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. A schematic illustration showing the concept of a space-time phase-modulated metasurface consisting of resonating dielectric nanoantennas operating in reflection mode. A traveling phase modulation in sinusoidal form is superposed on the designed phase gradient along the horizontal direction. Light impinging on the metasurface with frequency ω is converted to a reflecting beam with frequency ω – Δω due to the parametric process arising from dynamic phase modulation, while the back-propagating beam with frequency ω – Δω is converted to ω – 2Δω instead of ω, resulting in a nonreciprocal effect. Courtesy of Xuexue Guo, Yimin Ding, Yao Duan, and Xingjie Ni. The dynamic modulation technique exhibited flexibility in tuning both spatial and temporal modulation frequencies. Completely asymmetric reflections in forward and backward light propagations were achieved experimentally with a wide bandwidth of around 5.77 THz within a subwavelength interaction length of 150 nm. Light reflected by the space-time metasurface acquires a momentum shift induced by the spatial phase gradient as well as a frequency shift rising from the temporal modulation. The light exhibits asymmetric photonic conversions between forward and backward reflections. By exploiting the unidirectional momentum transfer provided by the metasurface geometry, the researchers found that selective photonic conversions could be freely controlled by designing an undesired output state to lie in the nonpropagative region. The researchers said that their approach exhibits excellent flexibility in controlling light both in momentum and energy space and could provide a new platform for exploring the physics of time-dependent material properties. Optical nonreciprocity was achieved at a subwavelength-scale , making this approach potentially compatible with integrated nanophotonic and quantum optical systems. The research was published in Light: Science & Applications (https://doi.org/10.1038/s41377-019-0225-z).