**2.4 The diffusion enhancement principle**

The principle of diffusion enhancement supplements previous ones and opens up new opportunities to combine properties of the framework and light-sensitizing agent, thus setting direction of the search for new substances and photochemical and photophysical processes in creation of volume light-sensitive materials. According to the principle, particles of light-sensitizing agent of medium-composite should have no binding with the rigid framework: when a latent image hologram is recorded, two antiphase gratings are formed in such medium, one of them being made up of photochemically transformed molecules (photoproduct), the other one (supplementary to the first) being made up of lightsensitive particles remaining unchanged (unexposed). The formation of two antiphase gratings is a necessary, but not a sufficient condition: the main special feature of the principle is that after the medium exposure the photoproduct particles should be rigidly bound to the framework (here, unexposed particles remain unbound). The hologram recorded, the particles, unbound to the framework (free), diffuse with the passage of time, spreading uniformly across the sample bulk and causing the supplementary grating to degrade and the photoproduct grating to "develop" (be enhanced), since particles are not subject to diffusion.

This principle was successfully implemented in practice by using polymeric medium on the base of polymethylmethacrylate with PQ (Cherkasov et al., 1991; Veniaminov et al., 1991, 1996; Steckman et al., 1998; Lin et al., 2000, 2006; Luo at al., 2008; Liu at al., 2010; Yu at al.,

Light-Sensitive Media-Composites for

its constant and thickness.

(Fig. 1c).

and estimation.

geometric dimensions of the sample and so on.

Recording Volume Holograms Based on Porous Glass and Polymer 49

In the presence of the amplitude component, the DE of an amplitude-phase transmission hologram is defined taking into account that the modulation amplitude of the first harmonic of hologram absorption index (α1) and the average hologram absorption index (α0). Fig. 1a presents the dependence (curve 3) for the case α1 = α0 = 0.05. The oscillatory nature of dependence η(φ1) is seen to persist. The data on Fig. 1a characterize variation of DE of holograms at their reconstruction under Bragg conditions. To describe the deviations from Bragg conditions, mismatch parameter (ξ) is used. Dependence of hologram DE, or diffracted beam intensity, on mismatch parameter ξ is the selectivity contour of a hologram. One recognizes the spectral selectivity contour of a hologram – dependence Id(λ) at θ = θBr and the angular selectivity contour of a hologram – Id (θ) at λ = λBr. The halfwidths of spectral and angular selectivity contours, Δλ and Δθ, are a measure of hologram selectivity. Comparison of selectivity of hologram-gratings with different DE by measured values of Δλ and Δθ is appropriately done in the variation range of phase modulation of a medium 0.1π < φ1 < 0.5π, where the halfwidth values for a selectivity contour are practically independent of the DE of a hologram and are defined by

As have been noted, the dependence of DE of transmission volume holograms on phase modulation value (φ1) is of oscillatory nature. Here, for phase holograms without absorption, φ1 = kπ ± sin-1 √η, where *k* = 0, 1, 2, 3; therefore, different sections of DE variation range (as indicated on Fig. 1a) are to correspond to different formulas for calculation of φ1 by measured values of η. Fig. 1b shows selectivity contours of transmission phase holograms with DE = 50% (at reconstruction under Bragg conditions), which were obtained at different values of phase modulation amplitude. It is clearly seen that φ1 for high-efficiency transmission holograms can be unambiguously found by DE values only with account of selectivity contour shape as opposed to thinlayer holograms, where, as a rule, φ1 < 0.5π. The study of amplitude-phase holograms should involve not only the hologram selectivity contour (the dependence of diffracted beam intensity on parameter ξ), but also the dependence of intensity of zero beam that passed the hologram without changing the direction, on parameter ξ

Dependences Id(ξ) and I0(ξ), given on Fig. 1 (b,c) for hologram-gratings with different values of phase modulation, display unsymmetrical nature of dependence I0(ξ) of amplitude-phase holograms (Fig. 1c) as opposed to phase holograms (Fig. 1b). Thus, symmetry of dependence I0(ξ) with respect to Bragg conditions is evidence of absence of the amplitude component in the hologram under study, which is necessary for correctness of calculations

When the coupled wave theory is used to analyze experimental results, account is to be taken of the measurements of hologram parameters taking place, as a rule, in the air, and the formula-defined relationship of the studied parameters being established inside the medium. Comparison of experimentally measured hologram parameters to those calculated theoretically for given experimental conditions allows finding the amplitude of modulation of optical parameters of the medium in a hologram and their variation in processes under study as well as estimating some other characteristics, e. g., the uniformity of the lightsensitive agent distribution in the sample bulk, the effective hologram thickness against the

2010). Attempts to implement the principle using other compositions was less successful. However, experts consider the principle today among the most promising in creation of volume recording materials for holographic memory (Ashley et al., 2000; Shelby, 2002; Liu at al., 2010).
