Photoinduced spontaneous patterning of azopolymer films using light controlled mass transport

During the last decade, large effort has been dedicated towards the miniaturization of devices for photonic applications. One of the key point towards this objectif is structuration at the nanoscale level. Recently, one-step photoinscription of surface relief gratings onto azo-polymers was demonstrated: when a polymer film containing azobenzene dyes is irradiated by an interference pattern between polarized laser beams at a wavelength near the chromophore absorption band, the film surface undergoes a direct, reversible and controlled topographic modification. Grating modulation amplitudes. as large as the film thickness can be obtained. In this work we provide experimental evidence of a new spontaneous light-assisted submicrometer hexagonal patterning process. More particularly, we show that uniform irradiation of an azo-dye polymer using a single laser beam with normal incidence onto the polymer film surface leads to a self structuration process resulting in the formation of a quasi hexagonal grating. The influence of parameters such as the laser intensity, the irradiation time, the thickness of the polymer film but also the irradiation wavelength is studied towards a better understanding of the process. Full control of the mechanisms at the origin of such process could further permit a complete manipulation of the molecular order and thus enable the generation of other spontaneous complex structures opening the way to the development of new easy-to-set micro and nano-structuration techniques.