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 , 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 for different concentrations. Experimental results complemented with theoretical predictions show that, in the low absorption and thin sample limits, the signal reproduces the 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.