Metasurface Reflects Light Only in One Direction

An illustration showing the concept of a space-time phase modulated metasurface consisting of resonating dielectric nanoantennas operating in reflection mode. (Source: X. Guo et al., PSU)

Light propagation is usually reciprocal meaning that the tra­jectory of light travelling in one direction is identical from 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.

Recently, scientists developed a new type of 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. For the first time, nonreciprocal light propa­gation in free space was realized experi­mentally at optical frequencies with such an ultrathin component. “This is the first optical metasurface with control­lable ultrafast time-varying properties that is capable of breaking optical reci­procity without a bulky magnet,” said Xingjie Ni, the Charles H. Fetter Assistant Professor in Department of Electrical Engi­neering at the Pennsylvania State University.

The ultrathin meta­surface consists of a silver back-reflector plate supporting block-shaped, silicon nanoantennas with large nonlinear Kerr index at near-infrared wave­lengths around 860 nm. Heterodyne interference between two laser lines that are closely spaced in frequency was used to create efficient travelling-wave refractive index modulation upon the nano­antennas, which leads to ultrafast space-time phase modulation with unpre­cedentedly large temporal modulation frequency of about 2.8 THz. This dynamic modulation technique exhibits great 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 around 5.77 THz within a sub-wavelength 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 arisen from the temporal modulation. It exhibits asymmetric photonic conversions between forward and backward reflections. In addition, by exploiting uni­directional momentum transfer provided by the meta­surface geometry, selective photonic conver­sions can be freely controlled by designing an undesired output state to lie in the forbidden, i.e. non-propa­gative, region.

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. (Source: CAS)

Reference: X. Guo et al.: Nonreciprocal metasurface with space–time phase modulation, Light: Sci. & Appl. 8, 123 (2019); DOI: 10.1038/s41377-019-0225-z

Link: Nanophotonics & Optoelectronics Laboratory, Dept. of Electrical Engineering, Pennsylvania State University, University Park, PA, USA

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