Photoinduced orientations of azobenzene chromophores in two distinct holographic diffraction gratings as studied by polarized Raman confocal microspectrometry

Optically isotropic thin films (0.5 mum) of an amorphous side-chain copolym er containing 10% azobenzene moieties and 90% meso-azobenzene groups (K1-10 sample) were irradiated by two interfering orthogonal (+/- 45 degrees) or parallel (p+p) linearly polarized laser beams; permanent holographic diffra ction gratings were thus inscribed. From atomic force microscopy (AFM) meas urements, relatively weak amplitudes (about 50-60 nm) in the surface relief modulation (SRM) are detected but strong perturbations in the profiles and new substructures are evidenced at half-period positions. Using a confocal microspectrometric technique we have recorded various pre-resonance enhanc ed polarized Raman spectra from a large grating area and obtained spatially resolved Raman images of the intensity variations. This yields new insight into the photoinduced orientation effects, the angular distributions of th e chromophore species, and finally the grating formation mechanisms. Differ ent theoretical equations of the polarized Raman scattering intensities, ta king account of the pump polarization directions and the high numerical ape rture objective used, are derived. Then, an analysis of the experimental re sults allows us to extract values of the second [P-2] and fourth [P-4] coef ficients in the chromophore orientation functions at several positions of t he SRM, in particular at the top and bottom regions, and to calculate the c orresponding normalized distribution functions via information entropy theo ry. In both gratings, somewhat broader distributions are found in the botto m regions, regions of greater polymer removal. Thus, mass-transport phenome na have perturbed, to a weak extent in the (+/- 45 degrees) first case and to a larger extent in the (p+p) second case, the primary photoinduced orien tations. So, two simultaneous and superimposed mechanisms appear to be resp onsible for the observed complex SRM in the highly birefringent K1-10 copol ymer under study and significant second-order contributions are suggested. The grating surface profiles are thus reproduced by fitting the SRM amplitu des at the fundamental and doubled frequency and the corresponding phase co nstant parameters: a good agreement between the observed (AFM) and simulate d profiles is obtained.