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 polarization power
-law dependence on
the external electric field.
- Mathematically it is defined as the electric dipole moment change per volume
resulting from absorption of radiation of optical frequencies, defined as
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
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
the field-induced motion of the electron can introduce
other frequency components on the electron motion, and this in turn leads to non-linear
The polarization component to the n
th-order in the field is denoted as
Thus, the following equations apply,
of the electric field strength and
is the usual 'susceptibility
absence of higher terms and
is the order of the field-induced
polarization in the material.
In an anisotropic medium,
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
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.
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 402
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. doi:10.1351/goldbook