Light-induced patterning of azopolymer films: observation of Wood's anomalies

It is now well established that, in azo-dye polymers repeated trans-cis-trans isomerization cycles induces molecular reorientation leading to quasi-permanent birefringence and dichroism. More recently, it has been shown that irradiation of an azo-polymer thin film with an interference pattern between polarized laser beams can lead to a direct and reversible topographic modification of the polymer film surface resulting in the induction of surface relief gratings in conjunction with the light interference pattern. Here, we provide experimental evidence of an original spontaneous light-assisted submicrometer patterning process. We show that uniform irradiation of an azo-dye polymer using a single collimated laser beam with normal incidence leads to the formation of organized hexagonal patterns. These structures geometry depends on the laser beam polarization : their period is about the irradiation wavelength and their modulation amplitude can reach one hundred nanometers. Possible origin of such process is discussed. Although showing similar features with models or experiments already described in the literature, the origin of light-assisted spontaneous patterning in azo-polymers is still unclear and further complementary investigations are needed. Indeed, accurate determination of the relevant parameters at the origin of such process should enable a full control of the process together with further possible generation of more complex structures.