Heteroanionic Control of Exemplary Second-Harmonic Generation and Phase Matchability in 1D LiAsS_2-Se

The isostructural heteroanionic compounds β-LiAsS_2-Se (x = 0, 0.25, 1, 1.75, 2) show a positive correlation between selenium content and second-harmonic response and greatly outperform the industry standard AgGaSe₂. These materials crystallize in the noncentrosymmetric space group Cc as one-dimensional ¹/_∞ [AsQ₂]^- (Q = S, Se, S/Se) chains consisting of corner-sharing AsQ₃ trigonal pyramids with charge-balancing Li⁺ atoms interspersed between the chains. LiAsS_2-Se melts congruently for 0 ≤ x ≤ 1.75, but when the Se content exceeds x = 1.75, crystallization is complicated by a phase transition. This behavior is attributed to the β- to α-phase transition present in LiAsSe₂, which is observed in the Se-rich compositions. The band gap decreases with increasing Se content, starting at 1.63 eV (LiAsS₂) and reaching 1.06 eV (β-LiAsSe₂). Second-harmonic generation measurements as a function of wavelength on powder samples of β-LiAsS_2-Se show that these materials exhibit significantly higher nonlinearity than AgGaSe₂ (d₃₆ = 33 pm/V), reaching a maximum of 61.2 pm/V for LiAsS₂. In comparison, single-crystal measurements for LiAsSSe yielded a d_eff = 410 pm/V. LiAsSSe, LiAsS_0.25Se_1.75, and β-LiAsSe₂ 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 AgGaSe₂, with S-rich materials slightly outperforming AgGaSe₂ and Se-rich materials slightly underperforming AgGaSe₂.