  Optical metasurfaces have been extensively investigated, demonstrating
diverse and multiple functionalities with complete control over the transmitted
and reflected fields. Most optical metasurfaces are however static, with only a
few configurations offering (rather limited) electrical control, thereby
jeopardizing their application prospects in emerging flat optics technologies.
Here, we suggest an approach to realize electrically tunable optical
metasurfaces, demonstrating dynamic Fresnel lens focusing. The active Fresnel
lens (AFL) exploits the electro-optic Pockels effect in a 300-nm-thick lithium
niobate layer sandwiched between a continuous thick and nanostructured gold
film serving as electrodes. We fabricate and characterize the AFL, focusing
800-900 nm radiation at the distance of 40 $\mathrm{\mu}$m with the focusing
efficiency of 15 % and demonstrating the modulation depth of 1.5 % with the
driving voltage of $\pm 10$ V within the bandwidth of $\sim\! 4$ MHz. We
believe that the electro-optic metasurface concept introduced is useful for
designing dynamic flat optics components.
