Absolute continuity of the spectrum of the periodic Maxwell operator in a layer

We study the Maxwell operator in a layer $\mathbb R^2\times(0,T)$. It is assumed that an electric permittivity $\varepsilon(\mathbf x)$ and a magnetic permeability $\mu(\mathbf x)$ are periodic along the layer. On the boundary of the layer, we impose conditions of ideal conductivity. Under wide assumptions on $\varepsilon(\mathbf x)$ and $\mu(\mathbf x)$, it is shown that the spectrum of the Maxwell operator is absolutely continuous.