Optical transitions due to native defects in nonlinear optical crystals LiGaS₂

LiGaS₂ (LGS) is a recently developed nonlinear optical crystal widely used for nonlinear conversion in the mid-infrared spectral region, but its applications are significantly influenced by the native defects present in the lattice. In this work, absorption and photoluminescence (PL) spectra are studied after annealing the as-grown LGS crystal in different chemical environments in order to reveal features related to anion vacancy (V_S) and cation antisite defect (Ga_Li). In addition, irradiation with fast electrons produces V_S centers and their complexes in LGS. The V_S migration and mutual transformations in these defects take place at room temperature whereas their complete annealing occurs at 670 K. Meanwhile, the first-principles calculations are adopted to identify the relevant optical transitions. A good agreement with the experimental results was obtained. It is established that the dominant defects V_S result in the optical absorption at 3.43 eV and violet PL emission, while Ga_Li is responsible for absorption at 2.95 eV. The intense PL peak at 1.92 eV is associated with the self-trapped excitons in LGS. The other weak PL bands are also related to the recombination of electrons or holes with the V_S and Ga_Li defect states.