Page 32 - Chalcogenide Glasses for Infrared Optics

Transmission of Light by Solids 11

such as found in chalcogenide glasses leads to an expression inde-
12
pendent of wavelength. Free carrier absorption for melt-formed
chalcogenide glasses used for optical applications has not been
observed but has been reported for highly conducting chalcogenide
15
glasses containing tellurium used in electronic devices.
The presence of impurity atoms such as oxygen, water, and carbon
bonded to constituent atoms leads to localized molecular absorption
bands. Classic examples are the 9-µm absorption in silicon due to
the Si—O bond, 16,17 the 11.6-µm absorption in germanium due to the
17
18
Ge—O bond, and a Si—C absorption for C in silicon at 16.5 µm. It is
12
19
interesting to note the fundamental for pure Si—C occurs at 12.6 µm.
Molecules such as water (H O), hydrogen sulfide (H S), or hydrogen
2 2
selenide (H Se) occur often in sulfur or selenium glasses. The impu-
2
rity molecules couple (in a weak bond) to the positive element in the
glass composition. An arsenic-selenide glass has only one absorption
band due to H Se at about 4.6 µm while an arsenic-germanium-
2
selenium glass has two absorption bands, one for arsenic at 4.5 µm
and one for germanium at 4.9 µm. A sulfur glass will have an absorp-
tion for water at 2.9 µm and one for hydrogen sulfide at 4 µm. Silicon-
oxygen (Si—O) absorption will occur at about 9.5 µm in glasses if
present as an impurity. Hydrocarbons present during the compound-
ing process for selenium glasses lead to the formation of H Se due to
2
the reaction of liquid selenium with a hydrocarbon.
A laboratory method used to generate small amounts of H Se is
2
to melt paraffin and selenium together. Localized absorption due to
low-level impurities in crystals or glasses produces narrow sharp
bands useful for determining the impurity concentration. Crystal
materials are often grown from a melt in near-perfect lattice form.
There is little chance of bubbles or particulate inclusions. Vapor-
grown polycrystalline materials may contain particulate matter from
the gases or vapors used in the growth. Glasses are formed from
melts as well. The melts may be in containers opened to the atmo-
sphere or sealed in evacuated chambers. Glasses are usually mixed
by some form of agitation of the melt to ensure complete reaction of
the components and to form a homogeneous melt from which the
solid forms. Bubbles may form during the process. Often, at the end
of the compounding mixing process, agitation will be stopped and a
period of time is allowed for the bubbles to rise to the surface and be
eliminated. Bubbles act as hollow spheres in the glass, scattering the
light. Most optical glass specifications allow bubbles with no larger
than 0.002-in diameter and only a few. Inert particulate matter may
also be present in the glass due to contaminate in the reactants. Such
particles are small and absorb light. Some glass compositions pro-
duce crystallites during processing. Crystallites may be distinguished
from particles on examination because they generally transmit light
and crystalline facets. Crystallites, as bubbles, cause transmission loss.