MgSiP₂: An Infrared Nonlinear Optical Crystal with a Large Non-Resonant Phase-Matchable Second Harmonic Coefficient and High Laser Damage Threshold

Superior infrared nonlinear optical (NLO) crystals are in urgent demand in the development of lasers and optical technologies for communications and computing. The critical challenge is to find a crystal with large non-resonant phase-matchable NLO coefficients and high laser damage threshold (LDTs) simultaneously, which however scale inversely. This work reports such a material, MgSiP_2, that exhibits a large second harmonic generation (SHG) coefficient of d₁₄≈d₃₆ = 89 ± 5 pm V⁻¹ at 1550 nm fundamental wavelength, surpassing the commercial NLO crystals AgGaS₂, AgGaSe₂, and ZnGeP₂. First principles theory reveals the polarizability and geometric arrangement of the [SiP₄] tetrahedral units as the origin of this large nonlinear response. Remarkably, it also exhibits a high LDT value of 684 GW cm⁻², which is six times larger than ZnGeP₂ and three times larger than CdSiP₂. It has a wide transparency window of 0.53–10.35 µm, allowing broadband tunability. Further, it is Type I and Type II phase-matchable with large effective SHG coefficients of d_eff,I ≈80.2 pm V⁻¹ and d_eff,II ≈73.4 pm V⁻¹. The outstanding properties of MgSiP₂ make it a highly attractive candidate for optical frequency conversion in the infrared.