Heteroanionic Control of Exemplary Second-Harmonic Generation and Phase Matchability in 1D LiAsS2–xSex

The isostructural heteroanionic compounds β-LiAsS2-xSex (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. LiAsS2-xSex 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-LiAsS2-xSex 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 deff = 410 pm/V. LiAsSSe, LiAsS0.25Se1.75, 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.