**5. Conclusion**

We have introduced here high-peak-power, narrow-linewidth, and continuously tunable terahertz-wave generation via wavelength conversion in a MgO:LiNbO3 crystal. These result 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 the terahertz wave in LiNbO3 could be increased (decreased) by cooling the crystal [31]. The 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 4 GW/sr·cm2 , 1019 K, 1 MW, and 2 MV/cm, respectively, from our narrowband and continuously 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.

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.
