Page 22 - Chalcogenide Glasses for Infrared Optics

CHAPTER 1

Transmission of

Light by Solids

1.1 Solids
In nature, material exists as gas, liquid, or solid. Gas atoms or mole-
cules are free to move within the confines of their container. Liquids
move to fill the shape of their container while solids are rigid in shape.
There is about a 1000-fold increase in density in going from gas to the
condensed state of liquid or solid. The atoms or molecules come
much closer together as a liquid and closer still as a solid. The dense
solid may have a precise three-dimensional spatial arrangement for
the atoms making up the solid that covers thousands of neighbors in
all three directions. When the long-range order is perfect, the solid
may be referred to as single-crystal. Or the order may be maintained
over limited atomic distances and be referred to as polycrystalline.
The atoms or molecules of liquids are free to move within their
arrangements continuously in any direction and are said to have no
long-range three-dimensional order. Order found is that of nearest
neighbors or second nearest neighbors or even more, but not long
range in the structural sense. Depending on elemental composition,
when the atoms or molecules of a solid come close together, they
begin to share their electronic bonding states, which results in forma-
tion of an energy structure for the solid. When excited, bonding
valence electrons are elevated into a higher conduction band state
and are free to travel through the solid as charge if an electric field is
applied. The energy difference between the valence state and the free
conduction state is called the bandgap of the solid. The vacancies left
in the valence band are called holes and can constitute charge flow
moving in the opposite direction to the field. The band structure is
well developed and precise in crystalline solids with good crystalline
perfection. Liquids and amorphous solid glasses are condensed states
but without long-range three-dimensional orders. A glass is referred
to as a disordered solid. The energy band structure may exist, but the

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