  Polymer thin films doped with azobenzene molecules do have the ability to
organize themselves in spontaneous surface relief gratings (SRG) under
irradiation with a single polarized beam. To shed some light in this still
unexplained phenomenon, we use a new method that permits us to access
experimentally the very first steps of the pattern formation process.
Decreasing the temperature, we slow down the formation and organization of
patterns, due to the large increase of the viscosity and relaxation time of the
azopolymer. As a result decreasing the temperature allows us to access and
study much shorter time scales,in the physical mechanisms underlying the
pattern formation, than previously reported. We find that the patterns organize
themselves in sub-structures which size increase with the temperature,
following the diffusion coefficient evolution of the material. That result
suggests that the pattern formation and organization is mainly governed by
diffusive processes, in agreement with some theories of the SRG formation.
Decreasing further the temperature we observe the emergence of small voids
located at the junction of the sub-structures.
