Two-Photon Induced Self-Structuring of Polymeric Films Based on Y-Shape Azobenzene Chromophore

We report on optically induced spontaneous formation of periodic two-dimensional patterns on newly synthesized polymers based on Y-shape azobenzene molecules. Thin films of this polymer are exposed to a single femtosecond laser beam in the near-infrared (wavelength = 800 nm), normally incident on the free surface of the polymer film. The spontaneous formation of periodic structures is observed after few seconds, both perpendicularly and parallel to the incident laser polarization direction, with a periodicity of about 470 and 250 nm, respectively, and amplitude up to 100 nm. The features of height and periodicity are analyzed as a function of the incident pulse energy, evidencing the presence of a threshold of 50 mu J/pulse. The spontaneously formed structures can be partially canceled by illuminating with incoherent UV light. This approach, allowing patterning areas of few mm(2) within a few minutes, constitutes a simple and straightforward method for the fabrication of polymer periodic nanostructures.