Abstract
The possibility of volumetric phase recording based on photo-oxidation of the side anthracene groups of a new polymer with a glass transition temperature of ~338 K by oxygen that enters the layer across the boundary with the atmospheric medium, was established experimentally. Volumetric holographic gratings with a maximum diffraction efficiency of 2.5–9.5% were recorded with the radiation from a He–Ne laser (λ = 633 nm) in layers 5–20-μm thick. It was shown that in layers thicker than 10 μm the diffraction efficiency is limited by saturation of growth in the modulation depth of the optical path difference. This effect is due to increase in the radius of the photorefraction sphere around the photosensitizer molecule with the increased photoconversion of the anthracene groups, which lowers the resolution of the holographic grating and its effective thickness.
Highlights
IntroductionPhoto-oxidation of anthracene in a polymeric medium forms the basis of the operation of the volume holographic medium reoxane, used for recording volume phase holograms and developed at the S. I. Vavilov State Optical Institute at the end of the 1970s [1,2,3,4]. The product of this photoreaction (transannular anthracene peroxide) has lower molecular refraction than the initial anthracene, and this reduces the refractive index of the material as a result of phototransformation. The photoreaction develops on account of transfer of oxygen molecules with which the photo layer was previously saturated into an electronically saturated singlet state by transfer of energy from a sensitizer. Various dyes that have absorption in the whole visible region of the spectrum can be used as sensitizer, and this makes it possible to vary the region of spectral sensitivity of the material in this range [3]. The need to saturate the reoxane layers with oxygen at high pressure creates technical difficulties in the application of the material and limits the time for which its photosensitivity is maintained. Another disadvantage of the photorecording material is the mobility of both the anthracene molecules and the molecules of its peroxide in the polymeric matrix, which reduces the stability of the recorded holograms with time [5]. It is possible in practice to stop translational diffusion of the anthracene molecules and their photooxidation products if the former are chemically bonded to the macromolecules during synthesis of the polymer. Here it is better to include the anthracene molecules in the side chain in order to avoid the formation of rigid-chain polymers with excessively high glass transition temperature and a tendency to crystallize. A material with a low glass transition point and, as a consequence, significant permeability for atmospheric oxygen [6] can secure photo-oxidation of the anthracene structures in layers of limited thickness without previous saturation with oxygen at high pressure
At the same time some decrease in the absorption intensity of the sensitizer is observed (Fig. 1b, curves 1 and 2), indicating that its consumption is insignificant compared with the anthracene groups
Possible volume phase recording by photo-oxidation of the anthracene side groups of polymer molecules by oxygen that enters the layer through the boundary with air was confirmed experimentally
Summary
Photo-oxidation of anthracene in a polymeric medium forms the basis of the operation of the volume holographic medium reoxane, used for recording volume phase holograms and developed at the S. I. Vavilov State Optical Institute at the end of the 1970s [1,2,3,4]. The product of this photoreaction (transannular anthracene peroxide) has lower molecular refraction than the initial anthracene, and this reduces the refractive index of the material as a result of phototransformation. The photoreaction develops on account of transfer of oxygen molecules with which the photo layer was previously saturated into an electronically saturated singlet state by transfer of energy from a sensitizer. Various dyes that have absorption in the whole visible region of the spectrum can be used as sensitizer, and this makes it possible to vary the region of spectral sensitivity of the material in this range [3]. The need to saturate the reoxane layers with oxygen at high pressure creates technical difficulties in the application of the material and limits the time for which its photosensitivity is maintained. Another disadvantage of the photorecording material is the mobility of both the anthracene molecules and the molecules of its peroxide in the polymeric matrix, which reduces the stability of the recorded holograms with time [5]. It is possible in practice to stop translational diffusion of the anthracene molecules and their photooxidation products if the former are chemically bonded to the macromolecules during synthesis of the polymer. Here it is better to include the anthracene molecules in the side chain in order to avoid the formation of rigid-chain polymers with excessively high glass transition temperature and a tendency to crystallize. A material with a low glass transition point and, as a consequence, significant permeability for atmospheric oxygen [6] can secure photo-oxidation of the anthracene structures in layers of limited thickness without previous saturation with oxygen at high pressure.
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