Investigations on the Photoanisotropy of
Polarization Sensitive Materials

Mario Ivanov


    The aim of the present thesis is developing of experimental methods for investigation of photoinduced anisotropy, and thus finding proper materials and conditions for realization of reversible and archive optical recording, and clarification of the mechanism of physical processes accompanying the optical recording. The following tasks are defined:
 

 The thesis consists of 133 pages, including 62 figures, 3 tables, 102 references, and 12 author's references.
 
 

Here is a review of the main chapters of the thesis:

 

Chapter I

The first chapter contains a review of photoanisotropic materials.
 

Chapter II

Polarimetric methods for measuring of photoinduced anisotropy in materials.
The measure of photoinduced anisotropy is closely related to the measurement of polarization of light. We have developed a photopolarimeter, based on the idea of Todorov and Nikolova [53]. Its output is connected to a 20 kHz, 12-bit analog-to-digital converter, from which the datΰ is passed to a personal computer. In this way, the change in time of the four Stokes parameters (S0,S1,S2,S3) of light  can be stored. 

The main advantages of our device are as follows:

  • It has no moving parts and its callibration needs only one measurement of the light with a known wavelength and linear polarization by the four detectors. As is known, the linear polarization can be defined more precisely than the elliptical one. Thus, the device matrix is more accurately determined, which results in more accurate measurements.
  • Time resolution of the device is estimated — 0.01 s.
  • The errors caused by the analog-to-digital conversion are estimated using a technique described by Azzam [66]. Their maxima are:  5΄10–6 for S1, 7΄10–3 for S2, 3.5΄10–6 for S3. 


On the basis of the photopolarimeter, few optical schemes are made. The basic scheme allows determination of linear birefringence and dichroism induced in the investigated materials, as well as of optical densities in directions parallel and orthogonal to the exciting light polarization. It works in the red region of the visible light. A similar scheme for measuring photoinduced circular birefringence and dichroism is also made. We have made also a scheme which additionally measures photoinduced dichroism in the blue region.
 

Chapter III.

Investigations of photoinduced anisotropy and optical recording of polarization diffraction Gratings in AgCl emulsion layers.
These materials are suitable for nonreversible holographic recording and the production of optical elements with interesting polarization properties. Such elements are polarization holographic diffraction gratings recorded with two circular polarizations. We have investigated layers synthesized in our laboratory, sensitized with chemical reducing agents — type A, with (hidrazinehidrocloride) and type B, with (NaNO3). In addition, a part of the samples were sensitized for the red spectral region. Applying the derived formula about the diffraction efficiency to the results from the real-time measurement of the birefringence and the dichroism, a curve reflecting the possible time behavior of the diffraction efficiency in such samples is obtained (in this way, recording with two orthogonal circular polarizations is simulated).
A method of producing stable holographic gratings in AgCl emulsions is proposed. 
 

 Chapter IV.

Photoinduced changes in rigid solutions of azodye/polymer.
These materials are suitable for reversible polarization optical recording and are especially interesting for the great photoanisotropy induced in them. It is shown that the birefringence is caused not only by the dye molecules reorientation (the kinetic curves of birefringence and dichroism are different). We believe that the polymer matrix undergoes changes.
Further, isotropic and anisotropic photoinduced changes in the refractive index of various dye/polymer systems are investigated. Polarization and nonpolarization diffraction gratings are recorded in those systems. The efficiency of the polarization gratings is higher. A conclusion is made that photoanisotropic changes are caused only by the relatively small dye molecules. The type of the matrix determines only the time constants of the photoprocesses.
A simplified theoretical model is considered,  in order to understand the influence of the type of polarization (linear or circular) on the quantity of the product in the photoinduced processes. It is shown that the average equilibrium optical density is higher when the exciting light is linearly polarized. Experiments with fluorescein in orthoboric acid and azodye/polymethylmethacrylate are conducted and they are in a good agreement with the theory.
 

 Chapter V.1.

Photoinduced anisotropy in liquid crystal polyesters with side chain azobenzene groups.
Linear and circular anisotropy in copolymer materials (azodye-liquid crystal) are investigated. These materials are suitable for archive recording, because photoinduced anisotropy is stable atroom temperature and has high values. On the other hand, erasure is possible by thermal treatment. We have observed for the first time photoinduced circular anisotropy (optical activity and circular dichroism) in such materials. Such high values of the optical activity (6°/mm) and circular dichroism (0.28) are, to our knowledge, not reported previously.
 

 Chapter V.2.

Polarization holographic gratings recorded in liquid crystal polyesters.
Theoretical analysis of the properties of holographic gratings recorded in materials with linear and circular anisotropy with two orthogonal circular polarizations is carried out. On this basis, a new holographic method for simultaneous real-time measurement of linear and circular birefringence is proposed.