Doubling of THz radiation frequency in nonlinear borate crystals

The use of nonlinear oxide crystals for radiation generation and conversion into terahertz (THz) range using the nonlinear crystal optics methods is a new promising direction that has actively been developed in the last few years. For the first time, the possibility is considered of effective frequency doubling of millimeter radiation in the wavelength range of 400–2500 m in borate crystals. The phase matching angles are calculated. It is shown that o + o → e and e + o → e interactions can be realized in LN and β-BBO crystals, and only o + o → e interactions are possible in LB4 crystals. The walk-off angle and the spectral and angular matching bandwidths are calculated. The maximal efficiency of the second harmonic generation is estimated taking into account the measured optical damage threshold of 150 TW/cm2 for the LB4 crystal and 100 TW/cm2 for the β-BBO crystal. This shows advantages of application of the oxide crystal in the THz range of the spectrum