**Acknowledgements**

**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

**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.

We have introduced here high-peak-power, narrow-linewidth, and continuously tunable

from the suppression of the SBS in a nonlinear crystal by using sub-nanoseconds pumping pulse. The high-brightness and continuously tunable source is important for the power calibration of terahertz-wave detectors. In general, the power calibration is based on calorimetry as a traceable international standard, but there is no power standard in the terahertz region. In this case, the power levels obtained from two kinds of pre-calibrated detectors, a calorimetric device, and a pyroelectric device, using the same terahertz beam were comparable [30]. Surprisingly, this was easily perceived directly by touch; the terahertz wave was felt to be similar to a 100 Hz (rep. rate) stimulation. Under our experimental conditions, the observed conversion efficiency is about 10−4 because the terahertz wave generated inside the crystal is absorbed by the nonlinear crystal itself while propagating to the crystal surface and is affected by Fresnel loss on the boundary surfaces. Furthermore, the parametric gain (absorption) of

conversion efficiency improves by a factor of at least 10 at liquid nitrogen temperatures. In this case, under the condition of a pumping energy of 50 mJ/pulse, the expected brightness, brightness temperature, peak power, and electric field of the terahertz wave are greater than

ously tunable is-TPG. Some applications require high-brightness terahertz waves, such as observing two- or multiphoton absorption to specific excitation states [32, 33]. The generation of the extremely high-brightness (megawatts (~ MW) peak power and narrow (~ GHz) linewidth) quasi-monochromatic terahertz-wave (several hundreds of cycles) pulses with field levels in the megavolt per centimeter (~ MV/cm) range will enable novel applications in the field of terahertz nonlinear optics. We also introduced how to optimize the tuning curve of the

is-TPG by controlling the pumping intensity and the interaction volume.

, 1019 K, 1 MW, and 2 MV/cm, respectively, from our narrowband and continu-

terahertz-wave generation via wavelength conversion in a MgO:LiNbO3

*) 2*

~ 0.2 GW/sr·cm2

could be increased (decreased) by cooling the crystal [31]. The

and 0.5 MV/cm at around 2.0 THz, respectively, at the focused point.

. The intensity and elec-

crystal. These result

value less than 1.1, and the brightness was *B = Pp/(λ M<sup>2</sup>*

tric field were 0.3 GW/cm2

38 High Power Laser Systems

**5. Conclusion**

the terahertz wave in LiNbO3

4 GW/sr·cm2

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