A Detailed Investigation of the Polarization-Dependent Surface-Relief-Grating Formation Process on Azo Polymer Films

We have carried out two-beam interference experiments to form surface-relief gratings (SRGs) on azo polymer films under different polarization combinations. The mechanism of SRG formation on azobenzene chromophore functionalized polymer films has been shown to depend on both the spatial variation of the intensity as well as the polarization state of the incident field pattern. The treatment presented in this paper clearly defines the intensity and polarization distribution across the two-beam interference region for different polarization configurations of the interfering beams. The efficient photoisomerization process due to the spatial variation of the optical field renders the material anisotropic and the surface mobile. The simultaneous presence of a component of electric field gradient in the grating vector direction results in large-modulation-depth SRGs. We have carried out this investigation in one class of prototypical amorphous epoxy-based azo polymer thin films. Our results confirm that efficient SRG formation on azo functionalized polymer film requires photoinduced anisotropy and a component of optical field gradient in the direction of the grating vector.