Page 25 - Chalcogenide Glasses for Infrared Optics

4 Cha pte r O n e

using arsenic trisulfide glass lenses to sense overheated bearings on
railroad cars. In further efforts, many chalcogenide glass compositions
6
were discovered and evaluated at Servo Corporation. Arsenic trisul-
fide glass is a red glass. Other sulfur glass compositions may show
visible light transmission. Most of the glasses based on selenium or
containing tellurium are opaque to the visible region and show metallic
luster.
As stated before, glasses have energy band structure similar to
that of crystalline materials employing the same elements. The chal-
cogenide glasses are electronic conductors with free electrons and
holes as are their crystalline counterparts. However, their disordered
nature leads to very low carrier mobility. Glasses based on sulfur
and/or selenium are very high in resistivity, almost semi-insulators.
Glasses containing tellurium have a more metallic nature and may
have fairly low resistivity. For selenium glasses, except those contain-
ing sulfur, transmission begins at their absorption edge, which for the
most part is 1 µm or more in wavelength. An example of the begin-
ning of transmission for a chalcogenide glass is shown in Fig. 1.2. The
glass is an arsenic selenide glass designated Amtir 4 by AMI, and the

1.0 µm 1.25 µm 1.5 µm 1.75 µm 2.0 µm
70 70
0.9 µm
0.8 µm
60

50 50

40 40

30 30

20 20

10 10
Amtir 4 absorption edge transmission
(2.6 cm thickness)
0 0

FIGURE 1.2 Absorption edge for Amtir 4, an arsenic-selenium glass.