The usefulness of infrared (IR) radiation has been recognized for many years and is today the basisof an expanding technology. The development of the laser, particularly the IR laser, has further fueled this technology expansion. Optical materials have always played a critical role in IR technology, primarily as transmissive and reflective optical components. Often, the lack Of an adequate IR optical material has delayed the implementation of applications Until appropriate materials or quality of materials were developed. While the developed material may satisfy the basic requirements of the application, often its other physical properties and/or cost are not desirable. The limited number of IR optical materials combined with the growing humber of applications has kept their development essential to our technological growth.

This paper reviews some current research and developmen t trend s in IR optical materials primarily for transmissive components. These IR components include geometric optics, windows and domes, optical fibers, high energy laser optics, and coatings. The emphasis is on optical materials for applications involving wavelengths in the 2-14 μm region.

Intrinsic Attenuation

Intrinsic attenuation defines the fundamental limits to light propagation in a transmissive material. It is composed of electronic or bandgap absorption, lattice vibration or multiphonon absorption, and Rayleigh and Brillouin scattering. Bandgap absorption results from the promotion of an electron from the valence to the conduction band by direct absorption of a photon or indirectly involving a phonon and the appropriate change in k-vector. The onset of absorption occurs at photon energies greater than or equal to the bandgap.