Extreme Huygens' Metasurfaces Based on Quasi-Bound States in the Continuum.

We introduce the concept of and a generic approach to realizing extreme Huygens' metasurfaces by bridging the concepts of Huygens' conditions and optical bound states in the continuum. This novel paradigm allows the creation of Huygens' metasurfaces with quality factors that can be tuned over orders of magnitude, generating extremely dispersive phase modulation. We validate this concept with a proof-of-concept experiment at the near-infrared wavelengths, demonstrating all-dielectric Huygens' metasurfaces with different quality factors. Our study points out a practical route for controlling the radiative decay rate while maintaining the Huygens' condition, complementing existing Huygens' metasurfaces whose bandwidths are relatively broad and complicated to tune. This novel feature can provide new insight for various applications, including optical sensing, dispersion engineering and pulse shaping, tunable metasurfaces, metadevices with high spectral selectivity, and nonlinear meta-optics.