In contrast to the conventional holographic process, in which intensity variations in an interference pattern between an object beam and a reference beam are recorded, polarization holography employs beams with two different polarizations for recording information. In this case, the polarization state of the resultant beam is recorded on a suitable medium. This process was discovered by Sh. D. Kakicheshvili, from Georgia (which was then part of the USSR). Currently there is only one monograph on polarization holography, namely that written in Russian by Kakicheshvili (Polyarizatsionnaya golografiya, Nauka, 1989). However, because of its complex presentation, this monograph is not easily amenable for application to practical work.

Polarization holographic storage has several unique properties: (1) it is possible to achieve theoretically 100% diffraction efficiency even in thin films; (2) the diffracted beams have unique polarization properties, depending on the polarization of the recording and read-out beams; (3) it is possible to fabricate polarization-sensitive optical elements; and (4) the optical elements fabricated with polarization holography are achromatic, allowing their use at all wavelengths.

Our book intends to fill a gap in the area of holography, and documents research done during the last two decades. High-capacity holographic storage remains a hot topic. This book is intended for scientists as well as students at graduate and postgraduate level. A basic undergraduate optics background is assumed, and a well-prepared undergraduate physics major will be able to appreciate the subtleties of polarization holography.

Polarization holograms have been shown to possess some extraordinary properties: they are capable of achieving a 100% diffraction efficiency; and they are able to reconstruct the polarization properties of the object beam in addition to providing intensity and wavelength information. Since the reconstructed images have different polarization from that of the undiffracted light, their signal-to-noise ratio is higher than in conventional holography. Polarization holograms also exhibit achromaticity. It is possible to fabricate a half-wave plate, or a polarization beamsplitter for linearly and circularly polarized light, independently of the wavelength of operation.

Polarization holograms in materials such as alkali halides, arsenic trisulfide and bacteriorhodopsin have been used to demonstrate several interesting properties. The most efficient material available today for polarization holography is based on azobenzene. Azobenzene-containing polymers have fast response, and the polarization holograms based on azobenzene polymers are stable at room temperature. However, the fast response depends on the intensity of the interfering beams. There are other drawbacks with this material. Azobenzene absorbs blue and green light. Thus any optical element based on azobenzene can be used only in the red and infrared. While polarization holograms fabricated in azobenzene polymers have been stable over many years under ambient conditions, they are not stable under high-temperature treatment. Amorphous polymers with high glass-transition temperatures have been shown to retain light-induced anisotropy until approximately 200°C. Liquid-crystalline polymers that depend on the reorientation of entire domains are more susceptible to degradation at temperatures around 100°C.

Photoinduced anisotropy in silver-halide materials

The appearance of dichroism in silver-halide emulsions exposed to linearly polarized light was first observed by F. Weigert [1–3] in 1919. This effect is called the Weigert effect (W-effect). Weigert discovered that the effect was most pronounced if the emulsion layer was first exposed to unpolarized short-wavelength light to form print-out silver and afterwards to long-wavelength linearly polarized light. Photodichroism was observed in layers only before photographical development. Later it was found that the effect could be reversible [4]. When a silver chloride (AgCl) emulsion layer was exposed to polarized red light and then the polarization direction was rotated by 90°, the degree of dichroism first diminished, then disappeared completely, and, with continued exposure, reappeared with opposite sign with respect to the first time. This reversible effect could be achieved several times by rotating the polarization direction. Photoinduced dichroism was later observed also in single crystals of AgCl. Hilsch and Pohl [5] and Cameron and Taylor [6] found that, when AgCl crystals are exposed first to unpolarized UV light and then to linearly polarized red light, they become dichroic. The effect was also observed by Zocher and Coper [7].

All these early investigators of the W-effect tried also to give a theoretical explanation. They all assumed that the first exposure created anisotropic particles distributed in all directions.

In this chapter, we shall examine first the principles of interference and holography and extend them to the concept of polarization holography. We shall study the specific character of the periodic anisotropic structures obtained by the holographic method, which we call polarization holograms. We shall show how their efficiency and their polarization properties depend on the choice of recording geometry and on the photoinduced anisotropy in the materials. Since the formation of linear anisotropy is more common and more pronounced in photoanisotropic materials known at the moment, we first consider polarization holography in materials with linear anisotropy only. Then we extend the consideration to materials with both linear and circular anisotropy. The appearance of relief gratings on the surface of polarization holograms is also taken into account.

Plane-wave interference and holography

The holographic method was first proposed by Denis Gabor in 1948 [1] as a method for the reconstruction of wavefronts. Gabor proposed a two-stage process. The first stage is a two-dimensional photographic storage of the intensity distribution in the interference pattern of the signal wave (S) with a reference wave (R). At the second stage the reference wave illuminates the obtained photograph and reconstructs both the amplitude and the phase of the signal wavefront. The method was called “holography”, that is, “whole writing”. Later it was shown by Denisyuk [2] that, if three-dimensional (volume) materials are used to fix the interference field, the wavelength of the signal wave could also be restored.