Polarization-twisting beam-driven rotational doppler effect with nonlinear metasurfaces

We present a dynamic metasurface driven by polarization-twisting beams to demonstrate the rotational Doppler effect. Polarization-twisting pulses, composed of left and right circularly polarized pulses with shifted center frequencies, generate a rapidly rotating linearly polarized field. We employed nanocylinders made of amorphous silicon as the building blocks of the metasurface. The rotating field alters the permittivity of the nanocylinders due to the nonlinear Kerr effect, thereby enabling the metasurface to function effectively as a fast-rotating waveplate. When a probe beam passes through this metasurface, both its frequency and spin state are altered due to the rotational Doppler effect. This phenomenon could be potentially used for developing magnetic field-free optical isolators.