Ultrahigh‐Q Fano Resonances in Terahertz Metasurfaces: Strong Influence of Metallic Conductivity at Extremely Low Asymmetry

Fano resonances in metasurfaces are important due to their low loss subradiant behavior that allows excitation of high‐quality ( Q ) factor resonances extending from the microwave to the optical regime. High‐ Q Fano resonances have recently enabled applications in the areas of sensing, modulation, filtering, and efficient cavities for lasing spasers. Highly conducting metals are the most commonly used materials for fabricating the metasurfaces, especially at the low‐frequency terahertz region where the DC, Drude, and perfect electric conductivity show similar resonant behavior of the subwavelength meta‐atoms. Here, it is experimentally and theoretically demontrated that the Q factor of a low asymmetry Fano resonance is extremely sensitive to the conducting properties of the metal at terahertz frequencies. Large differences in the Q factor and figure of merit of the Fano resonance is observed for perfect electric conductors, Drude metal, and a DC‐conducting metal, which is in sharp contrast to the behavior of the inductive–capacitive resonance of meta‐atoms at terahertz frequency. Identification of such a low asymmetry regime in Fano resonances is the key to engineer the radiative and nonradiative losses in plasmonic and metamaterial‐based devices that have potential applications in the microwave, terahertz, infrared, and the optical regimes.