Investigation of optical properties in La2−xSrxCoO4 (x = 0.5, 0.7, 0.9, 1.1, 1.3, and 1.5) thin films: a focus on the linear and nonlinear responses

This study investigates the linear and nonlinear optical properties of La₂₋ₓSrₓCoO₄ (x = 0.5, 0.7, 0.9, 1.1, 1.3, 1.5) thin films, prepared via electron beam evaporation. Structural and morphological characterizations were performed using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), confirming a layered perovskite structure. UV-Vis spectroscopy revealed a decrease in optical band gap from 3.25 eV (x = 0.5) to 2.25 eV (x = 0.9), followed by irregular variations for x > 0.9. Nonlinear optical properties, assessed via Z-scan, showed peak nonlinear absorption (14.57 × 10⁻⁵ cm/W) and refractive index (9.32 × 10⁻⁵ cm²/W) at x = 0.9, attributed to enhanced Co³⁺ populations. These properties make La₂₋ₓSrₓCoO₄ thin films promising for photonic devices, such as optical switches and modulators, offering advantages over nanoparticles due to improved crystallinity and tunable optical responses. This work advances the understanding of Sr doping effects on Ruddlesden-Popper perovskites for optoelectronic applications.