Scientists at The Pennsylvania State University (Penn State) developed a new type of optical metasurface that makes light reflect in one direction only. Leveraging both the spatial and temporal phase manipulation provided by an ultrathin nonlinear metasurface, the scientists experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm. The new metasurface could advance the development of scalable, magnetic-free, nonreciprocal devices that can be integrated with other optical components.
Optical devices that support the unidirectional flow of light, such as isolators and circulators, are currently almost exclusively based on the magneto-optic effect, making the devices bulky and difficult to integrate. “This is the first optical metasurface with controllable ultrafast time-varying properties that is capable of breaking optical reciprocity without a bulky magnet,” professor Xingjie Ni said.
The ultrathin metasurface consists of a silver back-reflector plate supporting block-shaped, silicon nanoantennas with a large, nonlinear Kerr index at near-infrared (NIR) wavelengths. The researchers used a heterodyne interference between two laser lines that were closely spaced in frequency to create efficient traveling-wave refractive index modulation on the nanoantennas, leading to an ultrafast space-time phase modulation with a large temporal modulation frequency of about 2.8 THz.