Bittner, Reinhard
(2003).
Basic Investigations on PVK-based Photorefractive Polymers Focussing on their Applicability as Mass Data Storage Media.
PhD thesis, Universität zu Köln.
Abstract
Poly(N-vinylcarbazole)-based PR polymer composites were investigated focussing on the particular features required for a potential application of this relatively new class of materials as optical holographic storage media in mass data storage devices. The main objective of this work was to get a more detailed insight into and a better understanding of the dynamic recording, erasure and dark decay behavior of holograms in this type of PR polymer. By means of a simplified model calculation the general diffraction properties of a hologram in PR polymers in the presence of strong beam coupling were examined. It could be proven that hologram bending due to strong beam coupling does not notably affect the diffraction properties of a holographic grating in organic PR devices within the range of the externally applied field experimentally possible. The dependence of the steady-state and the dynamic PR performance on the glass-transition temperature as well as on the doping level of electro-optic chromophores was investigated. The reduced temperature, which is the glass-transition temperature relative to the ambient temperature, was identified as a factor of outstanding importance. A steady-state performance optimum as a function of the reduced temperature was observed in materials containing large amount of chromophore. The hologram build up speed was found to be limited by the orientational mobility of the electro-optic chromophores for positive reduced temperatures. For negative reduced temperatures the dynamics of the space-charge field was identified as the rate-limiting step. The chromophore doping level only plays a role at positive reduced temperatures. Considering the influence of the sensitizer concentration on the steady-state and the dynamic PR behavior, strong indication was found that the active PR trap manifold consists of conformational traps on the one hand and of coulombic traps formed by charged sensitizers on the other. The first exist ab initio while the latter are formed during the grating recording process and add to the conformational traps. There is strong indication that the hologram build-up dynamics for typical sensitizer concentration is limited by the charge carrier mobility. The general erasure behavior of PR gratings was investigated in detail. For the dependence of the PR grating erasure on the reduced temperature, a relationship similar to the recording dynamics was found. In all cases, a strong correlation between the erasure dynamics of a hologram and its corresponding recording time was observed. The grating erasure process was found to slow down as a function of increasing recording time. For positive reduced temperatures this can be attributed to a viscous flow of the polymer matrix, whereas optical activation of trapping sites could be identified to cause this effect at negative reduced temperatures. The possible nature of the optically activated traps is discussed. It could be shown that an optimum reduced temperature must be anticipated for a potential application of the investigated materials in holographic multiplexing. Doping the materials with large amounts of extrinsic deep traps yielded a complicated erasure behavior. Applying short recording times and low light intensities, a further increase of the hologram strength was observed during the initial erasure process. A phenomenological mechanistic picture of the recording and the erasure process of a hologram in a material showing trap controlled charge transport is proposed, which can qualitatively explain the experimental observations. The dark decay of holograms was investigated in detail taking into account different glass-transition temperatures as well as extrinsic trap doping. Within the range of reduced temperatures investigated, the dark decay was found to be governed by the decay of the PR space-charge field. The phase shift of the PR grating turned out to be a crucial parameter yielding faster dark decay for smaller PR phase shifts. Eventually, holographic multiplexing in the investigated type of materials is demonstrated by means of peristrophic multiplexing experiments. Therefore an expanded numerical formalism for a multiplexing exposure schedule was to be devised, which accounts for the complicated dynamic behavior. In conclusion, the investigated type of holographic storage medium appears inapplicable for holographic mass data storage devices. This is mainly due to the complicated and unfavorable dynamic behavior, which appears to be an inherent feature of the investigated type of material, or possibly even the entire class of materials. However, this class of holographic storage materials may find application in any kind of volatile holographic storage like, among others, associative memories, buffer holograms, or time gated holographic imaging.
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