  Dynamic optical metasurfaces with ultrafast temporal response, i.e.,
spatiotemporal optical metasurfaces, provide attractive solutions and open
fascinating perspectives for modern highly integrated optics and photonics. In
this work, electro-optically controlled optical metasurfaces operating in
reflection and utilizing resonant waveguide mode excitation are demonstrated
from the viewpoint of free-space propagating light modulation. The modulation
of reflected light power with superior characteristics in comparison with prior
research is achieved by identifying a suitable low-loss waveguide mode and
exploiting its resonant excitation. The electro-optic Pockels effect in a
300-nm-thick lithium niobate (LN) film sandwiched between a continuous thick
gold film and an array of gold nanostripes, serving also as control electrodes,
is exploited to realize fast and efficient light modulation. The fabricated
compact (active area <1000 $\mu$m) modulators operate in the wavelength range
of 850-950 nm, featuring a maximum intensity modulation depth of $\sim\!$ 42 %
at the driving voltage of $\pm$10 V within the bandwidth of 13.5 MHz (with the
potential bandwidth of 6.5 GHz). The introduced nonlocal electro-optic
metasurface configuration opens new avenues towards the realization of
ultrafast, efficient and robust free-space light modulators based on an LN flat
optics approach.
