Optical nonlinear properties of gadolinium-doped In2S3 thin films prepared by nebulizer-aided spray pyrolysis method

Abstract

Nonlinear optical properties are of increasing importance in the development of optoelectronic devices. In the current study, undoped and gadolinium-doped In₂S₃ thin films have been deposited by nebulizer spray pyrolysis within a Gd doping range of 0–5 wt%. The focus of the investigation is on how the presence of rare-earth element affects the optical properties gadolinium has been chosen to be used as the dopant since the ionic radius of gadolinium is nearly equal to that of In³⁺, which would make it possible for gadolinium to occupy the same position as the In³⁺ ion without causing crystal structure distortion. Different analytical methods have been employed to investigate the physical properties of the prepared samples. It is noteworthy that the 3 wt% Gd sample exhibits the strongest overall nonlinear optical response among the studied samples, with the nonlinear absorption coefficient, nonlinear refractive index, and third-order susceptibility all peaking at this composition, alongside a direct bandgap of 2.49 eV identifying it as a promising candidate absorber layer, while the smallest crystallite size among the crystalline samples was instead observed at 4 wt% Gd (see Tables 1 and 3). The surface of the films had a tight nano-grain structure, forming dots throughout the film. Visible-to-NIR absorbance spectra (400–1000 nm) showed a progressive increase in absorbance with Gd doping up to 3 wt%, followed by a decrease at higher concentrations. The optical bandgap decreased from 2.76 eV (undoped) to a minimum of 2.49 eV at 3 wt% Gd, before increasing again at higher doping levels. Values ranging from 2.49 to 2.76 eV were obtained using the Tauc equation. Z-scan method was used to evaluate the values of nonlinear absorption coefficient, nonlinear refractive index, and third-order susceptibility in both undoped and doped thin films. The nonlinear absorption coefficient β, nonlinear refractive index n₂, and third-order susceptibility |χ⁽³⁾| all peaked at 3 wt% Gd, with β ranging from −3.98 × 10⁻² m/W (undoped) to −7.40 × 10⁻² m/W, n₂ ranging from −4.41 × 10⁻⁹ to −12.09 × 10⁻⁹ m²/W, and |χ⁽³⁾| ranging from 491.4 × 10⁻¹³ to 2149.5 × 10⁻¹³ m²/V². Results have shown that NLO properties make these thin films good candidates for optical switches and sensors.