Tsveta Petrova

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.

Bifocal-polarization holographic lens

We present a holographic lens with novel features recorded in an azopolymer film. Two holographic modulations, bulk birefringence and surface relief, are induced in the medium at the same time. The resultant holographic element has two focal planes, and the polarization of light in the focal points depends on the polarization of the incident light. Applications of this device for writing-reading information in two planes simultaneously or separately are described.

Compact electronic speckle pattern interferometer using a near infrared diode laser and a reflection holographic optical element

The complicated experimental set-ups required for existing electronic speckle pattern interferometers limit their application potential. To overcome the limitations we present a simple and very compact electronic speckle pattern interferometer. Our system does not use expensive and complicated optical elements. We used a near infrared reflection holographic optical element for the first time in an out-of-plane sensitive electronic speckle pattern interferometer operating with a near infrared diode laser. The use of a holographic optical element drastically reduces the number of optical elements; the final system only consists of a CCD camera, a HOE and a test object, with significant reductions in system costs and time spent aligning the system. Reflection holographic optical elements were recorded in HP-650 emulsions using a He?Ne laser. The hologram was reconstructed using a laser diode operating at 784?nm by swelling the emulsion. Current modulation of the laser diode was utilized to introduce the desired phase shifts for fringe analysis.

Lifetime of silver halide holographic materials comparative accelerating testing

Ultra fine grain panchromatic silver halide light sensitive material HP-P for RGB recording of reflection holograms have been recently developed. The average grain size is less than 10 nm, and recording energy for maximum diffraction efficiency for the spectral range 400-675 nm is in the frame of 0.5-2.5 mJ/cm2. To estimate the life time of the new material an accelerating testing at different temperatures (40°C, 60°C and 100°C) has been applied and compared with the results for the well known red sensitized silver halide emulsions HP-650, having average grain size about 10 nm. On the base of the obtained results according Arenious low at least three months life time of the new material at ambient conditions could be guaranteed, and more than six months at -5°C storage condition.

Polarization holographic gratings with surface relief in azobenzene-containing polymers

Polarization holographic gratings in azopolymers are due to three photoinduced effects: linear anisotropy, circular anisotropy, and surface relief. In our polymers the surface relief grating is with doubled frequency with respect to the volume anisotropic grating. A theoretical analysis is presented and compared with experimental data.

Light-controlled polarization switches in azobenzene-containing polymers

Three new side-chain cyan-azopolymers have been synthesized for the purposes of optical storage and processing of information. A very large birefringence (> 0.05) is induced in them on illumination with linearly polarized Ar laser beam (λ = 488 nm). It has been shown that the vaue of the photoinduced birefringence depends on the length of the side chains of the polymers. The effect has long memory time -- more than six months. We have used the self-induced rotation of the polarization ellipse in photobirefringent materials to induce chiral structures in the polymers. Total twist angles of the photoinduced spirals more than 90° have been achieved. The new polymers are suitable for the fabrication of light-controlled polarization switches.

Chemical Sensitization of Fine-Grain Silver Halide Emulsions for Holographic Recording

Silver halide materials for holographic recording have relatively low light sensitivity. This is due mainly to the small size of their emulsion microcrystals. The process of chemical sensitization of fine-grain emulsions is hampered by the following factors: 1. Microcrystals sized under 50 nm feature very high dimensional instability1. That is why when such emulsions are heated, processes of physical ripening ( dissolution of the small crystals and growth of the big ones) take place after which the sensitivity specks remain under the grain surface. The crystal growth is favoured in polydisperse systems, and such are almost all ultrafine-grain silver halide emulsions2. 2. The size of the microcrystals in the emulsions for holographic recording, that we investigate, is 10 nm. Electron micrographs show that grains of this size are spherical and do not have a clearly pronounced crystal form. There is a lot of equivalent, but not sufficiently deep defects on the surface. That is why, there are substantial difficulties for carrying out chemical sensitization whose objective is the formation of one or more efficient sensitivity specks on the surface of the microcrystals.

Thermally sensitized optical recording in azobenzene polymers

Optical storage in azobenzene-containing polymers is based on the alignment of the polymer chains initiated by the reorientation and ordering of the azobenzene groups. This process requires considerably high recording energies. We investigate photoinduced birefringence in two types of acrylic cyan-azo polymers - one amorphous and one liquid crystalline. We show that for both types the light energy required for the recording can be reduced by an order of magnitude if the photoalignment is done at elevated temperatures. The optimal temperatures depend on the polymer. In our case they are 52-55°C for the liquid-crystalline polymer and 50-60°C for the amorphous polymer. At these temperatures birefringence as high as 0.07 can be obtained at light intensities 30-40mW/cm2 for about 100s and these values are retained after cooling the polymer films.

Polarization holographic element using an azobenzene polymer

A holographic lens is recorded superimposing two beams with orthogonal linear polarizations on an azobenzene polymer film. The polarization pattern on the interference plane induces two modulations in the media: the volume and the surface modulation. The spatial frequency of the surface relief is twice the one for the volumetric modulation resulting in a holographic lens with two different focal lengths. Additionally, because of the modulated anisotropy induced in the medium, the polarization at the longer focus distance is orthogonal to the polarization at the shorter one. We propose this polarization element to send or detect information in two planes simultaneously or separately by using an analyzer behind the holographic lens.

Polarization Holographic Diffraction Grating

The aim of this work was to produce a new type of diffraction grating, having two diffraction orders only ( ±..1), whose intensities are proportional to the intensities of the left-hand and right-hand circularly polarized components of the light wave, incident on the grating. According to the existing theoretical considerations [1], such a grating can be recorded holographically by irradiating a recording material, possessing linear photoanisotropy, with an interference pattern, resulting from the interaction of two coherent wavefronts with orthogonal circular polarizations.