Nonreciprocal metasurface with space–time phase modulation

Abstract

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.

Recent advances and perspectives on space-time coding digital metasurfaces

“…One of the most interesting and promising applications of space-time metastructures is the possibility to break Lorentz reciprocity without the need of magnetic or nonlinear materials. Theoretical [41][42][43] and experimental [44][45][46] studies have shown the possibility to attain nonreciprocal reflection or transmission effects via a metasurface imparting a suitable space-time phase gradient. In a recent study [30], this concept was implemented and demonstrated by relying on a space-time coding digital metasurface.…”

Loss-induced nonreciprocity

“…Despite these achievements, the basic principles for realizing optical nonreciprocity remain limited as a result of the time-reversal symmetry and linear nature of Maxwell’s equations. The existing approaches can be grouped into three categories with the following requirements 1 – 3 : (i) magnetic-field-induced breaking of time-reversal symmetry 26 – 29 , (ii) spatiotemporal modulation of system permittivity 30 – 36 , and (iii) nonlinearity 37 , 38 . However, these principles either encounter difficulties in integration 39 , require stringent experimental conditions 40 , or have limited performance 38 .…”

Joint Multi‐Frequency Beam Shaping and Steering via Space–Time‐Coding Digital Metasurfaces

“…A variety of new ideas, including time‐modulated meta‐atoms, [ 5 ] temporal multilayers, [ 6–11 ] space–time‐periodic diffraction gratings, [ 12 ] Fresnel drag, [ 13 ] and time‐dependent epsilon‐near‐zero media, [ 14 ] just to mention a few, have been proposed and explored, also revisiting old results in the field of electrical engineering [ 15–17 ] and taking inspiration from emerging concepts in classical and quantum physics such as “time‐crystals.” [ 18,19 ] Arguably, one of the most promising application scenarios for space–time metastructures involves breaking Lorentz reciprocity without the need of magnetic bias, and several candidate strategies have been explored theoretically [ 20–24 ] and experimentally. [ 25–29 ]…”