Reinhard Bittner

Influence of the Glass-Transition Temperature and the Chromophore Content on the Grating Buildup Dynamics of poly(N-vinylcarbazole)-based Photorefractive Polymers.

The influence of the glass-transition temperature T(g) and the electro-optical chromophore content on the grating buildup dynamics in photorefractive polymer composites is investigated. The response times were found to be strongly dependent on both parameters. In the low-T(g) regime, composites of different chromophore content respond similarly quickly (200-500 ms), whereas significant differences occur for T(g) above the measurement (room) temperature. The composites with the highest chromophore content give the best steady-state performance; however, their response is much slower than that for those containing less chromophore.

Improved performance of photorefractive polymers based on merocyanine dyes in a polar matrix

Recently, chromophores with a large figure-of-merit for incorporation into photorefractive polymers with low glass-transition temperature have become available. However, their rather polar nature so far limited their use in typical nonpolar photoconducting matrices due to dye aggregation. By incorporation of an additional polar compound we were able to influence this situation favorably. The material we report here has a factor 4 to 5 (2) improved index modulation amplitude (gain coefficient) compared to the best previously known materials. The material is sufficiently resistant against phase separation and operates in the near infrared at typical wavelengths of commercially available high-power laser diodes.

Holographic multiplexing in photorefractive polymers

A new multiplexing schedule is derived for multiplexing holograms in photorefractive polymers which do not exhibit mono-exponential recording behavior. An M-number (M/#) of 0.3 was measured experimentally by recording 20 holograms of roughly equal strength in a single location of 125-μm-thick material using peristrophic multiplexing. The effects of hologram dark-decay on the time-evolution of the M/# and the relative strengths of individual holograms is investigated.

Field asymmetry of the dynamic gain coefficient in organic photorefractive devices

The performance of amorphous organic photorefractive materials in holographic two-beam coupling experiments in the typical tilted geometry was found to be asymmetric with respect to the applied electric field direction. For one field direction, light is coupled into the polymer layer and can be detected on the side of the devices. For the other, the originally Gaussian-shaped writing beams show a shoulder or even split into two. The strength of the asymmetry depends on the diameter of the beams writing the hologram. We demonstrate that this effect is due to beam fanning. As a result of the fanning, the apparent photorefractive gain coefficients take on unphysical values.

Competing photorefractive gratings in organic thin-film devices

Recently, amorphous organic photorefractive materials have generated great excitement because of their excellent performance, which permits applications in high-density holographic storage, real-time image processing, and phase conjugation. However, the heterostructure of the devices (consisting of glass cover slides, transparent electrodes, and the photorefractive material) and the tilted recording and readout geometry commonly used result in multiple reflected beams in addition to the normal object and reference beams. This result leads to several photorefractive gratings competing inside the photorefractive polymer device. We prove the coexistence of these gratings by two-beam coupling and four-wave mixing experiments and demonstrate how to distinguish between them.

Photoconductivity and charge-carrier photogeneration in photorefractive polymers

We have studied photogeneration, transport, trapping and recombination as the governing mechanisms for the saturation field strength and the time response of the photorefractive (PR) effect in PVK-based PR materials, utilizing xerographic discharge and photoconductivity experiments. Both the charge carrier photogeneration efficiency and the photocurrent efficiency were found to be independent of chromophore content, suggesting that the chromophore does not participate in carrier generation and trapping. The photoconductivity gain factor G defined as the number of charge carriers measured in photoconductivity in relation to the number of carriers initially photogenerated as determined by the xerographic experiments is found to be much smaller than unity, which indicates that the mean free path of the photogenerated charge carriers is less than the grating period. Photoconductivity data can be explained over 3 orders of magnitude in field, assuming a field-independent trap density. Based on the photoelectric data, PR response times have been predicted by Yehs model for the build-up of space or by calculating the time, which is necessary to fill all traps by photogenerated holes. Only the latter model can reasonably well explain the observed field dependence of the PR growth time, suggesting that trap-filling essentially controls the PR onset behavior.

Nonsteady-state photo-EMF effect in photorefractive polymers

The nonsteady-state photo-electromotive force (photo-EMF) effect has been observed in photorefractive (PR) polymer films based on poly(N-vinylcarbazole) (PVK). It was investigated in the typical tilted transmission configuration without and with application of moderate external direct current (dc) electric fields. The dependencies of the photo-EMF signals on the frequency and the amplitude of the phase modulation, the tilt angle, and the externally applied dc field are qualitatively explained using a simple model of the effect, developed earlier for unipolar photoconductive inorganic crystals without saturation of the trapping centers. The absolute value of the photo-EMF signal was two to three times larger than expected from this model and the experimental data on conventional photoconductivity of the polymer film. This was rationalized considering the presence of additional space-charge gratings appearing due to light reflectance from the rear surface of the thin-film device.

Optimization of the recording scheme for fast holographic response in photorefractive polymers

Abstract The influence of the recording scheme on the dynamic performance of a photorefractive polymer composite is investigated. In the low-T g composite studied here, the response time is at first limited by the formation of the photorefractive space-charge field and on a longer time scale by the orientation of the electrooptic chromophores. By in situ pre-poling the devices before writing the hologram a substantial speed enhancement was achieved compared with non-prepoled devices.

Dark decay of holograms in photorefractive polymers

The decay of holograms stored in photorefractive polymer composites based on poly(N-vinyl-carbazole) with and without extrinsic deep traps is investigated. The photorefractive phase shift is identified as one of the key parameters determining the dark decay dynamics. This has important implications for all kinds of photorefractive imaging applications including holographic data storage. A trade off will be required between accepting a certain degree of hologram distortion due to two-beam coupling on the one hand and achieving high hologram stability during idle periods in the dark with the external field applied on the other.

Improved Long-Term Stability and Performance of Photorefractive Polymer Devices Containing Eutectic Mixtures of Electrooptic Chromophores

Recently, amorphous organic photorefractive (PR) materials have attracted a lot of attention. However, since most of the high-performance PR organic materials are polymers blended with large amounts of low-molecular-weight components, their shelf-life time is limited due to phase separation. One way to improve the shelf-life time, which was proposed by us recently, is to use eutectic mixtures of two electrooptically active chromophores. However, in our earlier attempt in improved life time was achieved at the cost of a reduced performance due to increased absorption. In this paper, we apply this technique again with great success, not only improving the life time by a factor of 15, but at the same time improving the steady-state holographic performance by 30%.