  All-dielectric metasurfaces consisting of arrays of nanostructured
high-refractive-index materials, typically Si, are re-writing what is
achievable in terms of the manipulation of light. Such devices support very
strong magnetic, as well as electric, resonances, and are free of ohmic losses
that severely limit the performance of their plasmonic counterparts. However,
the functionality of dielectric-based metasurfaces is fixed-by-design, i.e. the
optical response is fixed by the size, arrangement and index of the
nanoresonators. A far wider range of applications could be addressed if
active/reconfigurable control were possible. We demonstrate this here, via a
new hybrid metasurface concept in which active control is achieved by embedding
deeply sub-wavelength inclusions of a tuneable chalcogenide phase-change
material within the body of high-index Si nanocylinders. Moreover, by strategic
placement of the phase-change layer, and switching of its phase-state, we show
selective and active control of metasuface resonances. This yields novel
functionality, which we showcase via a dual- to mono-band meta-switch operating
simultaneously in the O and C telecommunication bands.
