  Actively tunable and reconfigurable wavefront shaping by optical metasurfaces
poses a significant technical challenge often requiring unconventional
materials engineering and nanofabrication. Most wavefront-shaping metasurfaces
can be considered 'local' in that their operation depends on the responses of
individual meta-units. In contrast, 'nonlocal' metasurfaces function based on
the modes supported by many adjacent meta-units, resulting in sharp spectral
features but typically no spatial control of the outgoing wavefront. Recently,
nonlocal metasurfaces based on quasi-bound states in the continuum have been
shown to produce designer wavefronts only across the narrow bandwidth of the
supported Fano resonance. Here, we leverage the enhanced light-matter
interactions associated with sharp Fano resonances to explore the active
modulation of optical spectra and wavefronts by refractive index tuning and
mechanical stretching. We experimentally demonstrate proof-of-principle
thermo-optically tuned nonlocal metasurfaces made of silicon, and numerically
demonstrate nonlocal metasurfaces that thermo-optically switch between distinct
wavefront shapes. This meta-optics platform for thermally reconfigurable
wavefront-shaping requires neither unusual materials and fabrication nor active
control of individual meta-units.
