“…The most successful techniques for generating narrowband, highpower THz pulses have come from frequency downconversion of the nanosecond 1 and 10 m lasers in nonlinear crystals. Tunable difference frequency generation (DFG) in the THz range was demonstrated both in birefringent, e.g., ZnGeP 2 [2,3], LiNbO 3 [4,5], and GaSe [6], and isotropic, e.g., GaAs [7] nonlinear materials. According to Manley-Rowe relations the maximum power conversion efficiency for DFG at a frequency of 3 = 1 − 2 (where 1 and 2 are input frequencies) is limited by the 3 / 1 ratio.…”