Third-order nonlinearity and two-photon-induced molecular dynamics: Femtosecond time-resolved transient absorption, Kerr gate, and degenerate four-wave mixing studies in poly (p-phenylene vinylene)/sol-gel silica film

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Abstract
Femtosecond response and relaxation of the third‐order optical nonlinearity in a newly developed poly (p‐phenylene vinylene)/sol‐gel silica composite are investigated by time‐resolved forward wave degenerate four‐wave mixing, Kerr gate, and transient absorption techniques using 60 fs pulses at 620 nm. Using a theoretical description of two‐ and four‐wave mixing in optically nonlinear media, it is shown that the results obtained from simultaneous use of these techniques yield valuable information on the real and imaginary components of the third‐order susceptibility. In the composite material investigated here, the imaginary component is derived from the presence of a two‐photon resonance at the wavelength of 620 nm used for the present study. This two‐photon resonance is observed as transient absorption of the probe beam induced by the presence of a strong pump beam. It also provides fifth‐order nonlinear response both in transient absorption and in degenerate four‐wave mixing. The fifth‐order contributions are derived from the two‐photon generated excited species which can absorb at the measurement wavelength and therefore modify both the absorption coefficient and the refractive index of the medium.