Tuning the nonlinear optical properties of SnO2–rGO nanocomposites: exploration using conventional Z-scan and thermal lensing models

Abstract

This study investigates the nonlinear optical properties of SnO₂–rGO nanocomposites by employing a conventional Z-scan technique and a thermal lensing model using a CW laser with an excitation wavelength of 532 nm. The fabrication of three batches of SnO₂–rGO nanocomposites at three compositional ratios 1 : 1, 1.25 : 1, and 2.5 : 1 (three different concentrations for each), was achieved via a conventional hydrothermal method followed by chemical reduction. The fabricated samples were subjected to rigorous investigations and analyses using UV-visible spectroscopy, XRD, FESEM, Raman spectroscopy, FTIR, and photoluminescence (PL) methods to uncover precisely their morphological details, sizes, defects, charge transfer characteristics, and identities. At low intensity (I₀ = 1.0 kW cm⁻²), our open aperture (OA) data suggest that both SnO₂ and GO exhibit reverse saturation absorption (RSA), whereas rGO and SnO₂–rGO nanocomposites show tunable saturation absorption (SA) characteristics. But at a higher intensity (I₀ = 5.3 kW cm⁻²), the symmetrical nature of all the close aperture (CA) data suggests that at this intensity, the nonlinear refraction dominated, and the values of the nonlinear refractive index were found to be negative, indicating self-defocusing phenomena. We believe that the charge transfer mechanism and the induced defect state can play dominant roles in tuning the nonlinear optical characteristic of the nanocomposites. A DFT study also uncovers further theoretical insight into the charge transfer characteristic in SnO₂–rGO nanocomposites.