  Dynamic wavefront shaping with optical metasurfaces has presented a major
challenge and inspired a large number of highly elaborate solutions. Here, we
experimentally demonstrate thermo-optically reconfigurable, nonlocal
metasurfaces using simple device architectures and conventional CMOS-compatible
dielectric materials. These metasurfaces support quasi-bound states in the
continuum (q-BICs) derived from symmetry breaking and encoded with a spatially
varying geometric phase, such that they shape optical wavefront exclusively on
spectrally narrowband resonances. Due to the enhanced light-matter interaction
enabled by the resonant q-BICs, a slight variation of the refractive index
introduced by heating and cooling the entire device leads to a substantial
shift of the resonant wavelength and a subsequent change to the optical
wavefront associated with the resonance. We experimentally demonstrate a
metalens modulator, the focusing capability of which can be thermally turned on
and off, and reconfigurable metalenses, which can be thermo-optically switched
to produce two distinct focal patterns. Our devices offer a pathway to realize
reconfigurable, multifunctional meta-optics using established manufacturing
processes and widely available dielectric materials that are conventionally not
considered "active" materials due to their small thermo-optic or electro-optic
coefficients.
