Chalcogenide glasses for nonlinear photonics: Structure-nonlinearity relationship and fundamental tradeoff

Chalcogenide glasses have emerged as a key class of materials for nonlinear photonics due to their high refractive indices, strong third-order optical nonlinearity, and broad infrared transparency. The purpose of this work is to establish a unified framework linking Kerr nonlinearity, two-photon absorption (TPA), and their dependence on electronic band structure. This review correlates chalcogen substitution (S, Se, Te), network connectivity, and mean coordination number (MCN) with polarizability, bandgap, and nonlinear coefficients. The influence of heavy elements and lone-pair electrons on enhancing χ 3 is critically examined along with the impact of structural disorder and defect states. While Te-rich and highly polarizable systems exhibit enhanced nonlinear response, they are often accompanied by reduced band gaps and increased TPA, limiting their performance at telecom wavelengths, whereas wider-bandgap compositions offer lower loss but reduced nonlinearity. A central viewpoint of this review is that no single chalcogenide composition is universally optimal for all nonlinear photonic applications. Instead, device performance is governed by fundamental tradeoffs between nonlinear optical response, optical loss, infrared transparency, thermal stability, and fabrication compatibility, requiring material compositions to be tailored according to application-specific requirements. Despite advances in low-loss waveguides, As-free glasses, and CMOS-compatible photonic platforms, challenges associated with nonlinear absorption, material stability, reproducible fabrication, and large-scale integration remain. Future progress will rely on data-driven materials discovery, machine learning, advanced fabrication techniques, and heterogeneous photonic integration to accelerate the development of practical designs of next-generation chalcogenide glasses for ultrafast, broadband mid-IR, quantum, and integrated optical technologies.