Ultrafast, reconfigurable all-optical beam steering and spatial light modulation

Achieving spatiotemporal control of light at subwavelength and subcycle scales is an important milestone in the development of new photonic materials for signal processing, pulse shaping and ultrafast imaging. Spatiotemporal light modulation currently relies on electronic interband and intraband transitions that yield pronounced refractive index changes but typically suffer from slow, picosecond response times due to carrier relaxation. Here we show that by leveraging resonant light-matter interactions in a high-quality factor metasurface it is possible to use the optical Kerr effect, a weaker but subfemtosecond optoelectronic polarization effect, to achieve ultrafast, reconfigurable light modulation. By the subwavelength all-optical tuning of the refractive index of the dielectric metasurface unit cells with a spatially structured pump beam, we experimentally demonstrate pulse-limited beam steering with a 74-fs response time at angles up to ±13° in the near-infrared, where the deflection angles are programmable by the pump pattern. The steering originates from the Kerr effect, with a background contribution arising from a slower two-photon-excited free carrier absorption. Additionally, we observe pump self-modulation and self-diffraction, linear frequency conversion, and demonstrate arbitrary subpicosecond spatial light modulation in two dimensions.