Optical materials PDF

Introduction

This monograph is one of a series commissioned by the Institute of Materials. The series is aimed at providing a foundation for the better understanding of the science underlying today's technological society and as an introduction to each of the subjects in the series. It is hoped that not only will the reader find each of these monographs complete in themselves but that they will act as a spur for further study.

It has been written to act as the introduction to the subject of optical materials. It must be stressed that it is only an introduction and refers to a lot of work and further reading if the reader wants to go further in his or her understanding. Nevertheless it is hoped that it is complete enough to enable the interested reader to gain a better understanding of the topic.

When electromagnetic radiation impinges upon a material it interacts by reflection, transmission, absorption or scatter. The optical properties of materials are physical phenomena, in as far as the interaction of the electromagnetic radiation with the material is reversible, although they are firmly bound up with the chemical, atomic and crystallographic properties of the materials involved. The atomic, molecular and crystallographic arrangements also affect the other physical, thermomechanical and chemical properties of the materials and so it is not surprising that there are families of materials which exhibit particular properties, usually with a graduation of magnitude. For instance, most oxide materials are transparent in the visible and near infra-red and absorb above -2 J.Lmwhilst fluorides, sulphides and nitrides have wider transparency windows and most materials from the II -VI and III -V regions of the periodic table exhibit semi-conducting properties. Cubic crystals exhibit linear optical characteristics whilst hexagonal and non -symmetric structures (and particularly chain structures) exhibit non-linear effects. The optical properties range from linear optical behaviour through a range of non -linear optical effects and on to electro-optic, opto-electronic and acousto-optic effects.

The dependence between the optical properties and the atomic, molecular and crystallographic structure is outlined in Chapter 2 and the resulting optical and physical properties are outlined in Chapter 3. This monograph will limit itself to those properties which are reversible and will stop once the energy absorbed is great enough to melt the material or crystal under consideration, or the power density is enough to cause avalanche ionisation and electromagnetic breakdown in the material.

Although much of the theory is relevant to a wider selection of materials this monograph has specifically been limited to materials which are transmissive to radiation in the ultra-violet (u-v), visible and infra-red (i-r) regions of the electromagnetic spectrum, see Figure 1.1. An additional limitation is that it does not refer to 'glasses' in any detail as these have been the subject of a similar monograph (Rawson 1993). Therefore, although many optical materials are glassy and other materials which do not transmit in these regions still have optical properties (e.g. metals which absorb and reflect but do not transmit) no further specific mention will be made to these materials.

The treatment will include both single crystalline and polycrystalline, linear and non-linear, organic and inorganic materials. They include 'ordinary' (linear) optical materials (Chapter 4) but emphasis will be put on the great number of 'optically active' materials which are becoming increasingly available and which are leading the optical technology revolution which is in turn changing much of the commercial and military technology which is being introduced into the marketplace today. They include optically nonlinear materials (Chapter 5), laser host crystals (Chapter 6), detector materials (Chapter 7), integrated optics and fibres (Chapter 8) and liquid crystals (Chapter 9). The monograph finally ends with a discussion of the power handling limits of optical materials (Chapter 10).

Edited by: Roger. M. Wood