Photorefraction trapping

Fig. 14.a Amplitude of the space-charge field as a function of the value of the applied field calculated from Eq. (92) for different values of the photorefractive trap density b Phase of the space-charge field as a function of the value of the applied field calculated trom Eq. (93) for different values of the photorefractive trap density For aU calculations, the value of the grating spacing was A = 3 pm and the static dielectric constant was = 6.4... 

Nonlinear ion traps, 15 662 Nonlinear materials, 14 680 Nonlinear optical materials, 17 442-460 advantage of, 17 448 classification of, 17 443—444 economic aspects of, 17 457-458 photorefractive materials, 17 457 second-order, 17 444r-453 third-order, 17 453—457 Nonlinear optical phenomena, 17 443 Nonlinear optics... 

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

In order to optimize the photorefractive performance of BaTi03, it is necessary to control the centers involved in the ionization and trapping of photocarriers. This may be accomplished in part by intentionally doping the crystal with various transition metal or rare earth ions, which may exist in more than one valence state. 

The photorefractive gain, shown in Figure 2b, is predominantly determined by two factors the trap density, NE, and the relative conductivity factor, . The latter factor accounts for the minimum... 

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. 

Figure 4. The photorefractive effect with and without trapping. Top the intensity pattern on the material. Middle O, anion density +, cation density x, ideal distribution of trapping of mobile holes. Bottom comparison of the net charge distribution in the ideal case (no. of cations - no. of anions + no. of trapped holes, x) with the corresponding space charge field in the absence of any trapping or recombination (no. of trapped holes = 0),------).

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