Temperature Dependent Analysis of Grating Formation on Azobenzene Polymer Films

The temperature dependence of surface relief grating formation was studied using continuous and pulse like exposure. Surface relief gratings were inscribed on amorphous azobenzene polymer thin films using a holographic pattern of circularly polarized light at wavelength equal to 514 nm in a vacuum chamber to avoid the hot air turbulence. The efficiency of the surface relief grating formation was probed using a He-Ne laser of wavelength 633 nm by monitoring the first order diffraction peak (I1) as well as the specularly reflected intensity (Is). Under continuous exposure permanent grating formation was observed up to a temperature of about 100°C only. The same was found under pulse like exposure but grating still exists as long as the actinic light is on. Above 100°C it relaxes entirely after switching the light off. This elastic component disappears at about 115°C, i.e., close to the glass transition temperature. Our findings can be interpreted by the competition between light-induced ordering of azobenzene side chains and temperature induced disorder. Because the accumulated stress within the polymer decreases with temperature, permanent grating formation can only be observed when the light-induced stress is above the yield stress.