Permittivity tensor
= Permittivity tensor
{title2=$D_i=\sum_j\epsilon_{ij}E_j$}
The <permittivity tensor> is the linear material-response matrix relating <electric displacement field> to <electric field> at a specified <frequency>. An isotropic local medium has $\epsilon_{ij}=\epsilon\delta_{ij}$. A magnetically biased transparent medium can have an imaginary antisymmetric transverse contribution; its eigenmodes can have opposite <circular polarizations> and different <refractive indices>.