Investigations of third order nonlinear optical properties and density functional theory (DFT)-based computational analysis of optical properties for undoped and Sr-doped ZnO thin films

Abstract

In this work, we report the synthesis, characterization, and comprehensive investigation of third-order nonlinear optical properties of Sr-doped ZnO thin films (0-6 wt% Sr) fabricated via sol-gel spin coating. The structural analysis confirmed the wurtzite crystal structure with c-axis orientation and increased porosity at higher doping levels. Z-scan measurements at 532 nm revealed that Sr doping systematically enhances both nonlinear absorption and refraction, with the nonlinear absorption coefficient increasing from 0.94 × 10^-2 to 4.89 × 10^-2 cm/W and the nonlinear refractive index (n₂) increasing in magnitude from -8.97 × 10^-8 to -4.42 × 10^-6 cm²/W. The observed reverse saturation absorption and self-defocusing behavior, confirmed by the pre-focal peak and post-focal valley configuration in closed-aperture Z-scan, indicates negative nonlinearity suitable for optical limiting. DFT calculations using the MBJ potential validated the experimental trends, showing increased band gap (3.2 to 3.4 eV) and enhanced density of states near the conduction band due to the Burstein-Moss effect, which facilitates two-photon absorption. The systematic enhancement of the real and imaginary parts of the third-order susceptibility with Sr doping confirms the effectiveness of Sr incorporation for tailoring nonlinear optical properties. These findings demonstrate that Sr-doped ZnO thin films are promising candidates for all-optical photonic devices, optical limiters, and switches operating at 532 nm, with the doping concentration providing a practical handle for optimizing performance.