Page 26 - Chalcogenide Glasses for Infrared Optics

Transmission of Light by Solids 5

true transmission is about 65 percent at 1.5 µm, decreasing rapidly to
0 percent at about 0.84 µm. The shape of the curve changes with
thickness which in this case is 2.6 cm. A thicker sample would hit
zero sooner at a longer wavelength due to internal absorption. The
65 percent transmission would be essentially the same for a thinner
piece since the absorption in that wavelength is very slight. The limit
of 65 percent is due to Fresnel reflection loss, which is determined by
the refractive index of the glass. The optical constants for the glass or
any solid are discussed later.

1.3 Long-Wavelength Cutoff
The long-wavelength limit for an infrared optical material is usually
determined by a multiphonon lattice band, a combination band, or
some vibrational absorption involving constituent atoms of the solid.
A qualitative understanding of the factors involved in determining the
long-wavelength cutoff for materials may be obtained by considering
the expression for the frequency of a simple free diatomic vibration
/
1 k   12
f =  
o 2πµ
 
where f = fundamental frequency for vibration between atoms of
o
elements A and B
k = force constant for chemical bond between atoms A and B
µ= reduced mass for elements A and B, from
1 = 1 + 1
µ ma mb
where ma and mb are the atomic masses of elements A and B
When infrared energy with a wavelength corresponding to the fre-
quency of vibration is absorbed by the molecular pair, the pair is raised
to a higher vibrational energy level. Energy is increased in the solid. The
absorption strength depends upon the ionic character between atoms A
and B. If the atoms are the same, purely covalent, as in silicon-silicon or
germanium-germanium, the absorption is weak or nonexistent. A purely
covalent bond means the negative and positive centers of charge
between the atoms coincide—there is no separation. Linus Pauling 7
developed the concept of electronegativity values for each atom. If the
atoms are different, in a Pauling electro negativity sense, there is some
separation of charge between the atoms, some ionic character. Separa-
tion of charge constitutes an electric moment or dipole in the chemical
bond. The dipole couples with the electric field of the infrared light,
allowing energy to be transferred from the light to the molecule. For
crystalline solids, the absorption may be very intense which leads to
the presence of a strong infrared reflection peaks often referred to as
a Reststrahlen band. Examples will be shown in later discussions for
ionic solids, crystalline semiconductors.