Heteroanionic Control of Exemplary Second-Harmonic Generation and Phase Matchability in 1D LiAsSSe.

The isostructural heteroanionic compounds β-LiAsSSe ( = 0, 0.25, 1, 1.75, 2) show a positive correlation between selenium content and second-harmonic response and greatly outperform the industry standard AgGaSe. These materials crystallize in the noncentrosymmetric space group as one-dimensional / [AsQ] (Q = S, Se, S/Se) chains consisting of corner-sharing AsQ trigonal pyramids with charge-balancing Li atoms interspersed between the chains. LiAsSSe melts congruently for 0 ≤ ≤ 1.75, but when the Se content exceeds = 1.75, crystallization is complicated by a phase transition. This behavior is attributed to the β- to α-phase transition present in LiAsSe, which is observed in the Se-rich compositions. The band gap decreases with increasing Se content, starting at 1.63 eV (LiAsS) and reaching 1.06 eV (β-LiAsSe). Second-harmonic generation measurements as a function of wavelength on powder samples of β-LiAsSSe show that these materials exhibit significantly higher nonlinearity than AgGaSe ( = 33 pm/V), reaching a maximum of 61.2 pm/V for LiAsS. In comparison, single-crystal measurements for LiAsSSe yielded a = 410 pm/V. LiAsSSe, LiAsSSe, and β-LiAsSe show phase-matching behavior for incident wavelengths exceeding 3 μm. The laser-induced damage thresholds from two-photon absorption processes are on the same order of magnitude as AgGaSe, with S-rich materials slightly outperforming AgGaSe and Se-rich materials slightly underperforming AgGaSe.