Spatiotemporal Metasurface for Ultrafast All-Optical Wavefront Shaping

Optical metasurfaces have established themselves as exceptional nanophotonic platforms to control the properties of light at subwavelength scales. By properly designing the geometry and spatial arrangement of their constituent meta-atoms, engineered phase gradients can be imparted to an incoming field, thereby tailoring its wavefront with a virtually unlimited number of control knobs. However, once fabricated, a metasurface has a permanently fixed optical response. Here, we introduce a strategy for dynamic control, by theoretically predicting and experimentally demonstrating all-optical, reversible, and ultrafast wavefront shaping in a periodic semiconductor nanowire metasurface. A strongly astigmatic femtosecond optical pump pulse is used to photoexcite the resonant metasurface non-uniformly, inducing a transient, spatially inhomogeneous permittivity modulation that reshapes the wavefront of a delayed probe pulse. Ultrafast imaging measurements reveal transient defocusing of the transmitted field at selected probe wavelengths, with a switching time below one picosecond. Quantitative numerical modelling elucidates the defocusing mechanism as arising from the interplay between the unperturbed optical response of the resonant metasurface and the pump-induced spatial permittivity gradient. Our work establishes a fully all-optical and ultrafast route to the dynamic analogue of passive wavefront shaping in gradient metasurfaces, paving the way to all-optically reconfigurable metalenses.