Over the past two decades, surface relief gratings have attracted much interest owing to their potential applications in optical data storage and optical communication and holography. Azobenzene-containing polymer films show an interesting behavior under irradiation with light interference patterns. The inhomogeneous irradiation; e.g. due to the interference pattern, of the azo-polymer film causes mass movement of the polymer from bright to dark area, and the mechanism underlying the formation of SRG finds its origin in the photoisomerization force. The latter is due to an inhomogeneous light irradiation which causes photoisomerization and increases the polymer mobility in the bright area, and owing to the intensity gradient, due to light interference, the photoisomerization force moves the polymer from the bright area into the dark. In this paper we discuss our experiments of holographic recording in films of Poly (Disperse Red 1 methacrylate); e.g. azo-polymer films, and the recorded surface relief gratings were investigated by using atomic force microscopy. The dependence of the polarization state and intensity of the writing beams was studied, and the diffraction efficiency was monitored in real time during the process of inscription. A brief description of the photoisomerization force is given.
Photoisomerization induced molecular motion in azo polymers is an area of research that witnessed intensive studies owing to its potential in optical manipulation. In this paper, we give an overview of the theory of matter motion induced by photoisomerization. We show that besides photoisomerization, a gradient of light intensity is necessary to generate motion; e.g. generate a photoisomerization force to move matter. In concept, matter motion is due to competing forces, including viscous and photoisomerization forces, and possible radiation pressure and elastic forces, as well as a random force due to thermal fluctuations. In solid films of azo-polymers, the photoisomerization force overcomes other forces due to softening and decrease of viscosity of the material by photoisomerization.