Chromophore Orientations upon Irradiation in Gratings Inscribed on Azo-Dye Polymer Films: A Combined AFM and Confocal Raman Microscopic Study

Abstract

Diffraction gratings of weak efficiency (≈2−3%) were inscribed on thin films of the azobenzene p(DR1M) homopolymer using orthogonal linear (+45°,+135°) polarizations of two coherent laser beams. After an erasure process using a single beam of circular polarization (“RCP”), the surface relief profiles measured from atomic force microscopy (AFM) were compared on a grating inscribed during a short time period (≈200 s) and on similar gratings tentatively erased with a “RCP” irradiation during a short (100 s) or a long (800 s) period. In addition, confocal micro-Raman polarized experiments were performed on these gratings and, using a rigorous treatment of the Raman intensity variations, the two even parity Legendre'polynomials or parameter orders, 〈P2〉 and 〈P4〉, were estimated in the well, slope, and peak regions of the surface relief gratings; the corresponding forms of the most probable chromophore orientation distribution functions were determined. It is thus concluded that the variations in diffraction efficiency observed at short time are not due to any change in the amplitude of the surface relief modulation (equal to ≈112−110 nm) but to a significant increase in the birefringence contributions; the chromophore distribution functions become largely anisotropic. Then, the orientational anisotropy is partly maintained during the poorly efficient erasing process, and after a longer time irradiation, the final relief amplitude is found equal to 66−70 nm. In the p(DR1M) polymer system under study, the (+45°,+135°) linear polarization configuration thus appears very suitable for single beam duplication experiments and/or for further phase mask applications.