  Compact varifocal lenses are essential to various imaging and vision
technologies. However, existing varifocal elements typically rely on
mechanically-actuated systems with limited tuning speeds and scalability. Here,
an ultrathin electrically controlled varifocal lens based on a liquid crystal
(LC) encapsulated semiconductor metasurface is demonstrated. Enabled by the
field-dependent LC anisotropy, applying a voltage bias across the LC cell
modifies the local phase response of the silicon meta-atoms, in turn modifying
the focal length of the metalens. In a numerical implementation, a
voltage-actuated metalens with continuous zoom and up to 20% total focal shift
is demonstrated. The concept of LC-based metalens is experimentally verified
through the design and fabrication of a bifocal metalens that facilitates
high-contrast switching between two discrete focal lengths upon application of
a 3.2 V$_{\rm pp}$ voltage bias. Owing to their ultrathin thickness and
adaptable design, LC-driven semiconductor metasurfaces open new opportunities
for compact varifocal lensing in a diversity of modern imaging applications.
