Evidence of a density grating under light induced formation of surface relief gratings at polymers containing azobenzene moieties

The formation of a surface relief grating (SRG) on polymer films containing azobenzene moieties has been inspected during exposure with counter-rotating circularly polarized light of λwrite=488 nm by probing the diffracting efficiency of a second low power laser. In particular, the growing SRG was observed at films of different film thickness and with low and high light power of the writing laser. Using probe lasers of two different wavelength (λprobe=514 and 670 nm) the grating formation was inspected near the film surface and within the whole film, respectively. At low writing power (P=50 mW) and small film thickness (d 300 nm) a density grating is also created within the film. The observations confirm the trends suggested by finite element calculations.