Synthetic crystal rotation with spacetime metamaterials

The interaction of light with rotating bodies has been historically limited to rotation frequencies much smaller than optical frequencies. Here, we investigate synthetic crystal rotations, i.e., spatiotemporal modulations mimicking the rotation of an anisotropic crystal, which grant access to large rotation frequencies. Spatiotemporal modulations change the fundamental symmetries of the electromagnetic field, breaking temporal and rotation symmetries, but preserving a spatiotemporal rotation symmetry that enforces the conservation of a combination of energy and spin angular momentum (SAM). The scattering of optical pulses by synthetically rotating crystals results in spatiotemporal light with intra-pulse SAM changes. The frequency-domain response reveals sidebands with frequency/SAM locking, and negative frequency sideband transitions for large enough rotation frequencies. Our results highlight the qualitatively different light-matter interaction regimes accessed by synthetic rotations.