The relevant material property that couples with the radiation field.
May be called optical or
dielectric polarization. Optical spectroscopies may be
classified according to the
dielectric polarizationpower-law dependence on
the external electric field.
Notes:
- Mathematically it is defined as the electric dipole moment change per volume
resulting from absorption of radiation of optical frequencies, defined as
,
where
is the electric displacement,
the electric constant (vacuum permittivity) , and
the strength of the radiation electric field. A dielectric medium is characterized
by
the constitutive relation
where
is the linear 'susceptibility' for a transparent singly refracting medium.
Depending on the molecular or atomic restoring force on the electron with respect
to the
displacement
,
the field-induced motion of the electron can introduce
other frequency components on the electron motion, and this in turn leads to non-linear
optical effects.
-
The polarization component to the
nth-order in the field is denoted as
Thus, the following equations apply,
and
where
is the
i-th
component
of the electric field strength and
is the usual '
susceptibility'
in the
absence of higher terms and
is the order of the field-induced
polarization in the material.
In an anisotropic medium,
,
and
are the medium '
hyper-susceptibilities';
they are tensors of rank 2, 3, and 4, respectively.
Linear optical responses such as absorption, light
propagation, reflection, and refraction,
involving a weak incoming field, are related to
.
Non-linear techniques are connected to the non-linear
polarization
.
Low order non-linear techniques, such as
three-wave mixing, are related to the second order optical polarization
.
For a random
isotropic medium (such as a liquid)
or for a crystal with a centrosymmetric
unit cell,
is zero by symmetry and then the lowest order non-linear techniques, as well as the
higher
order, are related to the third-order optical polarization,
,
and the corresponding hyper-susceptibility.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 402
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.
