Formation of bragg gratings with large angular multiplicity by means of the photoinduced reorientation of azobenzene copolymers.
Optically transparent thick films of methacrylate copolymer containing donor-acceptor-type azobenzene and cyanobiphenyl moieties were prepared, and their optical properties were explored in terms of a photoinduced change in molecular alignment and the formation of Bragg gratings with angular and polarization multiplicity. Azobenzene moieties substituted with ethoxy, cyano, and nitro groups were employed, and their photoresponsive behavior was investigated. It was found by polarized UV-vis spectroscopy that the alignment of azobenzene moieties in the films was random in the initial state but became perpendicular to the direction of polarization of the writing beam after irradiation with a linearly polarized beam from an Ar+ laser at 514 nm. The film containing the cyanoazobenzene moieties showed the fastest formation of optical anisotropy due to an efficient change in molecular alignment. When two linearly polarized beams from the Ar+ laser were allowed to interfere in the films with a thickness of 200 microm, the diffraction beam was observed immediately and reached 97% upon photoirradiation at 200 mW/cm2 for 5 min. It was clearly observed that the response time became shorter with an increase in the light intensity of the writing beams, maintaining the high diffraction efficiency. Furthermore, it was clearly seen that the diffraction efficiency strongly depended on the direction of polarization of the reading beam. Holographic multiple data storage in a film was demonstrated, and 55 holograms with angular multiplicity were recorded successfully. In addition, multiple data storage using the polarization and angle selectivity simultaneously was first demonstrated.