In this study we have investigated the electrical poling effects in a p(DR1M) azopolymer thin film and in related diffraction gratings inscribed on other films. Their optical properties were determined using polarization measurements in UV-visible spectroscopy, in confocal Raman micro-spectrometry and in second harmonic generation (SHG) in the transmission mode. Near-field scanning optical microscopy (NSOM) experiments together with SHG responses in reflection were also performed in order to retrieve simultaneously the topography and the SHG contrasts of the surface gratings. This multitechnique approach led to the determination of the first even and odd parity Legendre polynomials, from < P-1> up to < P-4>, allowing the establishment of precise chromophore orientational distribution functions. In particular, from SHG near-field scanning optical microscopy (NSOM) experiments we have evidenced that poled gratings with large diffraction efficiencies and x((2)) nonlinear susceptibilities exhibit new sub-structures with puzzling differences from their topographical AFM images. A combination of all the linear and nonlinear results allows us, for the first time, to explain the peculiar orientational and mechanical properties in the peak areas of the surface relief modulations, at near each half-period grating (Delta/2 approximate to 0.7 mu m). An efficient poling of the azobenzene dye groups in such gratings thus leads to new interesting properties.