Violeta Dragostinova

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.

Polarization reflection holographic gratings in azobenzene-containing gelatine films.

We have written polarization reflection holograms in an azobenzene-containing material. Two waves with circular polarization were used to record the gratings; the light resulting from their overlap induces chirality in the samples. The holographic reflection of the polarization gratings has the properties of Bragg reflection in cholesteric liquid crystals.

Temperature-dependent light intensity controlled optical switching in azobenzene polymers

We have investigated the dependence of the photoinduced birefringence in a low-molecular azopolymer on the exciting light intensity at elevated temperatures. We have found that in the temperature range 44–56 °C the stationary photobirefringence is higher for a low pump intensity(1 mW) than for a higher intensity (20 mW). At some fixed temperatures, the difference between the two values of the birefringence is as high as 0.07. By alternating the light intensity, the anisotropic phase shift in the polymer films can be switched repeatedly between two values differing by more than 130 deg. This can be used for light-intensity controlled optical switching.

Investigations on photoinduced processes in a series of azobenzene-containing side-chain polymers

We present a spectrophotometric investigation of the processes in three azobenzene polymers induced by a linearly polarized Ar beam (488 nm). The polymers differ only in the length of the spacers attaching the chromophores to the main chain. The experimental results show that this difference determines not only the polymer structure; it also substantially influences the value of the photoinduced dichroism and its time behaviour after the illumination. We explain this on the basis of the different efficiencies of the two main photoinduced processes in the azobenzenes—the selective trans–cis isomerization and the reorientation of the trans-azobenzenes. The results from a real-time measurement of the photoinduced anisotropic changes in the refractive indices of the three polymers support this understanding.

Light-induced optical activity in optically ordered amorphous side-chain azobenzene containing polymer

Abstract We report the appearance of circular birefringence (optical activity) in amorphous side-chain azobenzene polymer films on illumination with circularly polarized light. The effect is observed only if an optical axis is previously created in the film with the help of linearly polarized light. The photoinduced optical activity is believed to be due to changes in the polymer structure initiated by a circular momentum transfer from the circularly polarized light to the azobenzene chromophores.