  In display technologies or data processing, planar and subwavelength
free-space components suited for flat photonic devices are needed.
Metasurfaces, which shape the optical wavefront within hundreds of nanometers,
can provide a solution for thin and portable photonic devices, e.g. as
CMOS-compatible modules. While conventional electro-optic modulators are
inconvenient to operate in free space configurations, its principle can largely
be applied to the development of active metasurfaces with the prospect of
modulation speeds up to the GHz region. Here, we use this concept to realize
fast and continuous modulation of light at low voltage and MHz speed with a
lithium niobate metasurface tuned by the linear electro-optic effect.
Furthermore, we exploit the resonance in the visible to enhance the modulation
of the transmitted light by two orders of magnitude, namely by a factor of 80,
compared to the unstructured substrate. This proof-of-concept work is a first
important step towards the use of lithium niobate metasurfaces for free space
modulation.
