Second-order nonlinearities of polycrystalline molecular films studied on a 20-fs time scale.

Second-harmonic generation in polycrystalline pentacene films of submicrometer thickness by pulses of 18-fs duration is studied. We observe a second-order nonlinearity of as much as d(eff) = 0.045 pm/V, which is resonantly enhanced at wavelengths near 790 nm. Off-resonance autocorrelation studies demonstrate a quasiinstantaneous nonlinear response of the material, whereas experiments on resonance point to a finite response time T(2) of the order of 15 fs.