Optical rectification and field enhancement in a plasmonic nanogap


Metal nanostructures act as powerful optical antennas1,2 because collective modes of the electron fluid in the metal are excited when light strikes the surface of the nanostructure. These excitations, known as plasmons, can have evanescent electromagnetic fields that are orders of magnitude larger than the incident electromagnetic field. The largest field enhancements often occur in nanogaps between plasmonically active nanostructures3,4, but it is extremely challenging to measure the fields in such gaps directly. These enhanced fields have applications in surface-enhanced spectroscopies5,6,7, nonlinear optics1,8,9,10 and nanophotonics11,12,13,14,15. Here we show that nonlinear tunnelling conduction between gold electrodes separated by a subnanometre gap leads to optical rectification, producing a d.c. photocurrent when the gap is illuminated. Comparing this photocurrent with low-frequency conduction measurements, we determine the optical frequency voltage across the tunnelling region of the nanogap, and also the enhancement of the electric field in the tunnelling region, as a function of gap size. The measured field enhancements exceed 1,000, consistent with estimates from surface-enhanced Raman measurements16,17,18. Our results highlight the need for more realistic theoretical approaches that are able to model the electromagnetic response of metal nanostructures on scales ranging from the free-space wavelength, λ, down to λ/1,000, and for experiments with new materials, different wavelengths and different incident polarizations.

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Figure 1: Measurement approach and layout.
Figure 2: Demonstration of optical rectification.
Figure 3: Further evidence for optical rectification.
Figure 4: Theoretical basis for validity of rectification.
Figure 5: Field (right axis) at the tunnelling region as a function of gap distance (top axis) for five devices (shown in different colours) measured a number of times at 80 K.


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D.N. and D.R.W. acknowledge support from the Robert A. Welch Foundation (grant C-1636) and the Lockheed Martin Advanced Nanotechnology Center of Excellence at Rice (LANCER). F.H. and J.C.C. acknowledge support from the Deutsche Forschungsgemeinschaft, the Baden-Württemberg Stiftung, the European Union through the Bio-Inspired Approaches for Molecular Electronics network (grant MRTN-CT-2006-035859) and the Spanish Ministry of Science and Innovation (Ministerio de Ciencia e Innovacion) (grant FIS2008-04209). F.P. acknowledges funding from a Young Investigator Group.

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D.R.W. fabricated the devices, performed all measurements and analysed the data. D.N. supervised and provided continuous guidance for the experiments and the analysis. F.P., F.H. and J.C.C. carried out the theoretical modelling and DFT calculations. The bulk of the paper was written by D.R.W. and D.N. All authors discussed the results and contributed to manuscript revision.

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Correspondence to Douglas Natelson.

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Ward, D., Hüser, F., Pauly, F. et al. Optical rectification and field enhancement in a plasmonic nanogap. Nature Nanotech 5, 732–736 (2010). https://doi.org/10.1038/nnano.2010.176

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