Cesium-doped zinc sulfate hexahydrate single crystals with enhanced third-order nonlinear optical properties for sustainable photonic applications

In this work, cesium-doped zinc sulfate hexahydrate (CZSH) single crystals were successfully grown at room temperature by a low-energy slow evaporation technique, offering a cost-effective and environmentally benign route for crystal fabrication. Transparent and structurally stable crystals with good optical quality were obtained. Structural analysis by single-crystal X-ray diffraction confirmed the retention of the monoclinic phase with slight lattice distortion due to Cs⁺ incorporation, indicating successful dopant accommodation without altering the host symmetry. FT-IR analysis verified the presence of sulfate groups, coordinated water molecules, and stable lattice bonding. UV–Visible studies revealed excellent optical transmission in the visible region with a sharp cut-off wavelength at 285 nm and a wide optical band gap of 5.3 eV, demonstrating low optical loss and suitability for high-power optical systems. Photoluminescence analysis combined with Gaussian deconvolution identified emission peaks at 362, 405, and 490 nm, corresponding to near-band-edge and defect-assisted radiative transitions. The calculated CIE chromaticity coordinates (0.214, 0.358) confirmed blue-cyan emission with a colour purity of 30.89%, indicating stable luminescent behaviour. Surface morphology analysis showed uniformly distributed submicron grains with an average feature size of 0.269 μm, reflecting controlled growth kinetics and good microstructural homogeneity. Third-order nonlinear optical studies using the Z-scan technique at 532 nm exhibited positive nonlinear refraction and reverse saturable absorption with β = 4.2 × 10⁻³ cm/W, n₂ = 3.1 × 10⁻⁸ cm²/W, and χ³ = 2.1 × 10⁻⁷ m²/V². The enhanced nonlinear response is attributed to Cs⁺-induced electronic polarization, localized defect states, and improved optical homogeneity. These findings demonstrate that CZSH is a promising low-cost inorganic material for optical limiting, laser protection, photonic switching, and sustainable optoelectronic technologies.