Correlation length relaxation times from hyper-Rayleigh scattering in thin films versus second-harmonic generation relaxation times

Abstract

Femtosecond hyper-Rayleigh scattering (HRS) has been used to prove the spatial orientational fluctuations between nonlinear optical chromophores as dopants in spincoated polymer films. The fluctuation in the second-order incoherently scattered light intensity upon micro- translating the solid sample is indicative of the degree of spatial correlation between the individual chromophores. The decay of the autocorrelation function of this fluctuating signal is characterized by a spatial correlation length. Electric-field poling of dipolar chromophores is shown to increase this correlation length. The longer correlation length after poling results from a higher degree of spatial orientational correlation between the individual chromophores. The temporal characteristics of this correlation length have been studied and compared with thermal relaxation times, obtained with coherent second- harmonic generation (SHG). A significant difference in relaxation time between coherent (SHG) and incoherent (HRS) measurements in electric-field poled thin polymeric films has been observed experimentally. Based on the intrinsic symmetry requirements for even-order nonlinear optical effects, rotational relaxation of the chromophore only was invoked in combination with translational diffusion of free volume in the polymer matrix over the coherence length for coherent second-harmonic generation and over the wavelength in the incoherent case.