Optical response functions

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The optical response functions couple an external electric field, \(E_\mathrm{ext}\), with the internal electric field arising from the response of the crystal. It is convenient to introduce the displacement field, \(\bf{D}\), which determines the electric field from external charges, \(\rho_\mathrm{ext}\). The displacement field is related to the internal electric field, \(\bf{E}\), through the polarization, \(\bf{P}\):

\[\bf {D} = \epsilon_0 \bf {E} + {\bf P},\]

where \({\bf -P}\) is the electric field from the internal charges, \(\rho_{int}\).

The polarization is related to the dipole moment of the material, \({\bf p}\),

\[{\bf P } \equiv {\bf p }/V,\]

where \(V\) is the volume of the material.

Linear response coefficients

The linear response coefficients \(\chi\) (susceptibility), \(\epsilon_r\) (dielectric constant), and \(\alpha\) (polarizability) relate the electrodynamic quantities outlined above to each other:

\[{\bf P } = \epsilon_0 \chi {\bf E },\]\[{\bf D } = \epsilon_r \epsilon_0 {\bf E },\]\[{\bf p } = \alpha {\bf E }.\]

Optical conductivity

For a perturbation \({\bf E}({\bf r}) = {\bf E}_0 \exp({\rm i}{\bf q} \cdot {\bf r} )\), the linear response current in the long wave-length limit (\(q \ll 1/a\) , where \(a\) is the lattice constant) is given by [Wis63]:

\[{\bf j}({\bf r}, \omega) = \sigma(q=0, \omega) {\bf E}({\bf r}, \omega),\]

where \(\sigma\) is the optical conductivity.

Units

The unit of the linear response coefficients are:

Coefficient Unit
\(\alpha\) C2/Nm5
\(\epsilon_r\) 1
\(\chi\) 1
\(\sigma\) C2/Nsm2

Relation between the linear response coefficients

All the response coefficients follow from the susceptibility, \(\chi\):

\[\epsilon_r(\omega) = (1 + \chi(\omega) ),\]\[\alpha(\omega) = V \epsilon_0 \chi(\omega),\]\[\sigma(\omega) = - i \omega \epsilon_0 \chi(\omega).\]

The derivation of the last relation can be found in [Mar04].

[Mar04]Richard M. Martin. Electronic structure: Basic theory and practical methods. Cambridge University Press, New York, 2004.
[Wis63]N. Wiser. Dielectric constant with local field effects included. Phys. Rev., 129:62–69, Jan 1963. doi:10.1103/PhysRev.129.62.