Photoinduced Light Assisted Patterning of Azopolymer Films: Towards a New Process for Nanostructuration

Abstract

Optical induction of quasi permanent birefringence and dichroïsm in azo-dye polymers has been the subject of many studies. It is now well established that azobenzene molecules, when excited with a polarized light beam whose wavelength lies in their absorption band, undergo a reversible trans-cis-trans isomerization process leading to a molecular quasi-permanent reorientation in a direction perpendicular to the light polarization direction. More recently, it has been shown that irradiation of an azo-polymer thin film with an interference pattern between polarized laser beams could 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. ^[1–3] The position of the maximum heights correspond to the light intensity minima. It has been demonstrated that gratings dynamics and modulation amplitude depend on several parameters among which the polymer film thickness, the grating pitch, the power and time of irradiation, or the writing beams polarization: p-: p-polarizations leading to diffraction efficiencies ten times larger than s-: s- polarizations, modulation amplitudes being larger as the grating pitch is higher. Maximum amplitudes of modulation can be of the order of the initial polymer film thickness. Saturation was shown to depend only on the total energy accumulated although saturation is reached faster as the irradiation power increases.