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Spontaneous Photo-Induced Structuration of the Surface of Azo-Benzene polymer Films by the Molecular Migration Effect
Cristophe Hubert, Celine Fiorini-Debuisschert, Paul Raimond, Jean-Michel Nunzi, Jean-Jacques Simon and Ludovic Escoubas

Spontaneous structuration of the surface of azo-benzene polymer films is observed under illumination with resonantly absorbed coherent collimated beams. The surface structure which is formed is regular: it is an array of submicron size hexagons when illumination beam has linear polarization. The saturated modulation depth is in the 100 nm range, independent of the power of the illumination beam. The phenomenon is rather universal as it is observed for several azo-polymer materials, illuminated at different wavelengths and under different conditions, as long as illumination is normal to the surface. When the phenomenon is set, incident light is strongly diffracted into the plane of the film. Calculation shows that the surface structure corresponds to optimum coupling of the incident light into the plane of the film: the surface self-organises into a holographic notch-filter structure. The reason is that azo-benzene molecules move under resonant excitation following their reversible trans-cis isomery. They move endlessly until they fall into a darker region. In the absence of any dark region, they create a diffraction structure which optimizes dissipation of the light. When the diffraction structure is set, the process crystallises.

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