Spatiotemporally Modulated Nonlocal Metasurfaces: Walking the Dispersion Curve

Interaction of light with spatiotemporally modulated structures enables dynamic transformation of both the momentum and frequency of light. However, the efficiency of a single-step transformation is limited by the strength of the perturbation. Here, we use a nonlocal mode in a semiconductor metasurface, modulated by two pump beams shifted relative to each other in both space and time, to demonstrate a ladder process─momentum conversion followed by frequency conversion─thus allowing the system to walk along its dispersion curve. Our results highlight both the prospects and the challenges of realizing a multistep frequency conversion cascade in optics.