Neutron Physics with Photorefractive Materials

When the subject of photorefractive effects began with the discovery of lightinduced refractive-index inhomogeneities in lithium niobate [1], neutron optics had already been established for more than 20 years [2, 3]. Both of the fields have evolved independently of each other into important branches of science and industry. In 1990 those dynamic areas were linked by an experiment in which cold neutrons were diffracted from a grating created by a spatially inhomogeneous illumination of doped polymethylmethacrylate (PMMA). A typical holographic two-wave mixing setup was used to record a refractiveindex pattern, a grating, in PMMA that was reconstructed not only with light, as usual, but also with neutrons [4]. Evidently, the illumination induced refractive-index changes for both light and neutrons! In analogy to light opties this phenomenon is called the photo-neutron-refractive effect. The chapter is organized as follows: Starting with a concise explanation of the relevant concepts in neutron optics, electrooptics, and photorefraction, as well as diffraction phenomena, we introduce PMMA and the electrooptic crystal LiNbO3 as examples of photo-neutron-refractive materials. The main part is concerned with neutron diffraction experiments performed on deuterated PMMA (d-PMMA). It is shown that this type of experiment can be useful in studying the polymerization process itself, when serving simply as a neutron-optical element, or when probing fundamental properties of the neutron. The latter is in particular true of electro neutron-optic LiNbO3, where the diffracted neutrons are inherently exposed to extremely high electric fields due to the light-induced charge transport. Corresponding experiments are presented and future perspectives of photo-neutron-refractive materials as well as their possible applications are discussed. Finally, atomic-resolution neutron holography is introduced and conducted experiments are presented.