Direct Energy, 2018a

56 3.3 Electro-Optics
KH 2 PO 4 [25] [26]. It has also been studied in chalcogenide glasses which are
suldes, selenides, and tellurides such as GeTe [25] [26]. When selecting a
pyroelectric material for an application, the pyroelectric coecient should
be considered. Thermal properties are important too. The material should
be able to withstand repeated heating and cooling, and it should have a
relatively high melting temperature to be useful.
The pyroelectric eect does not have many applications. Some optical
detectors designed to detect infrared light are made from pyroelectric materials
[41] [42]. However, most optical detectors are photovoltaic devices
made from semiconductor junctions, and this technology will be discussed
in Chapter 6. While sensors using the pyroelectric eect could be used to
measure temperature, other types of temperature sensors, such as thermocouples,
are typically used. Thermocouples, which operate based on the
thermoelectric eect which is discussed in Chapter 8, are more convenient
to build and operate. Additionally, in many pyroelectric materials, the effect
is nonlinear while linear sensors are easier to work with and calibrate.
3.3 Electro-Optics
3.3.1 Electro-Optic Coecients
Typically, the magnitude of material polarization in a dielectric is proportional
to the strength of an applied electric eld.
−→ −→ −→ −→ P = D − ɛ0 E = ɛ0 χ e E (3.3)
In this equation χ e is the electric susceptibility, and it is unitless. It is
dened in Sec. 2.2.3 and related to permittivity by Eq. 2.9. However in
other materials, the material polarization depends nonlinearly on the applied
electric eld. Materials for which the material polarization depends
linearly on the external electric eld are called linear materials while others
are called nonlinear or electro-optic materials. The electro-optic eect occurs
when an applied external electric eld induces a material polarization
in a material where the amount of polarization depends nonlinearly on the
external eld. The name involves the word optic because the external eld
is often due to a visible laser beam. However, the external eld can be
from any type of source at any frequency, and a material polarization will
occur even with a constant applied electric eld. A large enough external
electric eld will cause a material to melt or to crystallize in a dierent
phase, but this eect is not the electro-optic eect. Instead, the electrooptic
eect only involves a change in the material polarization, not the
crystal structure, and the change involved is not permanent.