Chromic Phenomena Involving the Manipulation of Light
This chapter is concerned mainly with colour phenomena which result from, or are used to influence, various physical effects, rather than specifically absorption and/or emission. Permitting or blocking the transmission of planes-polarised light is controlled by liquid crystals, and the effect is widely used in LC displays of flat screen TVs and computer monitors. Various reflection, scattering, refraction and diffraction phenomena, leading to light interference effects, are typified by many natural creatures, and now by special iridescent and opalescent colour variable pigments. These are used to colour cars and an increasingly wide range of consumer goods, cosmetics, and as a component of security systems for banknotes, passports and similar. In three dimensions, interference effects in colloidal crystals lead to synthetic opals and their inverses, with future applications in the all-light photonics technology. Light interference is also used as the basis of holography. Holograms have display applications, as well as providing the basis for one of the new proposed 3D optical data storage systems, and in securities. Organic materials, as well as more conventional semi conductors, are being used to devise an increasing variety of laser diodes. The final topic considered is nonlinear optics, where various dye-like molecules and materials are used to interface electronics with photonics, or even to start to underpin all-photon technology. NLO materials are also used as multi-photon absorbers, used in other types of high volume ODS systems, optical limiters for protection against lasers, second harmonic generation (frequency doubling), and SHG imaging in biomedicine. They are also a component in photorefractive polymers, again used in high volume ODS.