This study aims to investigate the nonlinear optical (NLO) characteristics of diluted crude oil samples from the Ramshir, Rag-e-Sefid, and Ahwaz-20 reservoir fields in the southwest region of Iran. Utilizing spatial self-phase modulation (SSPM) techniques, we measured the nonlinear refractive index ( \({n}_{2,th}\) ) by observing the intensity-dependent number of diffraction rings with continuous-wave laser beams at \(405 \text{nm}\) and \(532\text{ nm}\) wavelengths. This study focused on examining the \({n}_{2,th}/\alpha\) ratios of different sample concentrations to identify the role of thermal effects in the observed NLO responses. Our findings indicate a direct correlation between the concentration of crude oil in the samples and their NLO properties, with thermal effects significantly influencing the changes in NLO responses. In the thermal region, the nonlinear refractive index was approximately determined to be \({10}^{-5} {\text{cm}}^{2}/\text{W}\) . Additionally, the study explores the collapse of diffraction rings to further understand how the relative nonlinear refractive index varies with the sample concentration and incident intensity at both wavelengths. The results suggest that crude oil possesses promising NLO properties, making it potentially suitable for the development of new photonic devices and applications, especially in areas where thermal nonlinearities are crucial, such as sensing and all-optical technologies.
