An investigation of third-order nonlinear optical and limiting properties of spray pyrolysis-deposited Co:CdS nanostructures for optoelectronics

Cobalt (Co)-doped CdS thin films were deposited and successfully grown by the chemical reactive spray pyrolysis technique with different Co-doping levels [(Cd_1−xCo_xS) in ‘x’ ratios of 0.00, 0.01, 0.05 and 0.1 wt%] on heated glass substrates at 350 °C. The effect of Co content on nonlinear optical (NLO) properties was investigated by the DPSS continuous wave laser at 532 nm using the Z-scan technique. Powder-XRD analysis confirms the samples that have cubic structure with no impurity phases. The energy gap (E_g) of the prepared films was estimated and was found to be decreased by 2%, i.e., in the range from 2.50 to 2.30 eV. The room-temperature photoluminescence spectra were recorded at λexc = 275 nm and an intense blue emissions were observed at 430 ± 15 nm for undoped CdS and Co:CdS films. The Z-scan result reveals that reverse saturable absorption and self-defocusing nature are the attributed and observed nonlinearity of the prepared nanostructures. The nonlinear absorption coefficient, refractive index and third-order NLO susceptibility were determined and found in the range from 1.89 × 10⁻³ to 7.26 × 10⁻³ (cm W⁻¹), 2.03 × 10⁻⁸ to 2.96 × 10⁻⁸ (cm² W⁻¹) and 1.21 × 10⁻⁶ to 6.95 × 10⁻⁶ (esu) correspondingly. An optical limiting topographies of the prepared films were explored and the limiting thresholds are also calculated at the experimental wavelength. These important results of NLO parameters are due to the increase in localized defect states on grain boundaries with the increase in Co-doping and suggest that these films are promising material for optoelectronic device applications.