  Optical metasurfaces have shown to be a powerful approach to planar optical
elements, enabling an unprecedented control over light phase and amplitude. At
that stage, where wide variety of static functionalities have been
accomplished, most efforts are being directed towards achieving reconfigurable
optical elements. Here, we present our approach to an electrically controlled
varifocal metalens operating in the visible frequency range. It relies on
dynamically controlling the refractive index environment of a silicon metalens
by means of an electric resistor embedded into a thermo-optical polymer. We
demonstrate precise and continuous tuneability of the focal length and achieve
focal length variation larger than the Rayleigh length for voltage as small as
12 volts. The system time-response is of the order of 100 ms, with the
potential to be reduced with further integration. Finally, the imaging
capability of our varifocal metalens is successfully validated in an optical
microscopy setting. Compared to conventional bulky reconfigurable lenses, the
presented technology is a lightweight and compact solution, offering new
opportunities for miniaturized smart imaging devices.
