Microstructuring of polymers using a light-controlled molecular migration processes C. Hubert a,* , E. Malcor a , I. Maurin a , J.-M. Nunzi b , P. Raimond a , C. Fiorini a a CEA Saclay, LIST, DECS-SE2M, Groupe Composants Organiques, Bat. 451, 91191 Gif-Sur-Yvette Cedex, France b Laboratoire POMA, UMR-CNRS 6136, Universite  d'Angers, ERT Cellules Solaires Plastiques Photovoltaõ Èques, 2 Boulevard Lavoisier, F49045 Angers Cedex, France Abstract The manipulation and precise control of molecular order is a challenging prospect in the design and realization of devices for photonic applications. A new concept for microstructuration, based on photoinduced molecular migration in polymer ®lms containing an azobenzene dye, was recently demonstrated. Irradiation with an interference pattern of polarized laser beams leads to substantial reversible mass-transport. We now show evidence that under speci®c experimental conditions, the interaction between a uniform beam from an argon-ion laser and the polymer ®lm induces hexagonal structures on the surface of the polymer. The self-patterning process was observed to depend on several parameters such as the polarization of the laser beam, the angle of incidence and the wavelength, as well as the nature of the dye. Understanding the fundamental mechanisms of this photoinduced self-patterning process should open a new route towards optical molecular control of materials microstructures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Azo-dye polymers; Photoinduced reorientation; Surface relief gratings; Spontaneous patterning; Feedback effect 1. Introduction Complete manipulation of molecular order, micro- and nanostructuration are challenging issues towards the development of new applications for plastic optoe- lectronic devices such as photovoltaic cells, organic light emitting displays as well as distributed feedback lasers [1]. For these applications, polymer matrices containing azobenzene dyes appear as interesting candidates for material engineering based on light± matter interactions. Indeed, it is now well established that azobenzene molecules can undergo photoinduced reorientation through trans±cis±trans photo-isomeri- zationcycleswhenirradiatedbypolarizedlight.Inthis paper, the different photoinduced structuring pro- cesses in azo-dye polymers are examined and the in¯uence of the experimental and geometrical para- meters among which the polarization direction of the beams and the spacing of the interference light pattern are analyzed. We then report spontaneous patterning effects also observed in azo-dye polymer ®lms follow- ing their interaction with uniform laser irradiation under speci®c experimental conditions. 2. Photoinduced structuring processes in azo-dye polymers Thein¯uenceofpolarizedlightonazo-dyepolymers has been the subject of many studies. It is now well Applied Surface Science 186 2002) 29±33 * Corresponding author. Tel.: 33-1-69-08-85-43; fax: 33-1-69-08-76-79. E-mail address: hubert@ortolan.cea.fr C. Hubert). 0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0169-433201)00658-4