Chromophore orientations in a nonlinear optical azopolymer diffraction grating: Even and odd order parameters from far-field Raman and near-field second harmonic generation microscopies

A diffraction grating with a large diffraction efficiency (η = 25% on the first order) was inscribed on an azobenzene-containing polymer thin film, electrically poled, and characterized using spatially resolved Raman confocal microscopy and near-field scanning optical microscopy (NSOM) coupled with second-order nonlinear optical measurements. Linear (Raman) and nonlinear (second harmonic generation or SHG) polarized microscopic measurements were performed on a grating with ∼ 1.4 μm periodicity, revealing the molecular orientations in various regions of the sinusoidal relief surface. The most probable distribution functions f(θ) of the chromophore orientations were derived using the two first even parity Legendre polynomials 〈P ₂〉 and 〈P ₄〉 together with the two odd order parameters 〈P ₁〉 and 〈P ₃〉, as determined by Raman and SHG, respectively. These distribution functions show that the poling treatment is quite efficient and also emphasize the importance of determining both couples of the order parameters in such a surface-modulated optical element. In particular, new information is provided about the formation mechanism of the additional surface modulations of half periodicity that are revealed by the SHG-NSOM data but are not detected in the far-field Raman scattering measurements.