In our experience, we find that physical optics based codes are ideal for top level system design as well as setting various system requirements, such as beam size, intensity, time duration, etc. Once the top level system design is complete, ray-trace codes are ideal for precise specification of system components, such as the lens focal lengths, inter-component distances, and optical element's aperture. A code such as FRED is particularly attractive because it allows for easy calculation of coherent beam effects and can be used with various CAD packages.

include both sequential and non-sequential raytracing modes.

well as coherent physical optics propagation.

language based commercial code is GLAD.

elements, such as lenses, gratings, mirrors, and prisms.

crystal to frequency double 800 mJ pulse from 1064 nm to 532 nm with up to 40% conversion efficiency. Here, the pulse bandwidth is relatively narrow (≈0.2 nm) and group velocity walk-off is insignificant. The conversion efficiency is primarily limited by beam quality and temporal pulse shape. Generated 532 nm pulse energy is plotted versus the compressed input pulse energy in Figure 21. At maximum IR energy, the conversion efficiency unexpectedly decreases. This may indicate onset of crystal damage, degradation in pulse quality, or an increase in phase mismatch.

Fig. 21. Frequency doubling of the 10 ps T-REX ILS laser with a peak efficiency of 40%.

#### **6. Simulation tools**

A wide selection of proprietary, commercial and open source simulation tools exist for various aspects of laser design. Some are narrow in scope; others are optimized for system level design. The brief overview given here is not comprehensive and simply exposes the reader to some of the available choices. As with any modeling tool, its use requires in-depth familiarity with the subject and a good intuition of the expected results. The simulation should serve as a design guide and should be checked against experimental results. In general, simulation tools require verification, validation, and benchmarking. When verifying a code, we check that the underlying equations of the physical model properly describe the phenomena being studied. For example, a nonlinear frequency conversion code designed for nanosecond laser pulses may not properly describe frequency conversion with femtosecond pulses because it excludes pulse dispersion effects.

Benchmarking a simulation involves comparing the results from several different codes. This may be particularly important when modeling a new concept or utilizing a new code. Finally, validating a code involves comparing the results of the simulation to a real experiment. Ideally, the code can be validated for a certain range of bounding parameters. Code validation, when possible, is perhaps the most important aspect of ensuring the precision and accuracy of a particular modeling tool.

Various system level simulation codes can be divided into ray-tracing codes and physical optics codes. Ray-tracing assumes that the light wave can be modeled as a large number of

crystal to frequency double 800 mJ pulse from 1064 nm to 532 nm with up to 40% conversion efficiency. Here, the pulse bandwidth is relatively narrow (≈0.2 nm) and group velocity walk-off is insignificant. The conversion efficiency is primarily limited by beam quality and temporal pulse shape. Generated 532 nm pulse energy is plotted versus the compressed input pulse energy in Figure 21. At maximum IR energy, the conversion efficiency unexpectedly decreases. This may indicate onset of crystal damage, degradation in pulse

Fig. 21. Frequency doubling of the 10 ps T-REX ILS laser with a peak efficiency of 40%.

femtosecond pulses because it excludes pulse dispersion effects.

precision and accuracy of a particular modeling tool.

A wide selection of proprietary, commercial and open source simulation tools exist for various aspects of laser design. Some are narrow in scope; others are optimized for system level design. The brief overview given here is not comprehensive and simply exposes the reader to some of the available choices. As with any modeling tool, its use requires in-depth familiarity with the subject and a good intuition of the expected results. The simulation should serve as a design guide and should be checked against experimental results. In general, simulation tools require verification, validation, and benchmarking. When verifying a code, we check that the underlying equations of the physical model properly describe the phenomena being studied. For example, a nonlinear frequency conversion code designed for nanosecond laser pulses may not properly describe frequency conversion with

Benchmarking a simulation involves comparing the results from several different codes. This may be particularly important when modeling a new concept or utilizing a new code. Finally, validating a code involves comparing the results of the simulation to a real experiment. Ideally, the code can be validated for a certain range of bounding parameters. Code validation, when possible, is perhaps the most important aspect of ensuring the

Various system level simulation codes can be divided into ray-tracing codes and physical optics codes. Ray-tracing assumes that the light wave can be modeled as a large number of

quality, or an increase in phase mismatch.

**6. Simulation tools**

1-dimensional rays which propagate through various interfaces according to Snell's law. In their basic implementation, ray-tracing does not account for various wave effects such as diffraction or interference. The advantage of ray-tracing is that it is very fast and efficient for dealing with complicated interfaces. Ray-tracing can produce accurate results when dealing with large beams and features much larger than the light's wavelength, where wave-like effects can be disregarded. Ray-tracing can further be sub-divided into sequential and non-sequential variety. Sequential ray-tracing assumes that the order of each interface is predetermined. An example of sequential ray-tracing is a system involving a light source, a sequence of lenses, and an imaging plane. In non sequential ray-tracing, the sequence of the interfaces is determined during the raytrace, as the code tracks the position, angle and direction of each ray. An individual surface may be struck by the ray multiple times. An example of a nonsequential raytrace is a folded pulse compressor. While non-sequential raytracing approximates a real system better than a sequential raytrace, the performance is significantly slower. Most of the major codes include both sequential and non-sequential raytracing modes.

Raytracing codes are generally optimized for lens design in imaging applications. The user can select existing lenses from an extensive lens catalogue and design new lenses by specifying surface curvature and lens material. Program feature allow for rapid optimization of various system parameters, such as wavefront aberration or RMS spot size, as well as tolerance analysis. Some of the well known programs include ZEMAX, OSLO, and Code V. These codes also have advanced features allowing non-sequential analysis, as well as coherent physical optics propagation.

Ray-tracing can accurately calculate coherent wave-like effects by utilizing gaussian decomposition. Here, a beam is decomposed into a summation of TEM00 modes. Each mode is described by four waves that represent its waist and several additional rays that describe beam divergence. The rays are then propagated by geometrical optics but retain both the phase and intensity information of the beam. The Gaussian decomposition algorithm can accurately model beam propagation in both the near and the far-fields. An example of such code is FRED, which performs coherent beam propagation for a wide variety of optical elements, such as lenses, gratings, mirrors, and prisms.

As an alternative to ray-tracing, physical optics codes treat the wave properties of light by solving some simplified form of the nonlinear Schrodinger's equation typically using fast Fourier transform methods. A major advantage of the physical optics codes is their ability to model the time dependent light properties. These codes can often model nonlinear light properties, such as frequency conversion, and Raman scattering. The Fourier based codes are typically far better at simulating far field light propagation, coherent wave effects, and complex non-geometric optical elements. A disadvantage of the physical optics codes is that they do not handle light propagation through irregular refractive interfaces. Examples of sophisticated physical optics codes include PROP, developed at LLNL, MIRO, developed by CEA, and its commercial variant Commod Pro. Another script language based commercial code is GLAD.

In our experience, we find that physical optics based codes are ideal for top level system design as well as setting various system requirements, such as beam size, intensity, time duration, etc. Once the top level system design is complete, ray-trace codes are ideal for precise specification of system components, such as the lens focal lengths, inter-component distances, and optical element's aperture. A code such as FRED is particularly attractive because it allows for easy calculation of coherent beam effects and can be used with various CAD packages.

Laser Pulses for Compton Scattering Light Sources 117

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#### **7. Conclusion**

We have presented a brief overview of the fundamental physics and applications of Compton-scattering based compact mono-energetic gamma-ray sources, emphasizing the recently commissioned 2nd generation T-REX device, and the currently under construction, 3rd generation MEGa-ray Compton scattering light source at LLNL. We have also detailed the underlying laser technology and described several technological breakthroughs which enable development of Compton sources with the highest peak brightness in a compact footprint. Finally, we anticipate continued interest and applications in scientific and technological frontiers, in particular, in the field of nuclear photonics, for high energy Compton scattering light sources.

We are grateful to S.G. Anderson, D.J. Gibson, R.A. Marsh, M. Messerly, C.A. Ebbers, C.W. Siders, C.P.J. Barty, and the entire MEGa-ray team at Lawrence Livermore National Laboratory for their support and many useful discussions. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-ENG-48.

#### **8. References**

We have presented a brief overview of the fundamental physics and applications of Compton-scattering based compact mono-energetic gamma-ray sources, emphasizing the recently commissioned 2nd generation T-REX device, and the currently under construction, 3rd generation MEGa-ray Compton scattering light source at LLNL. We have also detailed the underlying laser technology and described several technological breakthroughs which enable development of Compton sources with the highest peak brightness in a compact footprint. Finally, we anticipate continued interest and applications in scientific and technological frontiers, in particular, in the field of nuclear photonics, for high energy

We are grateful to S.G. Anderson, D.J. Gibson, R.A. Marsh, M. Messerly, C.A. Ebbers, C.W. Siders, C.P.J. Barty, and the entire MEGa-ray team at Lawrence Livermore National Laboratory for their support and many useful discussions. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence

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Livermore National Laboratory under Contract W-7405-ENG-48.

**7. Conclusion**

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**6**

*Japan*

**Frequency-Tunable Coherent THz-Wave Pulse**

**One Nd:YAG Laser Pumping and Applications**

*Department of Materials Science, Graduate School of Engineering, Tohoku University*

The development of various original semiconductor devices, which act as electromagnetic wave generators in the THz region of the spectrum, has long been an active area of research in our group. The terahertz (THz) region, which lies between the microwave and infrared regions, offers a wealth of untapped potential. In most cases, the devices are based upon the utilization of THz lattice vibrations in compound semiconductors (*e.g.*, GaP and GaAs), which has recently become an important technology behind frequency-sweepable coherent THz-wave sources, following the invention of the semiconductor laser by Nishizawa in 1957 [1]. In 1963, Nishizawa was the first to predict the utility of the phonon and molecular vibration in semiconductors for optical communication and THz-wave generation. However, an important frequency gap between the microwave and optical frequencies remained, and presently is referred to as the THz region. In 1983, the semiconductor Raman laser was realized, which relied on the longitudinal optical phonon (LO phonon) mode of a GaP crystal. This work highlighted the generation of a 12-THz wave with a peak power as high as 3 W [2]. The output power of the Raman laser was increased by a phonon

A high-power frequency-tunable THz-wave was generated via excitation of phonon– polaritons mode in GaP [4–8]. The frequency range was approximately 0.3–7.5 THz, in which the peak power was greater than 100 mW over most of the tunable region [5–7]. The generated THz-wave power and frequency regions have been shown to depend on carrier

Furthermore, THz signal generators have been developed with various functions. Generation of narrow-linewidth THz waves has very useful applications in the fields of high-resolution spectroscopy, optical communications and *in-situ* security screening. The CW THz waves are generated from GaP by using semiconductor diode lasers. The linewidth is about 4 MHz. A 30 cm-long portable THz-wave generator is constructed using two

In this chapter, we review the photonic approaches of THz-wave sources and highlight the principles and performance of these THz-wave generating devices. Developments in THz

enhancement effect within the waveguide-structured GaP [3].

Cr:Forsterite lasers pumped using a single Nd:YAG laser.

densities within the GaP crystals.

**1. Introduction**

**for Non-Destructive THz Inspection**

Tadao Tanabe and Yutaka Oyama

**Generation Using Two Cr:Forsterite Lasers with**

### **Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection**

Tadao Tanabe and Yutaka Oyama

*Department of Materials Science, Graduate School of Engineering, Tohoku University Japan*

#### **1. Introduction**

118 Laser Systems for Applications

[30] R. Trebino, Frequency-Resolved Optical Gating: TheMeasurement of Ultrashort Laser

[31] Selcuk Akturk, Xun Gu, Pablo Gabolde, and Rick Trebino, "The general theory of first-

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of an arbitrary ultrashort pulse by using frequency-resolved optical gating," Opt.

of the intensity and phase of a femtosecond UV laser pulse with frequency-

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calcium oxyborate," Opt. Lett. 31, 1277-1279 (2006)

8642-8661 (2005)

FThL5.

(1995)

Lett. 18, 823-825 (1993)

The development of various original semiconductor devices, which act as electromagnetic wave generators in the THz region of the spectrum, has long been an active area of research in our group. The terahertz (THz) region, which lies between the microwave and infrared regions, offers a wealth of untapped potential. In most cases, the devices are based upon the utilization of THz lattice vibrations in compound semiconductors (*e.g.*, GaP and GaAs), which has recently become an important technology behind frequency-sweepable coherent THz-wave sources, following the invention of the semiconductor laser by Nishizawa in 1957 [1]. In 1963, Nishizawa was the first to predict the utility of the phonon and molecular vibration in semiconductors for optical communication and THz-wave generation. However, an important frequency gap between the microwave and optical frequencies remained, and presently is referred to as the THz region. In 1983, the semiconductor Raman laser was realized, which relied on the longitudinal optical phonon (LO phonon) mode of a GaP crystal. This work highlighted the generation of a 12-THz wave with a peak power as high as 3 W [2]. The output power of the Raman laser was increased by a phonon enhancement effect within the waveguide-structured GaP [3].

A high-power frequency-tunable THz-wave was generated via excitation of phonon– polaritons mode in GaP [4–8]. The frequency range was approximately 0.3–7.5 THz, in which the peak power was greater than 100 mW over most of the tunable region [5–7]. The generated THz-wave power and frequency regions have been shown to depend on carrier densities within the GaP crystals.

Furthermore, THz signal generators have been developed with various functions. Generation of narrow-linewidth THz waves has very useful applications in the fields of high-resolution spectroscopy, optical communications and *in-situ* security screening. The CW THz waves are generated from GaP by using semiconductor diode lasers. The linewidth is about 4 MHz. A 30 cm-long portable THz-wave generator is constructed using two Cr:Forsterite lasers pumped using a single Nd:YAG laser.

In this chapter, we review the photonic approaches of THz-wave sources and highlight the principles and performance of these THz-wave generating devices. Developments in THz

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

the beam spot size, *S*, based on the follow equation:

temperature or a liquid-helium-cooled Si bolometer.

*THz-wave*

long as *θin* << 1:

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 121

The THz-wave output power (*PTHz*) can be increased by exploiting the inverse proportion to

*THz* 1 2 *<sup>A</sup> P PP*

where *A* is the coefficient for generating THz waves from a GaP crystal pumped under noncollinear phase-matching conditions. Note that *A* is estimated to be 0.4 × 10–13 W–1 cm2 [15, 16], while *S* is the spatial overlap of the cross-sectional areas of the pump and signal beams,

The two lasers used for the DFG of THz waves via excitation of the phonon–polariton mode in GaP crystals were a 1.064 μm Nd:YAG source and a β-BaB2O4-based OPO system. They were set up in a non-collinear configuration with a very small angle between the two beams, and the GaP crystals were positioned as depicted in Fig. 1 [4–7]. The wavelength of the pump beam was varied between 1.035 and 1.062 μm, which corresponded to generated THz-wave frequencies between 8 and 0.5 THz. The THz-wave energy can be collected with parabolic reflectors and determined using a pyroelectric DTGS detector operating at room

and *P1* and *P2* are the effective powers of the pump and signal beams, respectively.

<110>

The THz-wave generation using two Cr:Forsterite lasers (Cr:F) was very similar to the OPObased system. The pump and signal beams were combined using a cubic polarizer placed on a rotating platform on top of a linear stage, which automatically produced a very small angle between the two beams to fulfill the phase-matching condition and achieve a spatial overlap of the incoming beams. The non-collinear phase-matching was satisfied under the following condition [4, 6]. The calculated angle between the pump and signal light beams (*θin*) inside the GaP crystal matched quite well with respect to the experimental results, as

<sup>2</sup> 2( / )( / ) *in I LS*

where ν*L*, *νS*, and ν are the frequencies of the pump, signal, and THz waves, respectively; (∆*q*/*q*) is the relative deviation of the wave-vector phonon–polaritons in the GaP crystal; *nI*, *nL* and *nS* are the refractive indexes of the THz wave, pump, and signal beams, respectively;

*q qn n n*

*L S*

Fig. 1. Schematic of the YAG and OPO beam paths within the GaP crystal.

<001> <110>

*<sup>S</sup>* , (1)

*in ext*

, (2)

*OPO*

*YAG*

technology allow spectroscopic investigation of low-energy excitations of macromolecules, such as molecular rotations, hydrogen bonding, and intermolecular interactions, with a broad frequency range and high resolution. Indeed, low-energy excitations are believed to be critical in understanding the complex behavior of biological molecules, cells, and tissues. For industrial applications, we have recently developed THz diagnosis technologies. Deformation of polyethylene can be monitored using polarized THz spectroscopy. The THz diagnoses the inside of object even with covered by materials.

THz wave has unique properties with high transparency for non-polarized materials such as with radio waves and easy handling as in the case of light. THz waves are expected to be a promising frequency for the non-destructive diagnosis of the interior of non-polarized materials [9-14]. For the THz inspection method, the spatial resolution is higher compared with that of conventional microwave techniques.

The energy of a THz wave is as low as room temperature, for example, whose energy level corresponds to hydrogen bonding, van der Waals interactions and free carrier absorption. Although the transparency of a THz wave is not as great as that of X-ray, a THz wave is sensitive to soft materials such as hydrate as well as to wet conditions. To date, our group has utilized THz waves for inspections of diffused water and defects in timber and concrete blocks using a 0.2 THz generator [15]. The phase transition of liquid crystal has been investigated based on molecular interactions using a GaP THz signal generator [16]. Polarization THz measurements are helpful for THz non-destructive diagnosis of the tensile strain in deformed UHMWPE [17]. Use of THz waves in conjunction with X-ray and -ray measurements shows promise for analysis macrostructures in organic materials and polymers. Recently, compact sized THz wave generators have been developed for practical use [18]. For single-frequency coherent THz waves, a geometrical optical design can be applied. A THz beam spot can be controlled to be as small as the wavelength of a THz wave. Such THz sources are suitable for spectroscopic imaging with spatial high-resolution.

As one application of the THz diagnosis, the study has focused on the interior copper conductors covered with insulating polyethylene. THz reflectivity of the copper surfaces was investigated by using the GaP THz wave generator. Surface evaluation of copper with various conditions was performed using THz diffused reflection spectroscopy. Copper is a basic metal. In particular, copper cables are used for a wide variety of electric components. Confirmation of copper conductors covered with plastics is essential for a social safety. A suitable way to evaluate the deterioration of interior copper conductors has not yet been fully established. A non-destructive method for the diagnosis of electric cables would be of value for quality evaluation in use of it.

#### **2. THz-wave generation from semiconductors**

Widely frequency-tunable high-power THz waves have been generated from GaP by pumping with a Q-switched Nd:YAG laser and an OPO, or two Cr:Forsterite lasers [4–7]. THz waves were generated in the frequency range from 0.3 to 7.5 THz using difference frequency generation (DFG) via the excitation of phonon–polaritons in GaP. Indeed, this process converts energy very efficiently, and resulted in a THz wave with an energy of 9 nJ/pulse (peak power of 1.5 W). Furthermore, frequency-tunable CW THz waves were generated by enhancing the power density of incident beams from semiconductor lasers [8].

technology allow spectroscopic investigation of low-energy excitations of macromolecules, such as molecular rotations, hydrogen bonding, and intermolecular interactions, with a broad frequency range and high resolution. Indeed, low-energy excitations are believed to be critical in understanding the complex behavior of biological molecules, cells, and tissues. For industrial applications, we have recently developed THz diagnosis technologies. Deformation of polyethylene can be monitored using polarized THz spectroscopy. The THz

THz wave has unique properties with high transparency for non-polarized materials such as with radio waves and easy handling as in the case of light. THz waves are expected to be a promising frequency for the non-destructive diagnosis of the interior of non-polarized materials [9-14]. For the THz inspection method, the spatial resolution is higher compared

The energy of a THz wave is as low as room temperature, for example, whose energy level corresponds to hydrogen bonding, van der Waals interactions and free carrier absorption. Although the transparency of a THz wave is not as great as that of X-ray, a THz wave is sensitive to soft materials such as hydrate as well as to wet conditions. To date, our group has utilized THz waves for inspections of diffused water and defects in timber and concrete blocks using a 0.2 THz generator [15]. The phase transition of liquid crystal has been investigated based on molecular interactions using a GaP THz signal generator [16]. Polarization THz measurements are helpful for THz non-destructive diagnosis of the tensile strain in deformed UHMWPE [17]. Use of THz waves in conjunction with X-ray and -ray measurements shows promise for analysis macrostructures in organic materials and polymers. Recently, compact sized THz wave generators have been developed for practical use [18]. For single-frequency coherent THz waves, a geometrical optical design can be applied. A THz beam spot can be controlled to be as small as the wavelength of a THz wave. Such THz sources are suitable for

As one application of the THz diagnosis, the study has focused on the interior copper conductors covered with insulating polyethylene. THz reflectivity of the copper surfaces was investigated by using the GaP THz wave generator. Surface evaluation of copper with various conditions was performed using THz diffused reflection spectroscopy. Copper is a basic metal. In particular, copper cables are used for a wide variety of electric components. Confirmation of copper conductors covered with plastics is essential for a social safety. A suitable way to evaluate the deterioration of interior copper conductors has not yet been fully established. A non-destructive method for the diagnosis of electric cables would be of

Widely frequency-tunable high-power THz waves have been generated from GaP by pumping with a Q-switched Nd:YAG laser and an OPO, or two Cr:Forsterite lasers [4–7]. THz waves were generated in the frequency range from 0.3 to 7.5 THz using difference frequency generation (DFG) via the excitation of phonon–polaritons in GaP. Indeed, this process converts energy very efficiently, and resulted in a THz wave with an energy of 9 nJ/pulse (peak power of 1.5 W). Furthermore, frequency-tunable CW THz waves were generated by enhancing the power density of incident beams from semiconductor lasers [8].

diagnoses the inside of object even with covered by materials.

with that of conventional microwave techniques.

spectroscopic imaging with spatial high-resolution.

**2. THz-wave generation from semiconductors**

value for quality evaluation in use of it.

The THz-wave output power (*PTHz*) can be increased by exploiting the inverse proportion to the beam spot size, *S*, based on the follow equation:

$$P\_{THz} = \frac{A}{S} \cdot P\_1 \cdot P\_2 \,\tag{1}$$

where *A* is the coefficient for generating THz waves from a GaP crystal pumped under noncollinear phase-matching conditions. Note that *A* is estimated to be 0.4 × 10–13 W–1 cm2 [15, 16], while *S* is the spatial overlap of the cross-sectional areas of the pump and signal beams, and *P1* and *P2* are the effective powers of the pump and signal beams, respectively.

The two lasers used for the DFG of THz waves via excitation of the phonon–polariton mode in GaP crystals were a 1.064 μm Nd:YAG source and a β-BaB2O4-based OPO system. They were set up in a non-collinear configuration with a very small angle between the two beams, and the GaP crystals were positioned as depicted in Fig. 1 [4–7]. The wavelength of the pump beam was varied between 1.035 and 1.062 μm, which corresponded to generated THz-wave frequencies between 8 and 0.5 THz. The THz-wave energy can be collected with parabolic reflectors and determined using a pyroelectric DTGS detector operating at room temperature or a liquid-helium-cooled Si bolometer.

Fig. 1. Schematic of the YAG and OPO beam paths within the GaP crystal.

The THz-wave generation using two Cr:Forsterite lasers (Cr:F) was very similar to the OPObased system. The pump and signal beams were combined using a cubic polarizer placed on a rotating platform on top of a linear stage, which automatically produced a very small angle between the two beams to fulfill the phase-matching condition and achieve a spatial overlap of the incoming beams. The non-collinear phase-matching was satisfied under the following condition [4, 6]. The calculated angle between the pump and signal light beams (*θin*) inside the GaP crystal matched quite well with respect to the experimental results, as long as *θin* << 1:

$$\frac{\theta\_{in}}{\nu} = \sqrt{\frac{2(\Delta q \,/\, q)(n\_l^2 \,/\, n\_L \cdot n\_S)}{\nu\_L \cdot \nu\_S}}\,,\tag{2}$$

where ν*L*, *νS*, and ν are the frequencies of the pump, signal, and THz waves, respectively; (∆*q*/*q*) is the relative deviation of the wave-vector phonon–polaritons in the GaP crystal; *nI*, *nL* and *nS* are the refractive indexes of the THz wave, pump, and signal beams, respectively;

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

prevent total internal reflection of the THz wave.

0

1E-3

GaSe (▲), CdSe (■), ZnGeP2-oee (◆), and ZnGeP2-eoo (◀).

0.01

0.1

oee

THz-wave power (mW)

1

10

100

calculated relationship described in more detail within the text.

2

4

6

Frequency (THz)

8

10

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 123

The THz-wave direction was compensated with a pair of off-axis parabolic reflectors, where one of them was carefully moved on a translation stage. The GaP crystal was rotated at a lower frequency in the Cr:F-based system when compared to the OPO-based source to

0 60 120 180 240 300

CW

*ext* (min.)

0.1 1 10 100

Fig. 3. Frequency dependence of the THz-wave output power from various semiconductor crystals. Note: the pump and signal energy was 3 mJ before incidence to the crystal: GaP (●),

Frequency (THz)

 theoretical (=1m) theoretical (=1.2m)

 Pulse Pumping (=1m) Pulse Pumping (=1.2m)

eoo

*in*

Fig. 2. The relationship between *θinext* and the THz-wave frequency at which the maximum output power was obtained in CW and pulse pumping. The solid line represents the

and Δ*q* is the deviation of *q* from the exact phase-matching value in the collinear configuration, which can be described as

$$
\left(\frac{\Delta q}{q}\right) = \frac{n\_I - n\_{eff}}{n\_I} \,. \tag{3}
$$

In the case of the collinear interaction, the difference between the pump and signal wave numbers can be described by the following equations:

$$k\_L - k\_S = k = \frac{n\_{\rm eff} \cdot 2\pi\nu}{c}, \ n\_{\rm eff} = n\_S + \nu\_L \cdot \frac{\delta n}{\delta \nu} \tag{4}$$

where *kL* and *kS* are the wave numbers of the pump and signal beams, respectively, and *δn/δν* is the frequency dispersion of the refractive index. Moreover, the dispersion relationship of phonon–polaritons in GaP can be given by the following equation:

$$\left(\frac{cq}{2\pi\nu}\right)^2 = n\_I^2 = \frac{\left(\varepsilon\_S \nu\_0^2 - \varepsilon\_\sigma \nu^2\right)}{\left(\nu\_0^2 - \nu^2\right)},\tag{5}$$

where *εs* and *ε∞* are the static and optical frequency dielectric constants, respectively (*εs* = 11.15, *ε∞* = 9.20), and *ν0* = 11.01 THz is the pure transverse optical phonon frequency [19]. In general, this equation holds true as long as the THz frequency, *ν,* is not nearly equal to *ν0*. The internal angle external to the GaP, *θinext*, is given by the relationship

$$
\theta\_{in} \approx \frac{\theta\_{in}^{ext}}{n\_S} \,\prime\,\tag{6}
$$

where *θin* is calculated to be 0.1° at 2 THz; therefore, the two near-IR beams overlapped sufficiently to interact with each other. The polarization of the pump and signal beams was adjusted to be in the <001> and <1 1 0> directions, respectively. Figure 2 illustrates the relationship between *θinext* and the THz-wave frequency at which the maximum output power was obtained. The slope of the curve declined noticeably, which was reflected in the measured dispersion curve of the phonon–polariton branch of GaP. The *θinext* increased with the wavelength of the pump and signal beams, while the propagation direction of the THz wave was related to (∆*q*/*q*). The output direction of the THz wave depends on the wavelength of the pump and signal beams, as well as the THz frequency. The angle of the propagation direction of the THz wave inside the crystal, *θI*, is given as

$$
\sin\left(\theta\_l - \theta\_{\rm in}\right) = \left(\frac{k\_S}{q}\right) \sin\theta\_{\rm in} \text{ with } \theta\_l >> \theta\_{\rm in}.\tag{7}
$$

From this, we obtain

$$
\Delta \sin \theta\_l \approx \sqrt{\frac{2\Delta q}{q}} \cdot \left(\frac{\nu\_S}{\nu\_L}\right) \cdot \left(\frac{n\_S}{n\_L}\right) \approx \sqrt{\frac{2\Delta q}{q}} \,. \tag{8}
$$

and Δ*q* is the deviation of *q* from the exact phase-matching value in the collinear

*q n n q n*

In the case of the collinear interaction, the difference between the pump and signal wave

where *kL* and *kS* are the wave numbers of the pump and signal beams, respectively, and *δn/δν* is the frequency dispersion of the refractive index. Moreover, the dispersion

> 2 2 2 0 2

where *εs* and *ε∞* are the static and optical frequency dielectric constants, respectively (*εs* = 11.15, *ε∞* = 9.20), and *ν0* = 11.01 THz is the pure transverse optical phonon frequency [19]. In general, this equation holds true as long as the THz frequency, *ν,* is not nearly equal to *ν0*.

*S*

*ext in*

*nS*

>

*L L q q n q n q*

where *θin* is calculated to be 0.1° at 2 THz; therefore, the two near-IR beams overlapped sufficiently to interact with each other. The polarization of the pump and signal beams was adjusted to be in the <001> and <1 1 0> directions, respectively. Figure 2 illustrates the relationship between *θinext* and the THz-wave frequency at which the maximum output power was obtained. The slope of the curve declined noticeably, which was reflected in the measured dispersion curve of the phonon–polariton branch of GaP. The *θinext* increased with the wavelength of the pump and signal beams, while the propagation direction of the THz wave was related to (∆*q*/*q*). The output direction of the THz wave depends on the wavelength of the pump and signal beams, as well as the THz frequency. The angle of the

2 *eff*

*c* , *eff S L*

relationship of phonon–polaritons in GaP can be given by the following equation:

*cq <sup>n</sup>*

The internal angle external to the GaP, *θinext*, is given by the relationship

propagation direction of the THz wave inside the crystal, *θI*, is given as

*I*

From this, we obtain

sin sin *<sup>S</sup> I in in k q*

2 2 sin *S S*

<sup>0</sup> 2

*I*

*in*

*n*

*I eff I*

*n n*

2 2

. (3)

(4)

*n*

, (5)

, (6)

, with θI >> θin. (7)

. (8)

configuration, which can be described as

numbers can be described by the following equations:

*L S*

*kkk*

The THz-wave direction was compensated with a pair of off-axis parabolic reflectors, where one of them was carefully moved on a translation stage. The GaP crystal was rotated at a lower frequency in the Cr:F-based system when compared to the OPO-based source to prevent total internal reflection of the THz wave.

Fig. 2. The relationship between *θinext* and the THz-wave frequency at which the maximum output power was obtained in CW and pulse pumping. The solid line represents the calculated relationship described in more detail within the text.

Fig. 3. Frequency dependence of the THz-wave output power from various semiconductor crystals. Note: the pump and signal energy was 3 mJ before incidence to the crystal: GaP (●), GaSe (▲), CdSe (■), ZnGeP2-oee (◆), and ZnGeP2-eoo (◀).

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

GaP crystals (7.4 10–13 W–1).

temporally and spatially for DFG in GaP.

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 125

conversion efficiencies were achieved as the waveguide size decreased. For example, in the case in which the waveguide cross section was 200 m 160 m, the conversion efficiency increased to 7.6 10–12 W–1. This value was an order of magnitude greater than that in bulk

In addition to the GaP crystal measurements, we also generated frequency-tunable coherent THz-waves using GaSe, ZnGeP2, and CdSe semiconductors based upon differencefrequency generation. GaSe had a high second-order nonlinear optical (NLO) coefficient (*d*<sup>22</sup> = 54 pm/V) [20]. Furthermore, GaSe crystals have merit because they can be used to construct a simple THz-wave generation system, since collinear phase-matched DFG eliminates the complexity of angle-tuning in both the input and output beams. THz waves were generated in a wide frequency range from the THz to the mid-infrared region as shown in Fig. 3. Note that Fig. 3 shows the frequency dependence of the THz-wave output power from each semiconductor crystal at various PM angles using the YAG and OPObased sources. Upon closer inspection of Fig. 6, it is apparent that the combination of GaP (0.3–7.5 THz) and GaSe (10–100 THz) had the widest tunable range with the highest power. In practical applications, THz-wave generation systems have been used for spectroscopic measurement and THz imaging. A portable THz-wave generator is necessary for practical use, by which it can be moved closer to sample for THz sensing in the field. For example, in organic and inorganic crystal fabrication processes, crystalline defects can be detected using a THz spectrometer. For this motivation, using only one small 30-cm-long Nd:YAG laser and two Cr:Forsterite crystals, we constructed two Cr:Forsterite lasers pumped with the YAG laser and generated THz waves with the compact device. We investigated the pulse duration and delay time to realize Cr:Forsterite lasers which are suitable for use as the THzwave generator because it requires overlapping of two Cr:Forsterite laser pulses both

A Cr:Forsterite (Cr:Mg2SiO4) laser is a solid state laser that is tunable between 1130 and 1370 nm. The laser properties have been investigated, leading to the CW and mode-locked pulse operations. A Cr:Forsterite crystal has Cr4+ in the tetrahedrally coordinated Si4+ site, which acts as the lasing ion. Crystal growth processes induce impurities such as Cr3+ and Cr2+. Those impurities can be decreased by annealing. Two Cr:Forsterite crystals (Cr:F-1, Cr:F-2) with different crystal properties were used for this study: Cr:F-1 is dark blue and Cr:F-2 is dark green. Respectively, they are a rectangular parallelepiped (-R) and a Brewster-cut crystal (-B) of 5 mm × 5 mm (cross-section) × 10 mm (length). Transmittance spectra of Cr:F-1 and Cr:F-2 were measured in the NIR region at room temperature and the absorption coefficient were estimated, respectively. The absorption peaks at 550, 660, 740, and 1060 nm are attributed to the Cr4+. In contrast, Cr3+ has absorption at 474 and 665 nm. At around 700 nm, the Cr3+ absorption is dominant compared to that of the Cr4+. The 732-nm absorption is considerably higher than that at 1064 nm, but the slope efficiency excited at 732 nm is less than 1064 nm. Consequently, a Nd:YAG laser was used for pumping Cr:Forsterite. The figure of merit (FOM) of Cr:F-1 and Cr:F-2 are 8.0 and 5.9, respectively. A Q-switched Nd:YAG laser with 10-ns pulse duration at 10 30Hz repetition was used to pump the Cr:Forsterite crystals. Selective frequency cavity oscillator systems were constructed using the Cr:F-1 and Cr:F-2. The Cr:Forsterite laser characteristics of the pulse duration and delay time were measured in terms of the Cr:Forsterite laser energy and cavity length. A THz-wave generation system was constructed using two Cr:Forsterite crystals and one Q-switched Nd:YAG laser. A plane-plane cavity oscillator was constructed for measurement of the slope efficiency in the Cr:Forsterite laser. The output coupler is of 6% transmittance around 1.2 m. The Cr:Forsterite

Figure 3 shows the frequency dependence of the maximum THz-wave output power at various *θinext*. The pulse energies of the YAG and OPO were both attenuated to 3 mJ before incidence to the GaP crystal. The THz-wave output power remained stable at approximately 100 mW over a wide frequency range (2.0–5.2 THz). The total tunable frequency range was 0.3–7.5 THz. In cases when the THz-wave frequencies were greater than 5.5 THz (*θinext* > 70°), a much higher power was obtained by rotating the GaP crystal in order to prevent total internal reflection. The THz-wave output power increased linearly with the pump and signal beam energy. Note that the frequency bandwidth is equivalent to the pump and signal beams.

Figure 4 shows the THz-wave power generated as a function of the measured pump beam energy using the Cr:F-based system. It can be seen that the THz-wave power was nearly proportional to the pump energy. When the pump and signal energies were 11.4 and 11.6 mJ, respectively, the THz power increased to 1.5 W for a 10-mm-long crystal without causing surface damage.

Fig. 4. Generated THz-wave power as a function of the pump beam (Ch1) energy/pulse in the 10-mm-long GaP crystal using a Cr:F source system. The signal beam (Ch2) energy/pulse was 11.6 mJ.

The continuous wave (CW) single-frequency THz waves were generated in a widely frequency-tunable pumping source consisting of an external cavity laser diode (ECLD) and a laser diode (LD)-pumped Nd:YAG laser combined with an ytterbium-doped fiber amplifier (FA) [8]. The estimated THz-wave output peak was 50 pW. In the automatic measurement of transmission spectroscopy, the wavelength of the ECLD was swept from 1.0538 to 1.0541 m with external cavity fine-tuning. The THz frequency can be shifted according to the wavelength of ECLD. The THz-wave generation efficiency is related to the THz absorption coefficient in GaP crystals. High-power THz-wave output requires a stoichiometric control of the GaP to reduce the THz absorption due to free carriers and phonons in crystal.

In the waveguide structure, efficient THz-wave generation was achieved. THz-wave generation was demonstrated in a GaP waveguide with the same size as the wavelength of THz-wave under a collinear phase-matching condition. The conversion efficiencies of THz wave generation from the rod-type waveguides were estimated. Interestingly, higher

Figure 3 shows the frequency dependence of the maximum THz-wave output power at various *θinext*. The pulse energies of the YAG and OPO were both attenuated to 3 mJ before incidence to the GaP crystal. The THz-wave output power remained stable at approximately 100 mW over a wide frequency range (2.0–5.2 THz). The total tunable frequency range was 0.3–7.5 THz. In cases when the THz-wave frequencies were greater than 5.5 THz (*θinext* > 70°), a much higher power was obtained by rotating the GaP crystal in order to prevent total internal reflection. The THz-wave output power increased linearly with the pump and signal beam energy. Note that the frequency bandwidth is equivalent to the pump and

Figure 4 shows the THz-wave power generated as a function of the measured pump beam energy using the Cr:F-based system. It can be seen that the THz-wave power was nearly proportional to the pump energy. When the pump and signal energies were 11.4 and 11.6 mJ, respectively, the THz power increased to 1.5 W for a 10-mm-long crystal without

Ch2: 11.6mJ

5 10

Ch1 input power (mJ)

Fig. 4. Generated THz-wave power as a function of the pump beam (Ch1) energy/pulse in

The continuous wave (CW) single-frequency THz waves were generated in a widely frequency-tunable pumping source consisting of an external cavity laser diode (ECLD) and a laser diode (LD)-pumped Nd:YAG laser combined with an ytterbium-doped fiber amplifier (FA) [8]. The estimated THz-wave output peak was 50 pW. In the automatic measurement of transmission spectroscopy, the wavelength of the ECLD was swept from 1.0538 to 1.0541 m with external cavity fine-tuning. The THz frequency can be shifted according to the wavelength of ECLD. The THz-wave generation efficiency is related to the THz absorption coefficient in GaP crystals. High-power THz-wave output requires a stoichiometric control of the GaP to reduce the THz absorption due to free carriers and

In the waveguide structure, efficient THz-wave generation was achieved. THz-wave generation was demonstrated in a GaP waveguide with the same size as the wavelength of THz-wave under a collinear phase-matching condition. The conversion efficiencies of THz wave generation from the rod-type waveguides were estimated. Interestingly, higher

the 10-mm-long GaP crystal using a Cr:F source system. The signal beam (Ch2)

signal beams.

causing surface damage.

energy/pulse was 11.6 mJ.

phonons in crystal.

0.5

0

1

THz peak power (W)

1.5

conversion efficiencies were achieved as the waveguide size decreased. For example, in the case in which the waveguide cross section was 200 m 160 m, the conversion efficiency increased to 7.6 10–12 W–1. This value was an order of magnitude greater than that in bulk GaP crystals (7.4 10–13 W–1).

In addition to the GaP crystal measurements, we also generated frequency-tunable coherent THz-waves using GaSe, ZnGeP2, and CdSe semiconductors based upon differencefrequency generation. GaSe had a high second-order nonlinear optical (NLO) coefficient (*d*<sup>22</sup> = 54 pm/V) [20]. Furthermore, GaSe crystals have merit because they can be used to construct a simple THz-wave generation system, since collinear phase-matched DFG eliminates the complexity of angle-tuning in both the input and output beams. THz waves were generated in a wide frequency range from the THz to the mid-infrared region as shown in Fig. 3. Note that Fig. 3 shows the frequency dependence of the THz-wave output power from each semiconductor crystal at various PM angles using the YAG and OPObased sources. Upon closer inspection of Fig. 6, it is apparent that the combination of GaP (0.3–7.5 THz) and GaSe (10–100 THz) had the widest tunable range with the highest power.

In practical applications, THz-wave generation systems have been used for spectroscopic measurement and THz imaging. A portable THz-wave generator is necessary for practical use, by which it can be moved closer to sample for THz sensing in the field. For example, in organic and inorganic crystal fabrication processes, crystalline defects can be detected using a THz spectrometer. For this motivation, using only one small 30-cm-long Nd:YAG laser and two Cr:Forsterite crystals, we constructed two Cr:Forsterite lasers pumped with the YAG laser and generated THz waves with the compact device. We investigated the pulse duration and delay time to realize Cr:Forsterite lasers which are suitable for use as the THzwave generator because it requires overlapping of two Cr:Forsterite laser pulses both temporally and spatially for DFG in GaP.

A Cr:Forsterite (Cr:Mg2SiO4) laser is a solid state laser that is tunable between 1130 and 1370 nm. The laser properties have been investigated, leading to the CW and mode-locked pulse operations. A Cr:Forsterite crystal has Cr4+ in the tetrahedrally coordinated Si4+ site, which acts as the lasing ion. Crystal growth processes induce impurities such as Cr3+ and Cr2+. Those impurities can be decreased by annealing. Two Cr:Forsterite crystals (Cr:F-1, Cr:F-2) with different crystal properties were used for this study: Cr:F-1 is dark blue and Cr:F-2 is dark green. Respectively, they are a rectangular parallelepiped (-R) and a Brewster-cut crystal (-B) of 5 mm × 5 mm (cross-section) × 10 mm (length). Transmittance spectra of Cr:F-1 and Cr:F-2 were measured in the NIR region at room temperature and the absorption coefficient were estimated, respectively. The absorption peaks at 550, 660, 740, and 1060 nm are attributed to the Cr4+. In contrast, Cr3+ has absorption at 474 and 665 nm. At around 700 nm, the Cr3+ absorption is dominant compared to that of the Cr4+. The 732-nm absorption is considerably higher than that at 1064 nm, but the slope efficiency excited at 732 nm is less than 1064 nm. Consequently, a Nd:YAG laser was used for pumping Cr:Forsterite. The figure of merit (FOM) of Cr:F-1 and Cr:F-2 are 8.0 and 5.9, respectively. A Q-switched Nd:YAG laser with 10-ns pulse duration at 10 30Hz repetition was used to pump the Cr:Forsterite crystals. Selective frequency cavity oscillator systems were constructed using the Cr:F-1 and Cr:F-2. The Cr:Forsterite laser characteristics of the pulse duration and delay time were measured in terms of the Cr:Forsterite laser energy and cavity length. A THz-wave generation system was constructed using two Cr:Forsterite crystals and one Q-switched Nd:YAG laser. A plane-plane cavity oscillator was constructed for measurement of the slope efficiency in the Cr:Forsterite laser. The output coupler is of 6% transmittance around 1.2 m. The Cr:Forsterite

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

delay time are decreased when the Cr:Forsterite laser energy increases.

understood according to the photon lifetime, which is

40

60

Delay time (ns)

conversion efficiencies are kept high.

80

100

8

12

Pulse duration (ns)

16

20

<sup>2</sup>

lifetime and long pulse duration.

where

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 127

energy changed the Cr:Forsterite laser energy to maximum. Both the pulse duration and

Figure 6 shows Cr:Forsterite laser characteristics of the pulse duration and delay time for the case in which the cavity length was changed from 12 cm to 30 cm at Cr:Forsterite laser energy of 3 mJ, 4 mJ, and 5 mJ. Both the pulse duration and delay time are increased as a function of the cavity length. The result of pulse duration at any cavity length can be

> (1 ) *L c R*

transmittance of an output coupler. Therefore, the cavity length produces a long photon

 Cr:Forsterite laser energy 5 mJ 4 mJ 3 mJ

10 15 20 25 30

Fig. 6. Cr:Forsterite laser characteristics of the pulse duration (a) and delay time (b) with the

According to these results, the delay time can be controlled according to the Cr:Forsterite laser energy and cavity length. The Cr:Forsterite laser energy is controllable according to the conversion efficiency and pumping energy. The conversion efficiency is changed by optical alignment. Each pumping energy of the two Cr:Forsterite lasers can be changed using a pair of a polarizer and a half-wavelength plate, even if single Nd:YAG laser is used. The method can generate a higher-powered THz wave because the two laser

cavity length of 12–30 cm. The Cr:Forsterite laser energy was 3 mJ, 4 mJ, and 5 mJ.

Cavity length (cm)

is the photon lifetime, *L* is the cavity length, *c* is the velocity of light, and *R* is the

(9)

(a)

(b)

laser energies using Cr:F-1-B, Cr:F-1-R, and Cr:F-2-B were measured according to changes in the Nd:YAG laser energy. Cr:F-1-B has the highest slope-efficiency of 8.4%, with the lasing threshold of 2.2 mJ/mm2. Slope efficiencies and lasing thresholds are respectively 6.8%, 2.0 mJ/mm2 in Cr:F-1-R and 4.8%, and 4.6 mJ/mm2 in Cr:F-2-B. These results suggest a relation of the slope efficiency to FOM and the crystal shape.

It is necessary to select a frequency for THz-wave generation. The cavity oscillator using a diffraction grating can select a frequency. The selective-frequency cavity was adjusted so that the first-order reflected light is directed to the direction of incident beam. The cavity can oscillate at 1250 nm when the incident beam angle is 31.2° normal to the grating of 830 lines/mm. The selective frequency Cr:Forsterite laser can be outputted from zero-order light. The pulse duration and delay time were optimized as a function of Cr:Forsterite laser energy and cavity length. The laser crystal was Cr:F-1-R. The delay time was measured as the time from the 50% rising edge of the Nd:YAG laser to the 50% rising edge of the Cr:Forsterite laser using photodiodes and an oscilloscope. The 200 points were recorded at the same optical condition; each result was plotted and fitted to a Gaussian function for measurement of the pulse duration and delay time.

Figure 5 shows the Cr:Forsterite laser characteristics of the pulse duration and delay time when the cavity length was fixed at 20 cm. The Nd:YAG pumping laser energy is changed to 66 mJ, 80 mJ, and 95 mJ. Changing the optical alignment at each Nd:YAG laser pumping

Fig. 5. Cr:Forsterite laser characteristics of the pulse duration (a) and delay time (b) at Cr:Forsterite laser energy of 3–8 mJ. The cavity length was fixed to 20 cm. Nd:YAG laser power was changed to 66 mJ, 80 mJ, and 95 mJ.

laser energies using Cr:F-1-B, Cr:F-1-R, and Cr:F-2-B were measured according to changes in the Nd:YAG laser energy. Cr:F-1-B has the highest slope-efficiency of 8.4%, with the lasing threshold of 2.2 mJ/mm2. Slope efficiencies and lasing thresholds are respectively 6.8%, 2.0 mJ/mm2 in Cr:F-1-R and 4.8%, and 4.6 mJ/mm2 in Cr:F-2-B. These results suggest a relation of

It is necessary to select a frequency for THz-wave generation. The cavity oscillator using a diffraction grating can select a frequency. The selective-frequency cavity was adjusted so that the first-order reflected light is directed to the direction of incident beam. The cavity can oscillate at 1250 nm when the incident beam angle is 31.2° normal to the grating of 830 lines/mm. The selective frequency Cr:Forsterite laser can be outputted from zero-order light. The pulse duration and delay time were optimized as a function of Cr:Forsterite laser energy and cavity length. The laser crystal was Cr:F-1-R. The delay time was measured as the time from the 50% rising edge of the Nd:YAG laser to the 50% rising edge of the Cr:Forsterite laser using photodiodes and an oscilloscope. The 200 points were recorded at the same optical condition; each result was plotted and fitted to a Gaussian function for

Figure 5 shows the Cr:Forsterite laser characteristics of the pulse duration and delay time when the cavity length was fixed at 20 cm. The Nd:YAG pumping laser energy is changed to 66 mJ, 80 mJ, and 95 mJ. Changing the optical alignment at each Nd:YAG laser pumping

**Nd:YAG laser Energy**

66 mJ 80 mJ 95 mJ

(b)

40

50

60

Delay time (ns)

power was changed to 66 mJ, 80 mJ, and 95 mJ.

70

80

10

Pulse duration (ns)

12

14

(a) <sup>8</sup>

345678

Cr:Forsterite laser energy (mJ)

Fig. 5. Cr:Forsterite laser characteristics of the pulse duration (a) and delay time (b) at Cr:Forsterite laser energy of 3–8 mJ. The cavity length was fixed to 20 cm. Nd:YAG laser

the slope efficiency to FOM and the crystal shape.

measurement of the pulse duration and delay time.

energy changed the Cr:Forsterite laser energy to maximum. Both the pulse duration and delay time are decreased when the Cr:Forsterite laser energy increases.

Figure 6 shows Cr:Forsterite laser characteristics of the pulse duration and delay time for the case in which the cavity length was changed from 12 cm to 30 cm at Cr:Forsterite laser energy of 3 mJ, 4 mJ, and 5 mJ. Both the pulse duration and delay time are increased as a function of the cavity length. The result of pulse duration at any cavity length can be understood according to the photon lifetime, which is

$$
\tau = \frac{2L}{c(1 - R)}\tag{9}
$$

where is the photon lifetime, *L* is the cavity length, *c* is the velocity of light, and *R* is the transmittance of an output coupler. Therefore, the cavity length produces a long photon lifetime and long pulse duration.

Fig. 6. Cr:Forsterite laser characteristics of the pulse duration (a) and delay time (b) with the cavity length of 12–30 cm. The Cr:Forsterite laser energy was 3 mJ, 4 mJ, and 5 mJ.

According to these results, the delay time can be controlled according to the Cr:Forsterite laser energy and cavity length. The Cr:Forsterite laser energy is controllable according to the conversion efficiency and pumping energy. The conversion efficiency is changed by optical alignment. Each pumping energy of the two Cr:Forsterite lasers can be changed using a pair of a polarizer and a half-wavelength plate, even if single Nd:YAG laser is used. The method can generate a higher-powered THz wave because the two laser conversion efficiencies are kept high.

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

degradation effect on UHMWPE.

perpendicular direction.

0.00 0.02 0.04 0.06 0.08 0.10

(a)

25%

the parallel polarization spectrum.

Integral intensity

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 129

it is harmless and easy to use it. Until now, chemical degradation diagnosis of UHMWPE was reported with THz Time Domain Spectroscopy [23]. THz spectra have been obtained for γ-ray irradiated UHMWPEs with or without vitamin E doping. Vitamin E has anti-

The UHMWPE plates were deformed at room temperature. One day after deformation, the polarized spectra were obtained with every strain. The measurement frequency is swept from1.5 to 3 THz with 15 GHz step. It takes about 10 minutes for measurement. The 2.2 THz absorption band is due to *B1u* lattice translational vibration mode of PE. For the spectra with the polarized direction of THz wave parallel to the deformed direction, the absorbance decreases drastically as a function of strain. For the spectra with the THz wave direction perpendicular to the deformed direction, the absorbance decreases gradually in the

The integral absorption intensities of the 2.2 THz band are plotted as a function of strain in Figure 8(a). In the parallel direction, up to 25% strain, absorption intensity decreases rapidly. Over 25% strain, the intensity decreases slowly. The degrees of orientation are estimated from XRD. In figure 8(b), the THz absorption intensities in parallel direction are plotted against the degrees of orientation from XRD. This relative is appeared linearity. This result indicates the correlative between THz spectroscopy and XRD. These results suggest

> 0.00 0.02 0.04 0.06 0.08 0.10

(b)

Integral absorption intensity

Fig. 8. (a) The integrated absorption intensities of the *B1u* band as a function of the degree of elongation strain. Incident THz wave was polarized parallel (●) and perpendicular (**■**) to the axial direction of the deformation. (b) THz absorption intensity vs. degree of orientation. THz wave electric polarization direction is perpendicular to the deformation direction.

These results are discussed with lamellar model. Dependent of THz spectral feature on the polarization direction as well as strain amount is explained. THz absorption intensities are inversely proportional to the degree of orientation. When lamellas are oriented along c-axis, the dipole moment direction of *B1u* vibration mode is perpendicular to c-axis. Then, the *B1u* absorption band cannot be observed with THz wave polarization direction to the c-axis. According to the IR selection rule, absorption intensity decreases in

For *in-situ* spectroscopic measurements of UHMWPE during uniaxial deformation, the UHMWPE plate was strained at every 10 minutes with 1 % step. It is appeared that the red

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Degree of orientation

the absorption intensity decrease of parallel polarization is caused by orientation.

0 20 40 60 80 100 120

Degree of orientation was evaluated by XRD measurements.

Elongation strain (%)

parallel

perpendicular

Fig. 7. Optical setup of THz-wave generation with two Cr:Forsterite lasers. The THz-wave detector was a liquid-He cooled silicon bolometer.

Figure 7 shows that two Cr:Forsterite laser oscillators with a 15-cm cavity for THz-wave generation were constructed to be 30 cm × 30 cm. The 30 cm-length Nd:YAG laser can be put diagonally under this Cr:Forsterite laser system. The two different frequency lasers were generated respectively using Cr:F-1-R and Cr:F-2-R with single Nd:YAG pumping of 95 mJ and 105 mJ. The Cr:Forsterite laser using Cr:F-1-R crystal energy is 5.0 mJ; the frequency is tuned to around 1238.4 nm. The Cr:Forsterite laser using Cr:F-2-R crystal energy is 1.5 mJ and the frequency is 1223.5 nm. The two pulses are overlapped temporally and spatially at the GaP crystal surface. The pulse timing was tuned using the conversion efficiency of Cr:Forsterite laser. The THz wave was generated with energy of 4.7 pJ around 2.95 THz. Both the Cr:Forsterite laser line widths were measured as less 0.07 nm using a spectrum analyzer. The linewidth of the generated THz wave is estimated as less than 30 GHz.

Changing the conversion efficiency and pump energy of a Cr:Forsterite laser controlled temporal overlap of two Cr:Forsterite laser pulses. A THz wave was generated using two Cr:Forsterite lasers pumped using a single Nd:YAG laser. The Cr:Forsterite laser system was built as 30-cm square for portable use in the field.

#### **3. Polarized THz spectroscopy for uniaxially deformed ultra high molecular weight polyethylene**

One research objective is developing THz evaluation technique of uniaxially deformed polyethylene. Polarized THz spectra have been measured for the deformed polyethylene at room temperature. The THz pulse has a linear polarization.

Ultra high molecular weight polyethylene (UHMWPE) has strength against impact force, friction, chemical attack, and coldness. Therefore this material has been used for many industrial applications, for example gears, gaskets and artificial joints. Several nondestructive tests for UHMWPE have been developed already. The conventional methods are XRD, FT-IR, Raman spectroscopy. These methods should be improved for easy and safe test of mechanical deformation of bulk polymer. The molecular vibration modes of polyethylene are well known. The 14 vibration modes of polyethylene are divided into two types: carbon – carbon vibration and CH2 plane vibration. In THz spectra of UHMWPE, absorption band is appeared around 2.2 THz, which is assigned to *B1u* translational lattice vibration mode [21, 22]. THz wave has high transparency for polymers comparing to mid-infrared. Furthermore,

30 cm Fig. 7. Optical setup of THz-wave generation with two Cr:Forsterite lasers. The THz-wave

Figure 7 shows that two Cr:Forsterite laser oscillators with a 15-cm cavity for THz-wave generation were constructed to be 30 cm × 30 cm. The 30 cm-length Nd:YAG laser can be put diagonally under this Cr:Forsterite laser system. The two different frequency lasers were generated respectively using Cr:F-1-R and Cr:F-2-R with single Nd:YAG pumping of 95 mJ and 105 mJ. The Cr:Forsterite laser using Cr:F-1-R crystal energy is 5.0 mJ; the frequency is tuned to around 1238.4 nm. The Cr:Forsterite laser using Cr:F-2-R crystal energy is 1.5 mJ and the frequency is 1223.5 nm. The two pulses are overlapped temporally and spatially at the GaP crystal surface. The pulse timing was tuned using the conversion efficiency of Cr:Forsterite laser. The THz wave was generated with energy of 4.7 pJ around 2.95 THz. Both the Cr:Forsterite laser line widths were measured as less 0.07 nm using a spectrum analyzer. The linewidth of the generated THz wave is estimated as

Changing the conversion efficiency and pump energy of a Cr:Forsterite laser controlled temporal overlap of two Cr:Forsterite laser pulses. A THz wave was generated using two Cr:Forsterite lasers pumped using a single Nd:YAG laser. The Cr:Forsterite laser system was

**3. Polarized THz spectroscopy for uniaxially deformed ultra high molecular**

One research objective is developing THz evaluation technique of uniaxially deformed polyethylene. Polarized THz spectra have been measured for the deformed polyethylene at

Ultra high molecular weight polyethylene (UHMWPE) has strength against impact force, friction, chemical attack, and coldness. Therefore this material has been used for many industrial applications, for example gears, gaskets and artificial joints. Several nondestructive tests for UHMWPE have been developed already. The conventional methods are XRD, FT-IR, Raman spectroscopy. These methods should be improved for easy and safe test of mechanical deformation of bulk polymer. The molecular vibration modes of polyethylene are well known. The 14 vibration modes of polyethylene are divided into two types: carbon – carbon vibration and CH2 plane vibration. In THz spectra of UHMWPE, absorption band is appeared around 2.2 THz, which is assigned to *B1u* translational lattice vibration mode [21, 22]. THz wave has high transparency for polymers comparing to mid-infrared. Furthermore,

Nd:YAG laser

30 cm

4K-Si Bolometer

less than 30 GHz.

**weight polyethylene**

mirror

GaP

Off-axis parabolic

detector was a liquid-He cooled silicon bolometer.

built as 30-cm square for portable use in the field.

room temperature. The THz pulse has a linear polarization.

it is harmless and easy to use it. Until now, chemical degradation diagnosis of UHMWPE was reported with THz Time Domain Spectroscopy [23]. THz spectra have been obtained for γ-ray irradiated UHMWPEs with or without vitamin E doping. Vitamin E has antidegradation effect on UHMWPE.

The UHMWPE plates were deformed at room temperature. One day after deformation, the polarized spectra were obtained with every strain. The measurement frequency is swept from1.5 to 3 THz with 15 GHz step. It takes about 10 minutes for measurement. The 2.2 THz absorption band is due to *B1u* lattice translational vibration mode of PE. For the spectra with the polarized direction of THz wave parallel to the deformed direction, the absorbance decreases drastically as a function of strain. For the spectra with the THz wave direction perpendicular to the deformed direction, the absorbance decreases gradually in the perpendicular direction.

The integral absorption intensities of the 2.2 THz band are plotted as a function of strain in Figure 8(a). In the parallel direction, up to 25% strain, absorption intensity decreases rapidly. Over 25% strain, the intensity decreases slowly. The degrees of orientation are estimated from XRD. In figure 8(b), the THz absorption intensities in parallel direction are plotted against the degrees of orientation from XRD. This relative is appeared linearity. This result indicates the correlative between THz spectroscopy and XRD. These results suggest the absorption intensity decrease of parallel polarization is caused by orientation.

Fig. 8. (a) The integrated absorption intensities of the *B1u* band as a function of the degree of elongation strain. Incident THz wave was polarized parallel (●) and perpendicular (**■**) to the axial direction of the deformation. (b) THz absorption intensity vs. degree of orientation. THz wave electric polarization direction is perpendicular to the deformation direction. Degree of orientation was evaluated by XRD measurements.

These results are discussed with lamellar model. Dependent of THz spectral feature on the polarization direction as well as strain amount is explained. THz absorption intensities are inversely proportional to the degree of orientation. When lamellas are oriented along c-axis, the dipole moment direction of *B1u* vibration mode is perpendicular to c-axis. Then, the *B1u* absorption band cannot be observed with THz wave polarization direction to the c-axis. According to the IR selection rule, absorption intensity decreases in the parallel polarization spectrum.

For *in-situ* spectroscopic measurements of UHMWPE during uniaxial deformation, the UHMWPE plate was strained at every 10 minutes with 1 % step. It is appeared that the red

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

**insulating polyethylene**

and safety.

been discussed.

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 131

The THz wave has a lot of characteristics, for example, high permeability for non polar materials, less-invasive, high reflectance on metal surfaces and safety. Our one research target is a THz application for monitoring of metal surface corrosion. Even a metal surface has a corrosion protective covering for practical uses, THz wave is transparent for covering polymers. Here at the metal surface THz wave is absorbed due to corrosion products. This application has many merits as following, high spatial resolution comparing to that of conventional microwave techniques, non-contact, non-destructive

Until now, the THz spectral data base has been developed mainly for organic materials. That data for metal and inorganic materials is under construction. However, of course, for THz non-destructive applications, the THz spectral data of metal compounds is very important. THz characteristics of metal compounds are essential. We have focused on copper for the non-destructive THz inspection of electric power lines. At present, inspection method of electric cupper cable is based on pealing of cover with visual observation. For prevention of unexpected break down, the non-destructive inspection is desired as a THz application. The non-destructive THz inspection can be applied even for live power lines. Thus THz spectra have been measured for inorganic chemicals of copper, such as Cu2O, CuO, Cu(COO)2・1/2H2O. THz reflection spectrum of natural cupper oxides on an electric line surface was measured for practical applications. The difference of spectral features between standard copper chemicals and natural copper oxides has

For sample preparation, Cu2O, CuO and Copper oxalate were purchased with 99 % purity. These samples were formed into pellets with PE powder under various concentrations for transmittance THz spectral measurements. The PE powder has high permeability for THz wave. Sample and PE powder are mixed and pressed, then a 1 mm thickness PE pellet was prepared with the diameter of 20 mm. In order to escape the interference in THz spectral

For the THz spectral measurement, THz wave is generated from GaP based on non linear optical effect. We use Nd:YAG laser as a pumping light. Two near-infrared (NIR) beams are from Cr:Forsterite lasers, and we use two detectors of DTGS in a double beam configuration. Two NIR laser beams from Cr:Forsterite lasers are introduced to GaP, then THz wave is generated via the difference frequency generation method. The THz-wave path was purged with dry air to eliminate water vapour absorptions. Absorbance is calculated by Lambert-Beer law. For practical applications, the diffused reflection spectrum of natural copper oxide was measured. In reflection spectral measurements, 4K cooled Si bolometer was used as a THz detector. Samples are copper electric line with oxidized surface and not oxidized surface. THz wave induced to sample surfaces are separated into two directions. One THz wave is reflected at sample surfaces and the other is transmitted and reflected at the

Figure 11(a) shows THz spectra of Cu2O with various concentrations. One sharp absorption peak is appeared at 4.43 THz. Over wide frequency region from 2.2 to 4.2 THz, the broad band is observed. Following the background correction, these absorption bonds are split

measurements, the pellet is formed with wedge shape.

interface between an oxide film and a copper substance.

**4. THz reflection spectroscopy for metal conductor surfaces covered with**

Fig. 9. The absorption peak position, FWHM, and integral absorption intensity of the *B1u* band as a function of the degree of elongation strain.

shift and widening of *B1u* absorption band as a function of strain. For perpendicular polarized spectra, the clear peak shift and widening are unclear. For the 2.2 THz band, the absorption peak position, full width at half maximum (FWHM) and integral absorption intensity are plotted in Figure 9 against the elongation strains, respectively. The peak position clearly shifts to lower frequency in the parallel polarized *in-situ* spectra. The peak shift rate is -7.6 % / strain. For absorption bands in higher frequency regions, Richard Wool and co-researchers reported that the peak shift rates of the CH2 rocking *B1u* band at 730 cm-1 and the CH2 *B2u* bending band at 1472 cm-1 are -3.0 % and -0.6 % / strain, respectively [24]. The relation between the peak shift rate and peak position is appeared linearity in Figure 10.

Fig. 10. The relation between peak shift rate and peak position of the *B1u* band during uniaxial deformation.

Polarized THz wave spectra were obtained for evaluation of polymer chains in mechanically extensional-deformed UHMWPEs. THz absorption band was seen around 2.2 THz which is due to the *B1u* translational lattice vibration mode. For the deformed UHMWPE, the dichroism is appeared in the 2.2 THz band intensity and peak position. The absorption intensity is smaller in spectra with the THz wave electric polarization direction parallel to the deformation direction than that with the polarization direction perpendicular. The peak position shift and absorption intensity is dependent on the amount of elongation strain, respectively. Based on these results, it is suggested that the THz nondestructive diagnosis of the tensile strain in deformed UHMWPE is possible based on the dichroism of *B1u* band intensities on polarized THz wave spectroscopy.

parallel

perpendicular

0.02 0.04 0.06 0.08 0.10 0.12

Integaral intensity

0 2 4 6 8 10 12 14 0.00

Elongation strain (%)

perpendicular

0.14 parallel

0 2 4 6 8 10 12 14 <sup>100</sup>

shift and widening of *B1u* absorption band as a function of strain. For perpendicular polarized spectra, the clear peak shift and widening are unclear. For the 2.2 THz band, the absorption peak position, full width at half maximum (FWHM) and integral absorption intensity are plotted in Figure 9 against the elongation strains, respectively. The peak position clearly shifts to lower frequency in the parallel polarized *in-situ* spectra. The peak shift rate is -7.6 % / strain. For absorption bands in higher frequency regions, Richard Wool and co-researchers reported that the peak shift rates of the CH2 rocking *B1u* band at 730 cm-1 and the CH2 *B2u* bending band at 1472 cm-1 are -3.0 % and -0.6 % / strain, respectively [24]. The relation between the peak shift rate and peak position is appeared linearity in Figure 10.

1600140012001000 800 600 400 200 0

Polarized THz wave spectra were obtained for evaluation of polymer chains in mechanically extensional-deformed UHMWPEs. THz absorption band was seen around 2.2 THz which is due to the *B1u* translational lattice vibration mode. For the deformed UHMWPE, the dichroism is appeared in the 2.2 THz band intensity and peak position. The absorption intensity is smaller in spectra with the THz wave electric polarization direction parallel to the deformation direction than that with the polarization direction perpendicular. The peak position shift and absorption intensity is dependent on the amount of elongation strain, respectively. Based on these results, it is suggested that the THz nondestructive diagnosis of the tensile strain in deformed UHMWPE is possible based on the dichroism of *B1u* band

Fig. 10. The relation between peak shift rate and peak position of the *B1u* band during

wave number (cm-1

)

Fig. 9. The absorption peak position, FWHM, and integral absorption intensity of the *B1u*

Elongation strain (%) Elongation strain (%)

FWHM (GHz)

0 2 4 6 8 10 12 14

perpendicular

parallel

band as a function of the degree of elongation strain.

0 -1 -2 -3 -4 -5 -6 -7 -8

peak shift (%/strain)

intensities on polarized THz wave spectroscopy.

uniaxial deformation.

2.14 2.15 2.16 2.17 2.18

Peak Position (THz)

#### **4. THz reflection spectroscopy for metal conductor surfaces covered with insulating polyethylene**

The THz wave has a lot of characteristics, for example, high permeability for non polar materials, less-invasive, high reflectance on metal surfaces and safety. Our one research target is a THz application for monitoring of metal surface corrosion. Even a metal surface has a corrosion protective covering for practical uses, THz wave is transparent for covering polymers. Here at the metal surface THz wave is absorbed due to corrosion products. This application has many merits as following, high spatial resolution comparing to that of conventional microwave techniques, non-contact, non-destructive and safety.

Until now, the THz spectral data base has been developed mainly for organic materials. That data for metal and inorganic materials is under construction. However, of course, for THz non-destructive applications, the THz spectral data of metal compounds is very important. THz characteristics of metal compounds are essential. We have focused on copper for the non-destructive THz inspection of electric power lines. At present, inspection method of electric cupper cable is based on pealing of cover with visual observation. For prevention of unexpected break down, the non-destructive inspection is desired as a THz application. The non-destructive THz inspection can be applied even for live power lines. Thus THz spectra have been measured for inorganic chemicals of copper, such as Cu2O, CuO, Cu(COO)2・1/2H2O. THz reflection spectrum of natural cupper oxides on an electric line surface was measured for practical applications. The difference of spectral features between standard copper chemicals and natural copper oxides has been discussed.

For sample preparation, Cu2O, CuO and Copper oxalate were purchased with 99 % purity. These samples were formed into pellets with PE powder under various concentrations for transmittance THz spectral measurements. The PE powder has high permeability for THz wave. Sample and PE powder are mixed and pressed, then a 1 mm thickness PE pellet was prepared with the diameter of 20 mm. In order to escape the interference in THz spectral measurements, the pellet is formed with wedge shape.

For the THz spectral measurement, THz wave is generated from GaP based on non linear optical effect. We use Nd:YAG laser as a pumping light. Two near-infrared (NIR) beams are from Cr:Forsterite lasers, and we use two detectors of DTGS in a double beam configuration. Two NIR laser beams from Cr:Forsterite lasers are introduced to GaP, then THz wave is generated via the difference frequency generation method. The THz-wave path was purged with dry air to eliminate water vapour absorptions. Absorbance is calculated by Lambert-Beer law. For practical applications, the diffused reflection spectrum of natural copper oxide was measured. In reflection spectral measurements, 4K cooled Si bolometer was used as a THz detector. Samples are copper electric line with oxidized surface and not oxidized surface. THz wave induced to sample surfaces are separated into two directions. One THz wave is reflected at sample surfaces and the other is transmitted and reflected at the interface between an oxide film and a copper substance.

Figure 11(a) shows THz spectra of Cu2O with various concentrations. One sharp absorption peak is appeared at 4.43 THz. Over wide frequency region from 2.2 to 4.2 THz, the broad band is observed. Following the background correction, these absorption bonds are split

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

indicates the thickness of the oxide layer was 3 m. The O-

123456

Frequency(THz)

4.5wt% 7.5wt%

polyethylene.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

temperature.

Transmittance

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 133

difference in each spectrum. The THz reflectance from the no oxide surface is larger than that from the oxide surface, which is attributable to the very thin surface oxide layer, even though the oxide layer thickness is very thin compared with the wavelength of THz wave. The difference is plotted at each frequency as an absorption spectrum of natural oxide, shown in Figure 14(b). The absorption band is split into 3 peaks with a Gaussian function fitting. The positions are 1.51, 1.77 and 1.95 THz, respectively. Every peak position is not in agreement with that of standard copper chemicals. This is due to the difference of composition ratio for Cu and O between the natural copper oxide and standard copper chemicals. The depth profiles of relative concentration are obtained on CuO, Cu and O by secondary ion mass spectroscopy (SIMS). Cs+ primary ions at an energy of 5 keV were used. The ion incidence angle is 60 ° relative to the surface normal. The ion current was 100 nA and the beam was scanned across areas of 300 m 300 m. To reduce the crater effect, the analysis beam with square of 90 m was positioned in the center of sputter scan area. CuO was used to monitor the distribution of oxygen. The monitoring CuO- cluster leads to change of dynamic range with half order of magnitude around 3 m. The depth profile

oxygen distribution is nearly uniform. Matrix effect affect on the Cu- yield. The difference of THz reflectivity is helpful for a non-destructive evaluation of the corrosion of metal surface. This result is one of killer applications of THz wave and greatly contributes in the field of non-destructive inspection for corroded metals, even covered with an insulating

Absorbance

For THz reflection spectroscopy, very thin surface oxide layer reveals serious difference on THz reflectivity at a specific frequency even when layer thickness is very thin compared with the wavelength of THz wave. This phenomenon contributes to detect the corroded surface of metals for non-destructive evaluation of corrosion. To realize such an evaluation, THz wave is the best, due to its high permeability for insulators and sensitive change of

Fig. 13. THz spectra of Cu(COO)2・1/2H2O with various concentrations at room

reflection intensity from metal surface even covered with insulators.

7.5wt%

0.2

①

(a) (b)

4.5wt%

yield is saturated so that the

123456

②③

Frequency(THz)

with a Gaussian function fitting. For each band the intensity are plotted against sample concentrations in a log-log graph, as shown in Figure 11(b). It is apparent that the each integrated intensity increase as a function of sample concentration.

Fig. 11. THz spectra of Cu2O with various concentrations at room temperature.

THz spectra of CuO are shown in Figure 12(a). A broad band is observed. The integrated intensity is plotted in Figure 12(b) as a function of sample concentrations in a log-log graph. This intensity increases against the concentration. The broad band is assigned to CuO. THz spectral feature of CuO is difference from that of Cu2O.

Figure 13 shows THz spectra of copper oxalate. An asymmetric band is observed around 3.3 THz. The band is split into 3 peaks with a Gaussian function fitting. The peak positions are 2.7, 3.4, and 3.5 THz, respectively. For bands at 2.7 and 3.4THz, the intensities are increased as a function of sample concentration. That means the 2.7 and 3.4 THz bands are assigned to copper oxalate. But, for 3.5 THz band, the intensity is near constant regardless of sample concentrations. The 3.5 THz band is not due to the sample.

Fig. 12. THz spectra of CuO with various concentrations at room temperature.

Diffused reflection THz spectra of copper surface covered with oxide and without oxides were shown in Figure 14(a), respectively. In the region from 1.3~2.5 THz, the reflectance is

with a Gaussian function fitting. For each band the intensity are plotted against sample concentrations in a log-log graph, as shown in Figure 11(b). It is apparent that the each

0.6 0.7 0.8 0.9 1.0 1.1 1.2 -1.0

THz spectra of CuO are shown in Figure 12(a). A broad band is observed. The integrated intensity is plotted in Figure 12(b) as a function of sample concentrations in a log-log graph. This intensity increases against the concentration. The broad band is assigned to CuO. THz

Figure 13 shows THz spectra of copper oxalate. An asymmetric band is observed around 3.3 THz. The band is split into 3 peaks with a Gaussian function fitting. The peak positions are 2.7, 3.4, and 3.5 THz, respectively. For bands at 2.7 and 3.4THz, the intensities are increased as a function of sample concentration. That means the 2.7 and 3.4 THz bands are assigned to copper oxalate. But, for 3.5 THz band, the intensity is near constant regardless of sample

Log(Integral intensity)

Diffused reflection THz spectra of copper surface covered with oxide and without oxides were shown in Figure 14(a), respectively. In the region from 1.3~2.5 THz, the reflectance is

Fig. 12. THz spectra of CuO with various concentrations at room temperature.

*Gradient 0.75*

**Log(wt%)**

@ 4.43 THz @2.2~4.2 THz

0.6 0.7 0.8 0.9 1.0 1.1 1.2

*Gradient 0.85*

Log(wt%)

@ 1.5~4.5 THz

**Log(Integral intensity)**

0.6 0.7 0.8 0.9 1.0 1.1 1.2

*Gradient 0.75*

**Log(wt%)**

integrated intensity increase as a function of sample concentration.

Fig. 11. THz spectra of Cu2O with various concentrations at room temperature.

**Log(Absorbance)**

4.43THz

spectral feature of CuO is difference from that of Cu2O.

concentrations. The 3.5 THz band is not due to the sample.

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Frequency(THz)

8.4wt%

4.2wt%

2.5wt%

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Frequency(THz)

Absorbance

0.2

15wt% 7.5wt% 5wt%

> 0.0 0.1 0.2 0.3 0.4 0.5

Absorbance

difference in each spectrum. The THz reflectance from the no oxide surface is larger than that from the oxide surface, which is attributable to the very thin surface oxide layer, even though the oxide layer thickness is very thin compared with the wavelength of THz wave. The difference is plotted at each frequency as an absorption spectrum of natural oxide, shown in Figure 14(b). The absorption band is split into 3 peaks with a Gaussian function fitting. The positions are 1.51, 1.77 and 1.95 THz, respectively. Every peak position is not in agreement with that of standard copper chemicals. This is due to the difference of composition ratio for Cu and O between the natural copper oxide and standard copper chemicals. The depth profiles of relative concentration are obtained on CuO, Cu and O by secondary ion mass spectroscopy (SIMS). Cs+ primary ions at an energy of 5 keV were used. The ion incidence angle is 60 ° relative to the surface normal. The ion current was 100 nA and the beam was scanned across areas of 300 m 300 m. To reduce the crater effect, the analysis beam with square of 90 m was positioned in the center of sputter scan area. CuO was used to monitor the distribution of oxygen. The monitoring CuO cluster leads to change of dynamic range with half order of magnitude around 3 m. The depth profile indicates the thickness of the oxide layer was 3 m. The O yield is saturated so that the oxygen distribution is nearly uniform. Matrix effect affect on the Cu- yield. The difference of THz reflectivity is helpful for a non-destructive evaluation of the corrosion of metal surface. This result is one of killer applications of THz wave and greatly contributes in the field of non-destructive inspection for corroded metals, even covered with an insulating polyethylene.

Fig. 13. THz spectra of Cu(COO)2・1/2H2O with various concentrations at room temperature.

For THz reflection spectroscopy, very thin surface oxide layer reveals serious difference on THz reflectivity at a specific frequency even when layer thickness is very thin compared with the wavelength of THz wave. This phenomenon contributes to detect the corroded surface of metals for non-destructive evaluation of corrosion. To realize such an evaluation, THz wave is the best, due to its high permeability for insulators and sensitive change of reflection intensity from metal surface even covered with insulators.

Frequency-Tunable Coherent THz-Wave Pulse Generation Using Two Cr:Forsterite

239 (2003).

(1995).

(2007).

36, pp. 953-957 (2003).

vol. 18, pp. 2008-2010 (2006).

*Mater*. vol. 1, pp. 26-33 (2002).

vol. 252, pp. 4226-29 (2006).

755-766 (1967).

*State Commun.* vol. 4, pp. 431-434 (1966).

Crystals (Berlin: Springer) pp. 68-240 (1997).

123109 1-3 (2005).

Lasers with One Nd:YAG Laser Pumping and Applications for Non-Destructive THz Inspection 135

[5] T. Tanabe, K. Suto, J. Nishizawa, K. Saito, T. Kimura, "Tunable Terahertz Wave

[6] T. Tanabe, K. Suto, J. Nishizawa, K. Saito, T. Kimura, "Frequency-tunable terahertz wave

[7] K. Suto, T. Sasaki, T. Tanabe, K. Saito, J. Nishizawa, M. Ito, "GaP THz wave generator

[8] J. Nishizawa, T. Tanabe, K. Suto, Y. Watanabe, T. Sasaki, Y. Oyama, "Continuous-wave

[9] B. B. Hu, M. C. Nuss, "Imaging with terahertz waves" *Opt. Lett.* vol. 20, pp. 1716-1719

[10] D. M. Mittleman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd, M. Koch, "Recent advances in terahertz imaging" *Appl. Phys. B* vol. 68, pp. 1085-94 (1999). [11] B. Ferguson, X. -C. Zhang "Materials for terahertz science and technology" *Nature*

[12] K. Kawase, Y. Ogawa, Y. Watanabe, H. Inoue "Non-destructive terahertz imaging of illicit drugs using spectral fingerprints" *Opt Express* vol. 11, pp. 2549-54 (2003). [13] J. Nishizawa, "Development of THz wave oscillation and its application to molecular

[14] M. Tonouchi, "Cutting-edge terahertz technology" *Nature Photo.* vol. 1, pp. 97-105

[15] Y. Oyama, L. Zhen, T. Tanabe, M. Kagaya, "Sub-Terahertz Imaging of Defects in

[16] J. Nishizawa, T. Yamada, T. Sasaki, T. Tanabe, T. Wadayama, T. Tanno, K. Suto,

[17] T. Tanabe, K. Watanabe, Y. Oyama, K. Seo, "Polarization sensitive THz absorption

[18] J. Nishizawa, T. Sasaki, T. Tanabe, N. Hozumi, Y. Oyama, Ken Suto "Single-frequency

[19] A. Mooradian and G. B. Wright, "First order Raman effect in III–V compounds", *Solid*

[20] V. G. Dmitriev, G. G. Gurzadyan, D. N. Nikogosyan, Handbook of Nonlinear Optical

[21] M. Tasumi, T. Shimanouchi, "Crystal Vibrations and Intermolecular Forces of

[22] M. Tasumi, S. Krimm "Crystal Vibrations of Polyethylene" *J. Chem. Phys.* vol. 46, pp.

[23] K. Yamamoto, M. Yamaguchi, M. Tani, M. Hangyo, S. Teramura, T. Isu, N. Tomita,

time-domain spectroscopy" *Appl Phys Lett* vol. 85, pp. 5194-5196 (2004).

"Degradation diagnosis of ultrahigh-molecular weight polyethylene with terahertz

Polymethylene Crystals" *J. Chem. Phys.* vol. 43, pp. 1245-58 (1965).

weight polyethylene" *NDT & E Int.* vol. 43, pp. 329-333 (2010).

one Nd:YAG laser" *Rev. Sci. Instrum.* vol. 79, p. 036101 (2008).

"Terahertz dichroism of MBBA liquid crystal on rubbed substrate" *Appl. Surf. Sci.*

spectroscopy for the evaluation of uniaxially deformed ultra-high molecular

coherent terahertz-wave generation using two Cr:forsterite lasers pumped using

sciences" *Proc. Jpn. Acad. Ser. B* vol. 80, pp. 74-81 (2004).

Building Blocks" *NDT & E Int.* vol. 42, pp. 28-33 (2009).

Generation in the 3- to 7-THz region from GaP", *Appl. Phys. Lett.*, vol. 83, pp. 237-

generation via excitation of phonon-polaritons in GaP", *J. Phys. D: Appl. Phys.*, vol.

and THz spectrometer using Cr:Forsterite lasers", *Rev. Sci. Instrum.*, vol. 76, pp.

frequency-tunable terahertz-wave generation from GaP", *IEEE Photo. Tech. Lett.,*

Fig. 14. THz spectra of copper surface covered with oxide and without oxides at room temperature.

#### **5. Conclusion**

Frequency-tunable terahertz (THz) wave pulse is generated by exciting a phononpolariton mode in a GaP crystal, which are based upon non-collinear phase-matched different frequency generation (DFG) of nonlinear optical effect. We have developed a compact THz-wave generator using two small Cr:Forsterite lasers with single Q-switched Nd:YAG laser pumping. A Cr:Forsterite laser was constructed with diffraction gratings, by which the pulse duration and delay time of the Cr:Forsterite laser depend on Cr:Forsterite laser energy and the cavity length. The Cr:Forsterite laser energy was tuned using the optical alignment and pumping energy. Temporal overlap of two Cr:Forsterite laser pulses was realized at the GaP crystal. A single-frequency THz wave was generated at energy of 4.7 pJ around 2.95 THz using a 30-cm-long Cr:Forsterite laser system. The tunable range was approximately 0.3–7.5 THz.

The THz waves are located between radio waves and light waves. Thus, a THz wave has unique properties with high transparency for non-polarized materials such as with radio waves and easy handling as in the case of light. THz waves are expected to be a promising frequency for the non-destructive diagnosis of the interior of non-polarized materials. THz non-destructive diagnosis are possible for tensile strain in deformed polyethylene and metal conductor surfaces covered with insulating polyethylene.

#### **6. References**

No oxide film

Fig. 14. THz spectra of copper surface covered with oxide and without oxides at room

Frequency-tunable terahertz (THz) wave pulse is generated by exciting a phononpolariton mode in a GaP crystal, which are based upon non-collinear phase-matched different frequency generation (DFG) of nonlinear optical effect. We have developed a compact THz-wave generator using two small Cr:Forsterite lasers with single Q-switched Nd:YAG laser pumping. A Cr:Forsterite laser was constructed with diffraction gratings, by which the pulse duration and delay time of the Cr:Forsterite laser depend on Cr:Forsterite laser energy and the cavity length. The Cr:Forsterite laser energy was tuned using the optical alignment and pumping energy. Temporal overlap of two Cr:Forsterite laser pulses was realized at the GaP crystal. A single-frequency THz wave was generated at energy of 4.7 pJ around 2.95 THz using a 30-cm-long Cr:Forsterite laser system. The

The THz waves are located between radio waves and light waves. Thus, a THz wave has unique properties with high transparency for non-polarized materials such as with radio waves and easy handling as in the case of light. THz waves are expected to be a promising frequency for the non-destructive diagnosis of the interior of non-polarized materials. THz non-destructive diagnosis are possible for tensile strain in deformed polyethylene and metal

[2] K. Suto and J. Nishizawa, "Low-threshold semiconductor Raman laser", *IEEE J. Quantum*

[3] K. Suto, T. Kimura, J. Nishizawa, "Fabrication and characteristics of tapered

[4] T. Tanabe, K. Suto, J. Nishizawa, T. Kimura and K. Saito, "Frequency–tunable high-

waveguide semiconductor Raman laser", *IEE Proc.-Optoelectron*, vol. 143, pp.

power terahertz wave generation from GaP", *J. Appl. Phys.*, vol. 93, pp. 4610-4615

Absorbance

0.2

0.5 1.0 1.5 2.0 2.5 3.0 3.5

tunable range was approximately 0.3–7.5 THz.

conductor surfaces covered with insulating polyethylene.

*Electron.*, vol. 19, pp. 1251-1254 (1983).

[1] J. Nishizawa, "Semiconductor Maser", Japanese Patent 273217, April 1957.

Frequency(THz)

1.7THz Oxide film

0

5

(a)

10 15 20

Reflectance(%)

temperature.

**5. Conclusion**

**6. References**

113-118 (1996).

(2003).

25

123

Freauency(THz)

(b)

**Part 2**

**Laser Beam Manipulation**

[24] R. P. Wool, R. S. Bretzlaff, B. Y. Li, C. H. Wang, R. H. Boyd, "Infrared and raman spectroscopy of stressed polyethylene " *J. Polym. Sci. B.* vol. 24, pp. 1039-1066 (1986).

## **Part 2**

**Laser Beam Manipulation**

136 Laser Systems for Applications

[24] R. P. Wool, R. S. Bretzlaff, B. Y. Li, C. H. Wang, R. H. Boyd, "Infrared and raman

(1986).

spectroscopy of stressed polyethylene " *J. Polym. Sci. B.* vol. 24, pp. 1039-1066

Sylvain Fourmaux, Stéphane Payeur, Philippe Lassonde,

**Scale Ti: Sapphire Laser Systems**

*Institut National de la Recherche Scientifique, Énergie, Matériaux et Télécommunications -*

**Laser Pulse Contrast Ratio Cleaning in 100 TW**

Extreme laser peak intensities can be produced with current laser technology, using both the chirped pulse amplification (CPA) technique (Strickland & Mourou, 1985) and Ti:Sapphire amplification crystals (Le Blanc et al., 1993). Laser systems using this technology are commercially providing instantaneous power in excess of 100 TW with a laser pulse duration ∼30 fs and energy per pulse of several Joules. By focusing these pulses to a few *μ*m spot size, high intensity laser matter interaction studies are now routinely performed at peak intensities

For fundamental physics research, increased laser intensities enhances current interaction processes and can lead to new and more efficient interaction regimes. A peak intensity above 10<sup>22</sup> W/cm2 has already been reported by Yanovski et al. (Yanovsky et al., 2008) and a facility such as the Extreme Light Infrastructure (ELI) (Gerstner, 2007) envisions peak intensities in the range of 10<sup>23</sup> W/cm2 which are needed for experiments on radiation reaction effects (Zhidkov

For applications development, recent progress of laser systems combining high intensity and high repetition rate have attracted considerable interest for the production of solid target based secondary sources where high mean brightness is required. In high field science, this includes bright x-ray sources (Chen et al., 2004; Schnürer et al., 2000; Teubner et al., 2003; Thaury et al., 2007), high energy particle acceleration (Fritzler et al., 2003; Steinke et al., 2010;

To illustrate this interest for high peak intensities, recently published scaling laws for laser based proton acceleration on thin film solid targets (Fuchs et al., 2006) have shown that an important increase of the on target laser intensity is necessary to reach the expected energy required for biomedical application in the proton therapy field (60 - 250 MeV). Moreover, intensities greater than 10<sup>20</sup> W/cm2 will allow access to the non collisional shock acceleration regime where >100 MeV maximum energy protons could be produced (Silva et al., 2004). For currently available peak intensities (1018 <sup>−</sup> <sup>10</sup><sup>20</sup> W/cm2 range), where the field strength is sufficient to accelerate particle to relativistic energies, the laser pulse contrast ratio (LPCR) is a crucial parameter to take into consideration. Considering the laser pulse intensity temporal profile, the LPCR is the ratio between its maximum (peak intensity) and any fixed delay before it. A low contrast ratio can greatly modify the dynamics of energy coupling between the

Zeil et al., 2010) and nuclear activation (Grillon et al., 2002; Magill et al., 2003).

**1. Introduction**

of 1018 <sup>−</sup> 1020 W/cm2.

et al., 2002).

Jean-Claude Kieffer and François Martin

*Université du Québec*

*Canada*

**7**

### **Laser Pulse Contrast Ratio Cleaning in 100 TW Scale Ti: Sapphire Laser Systems**

Sylvain Fourmaux, Stéphane Payeur, Philippe Lassonde, Jean-Claude Kieffer and François Martin *Institut National de la Recherche Scientifique, Énergie, Matériaux et Télécommunications - Université du Québec Canada*

#### **1. Introduction**

Extreme laser peak intensities can be produced with current laser technology, using both the chirped pulse amplification (CPA) technique (Strickland & Mourou, 1985) and Ti:Sapphire amplification crystals (Le Blanc et al., 1993). Laser systems using this technology are commercially providing instantaneous power in excess of 100 TW with a laser pulse duration ∼30 fs and energy per pulse of several Joules. By focusing these pulses to a few *μ*m spot size, high intensity laser matter interaction studies are now routinely performed at peak intensities of 1018 <sup>−</sup> 1020 W/cm2.

For fundamental physics research, increased laser intensities enhances current interaction processes and can lead to new and more efficient interaction regimes. A peak intensity above 10<sup>22</sup> W/cm2 has already been reported by Yanovski et al. (Yanovsky et al., 2008) and a facility such as the Extreme Light Infrastructure (ELI) (Gerstner, 2007) envisions peak intensities in the range of 10<sup>23</sup> W/cm2 which are needed for experiments on radiation reaction effects (Zhidkov et al., 2002).

For applications development, recent progress of laser systems combining high intensity and high repetition rate have attracted considerable interest for the production of solid target based secondary sources where high mean brightness is required. In high field science, this includes bright x-ray sources (Chen et al., 2004; Schnürer et al., 2000; Teubner et al., 2003; Thaury et al., 2007), high energy particle acceleration (Fritzler et al., 2003; Steinke et al., 2010; Zeil et al., 2010) and nuclear activation (Grillon et al., 2002; Magill et al., 2003).

To illustrate this interest for high peak intensities, recently published scaling laws for laser based proton acceleration on thin film solid targets (Fuchs et al., 2006) have shown that an important increase of the on target laser intensity is necessary to reach the expected energy required for biomedical application in the proton therapy field (60 - 250 MeV). Moreover, intensities greater than 10<sup>20</sup> W/cm2 will allow access to the non collisional shock acceleration regime where >100 MeV maximum energy protons could be produced (Silva et al., 2004).

For currently available peak intensities (1018 <sup>−</sup> <sup>10</sup><sup>20</sup> W/cm2 range), where the field strength is sufficient to accelerate particle to relativistic energies, the laser pulse contrast ratio (LPCR) is a crucial parameter to take into consideration. Considering the laser pulse intensity temporal profile, the LPCR is the ratio between its maximum (peak intensity) and any fixed delay before it. A low contrast ratio can greatly modify the dynamics of energy coupling between the

threshold is fluence dependent. It corresponds to the onset of a phase transition following energy deposition in the solid target and subsequent morphology change. This threshold is a function of the laser pulse duration and the nature of the target (bulk or thin film). For a low fluence, the value depends on the target material, the damage may affect only the first atomic layers such that the damage threshold may be difficult to characterize. A more precisely measured characteristic, used here, is the ablation threshold that corresponds to a strong solid to gas phase transition with ejection of matter. The ablation threshold has been covered in several papers; in metals, its value is close to a few J/cm<sup>2</sup> for laser pulse durations of approximately 1 ns (Cabalin et al., 1998; Corkum et al., 1988). Such a morphology change certainly affects the energy absorption process and may damage or destroy the target in some

Laser Pulse Contrast Ratio Cleaning in 100 TW Scale Ti: Sapphire Laser Systems 141

Pre-pulses incident on the target long before the peak intensity (several ns) can modify the target surface morphology or damage it. Thus, it is absolutely necessary to check the presence of any ns pre-pulses. This measurement has to be performed before every experimental measurement set as every alignment of the laser system can detune the Pockels cells or produce reflections that can increase laser pre-pulses. Usually the duration of the pre-pulses

Even if the ns pedestal is below the ionization threshold, the energy deposited over a few ns can be above the ablation threshold and can change the target morphology or damage it. Thus, it is important to check the pedestal temporal profile several ns before the laser peak intensity. A situation where the energy contained in the pedestal over a few ns is a problem is

The architecture of a typical laser system based on CPA Ti:Sapphire technology with its amplification stages is shown in figure 1. In this example, the laser consists of: an oscillator; a grating based stretcher which increases the laser pulse duration; a regenerative amplifier; three multi-pass amplification stages to reach the final maximum laser pulse energy; and a

Fig. 1. Architecture of a high intensity laser system. P.C. indicates the position of the Pockels cells in the laser system. Typical output energy and pulse duration (FWHM) are indicated at

The Ti:Sapphire oscillator operates at several 10's of MHz repetition rate and generates laser pulses with a central operating wavelength close to 800 nm. It typically produces an initial pulse duration of 18 fs with 80-100 nm spectral bandwidth. An acousto-optic programmable dispersive filter (AOPDF) is usually used to control the spectral phase distribution of the pulse and compensate for the gain narrowing that occurs in the different amplification stages. It is

are in the sub-ns time range, and the ablation and ionization threshold are similar.

**3. High intensity laser systems based on Ti:Sapphire technology**

grating compressor to restore the short laser pulse duration.

cases, such as thin foils or velvet targets (Kulcsár et al., 2000).

illustrated in section 5.1.

each location.

laser pulse and the initial target by producing a pre-plasma that can change the interaction mechanism. This issue will be even more important for future laser systems with higher intensities. Thus, LPCR characterization and improvement are of great importance for such laser systems.

In this chapter, the second section presents the basic concepts: Laser Pulse Contrast Ratio, ionization and damage thresholds definitions. Section 3 is devoted to the presentation of the typical structure and performances of a high power laser system based on CPA Ti:Sapphire technology which is currently the most popular way to reach high intensity at a high repetition rate. Section 4 presents the measurement techniques to characterize the pre-pulse, the Amplification of Spontaneous Emission (ASE), and the coherent contrast of a laser system. We can divide the pulse cleaning techniques that have been proposed to improve the LPCR into two categories: first those used before compression which mainly reduce pre-pulses and ASE; second those applied after compression which allow a coherent contrast enhancement. In the first category, we present in section 5 the technique based upon high energy injection through saturable absorbers before power amplification and the technique based on cross wave polarization (XPW). To improve the coherent contrast, only the cleaning techniques used after pulse compression are efficient (second category); we detail in section 6 the techniques of second harmonic generation and the technique using plasma mirrors. We conclude this chapter in section 7.

#### **2. Basic concepts**

#### **2.1 Laser Pulse Contrast Ratio (LPCR) definition**

We define the LPCR as the ratio R between the laser pulse peak intensity *Ipeak* and any pre-pulse or pedestal intensity *Ip*, i.e. *R* = *Ipeak*/*Ip*. A low contrast ratio laser pulse can generate a pre-plasma before the arrival of the laser pulse peak intensity and completely change the interaction mechanism between the laser pulse and the initial target (Workman et al., 1996; Zhidkov et al., 2000). For high intensity laser systems using the CPA technique, several overlapping time scales of the intensity temporal profile should be distinguished, each corresponding to a specific source of LPCR degradation (Konoplov, 2000). On the ns time scale, from a few ns up to approximately 20 ns, insufficient contrast of the amplified laser pulse relative to neighbouring pulses could occur in the oscillator or a regenerative amplifier cavity resulting in pre-pulses. Below several ns, amplified fluorescence produced by the pumped crystal during the amplification process may propagate down the amplifier system; this Amplification of Spontaneous Emission (ASE) produces a plateau shaped ns pedestal. On the ps time scale, below a few 10 s of ps, imperfect pulse compression due to deficient laser spectrum manipulations results in a LPCR degradation close to the peak intensity. On this timescale, the LPCR is called the coherent contrast and corresponds to the rising edge of the laser pulse toward the peak intensity.

#### **2.2 Target ionization and damage thresholds**

When considering the effect of low LPCR, two target parameters are of importance: the ionization threshold and the damage threshold. The ionization threshold is intensity dependent. It corresponds to the ionization of the atomic species present in the target and production of a plasma; for example, the ionization threshold is close to 10<sup>12</sup> W/cm2 for metals and close to 1013 W/cm2 for dielectrics targets. The presence of a pre-plasma near the target surface before the laser pulse peak intensity modifies the electron density gradient and the nature of the processes leading to laser energy absorption in the target. The damage 2 Will-be-set-by-IN-TECH

laser pulse and the initial target by producing a pre-plasma that can change the interaction mechanism. This issue will be even more important for future laser systems with higher intensities. Thus, LPCR characterization and improvement are of great importance for such

In this chapter, the second section presents the basic concepts: Laser Pulse Contrast Ratio, ionization and damage thresholds definitions. Section 3 is devoted to the presentation of the typical structure and performances of a high power laser system based on CPA Ti:Sapphire technology which is currently the most popular way to reach high intensity at a high repetition rate. Section 4 presents the measurement techniques to characterize the pre-pulse, the Amplification of Spontaneous Emission (ASE), and the coherent contrast of a laser system. We can divide the pulse cleaning techniques that have been proposed to improve the LPCR into two categories: first those used before compression which mainly reduce pre-pulses and ASE; second those applied after compression which allow a coherent contrast enhancement. In the first category, we present in section 5 the technique based upon high energy injection through saturable absorbers before power amplification and the technique based on cross wave polarization (XPW). To improve the coherent contrast, only the cleaning techniques used after pulse compression are efficient (second category); we detail in section 6 the techniques of second harmonic generation and the technique using plasma mirrors. We conclude this

We define the LPCR as the ratio R between the laser pulse peak intensity *Ipeak* and any pre-pulse or pedestal intensity *Ip*, i.e. *R* = *Ipeak*/*Ip*. A low contrast ratio laser pulse can generate a pre-plasma before the arrival of the laser pulse peak intensity and completely change the interaction mechanism between the laser pulse and the initial target (Workman et al., 1996; Zhidkov et al., 2000). For high intensity laser systems using the CPA technique, several overlapping time scales of the intensity temporal profile should be distinguished, each corresponding to a specific source of LPCR degradation (Konoplov, 2000). On the ns time scale, from a few ns up to approximately 20 ns, insufficient contrast of the amplified laser pulse relative to neighbouring pulses could occur in the oscillator or a regenerative amplifier cavity resulting in pre-pulses. Below several ns, amplified fluorescence produced by the pumped crystal during the amplification process may propagate down the amplifier system; this Amplification of Spontaneous Emission (ASE) produces a plateau shaped ns pedestal. On

spectrum manipulations results in a LPCR degradation close to the peak intensity. On this timescale, the LPCR is called the coherent contrast and corresponds to the rising edge of the

When considering the effect of low LPCR, two target parameters are of importance: the ionization threshold and the damage threshold. The ionization threshold is intensity dependent. It corresponds to the ionization of the atomic species present in the target and production of a plasma; for example, the ionization threshold is close to 10<sup>12</sup> W/cm2 for metals and close to 1013 W/cm2 for dielectrics targets. The presence of a pre-plasma near the target surface before the laser pulse peak intensity modifies the electron density gradient and the nature of the processes leading to laser energy absorption in the target. The damage

s of ps, imperfect pulse compression due to deficient laser

laser systems.

chapter in section 7.

**2. Basic concepts**

**2.1 Laser Pulse Contrast Ratio (LPCR) definition**

the ps time scale, below a few 10

laser pulse toward the peak intensity.

**2.2 Target ionization and damage thresholds**

threshold is fluence dependent. It corresponds to the onset of a phase transition following energy deposition in the solid target and subsequent morphology change. This threshold is a function of the laser pulse duration and the nature of the target (bulk or thin film). For a low fluence, the value depends on the target material, the damage may affect only the first atomic layers such that the damage threshold may be difficult to characterize. A more precisely measured characteristic, used here, is the ablation threshold that corresponds to a strong solid to gas phase transition with ejection of matter. The ablation threshold has been covered in several papers; in metals, its value is close to a few J/cm<sup>2</sup> for laser pulse durations of approximately 1 ns (Cabalin et al., 1998; Corkum et al., 1988). Such a morphology change certainly affects the energy absorption process and may damage or destroy the target in some cases, such as thin foils or velvet targets (Kulcsár et al., 2000).

Pre-pulses incident on the target long before the peak intensity (several ns) can modify the target surface morphology or damage it. Thus, it is absolutely necessary to check the presence of any ns pre-pulses. This measurement has to be performed before every experimental measurement set as every alignment of the laser system can detune the Pockels cells or produce reflections that can increase laser pre-pulses. Usually the duration of the pre-pulses are in the sub-ns time range, and the ablation and ionization threshold are similar.

Even if the ns pedestal is below the ionization threshold, the energy deposited over a few ns can be above the ablation threshold and can change the target morphology or damage it. Thus, it is important to check the pedestal temporal profile several ns before the laser peak intensity. A situation where the energy contained in the pedestal over a few ns is a problem is illustrated in section 5.1.

#### **3. High intensity laser systems based on Ti:Sapphire technology**

The architecture of a typical laser system based on CPA Ti:Sapphire technology with its amplification stages is shown in figure 1. In this example, the laser consists of: an oscillator; a grating based stretcher which increases the laser pulse duration; a regenerative amplifier; three multi-pass amplification stages to reach the final maximum laser pulse energy; and a grating compressor to restore the short laser pulse duration.

Fig. 1. Architecture of a high intensity laser system. P.C. indicates the position of the Pockels cells in the laser system. Typical output energy and pulse duration (FWHM) are indicated at each location.

The Ti:Sapphire oscillator operates at several 10's of MHz repetition rate and generates laser pulses with a central operating wavelength close to 800 nm. It typically produces an initial pulse duration of 18 fs with 80-100 nm spectral bandwidth. An acousto-optic programmable dispersive filter (AOPDF) is usually used to control the spectral phase distribution of the pulse and compensate for the gain narrowing that occurs in the different amplification stages. It is

time delay between these pre-pulses and the main amplified pulse is a multiple of a round trip inside the regenerative cavity, the first pre-pulse exits the cavity before the main pulse with one less round trip. The typical energy ratio between the amplified pulse maximum energy and the first pre-pulse is 3 <sup>×</sup> <sup>10</sup>4. The first pre-pulse duration is 500 fs corresponding to a LPCR of 4.7 <sup>×</sup> <sup>10</sup>5. The use of an additional Pockels cell after the regenerative cavity increases the LPCR of the pre-pulses by a factor 60 but has no effect on shorter timescales because of the

Laser Pulse Contrast Ratio Cleaning in 100 TW Scale Ti: Sapphire Laser Systems 143

**4.2 ASE and coherent contrast measurement using a high dynamic range third-order**

<sup>F</sup> TFP1 TFP2

L/2

D

Fig. 2. Experimental set-up for LPCR measurement of the laser system pulse pedestal and coherent contrast using a high dynamic range third order cross-correlator. The arrows illustrate the laser beam propagation path, the 800 nm beam is indicated in red, the 400 nm (second harmonic) in blue, and the 266 nm (third harmonic) in black. TFP1 and TFP2 are thin film polarizer, F is a set of calibrated filters, L/2 is a wave-plate; BBO1 and BBO2 are BBO crystals, D is a motorized delay line and CC is a corner cube, S is a slit to select the third

The LPCR for the time range of a few ns is characterized using a high dynamic range third-order cross-correlator. For this measurement, part of the laser beam is usually sampled while the laser system is working at nominal energy or close to it. During the measurement, the last amplification stage can only be partly pumped, this does not significantly affect the final result as long as most of the amplification has been obtained (Kiriyama et al., 2010). The experimental set-up is described on figure 2. The sampled beam is divided using a polarizer beamsplitter (TFP2). The reflected part is frequency doubled using a 200 *μ*m thick type I BBO crystal and reflected by a motorized delay line (D). The transmitted part at the fundamental frequency is directed toward a corner cube (CC). Both beams are then combined in a non-collinear geometry into a 100 *μ*m thick type II BBO crystal to generate the third harmonic. The 266 nm third harmonic is separated from the other wavelengths using a grating

The use of calibrated neutral density filters allows the cross-correlator to cover a dynamic range typically of 12 decades. The range of the motorized delay line (D) is limited to 600 ps in the standard commercial cross-correlator. In routine use, usually only the pedestal

BBO2

BBO1

CC

S

G

harmonic beam, G a grating and PM the photo-multiplier detector.

limited rise time of the Pockels cells.

PM

and recorded by a photomultiplier.

**cross-correlator**

normally placed after the stretcher to control the spectral phase and amplitude distribution of the laser pulse. The grating based stretcher increases the laser pulse duration to ∼ 250 ps in order to reduce the peak intensity on the optical components during the energy amplification process. A 55 nm bandwidth FWHM can typically be achieved after compression. In a typical high power laser system, the minimum pulse duration routinely produced after compression is currently 25-30 fs. Note that the grating based compressor has to be located inside a vacuum chamber as the laser pulse instantaneous power after compression on the last grating is high enough that self-focusing could occur during propagation in air leading to pulse distortion and potential damage to the optics.