Li_xAg_1-xGaS₂: A Tunable Crystal for Mid-IR Nonlinear Optics via Composition Control

Li_xAg_1-xGaS₂, a novel quaternary solid-solution nonlinear optical (NLO) crystal, has emerged as a promising material for mid-IR applications. The crystal exhibits a significantly enhanced nonlinear optical coefficient and a more optimally tuned bandgap compared to AgGaS₂. In this work, single crystals of Li_xAg_1-xGaS₂ (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1) with dimensions of Φ25 mm × 25-50 mm were first grown by a modified Bridgman method. It is found that Li_xAg_1-xGaS₂ crystals (x = 0-0.6) maintain the same tetragonal symmetry as AgGaS₂, while Li_0.8Ag_0.2GaS₂ and LiGaS₂ exhibit orthogonal symmetry. Due to the low transparency of as-grown crystals, we conducted a systematic thermal treatment experiment resulting in significant improvement in the transmittance and bandgap of Li_xAg_1-xGaS₂ crystal wafers. The photoluminescence spectrum of AgGaS₂ is characterized by a wide "green emission" centered at about 500 nm (2.50 eV) due to donor-acceptor pair recombination, and this emission wavelength decreases after Li⁺ replacement. Density functional theory calculations were performed to simulate the phonon spectrum and density of states, facilitating the study of phonon vibrational modes in the lattice. Finally, the second harmonic generation (SHG) response of Li_0.5Ag_0.5GaS₂ was measured to be approximately 2.5 times that of AgGaS₂, confirming its promising nonlinear optical properties.