Photorefraction in centrosymmetric crystals

Photorefractive materials, i.e. substances that change their refractive index upon illumination with light, have become an important tool in tailoring proper devices for photonic applications. Among them are crystals used for holographic data storage, as signal amplifiers in optical waveguides or for wavelength multiplexing and de- multiplexing. The photorefractive properties in most of those substances are based on a nonlinear optical effect, the so-called linear electrooptic effect. For symmetry reasons, however, the latter is forbidden in centrosymmetric crystals. Nevertheless, photorefraction was discovered in a few centrosymmetric substances only recently. Its origin and mechanism is not understood up to now. The aim of the present project is to verify a particular model which may be able to explain photorefraction in centrosymmetric crystals. We will tackle the problem by formulating minimal requirements for the occurrance of a photorefractive effect in centrosymmetric crystals and to experimentally verify the basic underlying ideas. This will be done by searching for and finding proper materials, growing this corresponding crystals, characterizing their properties by holographic experiments and hence demonstrating the validity of the assumption. An explanation for the experimental results will include the clarification of the phenomena observed in already discovered centrosymmetric crystals. The successful realization of the suggested project will have enormous impact not only on the scientific community but also on photonics industry.

Conventional photorefraction, i.e., changes of the refractive index upon illumination with light, is based on the linear electrooptic effect. The latter is symmetry-forbidden in centrosymmetric materials. In the present project we targeted on finding novel photosensitive materials, and on characterizing as well as understanding their peculiar properties. Three major scientific advances were made: (1) We could discover new centrosymmetric photosensitive materials, (2) establish a model to explain the particular kinetics during formation of holographic gratings, and (3) prove their applicability for neutron optical devices. A variety of materials with quite different origins of their photosensitive properties was investigated, e.g., doped garnets, molecular crystals or polymer-dispersed liquid crystals. At the beginning we formulated minimal preconditions for the existence of a photorefractive effect in centrosymmetric crystals. These attributes imposed certain peculiarities on the formation of holographic gratings in these materials. Experimental holographic investigations in centrosymmetric Sodiumnitroprusside (SNP) revealed accurately this behaviour. As a consequence we set up a phenomenological model based on a two-states-system, that could accurately explain the strange kinetics of grating formation, not only in SNP but also in a variety of known (DX-centre semiconductors) and newly discovered (doped Gadolinium-Gallium-garnet) photorefractive centrosymmetric materials. During the last year we initiated studies in polymer-dispersed liquid crystals, materials which consist of a homogeneous mixture of monomers (optically isotropic) and liquid crystals (optically highly anisotropic and easily electrically switchable). Upon illumination of light polymerization takes place and ends up with a polymer matrix in which liquid crystals are dispersed, resulting in strong light scattering. We discovered that those materials exhibit strong holographic scattering, i.e., scattered amplified light, in any direction according to their centrosymmetric nature. Furthermore, holographically prepared gratings in those materials constitute diffraction gratings of extraordinary efficiency for cold neutrons. The results of our investigations are of importance in the field of photonics (signal gain in optical fibres, electrically switchable diffraction gratings), neutron optics (diffraction gratings, interferometers), and in medicine where the NO-molecule present in optically switchable SNP offers important biological functions (neurotransmission, enzyme, immune and blood pressure regulation, and even inhibition of tumour growth). An important part of the results was obtained in close collaboration with groups of scientists from the Universität Osnabrück, Germany and the J. Stefan-Institute in Ljubljana, Slovenia.