  We introduce the concept and a generic approach to realize Extreme Huygens'
Metasurfaces by bridging the concepts of Huygens' conditions and optical bound
states in the continuum. This novel paradigm allows creating Huygens'
metasurfaces whose quality factors can be tuned over orders of magnitudes,
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.
