Integrated metasurfaces on silicon photonics for emission shaping and holographic projection.

The emerging applications of silicon photonics in free space, such as LiDARs, free-space optical communications, and quantum photonics, urge versatile emission shaping beyond the capabilities of conventional grating couplers. In these applications, silicon photonic chips deliver free-space emission to detect or manipulate external objects. Light needs to emit from a silicon photonic chip to the free space with specific spatial modes, which produce focusing, collimation, orbital angular momentum, or even holographic projection. A platform that offers versatile shaping of free-space emission, while maintaining the CMOS compatibility and monolithic integration of silicon photonics is in pressing need. Here we demonstrate a platform that integrates metasurfaces monolithically on silicon photonic integrated circuits. The metasurfaces consist of amorphous silicon nanopillars evanescently coupled to silicon waveguides. We demonstrate experimentally diffraction-limited beam focusing with a Strehl ratio of 0.82. The focused spot can be switched between two positions by controlling the excitation direction. We also realize a meta-hologram experimentally that projects an image above the silicon photonic chip. This platform can add a highly versatile interface to the existing silicon photonic ecosystems for precise delivery of free-space emission.