Infrared Optical Materials: Where Do We Stand?
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titffMfct& • 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 development trends 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 fjm region. Fundamentals of Optical Material Properties 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. Other intrinsic electronic absorption processes include free-carrier absorption and exciton formation. Bandgap absorption scales as an exponential function of energy and decreases with increasing wavelength. This is often called the Urbach tail. Therefore a large bandgap material will result in a greater range of optical transparency. Multiphonon absorption is the result of coupling of the incident radiation with the fundamental molecular vibrational modes of the material. Considering a simple diatomic molecule, the fundamental or reststrahl frequency can be approximated by the following:
Paul Klocek* Texas Instruments V = , / 2 ,('/u) v 2 where f = force constant u = reduced mass of the two atoms. This harmonic solution indicates the wellknown trend for IR transparency in weakly bonded (large interatomic spacing and/or large coordination number) or heavy mass materials. The more real case of anharmonic motion gives rise to overtones and combination bands at higher frequencies than the fundamental. The combination of the transverse and longitudinal optic and acoustic phonons generally gives rise to an exponential dependence of multiphonon absorption on frequency such that the absorption decreases with increasing frequency. The third contributor to atte
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