Dynamics of resonant third-order optical nonlinearity in perylene tetracarboxylic dianhydrlde studied by monitoring first- and second-order diffractions in subpicosecond degenerate four-wave mixing

We present a novel approach using simultaneous monitoring of temporal behavior of the usual phase conjugate signal and the second-order diffraction produced in a degenerate four-wave mixing experiment to obtain information about dynamics of resonant third-order nonlinear optical processes. The second-order diffraction is interpreted as arising from the presence of the appropriate Fourier component of the excited state grating. The higher Fourier components are expected to be generated in the presence of such excited state processes as bimolecular decay, two-photon absorption, saturation of absorption and diffusion of excitation. Third-order nonlinear optical properties of an organic dye: perylene tetracarboxylic dianhydride were studied by this approach using subpicosecond degenerate four-wave mixing at a wavelength of 602 nm. The dye is found to exhibit a strong resonant nonlinear effect with an effective χ⁽³⁾ of ≈2 × 10^-10 esu. The decay of the phase conjugate signal is power dependent and can be theoretically simulated by using a combination of monomolecular and bimolecular decay laws. The temporal behavior of the phase conjugate signal and the second-order diffraction are investigated at different intensities. The observed characteristics are well simulated by using a dominant bimolecular decay mechanism at higher excitation density.