Chapter 17
Photofabrication of Surface Relief Gratings 1
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D. Y. Kim , X. L. Jiang , L. Li , J. Kumar , and S. K. Tripathy Downloaded by STANFORD UNIV GREEN LIBR on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch017
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Department of Chemistry and Department of Physics, Center for Advanced Materials, University of Massachusetts, Lowell, MA 01854
Surface relief gratings were optically produced on a series of azobenzene-based polymer films. The surface grating formation was investigated by monitoring the diffraction efficiency and using atomic force microscopy. The effects of structure of the chromophores and polymer backbones on the surface grating formation were investigated. The surface deformation process depended on the polarization state of the writing beams. The localized variations of the light intensity and alteration of the resulting electric field polarization were essential writing conditions to the formation of the surface relief gratings.
It has been known over a decade that azobenzene groups in polymer matrices give rise to optical birefringence when excited by polarized light (7). This process involves repeated trans-cis photo-isomerization and thermal cis-trans relaxation of azobenzene groups, which result in orientation of the azobenzene groups perpendicular to the polarization direction of the incident beam. This photoinduced orientation of azobenzene groups has been studied in various polymer matrices and also employed to produce birefringence gratings by a number of research groups (26). However, surface relief grating formation in these polymers has only been reported very recently (7-10). We have recently reported direct formation of large amplitude (>1000 Â) holographic surface relief gratings on epoxy-based nonlinear optical (NLO) polymer films containing azobenzene groups (7, 8). The formation of surface relief gratings on an acrylate polymer with azobenzene groups was also recently reported by Rochon et al (9, 10). These surface relief gratings were produced upon exposure to an interference pattern of A r laser beams at modest intensities without any subsequent processing steps. This type of surface relief grating formation has not been observed before in the orientation birefringence studies of azo dye incorporated polymers. The gratings were very stable when kept below the glass transition temperature (TV) of the polymer. The gratings could be erased by heating the polymer above Tg. Since the amplitude of the surface variation is large and the relief gratings can be conveniently recorded on the polymer films, such polymers have significant potential applications for various optical devices and optical elements. The driving force and the mechanism of this process is not well understood at +
Corresponding author © 1997 American Chemical Society
In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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Downloaded by STANFORD UNIV GREEN LIBR on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch017
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PHOTONIC AND OPTOELECTRONIC POLYMERS
the present time. Large scale molecular motion and volume change due to reorientation seem to be occurring simultaneously. To understand this process, the role of the structural elements, chromophore side groups and polymer backbone, should be investigated. Especially, the structure of the chromophore side groups appears to be critical to the process. We have also observed that p-polarized recording beams induced much larger surface relief structures than s-polarized recording beams (8). It demonstrates that this large mass transfer process is not simply a thermal process. It also implies that the direction of orientation of the azobenzene groups has a very important role to this process. In this paper we report the effects of the structures of various chromophores and backbones on the writing process. A detailed investigation of the polarization dependent recording process was also carried out. Experimental Section Polymers were synthesized using the procedure described previously (11, 12). The chemical structures of the polymers are presented in Figure 1. The polymer, PBD03, was synthesized from 1,4-butanediglycidyl ether and Disperse Orange 3 by the same procedure. TgS of the polymers were measured from DSC (Dupont Thermal Analysis). Molecular weights (Mw) of the polymers were in the range of 5000-10000. Optical quality polymer films were prepared by spin coating of filtered polymer solutions on microscope glass slides. The typical thickness of the polymer films was approximately 0.8 μιη. The spin-coated polymer films were optically isotropic. The experimental setup for the grating recording is shown in Figure 2. A linearly polarized laser beam at 488 nm from an A r laser was used. The polarized laser beam passed through a halfwave plate, and was then expanded and collimated. Half of the collimated beam passed through another halfwave plate and was incident on the sample directly. The other portion of the beam was reflected from an aluminum coated mirror. Two sets of experiments were carried out. In the first set of experiments, the second halfwave plate was removed. Laser beams with different polarizations (defined by an angle a, with respect to s-polarization) was achieved by rotating the first halfwave plate. By replacing this halfwave plate with a quarter wave plate or a depolarizer, circularly polarized and unpolarized recording beams were obtained respectively. Due to the complex refractive index of aluminum, the polarization of the beam reflected from the mirror became elliptically polarized except for oc=0° (s-polarization) and
