A Raman confocal microspectroscopic study on azopolymer holographic diffraction gratings: Photo- and mass transport-induced effects on the molecular orientation

Using the confocal microspectrometric technique we have recorded various re sonance enhanced polarized Raman spectra from the surface profile of a perm anent holographic diffraction grating prepared by interfering two circularl y contra-rotating polarized laser beams on a thin amorphous copolymer film containing azobenzene moities (pDR1Mco-MMA). Different theoretical equation s of the Raman scattering intensities, including a treatment taking account for the effect of the high numerical aperture objective, are thus derived. Calculations and simulations of these equations allow for the first time t o extract the second <P-2> and fourth <P-4>, coefficients of the chromophor e orientation function in the various regions and to obtain the correspondi ng information entropy distribution functions. We then discuss both contrib utions of the photo- and mass transport- induced effects in the grating for mation mechanisms and confirm the existence of a significant dye concentrat ion gradient (Delta N/N approximate to 5.1+/-11.1%) in between the top and bottom regions. Al the results are consistent with the model of a viscoelas tic flow of the polymer involving large pressure gradients and translationa l diffusion dynamics.