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

Li x Ag 1- x GaSe 2 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, Li x Ag 1- x GaSe 2 ( x = 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 Li x Ag 1- x GaSe 2 ( x = 0.2-0.8) crystals keep the same tetragonal symmetry with AgGaSe 2 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 Li 0.6 Ag 0.4 GaSe 2 annealed in a LiGaSe 2 powder atmosphere increases from 2.35 to 2.56 eV, while the band gap of LiGaSe 2 annealed in vacuum decreases from 3.39 to 3.01 eV. Finally, the Li x Ag 1- x GaSe 2 shows an extreme SHG response, especially Li 0.8 Ag 0.2 GaSe 2 , which has about five times that of LiGaSe 2 , proving the promising NLO properties.