Ultrafast Wavefront Shaping via Space-Time Refraction

A myriad of metasurfaces have been demonstrated thatmanipulatelight by spatially structuring thin optical layers. Manipulation ofthe optical properties of such layers in both space and time can unlocknew physical phenomena and enable new optical devices. Examples includephoton acceleration and frequency conversion, which modifies Snell'srelation to a more general, nonreciprocal form. Here, we combine theoryand experiment to realize wavefront shaping and frequency conversionon subpicosecond time-scales by inducing space-time refractive indexgradients in epsilon-near-zero (ENZ) films with femtosecond lightpulses. We experimentally tune wavefront steering by controlling theincident angle of the beams and the pump-probe delay withoutthe need for nanostructure fabrication. As a demonstration of thisapproach, we leverage the ultrafast, high-bandwidth optical responseof transparent oxides in their ENZ wavelength range to create largerefractive index gradients and new types of nonreciprocal, ultrafasttwo-dimensional (2D) optics, including an ultrathin transient lens.