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.

Damage to optical materials under intense photon irradiation has always been a major problem in the design and operation of high-energy and high-average-power lasers. In short-wavelength lasers, operating at visible and ultraviolet wavelengths, the problem appears to be especially acute; presently attainable damage thresholds seriously compromise the engineering design of laser windows and mirrors, pulsed power trains and oscillator-amplifier systems architecture. Given the present interest in ultraviolet excimer lasers and in short-pulse, high-power free-electron lasers operating at visible and shorter wavelengths, the “optical damage problem” poses a scientific and technological challenge of significantdimensions. The solution of this problem even has significant implications outside the realm of lasers, for example, in large space-borne systems (such as the Hubble Telescope) exposed to intense ultraviolet radiation.

The dimensions of the problem are illustrated by the Large-Aperture krypton-fluoride laser amplifier Module (LAM) shown schematically in Figure 1. This device, now operating at the Los Alamos National Laboratory, is typical of current and planned large excimer lasers for fusion applications. The LAM has an active volume of some 2 m3, and optical surfaces (resonator mirror and windows) exceeding 1 m2 in size; the fabrication of these optical elements was the most expensive and time-consuming single item in the construction of the laser. During laser operation, a population inversion in an Ar-Kr-F2 mix ture is created through electron-beam excitation of the laser gas by two 400 kA beams of 650 keV electrons from a cold cathode discharge. The electron trajectories in the gas are constrained by a 4 kG magnetic field transverse to the optical axis produced by a pair of large Helmholtzcoils.

The Center for Laser Studies (CLS) is a research organization within the School of Engineering at the University of Southern California. Since its founding in 1973, CLS has grown to about a dozen full-time research scientists working with about 35 graduate students on projects funded by a wide range of government and industrial agencies. Because studies of and research with lasers are frequently interdisciplinary ventures, close ties are maintained with academic departments within the University such as Electrical Engineering, Physics, Materials Science, Chemistry, Mechanical Engineering and Chemical Engineering. The need to cross traditional university department lines was one of the reasons for establishing CLS. The CLS researchers share an extensive equipment and knowledge base, which also serves as a resource facility for cooperative research with industry. This approach has led to over 200 publications in major journals and has given the Center national prominence.

Six faculty members conduct a majority of their research at CLS and an approximately equal number have experiments in residence. Examples of some of the current research projects of the primary faculty are given below:

1. Professor Elsa Garmire, Director, is studying nonlinear optical devices in a variety of materials with particular emphasis on using semiconductor nonlinearities in optically bistable devices and in optical phase conjugation. This group first demonstrated mid-infrared bistability in In As with a 3 mW threshold, the lowest threshold for any bistability reported at that time. Optical bistability occurs when a semiconductor etalon is illuminated near its bandgap. The nonlinear refractive index combined with reflection feedback causes the transmission or reflection to have two stable outputs at a single value of the input, depending on the device irradiation history. Typical results are shown in Figure 1. Currently under investigation is optical phase conjugation by means of degenerate four wave mixing with two wave mixing recently observed in Cr-doped GaAs.

The 1986 MRS Fall Meeting will feature 22 symposia spanning developments in semiconductors, ceramics, polymers, nuclear waste management, cements, fly ash and coal conversion, optical fiber materials, fractals, and alloys. Contributed oral and poster papers are currently being sought to round out the program. The following synopses describe the scope and focus of each symposium. For further information on topical content, and preparation of abstracts, contact one of the symposium chairs for the particular symposium of interest to you. See the January/February issue of the MRS BULLETIN for further information on the meeting, and look for the complete Call for Papers brochure in the mail.

Many of the symposia will publish proceedings or extended abstract volumes. Ask the symposium chairabout publication plans for his/her symposium.

This symposium is devoted to fundamental aspects of beam-solid interactions, beam-induced crystal growth and other transient processes in materials, including semiconductors, metals, ceramics, and polymers. The symposium provides an interdisciplinary forum for scientists engaged in research on the interaction of photon, ion, and electron beams with solids, the thermodynamics and kinetics of rapid phase transformations between metastable and stable states induced by such interactions, and novel applications of these processes.