N. Tomova

Polarization holography. 1: A new high-efficiency organic material with reversible photoinduced birefringence.

A new organic material for polarization holographic recording--azo-dye methyl-orange, introduced in polyvinyl alcohol (PVA)--is fabricated and investigated. It possesses all the good characteristics of the known polarization-sensitive materials--high sensitivity, reversibility, etc.--but excels them substantially in the magnitude of the photoinduced birefringence: delta n > 10(-3). This makes possible the recording of reversible polarization gratings with diffraction efficiency over 30%. Depending on the conditions of production and subsequent mechanical and thermal treatments the layers of methyl orange/PVA may also have intrinsic birefringence. In this paper the results of experimental investigations into the properties of this new material are reported.

Polarization holography. 2: Polarization holographic gratings in photoanisotropic materials with and without intrinsic birefringence

Results are reported of experimental investigations on the diffraction efficiency and its polarization dependence of holographic phase polarization gratings in a photoanisotropic medium methyl orange/PVA with intrinsic birefringence. A comparative study of layers with different intrinsic birefringence was conducted. The conditions—initial birefringence and a type of polarization recording—were found to have high diffraction efficiency (>30%) and strong dependence on the direction of polarization of the linearly polarized readout beam. The results are in agreement with the theoretical considerations on such gratings.

High-sensitivity material with reversible photo-induced anisotropy

Abstract A new high-sensitivity material with reversible photo-induced anisotropy, suitable for real-time recording, is investigated. The material (methyl-red dye introduced in a polymer layer) requires low recording intensities (tens of mW cm-2) and possesses excellent reusability.

Polarization holography. 3: Some applications of polarization holographic recording

Possible applications of polarization holography are demonstratedrecording of pairs of superimposed holograms with one spatial frequency; logical operations on two data arrays recorded simultaneously in the material; double-exposure holographic interferometry.

Self-induced light polarization rotation in azobenzene-containing polymers

We report here a light-induced phenomenon—a self-induced rotation of the azimuth of elliptically polarized light passing through photobirefringent azopolymers. The experiments are carried out with films of amorphous and liquid-crystalline polymers. It has been shown that the induced rotation angle depends on the ellipticity of the input light. A theoretical analysis of the phenomenon has been done and it has been shown that light induces chiral structure in the polymer films.

Photochromism and dynamic holographic recording in a rigid solution of fluorescein

An investigation was made of the spectra, the kinetics of induction and thermal decay and the dichroism of the photoinduced triplet absorption band in a rigid solution of fluorescein in orthoboric acid. Dynamic holographic recording was possible on this material. An experimental study was conducted into the polarizing properties of the volume transmission holograms. The experiments on four-wave mixing with continous laser light revealed a very high resolution for this rigid solution when used as a medium for optical recording. As a result of the investigations, some possible applications of the rigid solution of fluorescein are pointed out.

Optically-controlled Photo-induced Birefringence in Photo-anisotropic Materials

Abstract A theoretical and experimental investigation is made of the possibility of controlling the photo-induced birefringence in photo-anisotropic materials by means of irradiation with a suitably chosen additional light. The applicability of the method is estimated by a consideration of the parameters of the optically and thermally activated processes taking place in the materials. The experimental results obtained in layers of rigid solutions of azo-dyes confirm the practical feasibility of such optical control.

Polarization-preserving wavefront reversal by four-wave mixing in photoanisotropic materials

A method of polarization (vector) wavefront reversal by four-wave mixing in a photoanisotropic material is described. Jones matrices, describing the dynamic holographic gratings recorded in the material at arbitrary polarization of the signal wave, are derived. They are employed to define the conditions for generating a wave that is fully conjugate (by polarization as well) to the signal wave. The way to achieving these conditions by optically controlling the anisotropy of the nonlinear medium is demonstrated experimentally.

Photoinduced changes in the refractive index of azo‐dye/polymer systems

The photoinduced isotropic and anisotropic changes in the refractive index of dye/polymer guest‐host systems are investigated. The values obtained are compared to the diffraction efficiencies of scalar and polarization holographic gratings, recorded in the same polymer systems. It is concluded that the induced anisotropy is related to the dye molecules involved in the photoprocesses, whereas the isotropic changes, which are much stronger, are related to the polymer matrix as well.

Photopolymers ― holographic investigations, mechanism of recording and applications

The holographic characteristics of a broad class of photopolymerization systems of the acryl type — metal (Ba, Pb, Ca, Sr) acrylates and acrylamide — are investigated. The conditions — composition, recording light intensity and spatial frequency — for self-fixing real-time recording are defined. Holographic techniques are employed to determine some basic characteristics of the photopolymerization reactions — order of the photochemical reaction and diffusion coefficients of the monomers used. Experiments are carried out proving the suitability of the investigated photopolymerization systems for optical correlation processing.