Abstract
Non-reciprocal devices—in which light is transmitted with different efficiencies along opposite directions—are key technologies for modern photonic applications, yet their compact and miniaturized implementation remains an open challenge. Among different avenues, nonlinearity-induced non-reciprocity has attracted significant attention due to the absence of external bias and the ease of integrability within conventional material platforms. So far, nonlinearity-induced non-reciprocity has been demonstrated only in guided platforms using high-quality-factor resonators. Here we demonstrate ultrathin optical metasurfaces with a large non-reciprocal response for free-space radiation based on silicon thermo-optic nonlinearities. Our metasurfaces combine an out-of-plane asymmetry—necessary to obtain non-reciprocity—with in-plane broken symmetry, which finely tunes the radiative linewidth of quasi-bound states in the continuum. Third-order thermo-optic nonlinearities, engaged by the quasi-bound state in the continuum, are shown to enable over 10 dB of non-reciprocal transmission and less than 3 dB of insertion loss, for impinging average intensities smaller than 3 kW cm–2. Numerical calculations suggest that the build-up and relaxation times of the non-reciprocal response can approach sub-microsecond scales, only limited by thermal dissipation. The demonstrated devices merge the field of non-reciprocity with ultrathin metasurface technologies, offering an exciting functionality for signal processing and routing, communications and protection of high-power laser cavities.
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Data availability
All data that support the findings of this work are available in this Article and its Supplementary Information. The raw data for all the experimental plots are available via Zenodo at https://doi.org/10.5281/zenodo.8408935.
Code availability
All numerical simulations were performed using a commercially available electromagnetic solver (COMSOL v. 6.0). All numerical results can be reproduced by following the descriptions provided in the Article and its Supplementary Information.
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Acknowledgements
We would like to thank D. Korobkin for experimental assistance in the earlier stages of this project. M.C. and A.A. acknowledge support from the Air Force Office of Scientific Research and the Simons Foundation. A.C. and A.P. acknowledge support from the research program of The Netherlands Organization for Scientific Research (NWO). Device fabrication was performed at the Nanofabrication Facility at the Advanced Science Research Center at The Graduate Center of the City University of New York.
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All authors conceived the idea and the corresponding experiment. M.C., A.C. and D.L.S. designed the device and performed the numerical analysis. M.C. fabricated the devices and performed the experimental measurements with assistance from A.C. All authors analysed the data and contributed to writing the manuscript. A.A. and A.P. supervised the project.
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Cotrufo, M., Cordaro, A., Sounas, D.L. et al. Passive bias-free non-reciprocal metasurfaces based on thermally nonlinear quasi-bound states in the continuum. Nat. Photon. 18, 81–90 (2024). https://doi.org/10.1038/s41566-023-01333-7
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DOI: https://doi.org/10.1038/s41566-023-01333-7
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