Surface-wave-assisted nonreciprocity in spatio-temporally modulated metasurfaces

Andrew E Cardin,Antoinette J Taylor,Wilton J M Kort-Kamp,Avadh Saxena,Shai R Vardeny,Sinhara R Silva,Abul K Azad,Hou-Tong Chen,Willie J Padilla,Diego A R Dalvit

doi:10.1038/s41467-020-15273-1

Abstract

Emerging photonic functionalities are mostly governed by the fundamental principle of Lorentz reciprocity. Lifting the constraints imposed by this principle could circumvent deleterious effects that limit the performance of photonic systems. Most efforts to date have been limited to waveguide platforms. Here, we propose and experimentally demonstrate a spatio-temporally modulated metasurface capable of complete violation of Lorentz reciprocity by reflecting an incident beam into far-field radiation in forward scattering, but into near-field surface waves in reverse scattering. These observations are shown both in nonreciprocal beam steering and nonreciprocal focusing. We also demonstrate nonreciprocal behavior of propagative-only waves in the frequency- and momentum-domains, and simultaneously in both. We develop a generalized Bloch-Floquet theory which offers physical insights into Lorentz nonreciprocity for arbitrary spatial phase gradients, and its predictions are in excellent agreement with experiments. Our work opens exciting opportunities in applications where free-space nonreciprocal wave propagation is desired.

Highlights

Emerging photonic functionalities are mostly governed by the fundamental principle of Lorentz reciprocity
Breakdown of Lorentz reciprocity as the very nature of the photonic modes is modified in the scattering process, and would lead to giant optical isolation as no propagative modes are radiated in reverse scattering
We exemplify the flexibility of our STMM platform by achieving dynamical beam steering in forward scattering and, when the forward reflection angle is above a certain threshold, we demonstrate nonreciprocity and complete free-space optical isolation by photon-to-surface wave conversion in reverse scattering

Summary

Introduction

Emerging photonic functionalities are mostly governed by the fundamental principle of Lorentz reciprocity. We propose and experimentally demonstrate a spatiotemporally modulated metasurface capable of complete violation of Lorentz reciprocity by reflecting an incident beam into far-field radiation in forward scattering, but into near-field surface waves in reverse scattering These observations are shown both in nonreciprocal beam steering and nonreciprocal focusing. A fully tailored electromagnetic response requires metasurfaces that can continuously alter their scattering properties simultaneously in time and space These functionalities can be achieved in spatio-temporally modulated metasurfaces[31] (STMMs), which have the potential to revolutionize fundamental and applied photonics, including nonreciprocity[32,33], through on-demand control of frequency and momentum harmonic contents of scattered light. We exemplify the flexibility of our STMM platform by achieving dynamical beam steering in forward scattering and, when the forward reflection angle is above a certain threshold, we demonstrate nonreciprocity and complete free-space optical isolation by photon-to-surface wave conversion in reverse scattering. This work can potentially impact emerging technologies benefiting from free-space dynamical wave-front shaping and nonreciprocity, including adaptive optics, Dopplerlike frequency translation, echo-immune antennas, and isolated on-chip communications enabled by robust one-way coupling to surface waves

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Results

Conclusion