Effect of a conductive layer on Fabry-Pérot resonances

Using terahertz spectroscopy, we investigate resonant transmission through a dielectric slab with a conducting layer of varied mobility on one side. We observe the well-known Fabry-P\'erot resonances in the transmission. Two fundamentally different regimes of the electrodynamic response of the system are established. In the clean limit, when the radiation frequency exceeds the carrier relaxation rate, $\ensuremath{\omega}\ensuremath{\tau}\ensuremath{\gg}1$, the system exhibits a plasmonic response. As a result, there is a continuous shift in the Fabry-P\'erot resonances with respect to the system parameters. By contrast, in the dirty regime, when $\ensuremath{\omega}\ensuremath{\tau}\ensuremath{\ll}1$, there is an abrupt $\ensuremath{\pi}$ shift in the resonance frequency, as the renormalized film resistance matches the impedance of free space. We find that under this condition, Fabry-P\'erot oscillations disappear entirely over a broad frequency range. Such a matching effect might be of interest in various terahertz applications, for example, involving broadband filters or absorbers.