https://dx.doi.org/10.1351/goldbook.P04712
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
whereis 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