Metal nanostructures act as powerful optical antennas[1, 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 nanostructures[3, 4], but it is extremely challenging to measure the fields in such gaps directly. These enhanced fields have applications in surface-enhanced spectroscopies[5-7], nonlinear optics[1, 8-10], and nanophotonics[11-15]. Here we show that nonlinear tunnelling conduction between gold electrodes separated by a subnanometre gap leads to optical rectification, producing a DC 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 1000, consistent with estimates from surface-enhanced Raman measurements[16-18]. 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 $\sim λ/1000$, and for experiments with new materials, different wavelengths, and different incident polarizations.
Seeing inside a Plasmonic Nanogap: Few-molecule Orientation and Preferential Adsorption
Ecole Polytechnique Fédérale de Lausanne (EPFL)（洛桑联邦理工学院）；Center for Quantum Science and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL)（洛桑联邦理工学院量子科学与工程中心）
Non-equilibrium quantum plasmonics in nanoparticle-on-mirror nanocavities
Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China（电子科技大学前沿科学技术研究院）；Photonics Research Centre, Universiti Malaya（马来亚大学光子研究中心）；Nanophotonics Centre, Cavendish Laboratory, Department of Physics, University of Cambridge（剑桥大学物理系卡文迪许实验室纳米光子学中心）
Tip-Enhanced Vibrational Ladder Climbing in Surface Molecular System
Institute for Molecular Science, National Institutes of Natural Sciences（国立自然科学研究所分子科学研究所）；Graduate Institute for Advanced Studies, SOKENDAI（综合研究大学院大学）
Investigating Analyte Co-Localization at Electromagnetic Gap Hot-Spots For Highly Sensitive (Bio)molecular Detection by Plasmon Enhanced Spectroscopies
