Change in electric dipole moment per unit volume of
dielectric material in response to its exposure to an electric field.
Notes: - The electric field causes limited reversible displacements of charged particles that cannot move freely in the dielectric material. Electronic polarization (displacements of electrons) takes a femtosecond. So it is in phase with an alternating electric field, such as that of light. Ionic polarization (of ions in ionic crystals) is also instantaneous. However orientation polarization based on reorientation of permanent dipoles is significantly slower (delayed) and thus often out of phase with the light field.
- Polarization density (unit \(\pu{C m-2}\)) is defined as \(P = \varepsilon E - \varepsilon_{0} E\), where \(\varepsilon E {=D}\) is so-called electric displacement, \(E\) the electric field strength, \(\epsilon_{0}\) the vacuum permittivity and \(\varepsilon = \varepsilon_{\rm{r}}\varepsilon_0\) permittivity of the dielectric material. The ideal dependence of \(P\) on \(E\) is linear: \(P = \varepsilon_{0}\chi E\), where \(\chi = \varepsilon_{\rm{r}} - 1\) is electric susceptibility. Non-linearity of this dependence gives rise to non-linear optical effects.
Source:
PAC, 2022, 94, 15. 'Glossary of terms relating to electronic, photonic and magnetic properties of polymers (IUPAC Recommendations 2021)' on page 42 (https://doi.org/10.1515/pac-2020-0501)