Our group developed a new type of time-variant optical metasurface with which phase modulation in both space and time is imposed on the reflected light, leading to different paths for the forward and backward light propagation. Light propagation is usually reciprocal meaning that the trajectory of light traveling in one direction is identical to that in the opposite direction. Breaking reciprocity can make light propagate only in one direction. Optical components that support such unidirectional flow of light, for example, isolators and circulators, are indispensable building blocks in many modern laser and communication systems. They are currently almost exclusively based on the magneto-optic effect, making the devices bulky and difficult for integration. It is in great demand to have a magnetic-free route to achieve nonreciprocal light propagation in many optical applications.
Here, for the first time, we realized nonreciprocal light propagation in free space experimentally at optical frequencies with an ultrathin component. This approach exhibits excellent flexibility in controlling light both in momentum and energy space. It will provide a new platform for exploring interesting physics arisen from time-dependent material properties and will open a new paradigm in the development of scalable, integratable, magnet-free nonreciprocal devices.
Reference: X. Guo, Y. Ding, Y. Duan, and X. Ni, Nonreciprocal metasurface with space–time phase modulation. Light Sci Appl 8, 123 (2019). https://doi.org/10.1038/s41377-019-0225-z