Effect of Aluminium doping on photoluminescence and third-order nonlinear optical properties of nanostructured CdS thin films for photonic device applications

The work presented here reports the influence of Aluminium (Al) doping on CdS thin films for the structural, morphological, optical and third-order nonlinear optical (NLO) properties. Thin films of Pure CdS and Al-doped CdS (Cd_1-xAl_xS) with x = 0, 0.01, 0.05 and 0.1 are prepared on the glass substrate at 350 °C using the spray pyrolysis technique. The observed X-Ray Diffraction (XRD) patterns of CdS films are found to a polycrystalline hexagonal structure and are not much affected by Al doping. Also the films have been examined by Field Emission Scanning Electron Microscopy (FESEM) images. The transmittance of the CdS films is observed to be 50–60% in the visible region and that decreased at higher doping concentrations and with higher Al doping the direct optical band gap is decreased from 2.52 to 2.38 eV. To understand the defect states characteristics, the corresponding room-temperature photoluminescence (RTPL) spectra have also been taken and found the nonlinear behavior in a band to band-edge emission in the prepared samples upon Al incorporation. The sign and the magnitude of the third-order NLO properties were determined using the Z-scan technique with a continuous wave laser as the excitation source. It is observed that the material exhibit strong two-photon absorption (2PA) with the nonlinear absorption (NLA) coefficient (β) in the range of 10⁻⁴ cmW⁻¹ and nonlinear refractive index (NRI) n₂ ∼10⁻⁹ cm²W⁻¹. The third-order NLO susceptibility has found to be enhanced from 3.12 × 10⁻⁵ esu to 6.36 × 10⁻⁵ esu upon Al incorporation. Optical limiting characteristics of the prepared films are studied at the experimental wavelength. The results suggest that the Cd_1-xAl_xS is a promising material for nonlinear optical devices at 532 nm and optical power limiting applications.