Impact of Microstructural Units Configuration and Arrangement on Phase-Matching in Nonlinear Optical KGa₅Se₈ Crystals

Achieving phase-matching (PM) behavior, which is typically derived from sufficient birefringence (Δ n ), is crucial for maximizing the nonlinear optics (NLO) laser power output. Δ n 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-KGa 5 Se 8 ( P 2 1, 1 ) and β-KGa 5 Se 8 ( P 1, 2 ), 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 n primarily stems from the different arrangements of tetrahedral units within the two phases. Specifically, the polarizability anisotropy of the tetrahedral units in β-KGa 5 Se 8 is 3.7 times that of α-KGa 5 Se 8 . This uniform packing improves the Δ n index from 0.015 (α-phase) to 0.023 (β-phase). Furthermore, β-KGa 5 Se 8 not only demonstrates a robust PM second-harmonic generation (SHG) response (2.1 × AgGaS 2 @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.