Page 23 - Chalcogenide Glasses for Infrared Optics

2 Cha pte r O n e

energy level states in the band structure are not nearly as precise as in

a crystalline solid.

1.2 Beginning of Transmission of Light—An Electronic
Transition
Generally speaking, infrared optical materials are insulators or semi-
conductors as judged by their bandgaps and resistivity. Photons of
light corresponding to energy greater than the bandgap of the solid
are strongly absorbed at the surface. As the wavelength is increased
and the photon energy decreased below the bandgap, light is trans-
mitted through the solid. The beginning of light transmission of a
solid occurs at the wavelength that corresponds to the bandgap
energy. The absorption of the photon is a very strong, quantized elec-
tronic transition. One may think of this energy as representing the
average ionization energy for the primary chemical bonds formed
between the atoms that make up the solid. If the required ionization
energy is large enough, transmission begins in the ultraviolet region
of the spectrum, as in the case for alkali halides or alkaline earth
halides. Then the solid appears water-clear or colorless. If it occurs in
the visible region, the solid appears colored. If the absorption edge
occurs in the infrared region, the solid appears metallic because all
visible light is strongly absorbed and reflected.
The use of infrared spectroscopy as an analytical tool to identify
and measure concentrations of organic compounds began in the late
1940s. Instruments, crude by today’s standards, used salt prisms to
disperse the light, salt windows for the instrument, and cells to contain
the samples being analyzed. Petroleum refineries used the infrared-
based technology for quality control in their laboratories. The band-
gaps for both the alkali halides and the alkaline earth halides occur in
the ultraviolet region and were not a factor in their infrared use. Most
of these ionic solids are soft, weak, and hygroscopic, making them
unsuitable for use outside of the laboratory.
The semiconductor revolution began in the early 1950s at Bell
Telephone Laboratories when Gordon Teal et al. developed the abil-
1
ity to grow high-purity germanium in single-crystal form. The result
was the germanium transistor. Later in the 1950s, Gordon Teal joined
Texas Instruments and under his direction accomplished the same
feat for silicon, resulting in the silicon transistor. Both germanium
and silicon found use as infrared optical materials and as infrared
light detectors. Germanium windows and lenses became the optical
material standard for the industry due to their wide transmission,
2 to 20 µm, with very little change in refractive index (low dispersion)
and good physical properties. Silicon found use as a missile dome
material due to its superior physical properties such as strength and
hardness. The transmission range was 2 to 14 µm again with little