  Thanks to the compact design and multi-functional light-manipulation
capabilities, reconfigurable metalenses, which consist of arrays of
sub-wavelength meta-atoms, offer unique opportunities for advanced optical
systems, from microscopy to augmented reality platforms. Although poorly
explored in the context of reconfigurable metalens, thermo-optical effects in
resonant silicon nanoresonators have recently emerged as a viable strategy to
realize tunable meta-atoms. In this work, we report the proof-of-concept design
of an ultrathin (300 nm thick) and thermo-optically reconfigurable silicon
metalens operating at a fixed, visible wavelength (632 nm). Importantly, we
demonstrate continuous, linear modulation of the focal-length up to 21% (from
165 $\mu$m at 20$\deg$C to 135 $\mu$m at 260$\deg$C). Operating under
right-circularly polarized light, our metalens exhibits an average conversion
efficiency of 26%, close to mechanically modulated devices, and has a
diffraction-limited performance. Overall, we envision that, combined with
machine-learning algorithms for further optimization of the meta-atoms,
thermally-reconfigurable metalenses with improved performance will be possible.
Also, the generality of this approach could offer inspiration for the
realization of active metasurfaces with other emerging material within field of
thermo-nanophotonics.
