Nonreciprocal metasurface with space–time phase modulation

Creating materials with time-variant properties is critical for breaking reciprocity that imposes fundamental limitations on wave propagation. However, it is challenging to realize efficient and ultrafast temporal modulation in a photonic system. Here, leveraging both spatial and temporal phase manipulation offered by an ultrathin nonlinear metasurface, we experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm. The metasurface, with travelling-wave modulation upon nonlinear Kerr building blocks, creates spatial phase gradient and multi-terahertz temporal phase wobbling, which leads to unidirectional photonic transitions in both the momentum and energy spaces. We observed completely asymmetric reflections in forward and backward light propagations over a large bandwidth around 5.77 THz within a sub-wavelength interaction length of 150 nm. Our approach highlights a potential means for creating miniaturized and integratable nonreciprocal optical components. Light propagating in one direction only could pave the way towards miniaturized components for faster, more efficient data transfer in optical circuits. Xingjie Ni and colleagues at the Pennsylvania State University demonstrated the unidirectional flow of near-infrared light in free space based on a 150-nanometre-thick nano-engineered surface, i.e., metasurface. Their approach adds to the body of research examining ways to disrupt the common wisdom that light travelling in one direction can also travel in the other. In the new system, the ultrathin metasurface consisting of a silver reflector plate supporting block-shaped, silicon nanoantennaswas dynamically modulated to generate time-varying parameters. The infrared light interacts with the ultrathin metasurface, ultimately resulting in a reflection that does not propagate in the opposite direction.