Page 30 - Chalcogenide Glasses for Infrared Optics

Transmission of Light by Solids 9

plot of Knoop surface hardness for crystalline semiconductors as a
function of molecular weight. Some of these materials are useful as
infrared (IR) optical materials. Note the low Knoop hardness values
with increasing mass.
To summarize, the transmission range of a solid is determined
by the bandgap of the material on the short-wavelength side and a
lattice-type absorption band involving constituent atoms on the long-
wavelength side. Both quantities are qualitatively predictable from
the location of the elements in the periodic table, as will be discussed
later. An example of Reststrahlen-like reflection bands for some chal-
cogenide glasses is shown in Fig. 1.6.
The percent reflections for As S glass, Ge S glass, a Si-As-Te
2 3 2 3
glass, and a Ge-P-Te glass, all measured in the far infrared using the
Perkin Elmer 301 infrared spectrophotometer at TI, are shown. Note
the percent reflection for these glasses is much smaller than that
shown by glassy quartz. The fundamental frequencies for the As—S
bond, the Ge—S bond, the Si—Te bond and the Ge—Te bond in the
glasses are all near the peak of their respective reflections. Their long
wavelength cutoff then is about one-half the wavelength of their
peaks.
Figure 1.7 depicts the transmission range of glasses based on
sulfur, selenium, or tellurium. We see the sulfur-based glasses show-
ing some visible light transmission but cutting off after 10 µm.
Notice transmission rates for sulfur-based glasses are the highest
because the Fresnel loss is less due to their lower refractive index.

50

Ge 15 P 15 Te 70
40

Si 10 As 10 Te 2 O
% Reflectivity 20 As 2 S 3
30

Ge 2 S 3
10

0
0 10 20 30 40 50 60
Wavelength (micrometers)
FIGURE 1.6 Far infrared refl ection spectra of some chalcogenide glasses.