Analogue of electromagnetically-induced transparency with ultra-narrow bandwidth in a silicon terahertz metasurface

Electromagnetically induced transparency (EIT) is a phenomenon in which the quantum interference effect occurs in the atomic three energy level system, which produces a sharp transparent window in a wide absorption spectrum [1]. Later, a phenomenon similar to the EIT was found in metamaterial [2-5] as well. The EIT-like response in metamaterial is generally realized by the coherent coupling between a bright mode and a dark mode [6-12] or between two bright modes [13-20]. The strong dispersion characteristic of the EIT can be used in slow-light equipment [21-23]. The main index of the slow-light effect is the group refractive index or group velocity, which is closely related to the bandwidth or Q factor of the EIT transparency window; the higher the Q value of the EIT, the larger is the group refractive index that can be obtained. To achieve high-Q EIT response in metasurfaces via bright-bright mode coupling, two resonances participating in coherent coupling are usually required to have large Q factor contrast (a high- and a low-Q resonance) and small wavelength detuning [24]. In recent years, high-Q resonances have been achieved by breaking the structural symmetry of all-dielectric metasurfaces Achieving an ultra-narrow bandwidth analogue of electromagnetically induced transparency (EIT) in bright-bright mode coupling metasurface requires a large contrast of the Q factor and small wavelength detuning between the two coupled modes. Here, by coupling a toroidal dipole (TD) high-Q Fano resonance and a low-Q magnetic dipole (MD) mode, we numerically demonstrated a high Q factor analogue of EIT on an all-silicon metasurface in the terahertz regime. The Q factor of Fano resonance and consequent EIT can be easily adjusted by the spacing between the air holes. By adjusting the radii of the air holes, the thickness of the silicon wafer, or the lattice constant of the metasurface, EIT-like response exhibiting a very high group refractive index and a large group delay was achieved. The proposed EIT metasurface is easy to fabricate and has potential applications in the fields of narrowband filtering and slow-light based devices. (c) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement