Levi-Civita tensor [@Tyldesley], $c$ is the speed of light, and $\omega$ is the angular frequency. Note that this angular frequency is not restricted to the optical range. For instance, it can also correspond to the 1-100 GHz frequency range. The $\gamma$ tensor provides the linear dependence of the dielectric permittivity on the wave vector ${\bf k}$ in the optically active material, that is: $$\label{eq15}
\varepsilon _{ik} \left( {\omega ,{\rm {\bf k}}} \right)=\varepsilon
_{ik}^{\left( 0 \right)} \left( \omega \right)+i\gamma _{ikl} k_l$$ Here, $k_l$ is the $l$-component of the wave vector; $\varepsilon _{ik} \left( {\omega ,{\rm {\bf k}}} \right)$ denotes the double Fourier transform in time and space of the dielectric tensor, with the long-wavelength components being denoted by $\varepsilon_{ik}^{\left( 0 \right)}$. Throughout this manuscript we adopt Einstein notation, in which one implicitly sums over repeated indices (as it happens, e.g., for the $l$ index in Eq. (\[eq15\])).

Thus, the calculation of the gyrotropy tensor can be reduced to the calculation of the tensor $\gamma$ responsible for the spatial dispersion of the dielectric permittivity.

Alternatively, one can use the following formula for the dielectric permittivity [@Landau; @Melrose]: $$\label{eq19}
\varepsilon _{ik}\left( {\omega ,{\rm {\bf k}}} \right) \\
=\delta _{ik} +\frac{4\pi i}{\omega }\sigma _{ik}\left( 