LiBX₂(B=Ga, In; X=S, Se,Te) crystal is one of the most promising frequency converters materials for IR due to extended transparency range, sufficiently large birefringence, high laser induced damage thresholds and small two-photon absorption coefficient. This article focuses on the optical properties, defect structures and applications of these crystals reported in the past few years. The promising applications on the phase-matched second harmonic generation (SHG), continuous-wave (cw) difference-frequency generation (DFG), optical parametric amplifier (OPA) and optical parametric oscillator (OPO) are summarized. The major native defects, defect concentration and optimal annealing temperature can be indicated by the transmission, absorption and photoluminescence (PL) spectra and irradiation with fast electrons. The relationship between color and composition and crystal defects is analyzed. Finally, the trends in the investigation on crystal growth, reduction of the residual losses and optimization of the antireflection (AR)-coating process are prospected.

Contents
1 Introduction
2 Structural characteristics
3 Polycrystalline synthesis and crystal growth
4 Optical properties and defect structures
4.1 LiInS₂ crystal
4.2 LiInSe₂ crystal
4.3 LiGaS₂ crystal
4.4 LiGaTe₂ and LiGaSe₂ crystal
5 Crystal applications
6 Conclusions and Outlook