Li x Ag 1– x GaS 2 : A Tunable Crystal for Mid-IR Nonlinear Optics via Composition Control

LixAg1-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 LixAg1-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 LixAg1-xGaS2 crystals (x = 0-0.6) maintain the same tetragonal symmetry as AgGaS2, while Li0.8Ag0.2GaS2 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 LixAg1-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 Li0.5Ag0.5GaS2 was measured to be approximately 2.5 times that of AgGaS2, confirming its promising nonlinear optical properties.