Enhanced birefringence and suppressed second harmonic generation response mechanism in nonlinear optical materials via structural fine-tuning

Abstract

The microscopic anionic groups with specific arrangement are crucial in determining the optical properties of optically functional materials. The responses of linear and nonlinear polarization from external fields change synchronously, while the opposite phenomenon rarely occurs. Here, a dynamic structural fine-tuning strategy is proposed to clarify how the fluorooxo-functional units BO_xF_4−x (x = 2, 3) affect birefringence and second harmonic generation (SHG) response for typical nonlinear optical (NLO) material fluorooxoborates. Birefringence exhibits an obvious enhancement under external pressure, but a countertrend in the SHG coefficient was observed for Li₂B₃O₄F₃. On regulating the structural changes by simulating pressure, the fluorooxo-functional units BO₃F and BO₂F₂ generate strong distortion, and the structural changes are more sensitive than that of BO₃. An apparent response electron distribution anisotropy (REDA) in BO₂F₂ and a large amount of change of BO₃F induce the enhancement of birefringence extending from 0.48 to 0.73 at 1064 nm. Here, the fluorooxo-functional units BO_xF_4−x (x = 2, 3) were proved to be the enhanced factor for birefringence. However, we were surprised to notice that with the decrease in the volume of the crystal structure, the SHG response tends to decrease. The offset arrangement of BO₂F₂ and BO₃F under pressure is the main reason for the reduced SHG response. This study provides a perspective to understand the mechanism of the microstructure on the macroscopic properties of functional materials.