Femtosecond self-diffraction as a measure of the nonlinear response spectrum

Self diffraction is a four-wave mixing process proportional to the square modulus of third-order nonlinearity susceptibility $\chi^{(3)}$, which is related to the material's electronic and thermal properties. In this study, we investigate the wavelength dependence of the self-diffracted signal generated by a femtosecond pulsed laser in a dye solution to directly evaluate the electronic third-order nonlinear susceptibility spectrum. By accounting for absorption effects and phase matching conditions, we determine the $\vert\chi^{(3)}\vert$ for different concentrations. Experimental results complemented with theoretical predictions, show that in the low absorption and thin sample limits, the signal reproduce the $\vert\chi^{(3)}\vert$ spectral profile. These findings demonstrate the feasibility of measuring nonlinear susceptibility spectra arising solely from the bound-electronic response across a wide spectral range and for various compounds.