KAg3Ga8S14: An Mid- and Far-infrared Nonlinear Optical Material Exhibiting High Laser-induced Damage Threshold

Nonlinear optical (NLO) crystals can produce tunable lasers due to their second-harmonic generation, sum-frequency generation, difference-frequency generation and optical parametric oscillation. The famous oxide-based NLO materials such as KH2PO4 (KDP), beta-BaB2O4 (BBO) and LiB3O5 (LBO) are widely used in ultraviolet-visible (UV-Vis) region. Nevertheless, they are not suitable for the mid- and far-infrared region because of the strong absorption there. Currently, commercially available IR NLO materials are rare, such as chalcogenides AgGaS2 (AGS), AgGaSe2 and phosphorus ZnGeP2, which have the advantages of large NLO coefficient and wide transmission range, but they have drawbacks, like low laser-induced damage threshold (LIDT). Discovering NLO crystals that exhibit simultaneously large NLO and high LIDT is a huge challenge. Here, the introducing electropositive alkali metal ionic K+ in chalcopyrite AGS successfully affords a new sulfide KAg3Ga8S14 by high temperature solid state reaction. Its crystal structure adopts a three-dimensional honeycomb-like open framework, in which all tetrahedral AgS4 and GaS4 units are arranged in a highly oriented manner, thereby producing about a medium phase-matching second harmonic generation (SHG) response of 0.4 times that of the benchmark AGS at the incident laser of 1910 nm. Remarkably, the compound possesses a wide band gap (2.95 eV), thus avoiding two-photon absorption of the incident 1064 nm laser, and exhibits a high LIDT of 4.6 times that of the AGS at the laser of 1064 nm. Moreover, KAg3Ga8S14 has a wide transmission range (0.25-25.0 mu m) that covers the two important atmospheric windows of 3-5 and 8-12 mu m. Furthermore, according to theoretical calculations, the conductive band is mostly composed of Ga-4s and S-3p states, mixing with small amounts of Ga-4p state, whereas the valence band near the Fermi level originates predominately from Ag-4p and S-3p states, mixing with small amounts of Ga-4p state, indicating that tetrahedral GaS4 and AgS4 units govern the optical and NLO properties of chalcopyrite KAg3Ga8S14.

Automated summary

The introduction of electropositive alkali metal ionic K+ in chalcopyrite AGS successfully affords a new sulfide KAg3Ga8S14 with wide transmission range and high laser-induced damage threshold. The compound exhibits a medium phase-matching second harmonic generation response and avoids two-photon absorption of the incident laser, making it promising for various nonlinear optical applications.