Ultrathin Perfect Absorbers for Normal Incident Waves Using Dirac Cone Metasurfaces With Critical External Coupling

We propose an ultrathin perfect absorber for normal incident waves using a Dirac cone metasurface with the thickness of 1/38 free-space wavelength. First, we prepare Dirac cone metasurfaces consisting of periodic metallic chain patterns on a dielectric substrate having the $\Gamma $ -point frequencies of approximately 30 GHz. The perfect absorber is designed by adjusting the external coupling of the Dirac cone metasurfaces with their chain densities. Based on the one-port resonator model, we clarify that the perfect absorber is realized at the critical coupling condition with identical internal and external $Q$ -factors. We experimentally demonstrate a perfect absorption with the reflectance as small as -37 dB by the metasurface with the critical external coupling.