Beam steering with a metasurface: Metasurfaces have been designed that function as many passive optical elements. Shaltout et al. created a time-dependent active element. A frequency-comb light beam (a light source consisting of a broad range of evenly spaced wavelengths) interacted with a specially designed passive metasurface so that the beam could be rapidly (<8 picoseconds) swept over a broad angle (25°). Such beam-steering capability could directly affect augmented and virtual reality systems. Science , this issue p. 374