Chromophore orientations in surface relief gratings with second-order nonlinearity as studied by confocal polarized Raman microspectrometry

Diffraction gratings of medium and strong efficiency (approximate to10-25%) were first inscribed on thin films of p(DR1Mco-MMA) copolymer, with a 12% azo mole fraction, and p(DR1M) azobenzene homopolymer using interferences between two linearly polarized (p + p) coherent laser beams. Then, the gratings were wirepoled at T approximate to T-g - 30 degreesC under an intense dc electric field, inducing a noncentrosymmetry to the embedded chromophores. The grating surface relief modulation was controlled by atomic force microscopy (AFM), and the optical properties were investigated using UV-vis spectroscopy and second harmonic generation (SHG) polarized measurements. In addition, confocal micro-Raman polarized experiments were performed on virgin (unpoled) gratings, on related poled gratings, and also on gratings inscribed into previously poled films. Using a rigorous treatment of the Raman intensity variations, the two even-parity order parameters or Legendre's polynomials, <P-2> and <P-4> are estimated in the depth and peak regions of the surface reliefs; the corresponding forms of the most probable chromophore orientation distribution functions are thus determined and compared in the various gratings. It is thus concluded that the chromophore distribution functions are generally broad with strong maxima near +/-90degrees in the bottom regions, and they are bimodal and asymmetric with sharp maxima at +/-56-60degrees in the peak regions. These orientational anisotropic properties are largely maintained during the poling process, and consequently, the final surface relief amplitudes do not show any significant change (+/-20 nm). Finally, for the highest efficiency poled gratings of the p(DR1M) homopolymer there appears a notable deficiency in the chromophore density in the vicinity of the peak regions, which could be related to the existence of an apparent new half-period structure as recently evidenced from SHG near-field scanning optical microscopic measurements.