Critical Absorption at Terahertz Frequencies through Fabry–Perot Cavity Resonators

Critical absorption in planar resonators is achieved when material losses match radiative losses. In this work, we show that this condition can effectively be satisfied by exploiting the higher-order leaky-mode resonances of a metasurface-based Fabry–Perot cavity. Remarkably, leaky-wave theory reveals how the condition of maximum power radiated at broadside (typically employed for leaky-wave antennas) is linked to that of maximum absorption (typically employed for absorbers): the two conditions almost coincide in low-loss systems, but progressively differ as the losses increase. To corroborate our design, the reflection spectrum of a Fabry–Perot cavity resonator—consisting of a grounded dielectric slab covered with a subwavelength two-dimensional lattice of metallic strip gratings—is measured through terahertz (THz) time-domain spectroscopy in reflection mode. Importantly, the matching condition for critical absorption is satisfied through the dielectric spacer losses and not the almost lossless metallic metasurfaces. The experiment recorded an impressive −40 dB reflectance spectrum and a nearly step-like π jump of the phase reflection spectrum at around 357 GHz, in agreement with the theoretical expectations for the first-order mode resonance. The proposed metasurface-based Fabry–Perot resonator offers significantly higher flexibility and reliability with respect to conventional absorbers based on thin conducting films, whose design is often hindered by material and fabrication uncertainties.