Page 31 - Chalcogenide Glasses for Infrared Optics

10 Cha pte r O n e

Oxides
100
S

%T or %R 50 Se S Se Te
Te

0
1.0 10 20 30 40 50 60
Wavelength (µm)

FIGURE 1.7 Pictorial representation of the transmission range for glasses
based on sulfur, selenium, or tellurium.

The selenium-based glasses start transmitting at about 1 µm and start
cutting off after about 12 µm. The tellurium glasses start transmitting
at about 2 µm and cutting off about 20 to 30 µm. Tellurium glasses
have the highest index and are the hardest to make without crystalliz-
ing. This depiction is for only one chalcogen in the composition. Mixed
chalcogen glasses such as sulfur-selenium or selenium-tellurium will
be somewhat different with regard to transmission, index, and tendency
to crystallize.

1.4 Extrinsic Loss within the Band, Impurities, Scatter,
and Quality
Electronic, vibrational, or physical defects related to purity or method
of preparation may affect the performance of a material within the
transmission range. Thus, in a general sense, the effects are consid-
ered extrinsic, not intrinsic, to the solid. Impurity atoms in crystalline
semiconductors may be electrically active in the host material, lead-
ing to charge carriers in excess of intrinsic levels. The free carriers
with high mobility classically in semiconductors may absorb infra-
red radiation in proportion to the infrared wavelength squared. 13
Inclusion of scattering terms may lead to a dependence greater than
wavelength squared. Intervalence band transitions may produce
absorption bands in P-type materials such as germanium and gallium
arsenide. Examples of such effects are described in standard texts such
as the one by Moss. The solution for low carrier mobility materials
14