  Metasurfaces consisting of electrically thin and densely packed planar arrays
of subwavelength elements enable an unprecedented control of the impinging
electromagnetic fields. Spatially modulated metasurfaces can efficiently tailor
the spatial distribution of these fields with great flexibility. Similarly,
time modulated metasurfaces can be successfully used to manipulate the
frequency content and time variations of the impinging field. In this paper, we
present time-modulated reflective metasurfaces that cause a frequency shift to
the impinging radiation, thus realizing an artificial Doppler effect in a
non-moving electrically thin structure. Starting from the theoretical analysis,
we analytically derive the required time modulation of the surface admittance
to achieve this effect, and present a realistic time-varying structure, based
on a properly designed and dynamically tuned high-impedance surface. It is
analytically and numerically demonstrated that the field emerging from the
metasurface is up-,down-converted in frequency according to the modulation
profile of the metasurface. The proposed metasurface concept, enabling a
frequency modulation of the electromagnetic field on-the-fly, may find
application in telecommunication, radar, and sensing scenarios.
