Crystal Growth, Characterization, and Properties of Nonlinear Optical Crystals of Li x Ag 1- x GaSe 2 for Mid-Infrared Applications.

LiAgGaSe is a new series of solid solution crystals that has a large nonlinear optical (NLO) coefficient and laser-induced damage threshold (LIDT). It has great application prospects in mid-infrared laser frequency conversion. In this work, LiAgGaSe ( = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) crystals (Φ 16 mm × 40 mm) were grown by the improved Bridgeman method in a four-zone furnace. It is found that the LiAgGaSe ( = 0.2-0.8) crystals keep the same tetragonal symmetry with AgGaSe and the melting and solidification temperature increase with the Li content. Because the as-grown crystals are almost opaque in the visible-NIR range, an annealing experiment is necessary. After annealing, the transmittance is improved significantly, which can meet the application requirements. The band gap is changed by annealing atmosphere; for instance, the band gap of LiAgGaSe annealed in a LiGaSe powder atmosphere increases from 2.35 to 2.56 eV, while the band gap of LiGaSe annealed in vacuum decreases from 3.39 to 3.01 eV. Finally, the LiAgGaSe shows an extreme SHG response, especially LiAgGaSe, which has about five times that of LiGaSe, proving the promising NLO properties.