Far-Infrared Spectral Dispersion Simulation of Refractive Indices and Absorption Coefficients in Typical Nonlinear Optical Crystals

Nonlinear optical (NLO) crystals for far-infrared (FIR) applications face a critical challenge: optimizing the inherent trade-off between low optical dispersion (essential for phase matching) and minimal phonon absorption (crucial for high transparency). In this study, we employ a first-principles approach to investigate the FIR dispersion of refractive indices and absorption coefficients in 10 typical NLO crystals (β-BaB 2 O 4, α-SiO 2, urea, LiNbO 3, α-LiIO 3, CdSe, GaAs, AgGaS 2, AgGaSe 2, and ZnGeP 2 ). Our simulations elucidate the correlation between optical responses and the structural characteristics of tetrahedral, octahedral, conical, and planar building units. The findings, which include the role of uniform atomic masses in shifting absorption edges and the inherent compromise between structural stability and optical performance, provide valuable insights for property tuning and structural design of FIR NLO crystals.