**4. Result and discussion**

**Figure 6** shows the tuning curves of two is-TPGs fabricated using our design obtained by scanning the wavelength of seeding beam. When the pumping beam diameter and energy are 1.5 mm and 20 mJ/pulse, and the seeding beam power is 800 mW (continuous wave), the tunable range of the terahertz wave is 0.7–3 THz (430–100 μm). The maximum output peak power is more than 55 kW (BT ~ 1018 K, brightness ~ 0.2 GW/sr·cm2 ) at around 1.8 THz. This source has a broad tuning range, with a flat region around 1.6–2.6 THz. The terahertz-wave output decreased in the low- and high-frequency regions (below 1.6 and above 2.6 THz) because of a low parametric gain and high absorption coefficient [29] in these regions, respectively. From the is-TPG, the pulsed terahertz waves are generated by 100 Hz (by 10 ms); however, the pyroelectric detector we used in the experiment only gives an average power. We therefore

**Figure 6.** The tuning curves of two is-TPGs with pumping beam diameters of 1.5 mm, represented by blue solid line and 2.2 mm by red one.

**Figure 7.** A beam profile of a terahertz wave from the is-TPG measured by a terahertz-wave imager (IRV-T0831, NEC).

High-Brightness and Continuously Tunable Terahertz-Wave Generation

http://dx.doi.org/10.5772/intechopen.75038

37

**Figure 8.** An example of the wavelength and linewidth measurements. In this case, the wavelength (frequency) of the terahertz wave is approximately 398 μm (0.754 THz), and the linewidth is less than 5 GHz. This linewidth is near the

Fourier transform limit in the sub-nanosecond pulse.

The terahertz wave is focused by the f = 50 mm lens. The spot size is less than 220 mm (FWHM) at 1.5 THz.

used an optical chopper to measure the average power. We estimated the energy/pulse from the calibrated average power, and the maximum energy/pulse was more than 5.5 μJ/pulse. When the thick glass plate as an IR pass filter was inserted, the output signal from the detector completely disappeared. We measured the duration of generated terahertz wave around 100 ps by the SBD, corresponding to peak power of more than 55 kW at 1.8 THz. The conversion efficiency in energy from pumping beam to terahertz wave is about 10−4 in this case. The tuning curve for a 2.2-mm-diameter pumping and seeding beam is also shown in **Figure 6**. In that case, the tunable range is 0.39–2 THz (760–150 μm). The tuning curve has an extremely broad bandwidth, and the lowest frequency (longest wavelength) of 0.39 THz (760 μm) was also the lowest frequency (longest wavelength) achieved in our experiment. The maximum terahertz-wave output is more than 7 kW (energy, 0.7 μJ/pulse; duration, 100 ps), which occurs near 330 μm (0.9 THz) when the pumping energy is 15 mJ/pulse and seeding power is 2400 mW. The conversion efficiency is about 10−5 in this case. These tuning curves depend on the gain curves, respectively. In the case of large-diameter pumping and seeding beams, the tuning curve has been shifted toward lower frequency (longer wavelength); the output peak power in the sub-terahertz range has been increased. These are because the parametric gain in the low-frequency (long-wavelength) region was increased by expanding the beam diameters as shown in **Figure 4**; that is, wavelength conversion in the region was effectively achieved. Meanwhile, in the high-frequency (short-wavelength) region, both tuning range and output peak power have decreased, because of the large absorption associated with increasing propagation distance in the crystal.

High-Brightness and Continuously Tunable Terahertz-Wave Generation http://dx.doi.org/10.5772/intechopen.75038 37

**Figure 7.** A beam profile of a terahertz wave from the is-TPG measured by a terahertz-wave imager (IRV-T0831, NEC). The terahertz wave is focused by the f = 50 mm lens. The spot size is less than 220 mm (FWHM) at 1.5 THz.

used an optical chopper to measure the average power. We estimated the energy/pulse from the calibrated average power, and the maximum energy/pulse was more than 5.5 μJ/pulse. When the thick glass plate as an IR pass filter was inserted, the output signal from the detector completely disappeared. We measured the duration of generated terahertz wave around 100 ps by the SBD, corresponding to peak power of more than 55 kW at 1.8 THz. The conversion efficiency in energy from pumping beam to terahertz wave is about 10−4 in this case. The tuning curve for a 2.2-mm-diameter pumping and seeding beam is also shown in **Figure 6**. In that case, the tunable range is 0.39–2 THz (760–150 μm). The tuning curve has an extremely broad bandwidth, and the lowest frequency (longest wavelength) of 0.39 THz (760 μm) was also the lowest frequency (longest wavelength) achieved in our experiment. The maximum terahertz-wave output is more than 7 kW (energy, 0.7 μJ/pulse; duration, 100 ps), which occurs near 330 μm (0.9 THz) when the pumping energy is 15 mJ/pulse and seeding power is 2400 mW. The conversion efficiency is about 10−5 in this case. These tuning curves depend on the gain curves, respectively. In the case of large-diameter pumping and seeding beams, the tuning curve has been shifted toward lower frequency (longer wavelength); the output peak power in the sub-terahertz range has been increased. These are because the parametric gain in the low-frequency (long-wavelength) region was increased by expanding the beam diameters as shown in **Figure 4**; that is, wavelength conversion in the region was effectively achieved. Meanwhile, in the high-frequency (short-wavelength) region, both tuning range and output peak power have decreased, because of the large absorption associated with increasing propa-

**Figure 6.** The tuning curves of two is-TPGs with pumping beam diameters of 1.5 mm, represented by blue solid line and

gation distance in the crystal.

2.2 mm by red one.

36 High Power Laser Systems

**Figure 8.** An example of the wavelength and linewidth measurements. In this case, the wavelength (frequency) of the terahertz wave is approximately 398 μm (0.754 THz), and the linewidth is less than 5 GHz. This linewidth is near the Fourier transform limit in the sub-nanosecond pulse.

**Figure 7** shows a beam profile of terahertz wave from the is-TPG measured by an imager for terahertz wave. The generated terahertz wave is collimated and focused by the Tsurupica cylindrical (f = 100 mm) and aspherical (f = 50 mm) lens. When the wavelength of the terahertz wave is 200 μm, the spot size is less than 220 μm (full width at half maximum). We estimated the M2 value less than 1.1, and the brightness was *B = Pp/(λ M<sup>2</sup> ) 2* ~ 0.2 GW/sr·cm2 . The intensity and electric field were 0.3 GW/cm2 and 0.5 MV/cm at around 2.0 THz, respectively, at the focused point.

In the future, we have to endeavor to generate higher-brightness beam and wider tuning range for applied researches. Since extreme high-brightness terahertz-wave generation has attracted attention in recent years as a method of enabling nonthermal free target energy-level control and measurement. When we realize such a terahertz-wave control and measurement system, new applications in the terahertz region would be possible, and various issues in modern society could potentially be overcome. This system could be powerful tools not only for solving real-world problems but also fundamental physics, such as remote sensing, realtime spectroscopic measurement/imaging, 3D fabrication, and manipulation or alteration of atoms, molecules, chemical materials, proteins, cells, chemical reactions, and biological processes. We expect that these methods will open up new fields and unique applications.

High-Brightness and Continuously Tunable Terahertz-Wave Generation

http://dx.doi.org/10.5772/intechopen.75038

39

The authors would like to thank Dr. Hiroshi Sakai of Hamamatsu Photonics K. K., Assoc. Prof. Takunori Taira of the Institute for Molecular Science, Dr. Chiko Otani and Prof. Hiromasa Ito of Riken, and all others from the Terahertz-Wave Research Group who facilitated this research. This work was supported in part by the Japan Science and Technology Agency and

1 Collaborative Research Laboratory of Terahertz Technology, Terahertz Technology Research Center, National Institute of Information and Communications Technology

3 Department of Electrical and Electronic Engineering and Information Engineering,

[1] Tonouchi M. Cutting-edge terahertz technology. Nature Photonics. 2007;**1**:97-105

[2] Chamberlain JM. Where optics meets electronics: Recent progress in decreasing the terahertz gap. Philosophical Transactions of the Royal Society of London/A. 2004;**362**:199-213

, Kodo Kawase3,2 and Hiroaki Minamide2

JSPS KAKENHI Grants-in-Aid for Scientific Research 25220606.

**Acknowledgements**

**Author details**

(NICT), Tokyo, Japan

**References**

Shin'ichiro Hayashi1,2\*, Kouji Nawata2

\*Address all correspondence to: hayashi@nict.go.jp

2 RIKEN Center for Advanced Photonics, Sendai, Japan

Graduate School of Engineering, Nagoya University, Nagoya, Japan

[3] Sherwin M. Terahertz power. Nature. 2002;**420**:131-132

**Figure 8** presents an example of wavelength and linewidth measurement by a scanning Fabry-Perot etalon consisting of two metal mesh plates. The horizontal axis represents the distance between metal meshes, and the vertical axis represents the energy of the transmitted terahertz wave. The metal meshes were made of nickel and had periods of 45 μm and reflectance of about 98% at 0.75 THz. As the distance between metal mesh plates increases, intensity peaks are observed periodically. In this case, the estimated wavelength (frequency) of the terahertz wave is about 398 μm (0.754 THz), and the linewidth is less than 5 GHz. This linewidth is a near the Fourier transform limit for the terahertz-wave pulse with a sub-nanosecond duration.
