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Revealing the quantum regime in tunnelling plasmonics

Abstract

When two metal nanostructures are placed nanometres apart, their optically driven free electrons couple electrically across the gap. The resulting plasmons have enhanced optical fields of a specific colour tightly confined inside the gap. Many emerging nanophotonic technologies depend on the careful control of this plasmonic coupling, including optical nanoantennas for high-sensitivity chemical and biological sensors1, nanoscale control of active devices2,3,4, and improved photovoltaic devices5. But for subnanometre gaps, coherent quantum tunnelling becomes possible and the system enters a regime of extreme non-locality in which previous classical treatments6,7,8,9,10,11,12,13,14 fail. Electron correlations across the gap that are driven by quantum tunnelling require a new description of non-local transport, which is crucial in nanoscale optoelectronics and single-molecule electronics. Here, by simultaneously measuring both the electrical and optical properties of two gold nanostructures with controllable subnanometre separation, we reveal the quantum regime of tunnelling plasmonics in unprecedented detail. All observed phenomena are in good agreement with recent quantum-based models of plasmonic systems15, which eliminate the singularities predicted by classical theories. These findings imply that tunnelling establishes a quantum limit for plasmonic field confinement of about 10−8λ3 for visible light (of wavelength λ). Our work thus prompts new theoretical and experimental investigations into quantum-domain plasmonic systems, and will affect the future of nanoplasmonic device engineering and nanoscale photochemistry.

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Figure 1: Formation and characterization of a nanoscale plasmonic cavity.
Figure 2: Onset of quantum tunnelling in sub-nm plasmonic cavities.
Figure 3: Evolution of the plasmonic modes through the quantum regime and the quantum limit of plasmonic confinement.

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Acknowledgements

This work was supported by EPSRC grants EP/G060649/1 and EP/H007024/1, EU grant CUBi-HOLE, and projects FIS2010-19609-C02-01 and EUI200803816 from the Spanish Ministry of Science and Innovation. J.J.B. also acknowledges support from the Ikerbasque Foundation, Jesus College Cambridge and the University of Cambridge, and M.M.H. acknowledges support from a Canadian NSERC post-doctoral fellowship.

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J.J.B. conceived of and planned the experiments. K.J.S. and M.M.H. designed, constructed and performed the experiments. J.A., A.G.B. and R.E. conceived of the theoretical approach, and carried out the calculations. All the authors contributed to analysing the results and writing the paper.

Corresponding author

Correspondence to Jeremy J. Baumberg.

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The authors declare no competing financial interests.

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Savage, K., Hawkeye, M., Esteban, R. et al. Revealing the quantum regime in tunnelling plasmonics. Nature 491, 574–577 (2012). https://doi.org/10.1038/nature11653

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