No abstract

High-Q metasurfaces have important applications in high-sensitivity sensing, low-threshold lasers, and nonlinear optics due to the strong local electromagnetic field enhancements. Although ultra-high-Q resonances of bound states in the continuum (BIC) metasurfaces have been rapidly developed in the optical regime, it is still a challenging task in the terahertz band for long years because of absorption loss of dielectric materials, design, and fabrication of nanostructures, and the need for high-signal-to-noise ratio and high-resolution spectral measurements. Here, a polarization-insensitive quasi-BIC resonance with a high-Q factor of 1049 in a terahertz all-silicon metasurface is experimentally achieved, exceeding the current highest record by 3 times of magnitude. And by using this ultra-high-Q metasurface, a terahertz intensity modulation with very low optical pump power is demonstrated. The proposed all-silicon metasurface can pave the way for the research and development of high-Q terahertz metasurfaces.

Bound states in the continuum (BICs) have attracted much attention due to their infinite Q factor. However, the realization of the analogue of electromagnetically induced transparency (EIT) by near-field coupling with a dark BIC in metasurfaces remains challenging. Here, we propose and numerically demonstrate the realization of a high-quality factor EIT by the coupling of a bright electric dipole resonance and a dark toroidal dipole BIC in an all-dielectric double-layer metasurface. Thanks to the designed unique one-dimensional (D)–two-dimensional (2D) combination of the double-layer metasurface, the sensitivity of the EIT to the relative displacement between the two layer-structures is greatly reduced. Moreover, several designs for widely tunable EIT are proposed and discussed. We believe the proposed double-layer metasurface opens a new avenue for implementing BIC-based EIT with potential applications in filtering, sensing and other photonic devices.

We realize and numerically demonstrate the analogue of electromagnetically induced transparency (EIT) with a high-Q factor in a metal-dielectric bilayer terahertz metamaterial (MM) via bright-bright mode coupling and bright-dark mode coupling. The dielectric MM with silicon dimer rectangular-ring-resonator (Si-DRR) supports either a bright high-Q toroidal dipole resonance (TD) or a dark TD with infinite Q value, while plasmonic MM with metallic rectangular-ring-resonator (M-RR) supports a low-Q electric dipole resonance (ED). The results show that the near-field coupling between the dark TD and bright ED behaves just as that between the two bright modes, which is dependent on the Q factor of the TD resonance. Further, due to the greatly enhanced near-field coupling between the bright ED and dark TD, the coupling distance is significantly extended to about 1.9 times of the wavelength (in media), and robust EIT with large peak value over 0.9 and high Q-factor is achieved. The proposed bilayer MM provides a new EIT platform for design and applications in high-Q cavities, sensing, and slow-light based devices.