal electrodes, electron affinity of the insulator material, thick‐ ness of the barrier, applied potential energy, etc.

**Figure 5.** MIM diode schematic diagram.

**4. Metal‐insulator‐metal (MIM) diodes**

**Figure 4.** Schematic diagram of geometric diode.

90 Terahertz Spectroscopy - A Cutting Edge Technology

representation of the MIM diode.

ness of the barrier, applied potential energy, etc.

MIM diodes are the successor of Schottky diodes. The Schottky diodes have been rou‐ tinely used for high‐frequency applications like rectification and mixing. MIM diodes are not influenced by parasitic capacitors as compared to Schottky diodes. Extremely broader bandwidth, minute size, room temperature operation, effortlessly integrability with CMOS innovation [28], and zero bias voltage requirements are different encouraging factors for MIM diodes to be investigated with increased interest from last two decades. It is an excep‐ tionally encouraging innovation for terahertz regime. The femtosecond quick exchanging time of these diodes makes them valuable in numerous low power and ultra‐fast appli‐ cations going from rectenna sunlight‐based cells as an energy harvesting device [29] to imaging, sensing, and hot electron transistors, also in numerous refined microelectronic products such as switching memories, display devices (LCD backplanes), field emission cathodes [30], etc. MIM diode is a thin‐film device in which the electrons tunnel through the insulator layer from the first metal layer to the second metal. **Figure 5** shows the schematic

Charge transport across the insulator occurs due to quantum‐mechanical tunneling of elec‐ trons. A similar concept applies to MIM diodes, the process in which a particle penetrates or tunnel via the energy barrier in place of jump over it in contrary to classical mechanics. When the electron reaches the barrier, the insulator in between the metal electrodes absorbs some part of the energy passes through the barrier reaching to the second electrode, but another part reflects back due to some irregularities in thee insulator. How much the energy absorbs and reflected originally depends upon the different characteristics of material used such as work‐function difference of metal electrodes, electron affinity of the insulator material, thick‐

Due to the tunneling phenomena involved, the energy beam absorbed makes it travel at ultra‐ high speeds. The speed is much higher than that on a normal conductor or a semiconduc‐ tor like silicon. Tunneling is a highly nonlinear phenomenon, and hence these diodes are exploited for the rectification, mixing, and detection of alternating currents at very high fre‐ quencies even in the THz spectral region. **Figure 6** shows the typical schematic diagram of the MIM band and Fowler‐Nordheim tunneling method.

The frequency response of the diode is governed by the resistor‐capacitor (RC) time constant. Higher the cutoff frequency, lower the capacitance and resistance. The impedance of the diode must be close enough to the impedance of the antenna for maximum power transfer, and the capacitance must be minimized for operation at THz frequencies. Thickness cannot be decreased to facilitate tunneling, hence only area is left to be altered. Krishnan et al. report

**Figure 6.** A typical schematic of MIM band diagram with FN tunneling.

the fabrication of terahertz MIM diode, the atto‐farad capacitance (10–18) is required, which further needs the device areas less than 100 nm × 100 nm for typical material systems which require e‐beam lithography. However, this challenge of fabricating such nanoscale geometry has been overcome to a large extent by using modern lithographic techniques. Miniature‐ size diodes can be fabricated with the modern ultra‐fine deposition techniques, such as e‐beam lithography, reactive sputtering, etc., with small size, higher cutoff frequency, and thin insulating layer. **Table 2** lists these features of the MIM devices fabricated using dif‐ ferent techniques.**Table 2** lists these features of the MIM devices fabricated using different techniques.

Trade‐off between asymmetry and diode junction resistance can be overcome using multi‐ layer structure [34]. In comparison to the MIM diode, the tunneling current can be easily engineered in such multi‐insulator MIM diodes by simply altering the type and number of insulators. This changes the number of barrier heights present at each interface and conse‐ quently the *I‐V* characteristics. Reasonably good results for asymmetry and nonlinearity have been obtained by means of such diodes [35]. Among multi‐insulator MIMs, the double insula‐ tor (MIIM) is a common preference [36].

## **4.1. Tunneling probability**

Probability of electrons penetrating the insulator layer is called as tunneling probability. It is also called as transport probability, it is the ratio of the number of electrons incident on the interface with the electrons that reflected back. In order to tunneling take place, the two metals having insulators in between must have different work functions. When a bias voltage is applied between the metal electrodes of MIM diode, the potential barrier is reduced and charge carriers start tunneling between the two electrodes through the thin‐film. Tunneling occurs in either direction, but by proper choice of metal electrodes with different work func‐ tions, the magnitude of the tunneling current will become larger in one direction than in the other direction. The tunneling current is calculated using Simmon's method [37]. This method assumes low‐temperature operation hence neglecting thermal currents and incorporating tunneling as the main electron transport mechanism.

**Table 2.** Comparison of MIM diodes fabricated using latest deposition methods.

Different algorithms such as TMM (transmission matrix method), WKB (Wentzel, Kramers, and Brillouin) approximation, and NEGF (nonequilibrium green function method) have been developed for calculating the tunneling probability. Simmon provides an approxima‐ tion of a finite probability of electron penetration in the course of the insulator using WKB method. But this method overvalued the transport current. NEGF formulism is the method used to calculate the tunneling transmission probability. Using NEGF approximation, the total number of electrons emitted from left electrode and right electrode can be taken into consideration by defining two quantities, namely, left electrode and right electrode coupling functions, **Figure 7** shows the application of NEGF formulism to MIM diode [38]. TMM is also a powerful fast method for the analysis of transport properties of nonhomogeneous systems [32]. These algorithms have been compared for the calculation of transport probability in [39].

the fabrication of terahertz MIM diode, the atto‐farad capacitance (10–18) is required, which further needs the device areas less than 100 nm × 100 nm for typical material systems which require e‐beam lithography. However, this challenge of fabricating such nanoscale geometry has been overcome to a large extent by using modern lithographic techniques. Miniature‐ size diodes can be fabricated with the modern ultra‐fine deposition techniques, such as e‐beam lithography, reactive sputtering, etc., with small size, higher cutoff frequency, and thin insulating layer. **Table 2** lists these features of the MIM devices fabricated using dif‐ ferent techniques.**Table 2** lists these features of the MIM devices fabricated using different

Trade‐off between asymmetry and diode junction resistance can be overcome using multi‐ layer structure [34]. In comparison to the MIM diode, the tunneling current can be easily engineered in such multi‐insulator MIM diodes by simply altering the type and number of insulators. This changes the number of barrier heights present at each interface and conse‐ quently the *I‐V* characteristics. Reasonably good results for asymmetry and nonlinearity have been obtained by means of such diodes [35]. Among multi‐insulator MIMs, the double insula‐

Probability of electrons penetrating the insulator layer is called as tunneling probability. It is also called as transport probability, it is the ratio of the number of electrons incident on the interface with the electrons that reflected back. In order to tunneling take place, the two metals having insulators in between must have different work functions. When a bias voltage is applied between the metal electrodes of MIM diode, the potential barrier is reduced and charge carriers start tunneling between the two electrodes through the thin‐film. Tunneling occurs in either direction, but by proper choice of metal electrodes with different work func‐ tions, the magnitude of the tunneling current will become larger in one direction than in the other direction. The tunneling current is calculated using Simmon's method [37]. This method assumes low‐temperature operation hence neglecting thermal currents and incorporating

> **Patterning technique**

lithography

lithography

lithography

sputtering

**Cut‐off freq. Ref.**

30 THz [31]

94 GHZ [32]

343 THz [33]

– [34]

techniques.

**MIM combination**

tor (MIIM) is a common preference [36].

92 Terahertz Spectroscopy - A Cutting Edge Technology

tunneling as the main electron transport mechanism.

**Active area Insulator**

Ni‐NiO‐Ni 110 × 110 nm2 3.5 nm E‐beam

Ni‐NiO‐Cr 1.6 μm2 3 nm E‐beam

Ni‐NiO‐Ag 3.1 × 10‐4 μm2 6 nm E‐beam

Cu‐CuO‐Cu 2 × 2 μm2 2 nm E‐beam/reactive

**Table 2.** Comparison of MIM diodes fabricated using latest deposition methods.

**thickness**

**4.1. Tunneling probability**

**Figure 7.** A typical schematic diagram showing the application of NEGF to MIM diode.

## **5. Modeling, fabrication and characterization of MIM diodes**

An efficient modeling of MIM diode enables an investigator to predict the optimum material com‐ bination and device geometry for desired current‐voltage characteristics. The accuracy of model‐ ing depends on correct estimation of modeling parameters such as applied bias voltage, barrier thickness, dielectric constants of the insulator layers, and effective mass (assumed to be unity).

Various research groups are working to design an accurate model for rectenna system using dif‐ ferent software such MATLAB, PSpice, Mathemaica, ADS, etc. [32, 34, 40]. We had not combined MIM diodes with antennas but modeled and fabricated MIM diodes for using two combinations (aluminum and chromium) of materials and assume the incoming energy to diode to be 100%.

## **5.1. Simulation**

The diodes are simulated using model [35] and fabricated diode's *I‐V* characteristics have been verified also. This simulator models the current density of MIM diode assuming a perfect insulator for a solely tunneling based analysis of current‐voltage characteris‐ tics. We simulated the MIM diode for (Al‐Al2 o3‐ Cr) in MATLAB for current density and nonlinearity.

## **5.2. Fabrication**

For the fabrication of MIM diode, the aluminum (work function of 4.28 eV) is used for the preparation of bottom electrode, and aluminum dioxide Al2 o3 (having electron affinity of 1.25 eV) is used as a barrier layer after plasma oxidation of aluminum. For top electrode, chro‐ mium (work function of 4.5 eV**)** is used having sufficiently higher work function comparative to other materials with similar work function like titanium, niobium, and silver [41]. The basic steps involved in the fabrication of MIM diodes are explained below.

## *5.2.1. Substrate preparation*

The substrate on which the MIM diode is fabricated is a microscopic glass slide, which is optically flat and smooth on both sides. The reason for using a glass substrate is that it is a transparent material, inexpensive, and readily available. Yet another reason is that, if silicon is used, it needs to be oxidized to form SiO2 before the metals could be deposited to prevent the shorting of metals with the substrate. Since glass as a substrate needs to be cleaned before fabrication. Complete process for cleaning has been followed as explained in Ref. [34].

#### *5.2.2. Metal deposition*

The metal deposition for MIM diode is done by thermal evaporation. Two fundamental requirements for the metal to be deposited are the adhesion to the substrate and a smooth uni‐ form surface. The fabricated MIM diode with the base metal made of aluminum and top metal made of chromium. The metal has been deposited using smart coat 3.0 by the thermal evapo‐ ration method. This method involves heating a material to the point of evaporation, usually at a pressure of around 10–6 mbar. The smart coat is cleaned properly by IPA or acetone. Using an automatic mode, the materials are loaded in the chamber and the deposition has been done. Thickness is accumulated at the program deposition rate. Basically, the potentiometer is controlling the current supply providing to the filament and accordingly controlling the rate of thickness Å/s, more the current more the rate of thickness. The rate of deposition can be monitored on the display panel. Mainly the current value is dependent on the material that are depositing, for example, for Al (aluminum) it will have 10—15 A current value and for Au (gold) it required > 70 A. Vibrations on the crystal holder produced due to external factors or due to internal vacuum pumps inside the smart coat 3.0 may vary the readings on thickness monitor. For the desired thickness (kÅ) continuous monitoring is required. **Figure 8** shows the smart coat 3.0 used for metal deposition.

**Figure 8.** (a) Smart coat machine for thin‐film deposition (b) Inside smart coat chamber.

## *5.2.3. Oxide formation*

**5.1. Simulation**

nonlinearity.

**5.2. Fabrication**

*5.2.1. Substrate preparation*

in Ref. [34].

*5.2.2. Metal deposition*

tics. We simulated the MIM diode for (Al‐Al2

94 Terahertz Spectroscopy - A Cutting Edge Technology

preparation of bottom electrode, and aluminum dioxide Al2

silicon is used, it needs to be oxidized to form SiO2

the smart coat 3.0 used for metal deposition.

steps involved in the fabrication of MIM diodes are explained below.

The diodes are simulated using model [35] and fabricated diode's *I‐V* characteristics have been verified also. This simulator models the current density of MIM diode assuming a perfect insulator for a solely tunneling based analysis of current‐voltage characteris‐

For the fabrication of MIM diode, the aluminum (work function of 4.28 eV) is used for the

1.25 eV) is used as a barrier layer after plasma oxidation of aluminum. For top electrode, chro‐ mium (work function of 4.5 eV**)** is used having sufficiently higher work function comparative to other materials with similar work function like titanium, niobium, and silver [41]. The basic

The substrate on which the MIM diode is fabricated is a microscopic glass slide, which is optically flat and smooth on both sides. The reason for using a glass substrate is that it is a transparent material, inexpensive, and readily available. Yet another reason is that, if

to prevent the shorting of metals with the substrate. Since glass as a substrate needs to be cleaned before fabrication. Complete process for cleaning has been followed as explained

The metal deposition for MIM diode is done by thermal evaporation. Two fundamental requirements for the metal to be deposited are the adhesion to the substrate and a smooth uni‐ form surface. The fabricated MIM diode with the base metal made of aluminum and top metal made of chromium. The metal has been deposited using smart coat 3.0 by the thermal evapo‐ ration method. This method involves heating a material to the point of evaporation, usually at a pressure of around 10–6 mbar. The smart coat is cleaned properly by IPA or acetone. Using an automatic mode, the materials are loaded in the chamber and the deposition has been done. Thickness is accumulated at the program deposition rate. Basically, the potentiometer is controlling the current supply providing to the filament and accordingly controlling the rate of thickness Å/s, more the current more the rate of thickness. The rate of deposition can be monitored on the display panel. Mainly the current value is dependent on the material that are depositing, for example, for Al (aluminum) it will have 10—15 A current value and for Au (gold) it required > 70 A. Vibrations on the crystal holder produced due to external factors or due to internal vacuum pumps inside the smart coat 3.0 may vary the readings on thickness monitor. For the desired thickness (kÅ) continuous monitoring is required. **Figure 8** shows

o3‐ Cr) in MATLAB for current density and

(having electron affinity of

before the metals could be deposited

o3

After loading the material in the smart coat chamber, the insulator layer is formed (using an aluminum mask) by the exposure of the oxygen (or argon) gas applied at a rate of 20–30 psi for a period of 10–12 minutes to form the plasma oxides on the base metal. Comparatively, thin layer with low impedance is formed in comparison with other deposition techniques. Similar procedure is used for the formation of top metal electrode as used for bottom electrode. After following the procedure completely, the final device is formed. **Figure 9** shows the fabricated diode and the mask used.

**Figure 9.** Fabricated MIM diode and mask used.

## **5.3. Electrical characterization**

#### *5.3.1. Experimental setup*

The setup to measure the *I*(*V*) characteristics of the fabricated MIM tunnel diode is shown in **Figure 10**. It includes a probe station and a source meter. The source meter used is Model 2450 from Keithley instruments, which can vary the voltage from ±5 μV to ±220 V and can measure current from ±10 pA to ±1 A. The source measurement unit is connected to a probe station, and the measurements were taken so that any interference affecting the device could be avoided. This way a test bed was set up in which the device testing has been conducted. **Figure 10** shows the DC setup used for characterization of diode. The voltage sweep range set to be in low voltage in the values of few millivolts and the maximum sweep was selected from 1 to +1 V in order to get a proper *I*‐*V* characteristics.

**Figure 10.** Setup for DC characterization.

Chromium is supplied with a negative bias and aluminum is supplied with a positive bias. The probes are lowered very slowly, so that the probe making contact with the top electrode is just in contact and the second probe on the bottom electrode is scraped a bit to ensure the proper contact with the aluminum.

#### *5.3.2. DC characteristics*

The DC characteristics of the fabricated diode MIM (Al‐Al2 o3 ‐Cr) include the *I*(*V*) character‐ istics and the nonlinear characteristics. The responses of the diode are obtained with a biased voltage of ±1 V. The *I*(*V*) characteristics of the fabricated MIM diode which are compared with the simulated one and are in good agreement with is shown in **Figure 11**. Maximum current density observed is 2 × 10‐4 A/cm2 at 0.8 V which is less in comparison with the theoretical Terahertz Detectors (THzDs): Bridging the Gap for Energy Harvesting http://dx.doi.org/10.5772/66347 97

**Figure 11.** Measured DC characteristics of (Al‐Al2 o3‐ Cr) MIM diode, (a) current versus voltage relationship (b) nonlinearity versus voltage.

value due to some imperfections in fabrication processes. However, the nonlinearity is approximately 3.1, which is close to the modeled value.

#### **6. Conclusion**

**5.3. Electrical characterization**

96 Terahertz Spectroscopy - A Cutting Edge Technology

1 to +1 V in order to get a proper *I*‐*V* characteristics.

proper contact with the aluminum.

**Figure 10.** Setup for DC characterization.

The DC characteristics of the fabricated diode MIM (Al‐Al2

A/cm2

*5.3.2. DC characteristics*

density observed is 2 × 10‐4

The setup to measure the *I*(*V*) characteristics of the fabricated MIM tunnel diode is shown in **Figure 10**. It includes a probe station and a source meter. The source meter used is Model 2450 from Keithley instruments, which can vary the voltage from ±5 μV to ±220 V and can measure current from ±10 pA to ±1 A. The source measurement unit is connected to a probe station, and the measurements were taken so that any interference affecting the device could be avoided. This way a test bed was set up in which the device testing has been conducted. **Figure 10** shows the DC setup used for characterization of diode. The voltage sweep range set to be in low voltage in the values of few millivolts and the maximum sweep was selected from

Chromium is supplied with a negative bias and aluminum is supplied with a positive bias. The probes are lowered very slowly, so that the probe making contact with the top electrode is just in contact and the second probe on the bottom electrode is scraped a bit to ensure the

istics and the nonlinear characteristics. The responses of the diode are obtained with a biased voltage of ±1 V. The *I*(*V*) characteristics of the fabricated MIM diode which are compared with the simulated one and are in good agreement with is shown in **Figure 11**. Maximum current

o3

at 0.8 V which is less in comparison with the theoretical

‐Cr) include the *I*(*V*) character‐

*5.3.1. Experimental setup*

Accurate detection and modeling of transport characteristics for any energy harvesting sys‐ tem are the main key for the calculation of *I‐V* characteristics of MIM diode for THz energy harvesting applications. Due mainly to some poor characteristics of detector system the most of the harvesting systems suffered a lot. The MIM diode is one of the most suitable candidates for terahertz energy harvesting applications. Most of the innovations are the results of anterior modeling and fabrication of the device that result in better performance of the detectors. Since the future MIM detectors may incorporate thin insulating layers, which may result in structural irregularities; therefore, its meticulous modeling and optimum fabrication methods are essential before the actual implementation of the device. Potential approaches and factors responsible for designing most critical rectifying diodes operating at terahertz frequencies are discussed, and one of the material combinations for MIM diode has been modeled and characterized. It will augment the capability of naive researchers in identifying the latest THzDs to be used, their limitations and solutions to derive new paths for energy harvesting.

#### **Acknowledgements**

The author is grateful for providing research facilities in support of this work by Director of University Institute of Engineering & Technology, Kurukshetra University, Kurukshetra. The author acknowledges the help of Prof. R.K. Aggarwal, Computer Engineering Department, National Institute of Technology, Kurukshetra for various supports.

## **Author details**

Kapil Bhatt\*, Shilpi Shriwastava, Sandeep Kumar, Sandeep and Chandra Charu Tripathi

\*Address all correspondence to<EMAIL_ADDRESS>
University Institute of Engineering & Technology, Kurukshetra University, Kurukshetra, Haryana, India

## **References**

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**Author details**

98 Terahertz Spectroscopy - A Cutting Edge Technology

Haryana, India

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Kapil Bhatt\*, Shilpi Shriwastava, Sandeep Kumar, Sandeep and Chandra Charu Tripathi

University Institute of Engineering & Technology, Kurukshetra University, Kurukshetra,

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#### **THz Metamaterial Characterization Using THz-TDS THz Metamaterial Characterization Using THz-TDS**

Christopher H. Kodama and Ronald A. Coutu, Jr. Christopher H. Kodama and Ronald A. Coutu,Jr.

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67088

#### **Abstract**

The purpose of this chapter is to familiarize the reader with metamaterials and describe terahertz (THz) spectroscopy within metamaterials research. The introduction provides key background information on metamaterials, describes their history and their unique properties. These properties include negative refraction, backwards phase propagation, and the reversed Doppler Effect. The history and theory of metamaterials are discussed, starting with Veselago's negative index materials work and Pendry's publications on physical realization of metamaterials. The next sections cover measurement and analyses of THz metamaterials. THz Time-domain spectroscopy (THz-TDS) will be the key measurement tool used to describe the THz metamaterial measurement process. Sample transmission data from a metamaterial THz-TDS measurement is analyzed to give a better understanding of the different frequency characteristics of metamaterials. The measurement and analysis sections are followed by a section on the fabrication process of metamaterials. After familiarizing the reader with THz metamaterial measurement and fabrication techniques, the final section will provide a review of various methods by which metamaterials are made active and/or tunable. Several novel concepts were demonstrated in recent years to achieve such metamaterials, including photoconductivity, high electron mobility transistor (HEMT), microelectromechanical systems (MEMS), and phase change material (PCM)-based metamaterial structures.

The views expressed in this paper are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government.

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

**Keywords:** terahertz, THz, time domain spectroscopy, TDS, metamaterials

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is 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.

## **1. Introduction to metamaterials**

In the field of metamaterials, "meta-atoms" are designed to resonate and respond to stimuli in a similar manner as normal atoms [1]. From a macroscopic perspective, a matrix of meta-atoms can be viewed as a single homogeneous material—a metamaterial—with altered material properties. Meta-atoms designed to have electric and magnetic resonances are the base units of electromagnetic metamaterials. These have altered values of electric permittivity and magnetic permeability, which in turn results in a modified refractive index. Acoustic metamaterials, with altered mechanical material properties such as Poisson's ratio, have also been demonstrated [2]. In order to rightfully be considered a homogenous material, the metaatoms constituting the metamaterial must be sufficiently small. For electromagnetic metamaterials, the meta-atoms need to be at least an order of magnitude smaller than the wavelengths of the desired operating frequencies [1].

Metamaterials are appealing because they offer the ability to artificially create custom material characteristics without having to depend on the fixed characteristics of existing materials. These metamaterials can further be designed to have exotic properties that are not seen in any naturally existing material. V.G. Veselago, in a paper in 1968, first considered and described the physical properties that would result from a material with simultaneously negative values of permittivity and permeability. Such materials are known as left-handed media [1].

In left-handed media, electromagnetic waves have a negative phase velocity; the waves seemingly follow the left-handed rule instead of the normal right-handed one. In addition, negative refraction occurs on a boundary between an ordinary and a left-handed medium. Negative refraction also causes left-handed concave and convex lenses to have reversed behavior: the concave lens converges light, and the convex lens diverges it. Surprisingly, a flat slab of lefthanded material can even be used as a focusing lens [3]. Other exotic effects include inverse versions of the Doppler effect, Cerenkov radiation (i.e., radiative shock waves induced by high-speed particles traveling through a dielectric medium), and the Goos-Hänchen shift (i.e., a finite lateral shift between an incident beam of finite extent and its reflected counterpart) [1].

Although Veselago had established theoretical groundwork for understanding the nature negative-index materials, experimental demonstration of these materials would not occur for a few decades [3, 4].

J.B. Pendry in the late 1990s was the first to examine the fabrication of materials with customizable permeabilities and permittivities. In 1998, he demonstrated that a thin wire mesh would excite low frequency plasmons, meaning that the mesh would have customized, effective homogeneous permittivity values at low frequencies; these values could even be negative [4]. Soon after, he proposed a split-ring resonator (SRR) structure, which could analogously produce a desired effective permeability (including negative values) in the GHz range [5]. The structures he proposed, especially the split-ring resonator structure, are now ubiquitous in metamaterial devices at all frequencies.

The unique properties of metamaterials make them prime targets for research in the RF and photonics fields, where metamaterials can be used for enhanced lensing, beam steering, and phase shifting. Metamaterials have also greatly advanced the field of cloaking by freeing up the restriction of only being able to use naturally occurring materials and their material properties. Several different cloaking techniques have been demonstrated using metamaterials [6–9].

The THz spectrum has much potential for future applications, ranging from communications to bomb screening. Unfortunately, since most materials are transparent in the THz frequency range, controlling THz waves via modulation and other techniques is much more difficult when compared to electronic or optical devices and systems; metamaterials can help fill this technology gap. Since metamaterials can, in theory, be specifically designed to have certain material properties, they can interact with THz waves in ways that normal materials cannot [10, 11].

## **2. Metamaterials analysis and measurement**

**1. Introduction to metamaterials**

104 Terahertz Spectroscopy - A Cutting Edge Technology

of the desired operating frequencies [1].

a few decades [3, 4].

metamaterial devices at all frequencies.

In the field of metamaterials, "meta-atoms" are designed to resonate and respond to stimuli in a similar manner as normal atoms [1]. From a macroscopic perspective, a matrix of meta-atoms can be viewed as a single homogeneous material—a metamaterial—with altered material properties. Meta-atoms designed to have electric and magnetic resonances are the base units of electromagnetic metamaterials. These have altered values of electric permittivity and magnetic permeability, which in turn results in a modified refractive index. Acoustic metamaterials, with altered mechanical material properties such as Poisson's ratio, have also been demonstrated [2]. In order to rightfully be considered a homogenous material, the metaatoms constituting the metamaterial must be sufficiently small. For electromagnetic metamaterials, the meta-atoms need to be at least an order of magnitude smaller than the wavelengths

Metamaterials are appealing because they offer the ability to artificially create custom material characteristics without having to depend on the fixed characteristics of existing materials. These metamaterials can further be designed to have exotic properties that are not seen in any naturally existing material. V.G. Veselago, in a paper in 1968, first considered and described the physical properties that would result from a material with simultaneously negative values

In left-handed media, electromagnetic waves have a negative phase velocity; the waves seemingly follow the left-handed rule instead of the normal right-handed one. In addition, negative refraction occurs on a boundary between an ordinary and a left-handed medium. Negative refraction also causes left-handed concave and convex lenses to have reversed behavior: the concave lens converges light, and the convex lens diverges it. Surprisingly, a flat slab of lefthanded material can even be used as a focusing lens [3]. Other exotic effects include inverse versions of the Doppler effect, Cerenkov radiation (i.e., radiative shock waves induced by high-speed particles traveling through a dielectric medium), and the Goos-Hänchen shift (i.e., a finite lateral shift between an incident beam of finite extent and its reflected counterpart) [1]. Although Veselago had established theoretical groundwork for understanding the nature negative-index materials, experimental demonstration of these materials would not occur for

J.B. Pendry in the late 1990s was the first to examine the fabrication of materials with customizable permeabilities and permittivities. In 1998, he demonstrated that a thin wire mesh would excite low frequency plasmons, meaning that the mesh would have customized, effective homogeneous permittivity values at low frequencies; these values could even be negative [4]. Soon after, he proposed a split-ring resonator (SRR) structure, which could analogously produce a desired effective permeability (including negative values) in the GHz range [5]. The structures he proposed, especially the split-ring resonator structure, are now ubiquitous in

The unique properties of metamaterials make them prime targets for research in the RF and photonics fields, where metamaterials can be used for enhanced lensing, beam steering, and

of permittivity and permeability. Such materials are known as left-handed media [1].

The split-ring resonator (SRR) is the fundamental unit cell used in a multitude of metamaterial designs. SRRs are split metallic rings meant to resonate with electromagnetic waves with wavelengths much larger than the SRR structure itself. The first SRRs proposed by Pendry consisted of two circular, concentric rings with oppositely oriented splits, as shown in **Figure 1(a)**. For the sake ease of fabrication, SRRs fabricated in the THz range typically have a much simpler geometry, like shown in **Figure 1(b)**.

 **Figure 1.** (a) Original SRR proposed by Pendry [5]; (b) a typical SRR shape used in THz research [11], [12].

SRRs are not the only method of generating resonant responses. In theory, pieces of metal in any geometrical shape can act as a resonator, albeit at frequencies determined by the geometry. For example, simple structures like rectangular hole arrays support surface wave polariton (SPP) resonant modes that can also strongly enhance incident radiation [13]. However, SRRs have two characteristics that make them desirable for use in metamaterials. SRRs couple well with the magnetic portion of electromagnetic waves due to its loop-like structure. This potentially allows SRR-containing metamaterials to have a frequency band with negative permeability. Second, the SRR is easily conceptualized as a simple LC resonator circuit, which allows one to quickly predict its lowest order resonant frequency.

As a basic approximation, the entire loop (ignoring the gap) can be viewed as a one-turn inductor and the gap as a parallel-plate capacitor. These quasi-static assumptions require that the dimensions of the SRR be much smaller than the wavelength of electromagnetic fields assumed to be passing through them (which conveniently is the same requirement needed to produce effective material properties from metamaterials). The wavelength of electromagnetic waves in free space at 1 THz is 300 μm, so metamaterial unit cells designed to operate in this range should have maximum dimensions of around 30 μm or less. The basic equations for the inductance, capacitance, and estimated resonant frequency of an SRR are

$$L \approx \mu\_0 \frac{l^2}{l'}, \quad C \approx \varepsilon\_r \varepsilon\_0 \frac{tw}{\mathcal{S}}, \quad f\_{\text{ideal}} \approx \frac{1}{2\pi} \sqrt{\frac{1}{LC}} = \frac{1}{2\pi} \frac{c}{\sqrt{\varepsilon\_r}} \sqrt{\frac{\mathcal{S}}{w}} \tag{1}$$

where *t* is the thickness of the SRRs, *μ*<sup>0</sup> is the vacuum permeability constant, *l* is the SRR side length, *ε<sup>r</sup>* is the relative dielectric constant, *ε*<sup>0</sup> is the vacuum permittivity constant, *w* is the line width of the SRR, *g* is the gap width of the SRR, and *c* is the speed of light in vacuum [14–16].

The lowest order resonance of a SRR can be adequately described by a quasistatic model incorporating capacitors and resistors. The higher order resonances, however, cannot be modeled in such a manner. The original SRR in **Figure 1(a)** can be modeled as a transmission line due to the presence of two distinct conductors [17]. The single SRR in **Figure 1(b)** has only one conductor and thus cannot support any transmission line modes. Single SRRs can, however, produce evanescent modes confined to the surface, known as plasmon resonances. These plasmonic resonances occur at frequencies that have half-wavelengths, which are multiples m of the perimeter of the SRR:

$$f\_{\rm dip} = \frac{c}{\sqrt{\overline{\xi\_{\rm ref}}}} \frac{1}{\{4l - g\}} \quad \frac{m}{2} \,\, l \tag{2}$$

where c is the speed of light, *ε<sup>r</sup>*. *eff* is the effective permittivity constant, l is the side length of the square SRR, g is the gap length of the SRR, and m is a positive integer greater than or equal to 1 [1]. The LC resonance can also be interpreted as the lowest order plasmon resonance, with *m* = 1 in the above equation [18].

These equations can only serve as basic approximations for the resonant frequency of a fabricated SRR. This is especially true in the THz range, where limits on feature size lead to nonidealities such as significant wire widths and gap widths. If more accuracy is desired, more precise equations need to be utilized for the inductance and capacitance terms for the LC resonant frequency, and the effective SRR perimeter seen by higher order resonances will differ from simply (4*l* − *g*), depending on the current distribution through the width of SRR. As an alternative to an analytical solution, SRRs can instead be modeled using electromagnetic simulation software such as CST Microwave Studio, ANSYS HFSS, or COMSOL.

Fabricated THz SRRs can be measured and characterized using THz time-domain spectroscopy (THz-TDS). In THz-TDS, broadband THz pulses are passed through a sample and then are Fourier transformed to obtain spectral data. A simple planar SRR structure is shown in **Figure 2(a)**. The SRR is patterned in a layer of gold, atop an intrinsic silicon substrate; the alignment of the incident THz pulse is shown in the inset. The solid line in **Figure 2(b)** represents the transmission coefficient of the THz pulse passed through the sample. The LC resonance can be seen at approximately 0.8 THz, and a higher order resonance can be seen near 2.2 THz. The dashed orange line represents simulated data from models in CST Microwave Studio. The simulated data agree well with measured data, except in the region near 2.5 THz and above. In this region, the spectral power in the measured signal is not strong enough to fully resolve the sharp, higher order resonances of the SRR [19].

**Figure 2.** Gold SRRs on silicon substrate are shown in (a). The SRRs are 20-μm squares with 5-μm line widths, 3-μm gap widths, and a 39-μm periodicity. The plot in (b) displays the transmission response of the SRRs [19].

## **3. Fabrication processes**

the dimensions of the SRR be much smaller than the wavelength of electromagnetic fields assumed to be passing through them (which conveniently is the same requirement needed to produce effective material properties from metamaterials). The wavelength of electromagnetic waves in free space at 1 THz is 300 μm, so metamaterial unit cells designed to operate in this range should have maximum dimensions of around 30 μm or less. The basic equations for

> ideal <sup>≈</sup> \_\_\_1 2*π* √

width of the SRR, *g* is the gap width of the SRR, and *c* is the speed of light in vacuum [14–16]. The lowest order resonance of a SRR can be adequately described by a quasistatic model incorporating capacitors and resistors. The higher order resonances, however, cannot be modeled in such a manner. The original SRR in **Figure 1(a)** can be modeled as a transmission line due to the presence of two distinct conductors [17]. The single SRR in **Figure 1(b)** has only one conductor and thus cannot support any transmission line modes. Single SRRs can, however, produce evanescent modes confined to the surface, known as plasmon resonances. These plasmonic resonances occur at frequencies that have half-wavelengths, which are multiples

\_\_\_ \_\_\_1 *LC* <sup>=</sup> \_\_\_1 2*π* \_\_*c* √ \_\_ *ε<sup>r</sup>* √

\_\_\_ *g*\_\_\_ *w l*

is the vacuum permeability constant, *l* is the SRR side

is the vacuum permittivity constant, *w* is the line

<sup>2</sup> , (2)

<sup>2</sup> , (1)

the inductance, capacitance, and estimated resonant frequency of an SRR are

\_\_\_ *tw <sup>g</sup>* , *f*

dip <sup>=</sup> \_\_\_\_ *<sup>c</sup>* √ \_\_\_\_ *ε<sup>r</sup>*.eff

simulation software such as CST Microwave Studio, ANSYS HFSS, or COMSOL.

\_\_\_\_\_ 1 (4*<sup>l</sup>* <sup>−</sup> *<sup>g</sup>*) \_\_ *m*

where c is the speed of light, *ε<sup>r</sup>*. *eff* is the effective permittivity constant, l is the side length of the square SRR, g is the gap length of the SRR, and m is a positive integer greater than or equal to 1 [1]. The LC resonance can also be interpreted as the lowest order plasmon resonance, with

These equations can only serve as basic approximations for the resonant frequency of a fabricated SRR. This is especially true in the THz range, where limits on feature size lead to nonidealities such as significant wire widths and gap widths. If more accuracy is desired, more precise equations need to be utilized for the inductance and capacitance terms for the LC resonant frequency, and the effective SRR perimeter seen by higher order resonances will differ from simply (4*l* − *g*), depending on the current distribution through the width of SRR. As an alternative to an analytical solution, SRRs can instead be modeled using electromagnetic

Fabricated THz SRRs can be measured and characterized using THz time-domain spectroscopy (THz-TDS). In THz-TDS, broadband THz pulses are passed through a sample and then are Fourier transformed to obtain spectral data. A simple planar SRR structure is shown in **Figure 2(a)**. The SRR is patterned in a layer of gold, atop an intrinsic silicon substrate; the alignment of the incident THz pulse is shown in the inset. The solid line in **Figure 2(b)** represents the transmission coefficient of the THz pulse passed through the sample. The LC resonance can be seen at approximately 0.8 THz, and a higher order resonance can be seen near 2.2 THz. The dashed orange line represents simulated data from models in CST Microwave Studio. The simulated data agree well with measured data, except in the region near 2.5 THz

*<sup>t</sup>* , *C* ≈ *ε<sup>r</sup> ε*<sup>0</sup>

*l* 2 \_\_

is the relative dielectric constant, *ε*<sup>0</sup>

*L* ≈ *μ*<sup>0</sup>

m of the perimeter of the SRR:

*m* = 1 in the above equation [18].

*f*

length, *ε<sup>r</sup>*

where *t* is the thickness of the SRRs, *μ*<sup>0</sup>

106 Terahertz Spectroscopy - A Cutting Edge Technology

Since THz metamaterial unit cells need to be electrically small compared to THz wavelengths, these unit cells have widths of around 30 μm or less. This requirement makes microfabrication and micro-electromechanical systems (MEMS) processes suitable choices for creating THz metamaterials.

There are three main techniques that can be used to make microelectronic and MEMS devices: bulk micromachining, surface micromachining, and microforming. These techniques are often used interchangeably to create complete devices. In all three of these techniques, the process of photolithography plays an important part in creating devices out of designs.

In photolithography, a layer of photoresist—a polymer sensitive to certain wavelengths of light, typically in the ultraviolet range—is selectively exposed to light screened by a patterned mask. The exposed sections of photoresist undergo a chemical change, depending on the type of polymer. For "positive" photoresist, the exposed photoresist weakens and will dissolve in a developer solution, leaving behind a patterned layer of unexposed photoresist. This patterned photoresist can then act as a mask for other processing steps.

For example, the gold MMs shown in **Figure 2(a)** could have been fabricated using the process shown in **Figure 3**, if a suitably thick photoresist was used along with an evaporation deposition process, which would deposit highly nonconformal material to the edges of the resist. Otherwise, there will be difficulty stripping the photoresist in the fourth step. More advanced, dual-layer photoresist patterning techniques can also be used to create crisper patterns or to be used in conjunction with conformal depositions like sputter deposition. Processes involving the incremental addition and/or etching of thin layers of material on a substrate to create a device are known as surface micromachining techniques.

**Figure 3.** An example of the use of photolithography to deposit gold.

In bulk micromachining techniques, a device is fabricated primarily through etching processes into the bulk of a substrate. Depending on the crystallography of the substrate and the etchant type, etching processes can be isotropic or highly anisotropic in certain crystallographic directions. There are also more involved methods, such as deep reactive ion etching (DRIE), which can create high aspect ratio structures through sequential reactive ion etching (RIE) of a sidewall-protected cavity [20].

Microforming processes, also known as high aspect ratio micromachining (HARM) processes, use especially thick, patterned photoresist layers as molds for metal layers deposited via electroplating. In the LIGA microforming method, X-ray-based photoresist PMMA can be patterned in thick layers without worry of diffraction to create very high aspect ratio devices. LIGA is an acronym for the three German words lithographie, galvanoformung, and abformung, which, respectively, stand for lithography, electroplating, and polymer replication [21–23]. Other thick photoresists such as SU-8, or multiple layers of SF-11, can also be used as a lower cost alternative to create high aspect ratio structures.

In addition to micromachining and MEMS processes, additive manufacturing techniques show promise for THz metamaterials. One advantage to additive manufacturing is the ability to create devices without requiring labor-intensive cleanroom methods. However, the present capability of many of these techniques does not allow for the sub-millimeter resolution needed to create THz metamaterials, with a few exceptions. In inkjet printing, droplets of material are deposited onto a substrate via moving, microscopic nozzle heads. This method has been shown to feasibly fabricate layers of SRRs in the low THz range [24–26]. Inkjet-printed SRRs display resonances that are similar but slightly degraded compared to their cleanroom-fabricated counterparts; the degradation is largely caused by print variation among the individual printed metamaterial unit cells. This can be seen as a tradeoff to the advantages of less labor-intensive processes and lower cost manufacturing offered by inkjet printing. Another additive manufacturing method used to create THz metamaterials is laser decal transfer [27]. In this process, a laser pulse ejects a small amount of material from a donor substrate and transfers it onto an underlying substrate. The process is analogous to a pen (the laser) writing on top of a piece of carbon paper. Using a digital micromirror device (DMD), the laser can be spatially modulated to transfer more complex shapes onto a target substrate. With this process and an x-y stage manipulator, metal SRRs were printed with a minimum feature size of 6 μm [28]. The printed SRRs were highly uniform and produced a transmission response nearly identical to that of a comparable lithographically prepared sample. The main limitation of this method is that the deposited material needs to be cured at around 150°C after laser transfer, so it may not be suitable for some temperature-sensitive applications.

## **4. Active metamaterials**

dual-layer photoresist patterning techniques can also be used to create crisper patterns or to be used in conjunction with conformal depositions like sputter deposition. Processes involving the incremental addition and/or etching of thin layers of material on a substrate to create

In bulk micromachining techniques, a device is fabricated primarily through etching processes into the bulk of a substrate. Depending on the crystallography of the substrate and the etchant type, etching processes can be isotropic or highly anisotropic in certain crystallographic directions. There are also more involved methods, such as deep reactive ion etching (DRIE), which can create high aspect ratio structures through sequential reactive ion etching

Microforming processes, also known as high aspect ratio micromachining (HARM) processes, use especially thick, patterned photoresist layers as molds for metal layers deposited via electroplating. In the LIGA microforming method, X-ray-based photoresist PMMA can be patterned in thick layers without worry of diffraction to create very high aspect ratio devices. LIGA is an acronym for the three German words lithographie, galvanoformung, and abformung, which, respectively, stand for lithography, electroplating, and polymer replication [21–23]. Other thick photoresists such as SU-8, or multiple layers of SF-11, can also be used as

In addition to micromachining and MEMS processes, additive manufacturing techniques show promise for THz metamaterials. One advantage to additive manufacturing is the ability to create devices without requiring labor-intensive cleanroom methods. However, the present capability of many of these techniques does not allow for the sub-millimeter resolution needed to create THz metamaterials, with a few exceptions. In inkjet printing, droplets of material are deposited onto a substrate via moving, microscopic nozzle heads. This method has been shown to feasibly fabricate layers of SRRs in the low THz range [24–26].

a device are known as surface micromachining techniques.

108 Terahertz Spectroscopy - A Cutting Edge Technology

(RIE) of a sidewall-protected cavity [20].

**Figure 3.** An example of the use of photolithography to deposit gold.

a lower cost alternative to create high aspect ratio structures.

Metamaterials with magnetic resonances in the THz range were first demonstrated by Yen et al. in 2004 [29]. They were able to couple into the strong magnetic modes of the SRRs by sending incident waves at oblique angles in ellipsometry experiments. There is much ongoing work on developing active THz metamaterials through several methods [10, 30–32]. In order for metamaterials to be useful in dynamic applications like modulation, they need an incorporated active component that can modify its resonance [33].

An active metamaterial was first demonstrated by Padilla et al. in 2006 [34]. They successfully shunted the SRR response of single SRRs on a GaAs substrate by utilizing an optical pump. Without any optical excitation, the SRRs exhibited a typical LC resonance. When the optical pump was activated, photo-excited carriers spontaneously formed across the entire surface of the GaAs wafer, which made the entire surface metallic. This in turn nullified the effects of the SRRs and their LC resonance. Nonlinear response can also be achieved through SRRs fabricated on doped GaAs layers [35]. With this technique, incident THz radiation of zero to low levels causes a metallic response due to the doped GaAs layer. However, with increasing THz field strengths, inter-valley scattering becomes more frequent, causing decreased carrier mobility and decreased conductivity. With the disappearance of the suppressing GaAs conductivity, the SRR's LC resonance becomes more prominent, and a notch appears in the transmission. At even higher incident radiation strengths, impact ionization in the gaps of the SRRs causes the conductivity to increase again, suppressing SRR response.

Building on those results, Chen and Padilla demonstrated modulation through voltage-biased SRRs on n-GaAs, with the SRR-nGaAs junction acting as a Schottky barrier [30, 31]. This design is shown in **Figure 4**. Without the presence of a bias voltage, the doped layer effectively suppresses the resonant response. When a high enough voltage is applied between the SRRs and the n-GaAs layers, a depletion region will form underneath the SRRs. This depletion region both electrically isolates the SRRs from the n-GaAs layer and reduces the overall conductivity of the n-GaAs layer, which restores the suppressed LC resonance of the SRRs.

**Figure 4.** Schottky-based modulation of split-ring resonator resonances [31].

In 2007, Chen et al. demonstrated modulation via frequency shifting by using SRRs enhanced with silicon in the capacitor gaps, as shown in **Figure 5** [36]. When the SRRs are exposed to laser pulses, photoexcited carriers in the silicon layer effectively lengthen the internal SRR capacitor, which alters the resonant frequency of the structure.

**Figure 5.** Split-ring resonator modulated through photoexcited silicon [36].

Shrekenhamer and Rout created a novel THz metamaterial design that incorporated high electron mobility transistors (HEMTs) in the gaps of SRRs, as shown in **Figure 6** [37]. At zero volts gate bias, a 2D electron gas channel naturally forms between the source and drain of the pHEMT, which is connected to opposite sides of the SRR gap. The electron channel shorts the SRR gap and removes the LC resonance from the structure's response. When a negative bias is applied to the gate, the channel is eliminated, and the SRR LC resonance is restored.

**Figure 6.** High electron mobility transistor–based modulation of split-ring resonators [37].

both electrically isolates the SRRs from the n-GaAs layer and reduces the overall conductivity

In 2007, Chen et al. demonstrated modulation via frequency shifting by using SRRs enhanced with silicon in the capacitor gaps, as shown in **Figure 5** [36]. When the SRRs are exposed to laser pulses, photoexcited carriers in the silicon layer effectively lengthen the internal SRR

Shrekenhamer and Rout created a novel THz metamaterial design that incorporated high electron mobility transistors (HEMTs) in the gaps of SRRs, as shown in **Figure 6** [37]. At zero volts gate bias, a 2D electron gas channel naturally forms between the source and drain of the pHEMT, which is connected to opposite sides of the SRR gap. The electron channel shorts the SRR gap and removes the LC resonance from the structure's response. When a negative bias is applied to the gate, the channel is eliminated, and the SRR LC resonance is restored.

capacitor, which alters the resonant frequency of the structure.

**Figure 5.** Split-ring resonator modulated through photoexcited silicon [36].

**Figure 4.** Schottky-based modulation of split-ring resonator resonances [31].

110 Terahertz Spectroscopy - A Cutting Edge Technology

of the n-GaAs layer, which restores the suppressed LC resonance of the SRRs.

The four previously mentioned active metamaterial structures used electrical properties, namely semiconductors and semiconductor junctions, to achieve tunability in the THz frequency range. Mechanically tunable SRRs have also been demonstrated using MEMS processes. For example, Coutu et al. used MEMs cantilever beams arrays to create electrostatically tunable meta-atoms for the RF frequency range [38]. There have since been several demonstrations of THz-range meta-atoms using MEMS. In one such design, bi-material cantilevers are used to alter the resonance of SRRs, shown in **Figure 7(a)** [39]. In the rest state, the cantilevers are bent downward due to residual stresses intentionally introduced during the fabrication process. This shorts the SRR gaps and eliminates the LC resonance. When the device is heated, thermal expansion coefficient difference between the top and bottom half of the cantilever causes the beam to straighten out, which removes the electrical contact and restores the LC resonance. In [40], the capacitor arms themselves are treated as bi-material cantilevers, which are designed to have an initial deflection and can be lowered with an external voltage. This is shown in **Figure 7(b)**.

Bi-material cantilevers can also be used to actively change the in-plane orientation of SRRs, as shown in **Figure 7(c)** [42]. By changing the angle of incidence of the incoming electromagnetic wave with respect to the metamaterial unit cell, the magnetic coupling between the SRRs and the incident wave can be enhanced, strengthening the measured resonance.

Another method of modulating metamaterials is through the use of phase change materials (PCMs) and metal-insulator transition (MIT) materials. PCMs and MIT materials are types of materials that have vastly different electrical and/or optical properties based on their crystallographic alignment [43, 44]. Transitions between crystalline and amorphous states can be rapidly achieved, typically through thermal, optical, or electrical methods.

Bouyge et al. demonstrated reconfigurable bandpass filtering by using vanadium dioxide (VO<sup>2</sup> ) switches on microwave-range SRRs [45]. Vanadium dioxide is a MIT material with a transition temperature of 340 K and is volatile, meaning that it will revert back to its original phase after cooling back down [44]. With the VO<sup>2</sup> acting as a reconfigurable switch, patches of metal could be electrically connected or disconnected to the SRR, thus altering the resonance of the structure.

**Figure 7.** Various microelectromechanical systems (MEMS) incorporated, terahertz (THz) frequency range, tunable splitring resonator (SRR) elements. In (a), THz SRRs are modulated using bi-material cantilevers [39]. The design in (b) uses bi-material cantilevers for the inner capacitive gap arms of THz SRRs, modulating the SRR resonances when these arms are deflected [40]. The third design in (c) shows structurally tunable THz SRRs which use bi-material cantilevers to bend the entire array in different directions [41].

Active metamaterial structures designed for the near-infrared range, like the one shown in **Figure 8**, have also been demonstrated using vanadium dioxide [46]. The planar hybrid structure shown can switch between electrically appearing as a normal SRR surface when the VO<sup>2</sup> is insulating or a full metal plane when the VO<sup>2</sup> has transitioned to the conductive state. The VO<sup>2</sup> in the self-aligned hybrid SRR causes a slight shift in the resonant frequency of the SRR before and after phase transition.

**Figure 8.** Near-infrared metamaterials with vanadium dioxide (VO<sup>2</sup> ). In (a) and (b), gold split ring resonators (SRRs) are fabricated on a layer of VO<sup>2</sup> . In (a), before transitioning the VO<sup>2</sup> layer, the VO<sup>2</sup> acts like a dielectric and the SRR response is dominates the behavior of the device. In (b), while the VO<sup>2</sup> layer is transitioned, the VO<sup>2</sup> layer is metallic and the overall response of the material is that of a flat metal plane. In (c) and (d), the SRRs have two materials in their thickness, a metal layer and a VO<sup>2</sup> layer. Before transitioning, as shown in (c), the VO<sup>2</sup> is a dielectric, which only affects the response of the SRR with a small red-shift. In (d), during the VO<sup>2</sup> transition to a metallic phase, the effective metal thickness of the SRR doubles, which slightly increases the resonant frequency of the device [46].

Phase change materials have also been applied in some rudimentary metamaterial and plasmonics designs. Phase change materials differ from MIT materials in the fact that their phase transitions are nonvolatile, meaning that an amorphous PCM will crystallize on heating but would not reamorphize on slow cooling. To successfully reamorphize a PCM, a fast heat pulse (from a laser, via Joule heating, etc.) accompanied by a strong heat sink is needed to "melt-quench" the material back into an amorphous phase. A dynamic metamaterial switch using germanium antimony telluride (GST) is shown in **Figure 9**. [47]. This switch is controlled by a short laser pulse, which can transition the GST between the conductive and insulating states. In addition to GST, the chalcogenide gallium lanthanum sulfide (GLS) has also been shown to have potential for an active, plasmonics-based switch [48] and tunable layer for metamaterials in the infrared range [49].

cooling back down [44]. With the VO<sup>2</sup>

112 Terahertz Spectroscopy - A Cutting Edge Technology

is insulating or a full metal plane when the VO<sup>2</sup>

**Figure 8.** Near-infrared metamaterials with vanadium dioxide (VO<sup>2</sup>

is dominates the behavior of the device. In (b), while the VO<sup>2</sup>

of the SRR with a small red-shift. In (d), during the VO<sup>2</sup>

. In (a), before transitioning the VO<sup>2</sup>

SRR doubles, which slightly increases the resonant frequency of the device [46].

layer. Before transitioning, as shown in (c), the VO<sup>2</sup>

before and after phase transition.

fabricated on a layer of VO<sup>2</sup>

a metal layer and a VO<sup>2</sup>

the entire array in different directions [41].

VO<sup>2</sup>

acting as a reconfigurable switch, patches of metal could

has transitioned to the conductive state. The

). In (a) and (b), gold split ring resonators (SRRs) are

transition to a metallic phase, the effective metal thickness of the

acts like a dielectric and the SRR response

is a dielectric, which only affects the response

layer is metallic and the

be electrically connected or disconnected to the SRR, thus altering the resonance of the structure.

Active metamaterial structures designed for the near-infrared range, like the one shown in **Figure 8**, have also been demonstrated using vanadium dioxide [46]. The planar hybrid structure shown can switch between electrically appearing as a normal SRR surface when the VO<sup>2</sup>

**Figure 7.** Various microelectromechanical systems (MEMS) incorporated, terahertz (THz) frequency range, tunable splitring resonator (SRR) elements. In (a), THz SRRs are modulated using bi-material cantilevers [39]. The design in (b) uses bi-material cantilevers for the inner capacitive gap arms of THz SRRs, modulating the SRR resonances when these arms are deflected [40]. The third design in (c) shows structurally tunable THz SRRs which use bi-material cantilevers to bend

in the self-aligned hybrid SRR causes a slight shift in the resonant frequency of the SRR

layer, the VO<sup>2</sup>

overall response of the material is that of a flat metal plane. In (c) and (d), the SRRs have two materials in their thickness,

layer is transitioned, the VO<sup>2</sup>

**Figure 9.** Metamaterial absorber with incorporated phase change material layer for dynamic applications [47].

Kodama et al. have demonstrated the use of germanium telluride (GeTe), another PCM, for achieving tunable SRRs. As shown in **Figure 10**, GeTe layer was placed in the gaps of SRRs. When the GeTe is amorphous and insulating, the SRRs exhibit their typical LC resonance; when the GeTe is crystalline and conductive, the SRR gap becomes electrically shorted, and the LC resonance is eliminated.

**Figure 10.** Tunable split-ring resonators using germanium telluride in the split gaps [19].

## **5. Conclusion**

This chapter was designed to familiarize the reader with terahertz metamaterials. Starting with fundamental metamaterial unit, cell concepts were outlined, methods for producing metamaterial samples were detailed, and the latest developments in active THz metamaterial research have been reviewed. We hope we have been able to convey the potential that metamaterial research has to offer for terahertz technologies and systems.

## **Acknowledgements**

The authors thank the Air Force Office of Scientific Research (AFOSR) for funding this effort (F4FGA06141J001) and the Air Force Research Laboratory (AFRL) Sensors Directorate for their assistance. The authors also thank the Air Force Institute of Technology (AFIT) cleanroom staff, Mr. Richard Johnston and Mr. Adam Fritche.

## **Author details**

Christopher H. Kodama and Ronald A. Coutu, Jr.\*

\*Address all correspondence to<EMAIL_ADDRESS>
Marquette University, WI, USA

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**5. Conclusion**

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**Acknowledgements**

**Author details**

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This chapter was designed to familiarize the reader with terahertz metamaterials. Starting with fundamental metamaterial unit, cell concepts were outlined, methods for producing metamaterial samples were detailed, and the latest developments in active THz metamaterial research have been reviewed. We hope we have been able to convey the potential that meta-

The authors thank the Air Force Office of Scientific Research (AFOSR) for funding this effort (F4FGA06141J001) and the Air Force Research Laboratory (AFRL) Sensors Directorate for their assistance. The authors also thank the Air Force Institute of Technology (AFIT) clean-

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## **Determining the Complex Refractive Index of Materials in the Far-Infrared from Terahertz Time-Domain Data Determining the Complex Refractive Index of Materials in the Far-Infrared from Terahertz Time-Domain Data**

Maxime Bernier, Frédéric Garet and Jean-Louis Coutaz Maxime Bernier, Frédéric Garet and Jean-Louis Coutaz

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66348

#### **Abstract**

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Terahertz time‐domain spectroscopy is a well‐established technique to study the far‐infra‐ red electromagnetic response of materials. Measurements are broadband, fast, and per‐ formed at room temperature. Moreover, compact systems are nowadays commercially available, which can be operated by nonspecialist staff. Thanks to the determination of the amplitude and phase of the recorded signals, both refractive index and absorption coefficient of the sample material can be obtained. However, determining these electro‐ magnetic parameters should be performed cautiously when samples are more or less transparent. In this chapter, we explain how to extract the material parameters from terahertz time‐domain data. We list the main sources of error, and their contribution to uncertainties. We give rules to select the most adapted technique for an optimized characterization, depending on the transparency of the samples, and address the case of samples with strong absorption peaks or exhibiting scattering.

**Keywords:** terahertz time‐domain spectroscopy, transmission TDS, reflection TDS, attenuated total reflection, extraction precision, Kramers‐Kronig relations, scattering

## **1. Introduction**

Up to the end of the 1980s, the far‐infrared electromagnetic response of materials was mostly investigated thanks to Fourier transform infrared (FTIR) spectroscopy, which exhibits sev‐ eral advantages. During one scan, the recorded time‐equivalent waveform is built from infor‐ mation delivered by the entire spectrum, whereas other dispersive prism‐ or grating‐based spectrometers receive at any time only signal from a narrow band, i.e., a weaker signal with a smaller signal‐over‐noise ratio (SNR). Second, and oppositely to dispersive spectrometers,

© 2016 The Author(s). Licensee InTech. This chapter is 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. © 2017 The Author(s). Licensee InTech. This chapter is 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.

the resolution of FTIR instruments is not limited by the size of the source. These are respec‐ tively known as the multiplex and *étendue* advantages [1]. Nevertheless, FTIR instruments display some drawbacks mostly due to the lack of efficient sources and detectors of far‐ infrared waves. The sources must be broadband in view of achieving narrow waveforms and thus performing broad spectral measurements. Generally, blackbody‐like sources are implemented: they are rather powerless and deliver incoherent light. Thus, a long integra‐ tion time is necessary to get a high SNR. Moreover, to obtain a high‐frequency resolution, the waveform must be recorded over a long equivalent time window, during which noise is also recorded. Sensitive detectors, like Si bolometers, operate only at cryogenic temperatures. These experimental problems, i.e., long recording times and cryogenic temperature, were solved by Auston and Chueng [2] in 1985 who introduced and demonstrated the new concept of coherent time‐domain far‐infrared spectroscopy, known today as terahertz time‐domain spectroscopy (THz‐TDS). This initial work was completed by researchers at IBM Corp. [3, 4], who definitively installed THz‐TDS as a very competing tool to study the far‐infrared properties of materials and devices. Since these pioneering researches, a strong effort has been devoted by numerous laboratories worldwide to improve THz‐TDS equipment and techniques. Today, several books describe this technology [5–9] and commercial systems are available [10]. THz‐TDS typically permits to investigate the range 0.1–5 THz, but some recent systems allow one to reach the mid‐infrared, i.e., frequencies larger than 10 THz [11–13]. In addition, the time‐domain technique makes possible to perform optical‐pump and THz‐ probe time‐resolved experiment to study the carrier dynamics in semiconductors [14, 15] or the kinetics of photo‐induced chemical reactions [16, 17]. Moreover, nonlinear THz effects can be observed, thanks to the huge THz peak power in THz‐TDS systems fed by amplified mode‐locked lasers [18].

In this chapter, we describe the principles of THz‐TDS and we explain how to extract the refractive index and the coefficient of absorption of materials from THz‐TDS data. We study the precision of this determination versus different error sources. We give rules to choose, depending on the samples under test, the most adapted THz‐TDS technique among transmis‐ sion or reflection ones. We also treat the case of materials that exhibit strong absorption bands, and heterogeneous materials that scatter and/or diffract the THz beam.

## **2. Principles and basics of THz‐TDS**