Photorefractive measurements

Figure 3.39. Holographic setup for photorefractive molecular glasses. The sample is tilted toward the grating, allowing an applied external field to support the motion of the mobile charges. The phase shift of the refractive index grating can be determined by measuring the transmitted writing beam intensities (two-beam coupling).

Figure 9 further illustrates that the magnitude of photorefractivity for samples with intramolecular dopants 1 through 4 is dependent upon the lifetime of the charge separated state. 1 has no measurable photorefractivity and a charge separated lifetime of only 530 ns in the liquid crystal. 2 has a charge separated... 

The photorefractive rise (t r) and decay (tpd) times versus A, as measured by the four-wave mixing experiments, are shown in Fig. 14a and b for the polymerized and unpolymerized samples, respectively. The decay times are... 

A truly remarkable feature of this work is that the photorefractive recording was done in the absence of any applied electric fields and with the writing beams bisected by the normal to the cell. Nonetheless, the authors claim to have measured a phase-shift of n/2. They propose that the symmetry is broken by the internal structure of the PDLC and perhaps also by local poling of the polymer by the light gradients during exposure. 

Dipole moments of l,3-dimethyl-2,2-tetramthylene-5-nitrobenzimidazoline, perspective for photorefractive polymeric materials [1276], and some nitrobenzo-furazans [1206, 1277] have been measured. 

The major advantages of the HTOF technique are that it is not subject to trapping constraints nor the restrictions concerning the absorption depth of conventional photocurrent transient measurements. The principal limitation is that it is limited to photorefractive materials. Malliaras et al. (1995) used the HTOF method to measure mobilities of ternary mixtures of poly(N-vinylcarbazole), 2.4.7-trinitro-9-fluorenone, and 4-(hexyloxy)nitrobenzene. Results obtained by the HTOF method were in good agreement with those obtained by conventional photocurrent transient measurements. 

The field of photorefractivity in organic polymers and glasses has been in existence for less than a decade. The understanding of charge generation in these materials (which are often composites) is not yet mature, and the behavior of some of the more common constituents is understood better. Much of the literature on photo-refraetivity deseribes free earrier generation quantum efficiency measurements only briefly, before a more detailed discussion of other factors such as mobility and electro-optic response. Some of the relevant information pertinent to free carrier generation in these materials is presented here, to be followed by a review of this aspect of the amorphous photorefractives literature. 

Figure 16. Transient photocurrent signal from a time-of-flight measurement of a photorefractive polymer composite containing 47.5 % electro-optic dye (EHDNPB), 1 % TNF with PVK polymer making up the remainder 21 V was applied across the 100 nm polymer film and a 10 nm thick rhodamine 6G charge generation layer was used. The hole mobility in this material is thus...

Fig.25. Normalized diffraction efficiency vs applied field measured in 105 pm-thick samples of the polymer-dispersed liquid crystal TL202 PMMA ECZ TNFDM (circles) and the photorefractive polymer DHADC-MPN PVK ECZ TNF (squares)...

Fig. 27. Field dependence of the two-beam coupling gain measured in the photorefractive polymer composite PDCST PVK BBP C5o squares single sample (140 pm-thick) circles a two-layer stack triangles a three-layer stack... 

I)i94 i94a were investigated as photorefractive materials. Substantial photorefractive responses were measured in both doped and undoped samples, as pure materials and as materials dispersed within polymers. 

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