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 χ⁽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 |χ⁽³⁾| for different concentrations. Experimental results complemented with theoretical predictions show that, in the low absorption and thin sample limits, the signal reproduces the |χ⁽³⁾| 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.