Impact of Microstructural Units Configuration and Arrangement on Phase-Matching in Nonlinear Optical KGaSe Crystals.

Achieving phase-matching (PM) behavior, which is typically derived from sufficient birefringence (Δ), is crucial for maximizing the nonlinear optics (NLO) laser power output. Δ is largely attributed to the configuration and arrangement of microstructural units, and phase transitions will significantly alter the stacking method of structural units. In this study, we successfully synthesized two diamond-like isomers of α-KGaSe (2, ) and β-KGaSe (1, ), which consist of open-honeycomb-like anionic frameworks interspersed with embedded K cations. Importantly, our analysis of frequency doubling experiment data revealed a significant finding at a laser wavelength of 1910 nm: the α-phase lacks PM capability, whereas the β-phase exhibits PM behavior. Notably, through structural and computational analysis, the distinct Δ primarily stems from the different arrangements of tetrahedral units within the two phases. Specifically, the polarizability anisotropy of the tetrahedral units in β-KGaSe is 3.7 times that of α-KGaSe. This uniform packing improves the Δ index from 0.015 (α-phase) to 0.023 (β-phase). Furthermore, β-KGaSe not only demonstrates a robust PM second-harmonic generation (SHG) response (2.1 × AgGaS @1910 nm) but also possesses a wide band gap of 2.56 eV, surpassing the 2.33 eV threshold that corresponds to the energy of doubly frequency light using the 1064 nm laser, a characteristic often not exceeding in most selenides. This work underscores the importance of the configuration and arrangement of microstructural units for achieving PM in NLO materials.