Andrey V. Veniaminov

Postexposure Evolution of a Photoinduced Grating in a Polymer Material with Phenanthrenequinone

The specific features of the postexposure evolution of photoinduced (holographic) gratings in a model medium that consists of poly(methyl methacrylate) with distributed phenanthrenequinone and is characterized by a diffusion enhancement are investigated using the holographic relaxation technique. It is established that the evolution process occurs in four stages controlled by different mechanisms: (i) initial nonmonotonic changes governed by the diffusion of an intermediate radical photoreaction product and its transformation into a stable product due to attachment to the macromolecules, (ii) hologram enhancement through the diffusion of phenanthrenequinone molecules, (iii) rapid partial degradation associated with the spatially confined motion of polymer chain segments, and (iv) slow degradation as a result of macromolecular diffusion. In the course of polymer relaxation after a temperature jump, the processes associated with the mobility of molecules are retarded and the depth of rapid degradation decreases. The effective diffusion coefficient characterizing the destruction of reflection gratings is smaller than that for transmission gratings. This can be explained by the spatial inhomogeneity of the polymer.

Transformation of color centers during hologram recording in an additively colored CaF2 crystal

When holograms are recorded on color centers in calcium fluoride crystals, these centers undergo spatial redistribution in the crystal bulk, which is accompanied by their transformation. The nature of this transformation has been investigated by optical spectroscopy and confocal scanning microscopy. It is shown that, under the recording conditions we used, the degree of center aggregation increases in both minima and maxima of the interference field in which the recording performed. The enhanced aggregation in field minima is caused by the increase in the concentration color centers, while the additional aggregation in maxima is determined by the specific conditions of hologram recording: the wavelength and power density of recording radiation and the crystal temperature.

Photophysical mechanisms of effective phase holograms recording in polymers with chemically attached photoactive centers

A sensitized photooxidation has been studied in polymer systems based on polymethacrylates and polyimides with anthracene nuclei chemically attached to or include into main chains. Using thermostable polyimide compositions the possibility of reversible photooxidation due to thermal decomposition of bisphenylanthracene was shown.

Rigid polymer materials with hologram enhancement by molecular diffusion

The principle of diffusional enhancement has been embodied in the rigid glassy polymer with phenanthrenequinone able to photochemically attach to surrounding macromolecules, thus forming a permanent grating. Owing to material stiffness, it does not suffer from shrinkage and can be made very thick; serving a basis for very stable spectrally selective elements. Replacement of commonly used acrylic glass by polycarbonate ensures further significant improvement of performance and stability of 3D holographic optical elements and memories.

Photochemical and diffusional apodization of high-efficiency thick phase holograms

The strength of volume holographic grating being uniform throughout material depth, the efficiency of such a grating as a function of angle or wavelength has several side-maxima with magnitude comparable to that of main maximum. The side-maxima can be the origin of noises and lack of selectivity of holographic optical elements and devices such as demultiplexers, filters, etc.

Holographic recording in polymeric materials with diffusional amplification

The volume holographic gratings in polymer materials are promising candidates for spectrally selective optical elements, dense data storage, optical communication, and spectral selective imaging. Recently new application of volume reflection-type grating for fast all-optical temporal/spatial light modulators was suggested. Spatial modulation of diffracted red beam by green laser was demonstrated. We also suggest and analyze theoretical model for material with diffusional amplification for different modes of recording: (1) Fast recording with following annealing; (2) Slow recording (real-time grating).

Properties of PDA as a thick holographic recording medium

Our experience of work with the photopolymer with diffusive amplification (PDA) as an extremely thick holographic recording medium evidences many attractive properties of this material, especially for such particular applications, as very selective holographic optical elements. However, sometimes the experimentally obtained characteristics of recorded holograms differ from the expected ones. This may be caused by the insufficient knowledge on the material behavior at different regimes and its specific peculiarities. In the present paper we analyze some parameters of PDA, in particular the noise source, and kinetics of the refractive index modulation and diffraction efficiency, as compared with the dynamic photopolymer medium Reoxan. As well as discus the possibilities of application of the PDA material.

Dark stability of holograms and holographic investigation of slow diffusion in polymers

A holographic method was used for investigation of very slow dif fusion of anthracene compounds in glassy polymers. The results show that properties of large organic molecules diffusion differ greatly from those known for small molecules. The diffusion of substituted anthracenes determines mainly the stability of holograms registered in volume phase medium Reoxan. I .

Spectral analysis of volume holograms in materials with diffusion-based formation mechanisms by means of Coupled wave theory and Kramers-Kronig relations

The article is devoted to the spectral analysis of volume holograms recorded in materials with diffusion-based formation mechanisms. Two media of different nature were examined: polymer material with photosensitive macromolecules (PQ-PMMA) and additively colored solid-state crystal (CaF2). Differential spectra of holograms optical parameters were determined by means of two approaches: numerical approximation of both spectral and angular hologram response (selectivity hypercontour) by Coupled wave theory and processing hologram spectra by Kramers-Kronig dispersion relations. The operation principles, experimental performance, determination results, advantages and disadvantages, as well as validity limits were discussed for both the approaches. Kramers- Kronig relations are operable tool in cases where the nature of holograms formation is well studied, whereas hypercontour approach is indispensable for investigation of hologram formation mechanisms.

Holographic grating relaxation technique for soft matter science

The holographic grating relaxation technique also known as forced Rayleigh scattering consists basically in writing a holographic grating in the specimen of interest and monitoring its diffraction efficiency as a function of time, from which valuable information on mass or heat transfer and photoinduced transformations can be extracted. In a more detailed view, the shape of the relaxation curve and the relaxation rate as a function of the grating period were found to be affected by the architecture of diffusing species (molecular probes) that constitute the grating, as well as that of the environment they diffuse in, thus making it possible to access and study spatial heterogeneity of materials and different modes of e.g., polymer motion. Minimum displacements and spatial domains approachable by the technique are in nanometer range, well below spatial periods of holographic gratings. In the present paper, several cases of holographic relaxation in heterogeneous media and complex motions are exemplified. N...