Molecular migration mechanism for laser induced surface relief grating formation

Direct and reversible holographic recording of surface-relief gratings in azo-dye polymers was recently evidenced using atomic force microscopy. Irradiation with an interference pattern of polarized laser beams was observed to lead to substantial mass-transport. In typical experiments, the wavelength of the laser was chosen to be near the absorption of the chromophores. The surface gratings have a negative amplitude which can be as large as twice the polymer film thickness. The origin of such photo-driven mass transport is still unclear. We provide here experimental evidence of a migration of the chromophores from high to low intensity regions. It indicates that, in such doped or grafted guest-host systems, polymer chain migration is a result of the guest chromophores photoinduced movements. This is in good agreement with our recently developed model of azo-dye photoinduced translation diffusion.