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 (Δn), is crucial for maximizing the nonlinear optics (NLO) laser power output. Δn 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-KGa₅Se₈ (P2₁, 1) and β-KGa₅Se₈ (P1, 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 Δn primarily stems from the different arrangements of tetrahedral units within the two phases. Specifically, the polarizability anisotropy of the tetrahedral units in β-KGa₅Se₈ is 3.7 times that of α-KGa₅Se₈. This uniform packing improves the Δn index from 0.015 (α-phase) to 0.023 (β-phase). Furthermore, β-KGa₅Se₈ not only demonstrates a robust PM second-harmonic generation (SHG) response (2.1 × AgGaS₂ @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.