Selenium-Based Molecular Cage Crystal β-P₄Se₃Br₂: A Promising Candidate for Infrared Nonlinear Optical Applications

Nonlinear optical (NLO) crystals play a vital role in a wide range of applications including all-solid-state lasers and infrared (IR) NLO devices. However, existing commercially available IR NLO crystals, such as AgGaS₂ (AGS), AgGaSe₂ (AGSe), and ZnGeP₂ (ZGP), face limitations due to low laser-induced damage thresholds and undesirable two-photon absorption. This has spurred interest in developing new materials that perform well in the IR atmospheric window (2.5-25 μm). In this study, we focus on selenium-based molecular crystals, which offer advantages over sulfur-containing analogs, including a wider IR transmission range and enhanced second-harmonic generation (SHG) responses. Specifically, we synthesized β-P₄Se₃Br₂, a new IR NLO material derived from the centrosymmetric (CS) α-P₄Se₃Br₂ phase. The crystal features molecular cages interconnected by van der Waals (vdW) forces, exhibiting near-uniform alignment that enhances its NLO properties. Experimental and theoretical investigations reveal that β-P₄Se₃Br₂ exhibits a promising SHG response (1.1 × AGS), a large band gap (2.31 eV), and a broad transmission window (3-25 μm), making it a promising candidate for IR NLO applications. These findings highlight the potential of selenium-based molecular crystals for advancing the field of IR NLO materials.