Dark decay of holograms in photorefractive polymers

Abstract

The decay of holograms stored in photorefractive polymer composites based on poly͑N-vinyl-carbazole͒ 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. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1492848͔The photorefractive ͑PR͒ effect is one of the most promising reversible holographic storage mechanisms. 1 Under nonuniform illumination, the refractive index of the photosensitive material is modulated due to the generation of mobile charge carriers in the bright regions, their subsequent redistribution, and eventual trapping in the dark areas. This gives rise to a space-charge field E SC , which modulates the refractive index of the material through the linear electrooptic effect and orientational effects. 2,3 Photorefractivity in amorphous polymers has been intensively investigated, 4,5 and these systems have been widely recognized as potential active media in rewritable holographic optical memories for security applications, 6 in associative memories, 7 or in adaptive ultrasound sensors. 8 Due to the rather low dielectric constants of polymers ( Ͻ10), oppositely charged carriers show a rather strong tendency to recombine. As a result, only rather short storage times are anticipated. However, so far, the dark decay ͑referred to as ''dd'' hereafter͒ of the holograms in periods when the system is idle, i.e., held in the dark with the external field still applied, has been mostly neglected in literature on organic PR materials, even though it is important for the aforementioned applications. In this letter, we present systematic investigations of the dd of PR gratings in PR polymers. Our results will give evidence that the phase shift between the interference pattern and the recorded index grating, the commonly accepted fingerprint of photorefractivity, is one of the key parameters, yielding slower dd for a larger phase shift.The investigated materials contained the photoconductor poly͑N͒vinylcarbazole ͑PVK, 39 wt %͒, the plasticizer N-ethylcarbazole ͑10 wt %͒, the eutectic mixture of two EO chromophores 2,5-dimethyl-4͑p-nitrophenylazo͒-anisole ͑25 wt %͒, and 3-methoxy-4͑p-nitrophenylazo͒-anisole ͑25 wt %͒, and finally the sensitizer 2,4,7-trinitro-fluorenone ͑TNF, 1 wt %͒. We also prepared a similar material doped with 0.82 wt % ͑replacing PVK͒ of the commonly used hole conductor N,NЈ-bis͑3-tolyl͒-N,NЈ-diphenyl-benzidine ͑TPD͒, whose highest occupied molecular orbital levels are situated about 0.5 eV below those of PVK. Thus, TPD moieties constitute deep traps within the carbazole transport manifold, and therefore a longer storag...

A Convenient Synthesis of Benzonitriles via Electrophilic Cyanation with N‐Cyanobenzimidazole

“…Dopants that may further affect the photoconductivity of PR composite were introduced into the material blends. The addition of dopants with lower oxidation potential (TPD) than the holeconductor (PVK) was investigated by Bittner et al158 The intention in this particular case was to increase the hologram dark‐decay times and to enhance the multiplexing capability of PR composites 159…”

1064‐nm Sensitive Organic Photorefractive Composites

“…The phase‐shift for 3 wt % and 5 wt % [84]PCBM was calculated according to Ref. [22 from Δ n p and Γ s to be 18° ± 1° and 17° ± 1°, respectively. By contrast, for the TT‐2324 materials the gain was found to be independent of the sensitizer content.…”

Organic Photorefractives: Mechanisms, Materials, and Applications

“…In addition to measurements of steady-state diffraction efficiency which is an important parameter for applications, the FWM experiment is widely used for temporal studies (e.g., formation and erasure) of the PR grating. Electric field and intensity dependence of the PR dynamics can yield much information about the material parameters (section 5.3.1). ,,,,, Furthermore, the FWM experiment is ideally suited to measure the dark decay of gratings, i.e., the decay after both writing beams are turned off. ,, This parameter is especially of interest for data-storage applications for which long dark-lifetimes are desirable. Moreover, at small dark currents, the dark decay time constant can provide information about detrapping rates (e.g., β 1,2 of eq 3) 58,105 (section 5.3.2).…”