70 MW-level picosecond mid-infrared radiation generation by difference frequency generation in AgGaS2, BaGa4Se7, LiGaSe2, and LiGaS2
Comparative study of nonlinear crystals for picosecond difference frequency generation in mid-IR is presented. Nonlinear crystals of AgGaS$_2$, BaGa$_4$Se$_7$, LiGaSe$_2$, and LiGaS$_2$ were studied. Samples of AgGaS$_2$, BaGa$_4$Se$_7$, LiGaSe$_2$, and LiGaS$_2$ were tested in thee sets having lengths of 2, 4, or 8 mm. In order to investigate the dependence of efficiency on the crystal length, three sets of crystals with lengths of 2, 4, or 8 mm were tested. The developed tunable DFG system was driven by the 1.03 $\mu$m, 1.8 ps, Yb:YAG thin-disk laser system operated at the repetition rate of 10 or 100 Hz. As the best result, picosecond mid-IR pulses at a wavelength of $\sim$7 $\mu$m with the energy up to 130 $\mu$J corresponding to the peak power of $\sim$72 MW were generated using the 8 mm long LiGaS$_2$ crystal. Using the BaGa$_4$Se$_7$ crystal, DFG tunability in the wavelength range from 6 up to 13 $\mu$m was achieved.