Design and synthesis of a nonlinear optical material BaAl4S7 with a wide band gap inspired from SrB4O7

Abstract

The most convenient method to generate mid-infrared (mid-IR) high-power lasers is through nonlinear optical (NLO) process using NLO materials with high laser damage threshold (LDT). The LDT is intrinsically dependent on the energy band gap, but often inverse of the NLO effect. Therefore, achieving a good balance between band gap (E_g > 3.0 eV) and NLO response (d_ij > 10 × KDP) in a good mid-IR NLO material is necessary. In this study, inspired by the oxide SrB₄O₇ with the highest LDT among all known borate crystals, we propose that the chalcogenide BaAl₄S₇ with similar crystal structure may have attractive NLO properties. The experiments revealed that BaAl₄S₇ possesses the broadest band gap (>3.95 eV) among all known mid-IR NLO chalcogenides, and its LDT is above 10 times that of the commercial material AgGaS₂. Moreover, its NLO coefficient is above 10 × KDP (half of that of AgGaS₂). All these features demonstrate that BaAl₄S₇ is a potential candidate as a good mid-IR NLO material.