**1. Introduction**

44 Holograms – Recording Materials and Applications

Norland Products Incorporated, Norland Optical Adhesive 65 (1996, 1999). 695 Joyce Kilmer

Volume holography, or holography in three-dimensional media, dates back to Yu. N. Denisyuk's works (Denisyuk, 1962), who implemented the idea of hologram recording in a three-dimensional medium by means of recording a hologram in counterpropagating beams, using traditional silver-halide light-sensitive materials. Adaptation of traditional photomaterials for purposes of image (pictorial) holography consisted in reconstruction of Lippman photographic layers with the size of light-sensitive grains less than 25 nm and use of photochemical processing techniques that allow obtaining amplitude-phase high-efficiency holograms in the visible region (Denisyuk & Protas, 1963).

Three-dimensional holograms with Klein parameter (Q) that describes the degree of threedimensionality of a hologram grating on order of 10 (Kogelnik, 1969), obtained on traditional photomaterials of thickness on order of 10 μm, are referred to as 3D-thin holograms. Hologram gratings with Q > 1000 are commonly considered 3D-volume holograms. To meet the condition, the thickness of the recording medium should be by 2-3 orders of magnitude greater than in the case of 3D-thin holograms and amount to a value on order of millimeters.

Recording 3D-volume holograms made use at different times of different recording media: crystals, photochromic glasses, etc., their main features in high demand in 3D-volume holography being large thickness and negligible shrinkage. Yet it became clear that the available media fail to meet the set of requirements placed on media for hologram recording, and foremost, for recording static holograms, intended to be used as hologram optical elements and holograms for long-term information storage. The need to develop the theoretical and experimental research in the field of volume holography called for creation of recording media of great thickness and with corresponding properties. Creating such media required new approaches to their development and corresponding measurement techniques for parameters.

The present section introduces media for 3D-volume holography, which were created on the basis of principles developed in 80s-90s of the ХХ century (Sukhanov, 1994a). The media demonstrated the potentiality of the implementation of the proposed theoretical principles of creation of volume recording media in practice and also proved conductive to refining parameter measurement techniques for volume holograms, to studying and understanding

Light-Sensitive Media-Composites for

**2.3 The dispersion refraction principle** 

**2.4 The diffusion enhancement principle** 

holograms.

range.

subject to diffusion.

Recording Volume Holograms Based on Porous Glass and Polymer 47

time. The manner was implemented by using organic dye phenanthrenequinone (PQ) as a light-sensitizing agent and forming the rigid framework by means of radical polymerization of composite on the base of methylmethacrylate monomer (MMA). The properties of the framework itself in such a RM are not limited only to maintenance of sample rigidity; the frame plays a certain part in recording, enhancement and fixation of

The approach to creation of volume light-sensitive media, proposed in work (Lashkov & Sukhanov, 1978) and termed the dispersion refraction principle, consists in the following. It is necessary that the impact of light caused a sharp change of the absorption band of the material in the spectral region very far away from the operating wavelength range of recording radiation. The change of the absorption band is accompanied by change of dispersion, which leads to photo-induced refractive index appearing in the operating spectral range. Sensitivity of created RM in given wavelength interval can be ensured by using sensitized phototransformations. The change of refractive index due to the above causes was proposed to term as sensitized dispersion refraction. Purposeful use of given principle allows selecting substances with corresponding spectral parameters and calculating the possible modulation amplitude of constructed hologram. The principle became the base for realization of the first volume polymeric medium "Reoksan", whose light sensitivity is due to sensitized photo-oxidation of compounds of anthracene structure (Sukhanov, 1986, 1994a). It was used to create a medium with diffusion enhancement and is applicable to description and analysis of photophysical processes in most media intended for recording and reading the information in the optical spectral

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

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.,

the relationship between physical-chemical processes of manufacturing technology for lightsensitive samples of volume recording medium and parameters of obtained holograms.
