Читать книгу Photorefractive Materials for Dynamic Optical Recording - Jaime Frejlich - Страница 9

This book is a corrected and largely extended version of my former one (Photorefractives, John Wiley & Sons, 2007). The objective of this book is mainly focused on photorefractive materials, their properties and their technological possibilities. These materials are still the most interesting ones for dynamic optical recording, not only because their good photoconductivity and the good photovoltaic effects of some of them allow thinking about photoelectric conversion applications as well.

The first part of this book is devoted to the analysis of the fundamental properties of this materials: electro‐opticity and photoconductivity as well as other effects that some of them may exhibit and which should be taken into account while operating with them – photovoltaicity, light‐induced absorption, luminescence and the Dember effect.

Part II is focused on the dynamic recording of a spatial distribution of electric charge and the associated spatial electric field distribution leading to a corresponding index‐of‐refraction (and sometimes also light absorption coefficient) modulation in the material volume as a consequence of their electro‐optic properties. Most of the recording is carried out using a spatially modulated interference (holographic) pattern of light, an index‐of‐refraction and sometimes associated absorption coefficient volume grating results. The real‐time diffraction of the recording beams by the grating being built up results in complex wave coupling effects that should be taken into account to mathematically describe the dynamics of this recording process. Electrical coupling among charge carriers (electrons and/or holes) during recording allows the possibility that more than one photoactive type of defect (the Localized State in the material Band Gap) should be also taken into account. The recording of an interference pattern of light or hologram is usually subject to serious environmental perturbations that may undermine the recording quality, mainly for the rather long recording time processes that are usually the case with photorefractives. To cope with this problem, we describe here some dynamically stabilized setups that actively compensate the environmental phase perturbations on the interference pattern of light during recording. Some of these setups use, when possible, their own grating being recorded as a reference for the stabilization process, which is therefore labeled “self‐stabilized recording”. Running holograms and self‐stabilized running holograms are also discussed here.

Part III is devoted to the characterization of photorefractives using holographic, nonholographic optical methods and electrical techniques, reporting a large number of actual experimental results on a variety of materials.

Some practical applications including holographic real‐time measurement of out‐of‐plane mechanical vibration modes in 2D and in‐plane amplitude mechanical vibration using backscattered (“speckle” pattern) laser light are discussed in Part IV. Also, the possibility of using thin photorefractive crystal plate devices for photoelectric conversion is discussed in detail. As recording on photorefractive crystals is essentially reversible (recorded holograms may also be erased by the same light used for recording), we discuss here some fixing techniques that may even allow the production of permanent micro‐ and sub‐microscopic structures using different holographic techniques.

Part V is an appendix where the physical meaning of some quantities closely related to photorefractives, such as Debye length, diffusion and mobility, as well as detailed practical techniques, such as how to measure diffraction efficiency of reversible holograms (which is a far from obvious matter), and even how to operate photodiodes and operational amplifiers for different light detection practical tasks, are discussed.

Campinas‐SP, April 2019