Modulation of nonlinear optical response of doped SiO₂ gel glasses by metal ion radii

Heavy metals and their compounds exhibit advantageous properties in the field of nonlinear optics, including high polarizability, broadband response, and tunability. These characteristics make them promising candidates for applications such as optical limiting, ultrafast modulation, and frequency conversion. Despite the challenges associated with toxicity and stability, incorporating these materials into solid-state matrices has been shown to enhance their stability and expand the technological application space of heavy-metal nonlinear optics in photonics. This study focuses on the incorporation of four heavy metal ions—zirconium ions (Zr⁴⁺), cadmium ions (Cd²⁺), cerium ions (Ce³⁺), and lead ions (Pb²⁺)—with increasing ionic radii, into a silica (SiO₂) gel network. The sol-gel technique was utilized, resulting in the preparation of environmentally stable heavy metal ion-doped SiO₂ gel glasses. The presence of heavy metal ions modulated the material band gap by introducing intermediate energy levels. The strong polarization ability of Pb²⁺ led to a significant red shift in the absorption edge of the Si-Pb system. Z-scan tests based on a 532 nm laser demonstrated that all samples exhibit reverse saturable absorption (RSA) and self-scattering properties. Furthermore, the nonlinear optical response was strongly and positively correlated with the radius of the heavy metal ions. The Si-Pb system exhibited optimal performance due to the high polarizability of Pb²⁺, with its nonlinear refractive index enhanced by a factor of 3.3 compared to that of the Si-Zr system. Modulating the heavy metal ion species optimized the nonlinear optical properties of the gel glass, providing both experimental basis and theoretical support for solid-phase photonics applications, such as high-energy laser protection and optical limiting devices.