LETTER doi:10.1038/nature11653 Revealing the quantum regime in tunnelling plasmonics Kevin J. Savage1, Matthew M. Hawkeye1, Rube´n Esteban2, Andrei G. Borisov2,3, Javier Aizpurua2 & Jeremy J. Baumberg1 When two metal nanostructures are placed nanometres apart, their probe dynamically controlled plasmonic cavities and reveal the onset of optically driven free electrons couple electrically across the gap. The quantum-tunnelling-induced plasmonics in the subnanometre regime. resulting plasmons have enhanced optical fields of a specific colour Two gold-nanoparticle-terminated atomic force microscope (AFM) tightly confined inside the gap. Many emerging nanophotonic tech- tips are oriented tip-to-tip (Fig. 1a). The tip apices define a cavity nologies depend on the careful control of this plasmonic coupling, supporting plasmonic resonances created via strong coupling between including optical nanoantennas for high-sensitivity chemical and localized plasmons on each tip7,27. This dual AFM tip configuration biological sensors1, nanoscale control of active devices2–4, and provides multiple advantages. First, independent nanometre-precision improved photovoltaic devices5. But for subnanometre gaps, co- movement of both tips is possible with three-axis piezoelectric stages. herent quantum tunnelling becomes possible and the system enters Second, conductive AFM probes provide direct electrical connection a regime of extreme non-locality in which previous classical treat- to the tips, enabling simultaneous optical and electrical measurements. ments6–14 fail. Electron correlations across the gap that are driven Third, the tips are in free space and illuminated from the side in a by quantum tunnelling require a new description of non-local dark-field configuration (Fig. 1b, c and Supplementary Information). transport, which is crucial in nanoscale optoelectronics and sin- This arrangement provides (for the first time, to our knowledge) back- gle-molecule electronics. Here, by simultaneously measuring both ground-free broadband spectroscopic characterization of the tip–tip the electrical and optical properties of two gold nanostructures plasmonic nanocavity throughout the subnanometre regime. A super- with controllable subnanometre separation, we reveal the quan- continuum laser (with polarization parallel to the tip–tip axis) is used tum regime of tunnelling plasmonics in unprecedented detail. All as an excitation source, providing high-brightness illumination over a observed phenomena are in good agreement with recent quantum- wide wavelength range (450–1,700 nm) and reducing integration 15 based models of plasmonic systems , which eliminate the singu- times to a few milliseconds. The tips are aligned using a recently larities predicted by classical theories. These findings imply that developed electrostatic-force technique28. The inter-tip separation d tunnelling establishes a quantum limit for plasmonic field confine- 28 3 is initially set to 50 nm and then reduced while recording dark field ment of about 10 l for visible light (of wavelength l). Our work scattering spectra and direct currents simultaneously. thus prompts new theoretical and experimental investigations into quantum-domain plasmonic systems, and will affect the future of nanoplasmonic device engineering and nanoscale photochemistry. Subwavelength metallic structures can concentrate light into nano- a White-lightte-light d 16 laseraser A scale dimensions well below the diffraction limit owing to reduced x 8 field penetration through a dense electron sea. Nanocavities formed 100× Objective inside a nanogap control the coupling of localized plasmons, thus allow- z ing cavity-tuning6–8 targeted to desirable applications that exploit the enhanced optical fields. But as the cavity gaps shrink to atomic length AFM tip 1 AFM tip 2 6 d scales, quantum effects emerge and standard classical approaches to (nm) describe the optics of these systems fail. One effect of confining elec- 40 tronic wavefunctions to small metallic nanoparticles is to slightly VIV d I modify the screening that tunes the plasmons17.However,tunnelling 4 30 plasmonics in the quantum regime has more profound effects that b A cannot be explained through hydrodynamic models that account for 20 18 quantum effects through smearing of the electronic localization . intensity (a.u.) Scattered 2 1 μm B 10 Recent theories show that quantum tunnelling across the cavity A 19,20 c strongly modifies the optical response , but computational limits 200 nm B restrict these quantum simulations to very small systems below a few C ~1 nanometres in size. Furthermore, the extreme difficulty of creating and 0 600 800 1,000 probing subnanometre cavities has limited experimental investigations Wavelength (nm) of plasmonics in the quantum regime. Top-down and self-assembly nanofabrication achieves gaps as small as 0.5 nm between plasmonic nanoparticles21,22, but these fail to reach the quantum tunnelling Figure 1 | Formation and characterization of a nanoscale plasmonic cavity. a, Scheme for simultaneous optical and electrical measurements of plasmonic regime. Small cavities are also accessed in scanning tunnelling micro- cavity formed between two Au-coated tips, shown in dark-field microscope scopes and electro-migrated break-junctions which show optical mixing, images (b) and false-colour scanning electron microscope image (c) of a typical 23–26 emission and rectification phenomena , but the effect of quantum tip, end radius R 5 150 nm. d, Measured dark field scattering spectra from one tunnelling on optical plasmon coupling across subnanometre cavities inter-tip cavity at different cavity widths d. Plasmonic cavity resonances are remains unexplored. Here we present broadband optical spectra that labelled A–C. 1Nanophotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK. 2Material Physics Center CSIC-UPV/EHU and Donostia International Physics Center DIPC, Paseo Manuel de Lardizabal 5, 20018 Donostia-San Sebastia´n, Spain. 3Institut des Sciences Mole´culaires d’Orsay – UMR 8214, CNRS-Universite´ Paris Sud, Baˆtiment 351, 91405 Orsay Cedex, France. 574 | NATURE | VOL 491 | 22 NOVEMBER 2012 ©2012 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH ab d Each piezoelectric scan investigates three interaction regimes: capa- 0.5 1.0 1.5 2.0 Quantum 0.4 0.6 0.8 Experiment theory citive near-field coupling (50 nm . d . 1 nm), quantum regimes 12 DE E D (1 nm . d . 0 nm) and physical contact with conductive coupling 0.0 CC d = 0 (d # 0 nm). Crucially, this set-up allows us to resolve the gradual d = 0 10 transition between each regime dynamically. The measured dark-field d 0.2 < QR d scattering spectra (Fig. 1d) within the capacitive regime (d 40 nm) QR show a single plasmonic scattering peak centred near l 5 750 nm 8 (mode A). As d is reduced, this peak redshifts owing to increasing 0.4 6 near-field interactions between the tip plasmons. As the cavity shrinks (nN) Applied force below 20 nm, a second scattering peak emerges at shorter wavelengths Intertip separation (nm) 0.6 (mode B, l 5 550 nm) and quickly increases in intensity. Modes A 4 ABC ABC 0.8 and B smoothly redshift until an estimated separation d 5 5 nm, 0 50 500 600 700 800 900 600 800 1,000 1,200 G G whereupon attractive inter-tip forces overwhelm the AFM-cantilever / 0 Wavelength (nm) Wavelength (nm) 29 restoring force and snap the tips into close proximity . However no c e Classical 0.4 0.6 0.8 Experiment theory current flow is detectable because this snap-to-contact point does not 8 E D coincide with conductive contact and the metal surfaces remain sepa- 0.0 rated. Snap-to-contact reduces d to ,1 nm, significantly increasing d = 0 6 CC the plasmonic interaction and dramatically changing the plasmonic 0.2 d scattering resonances (blue curve, Fig. 1d). This increased coupling fur- QR ther redshifts modes A and B and reveals a new higher-order resonance 4 d 0.4 (mode C) at l 5 540 nm. After snap-to-contact, increased piezoelectric QR displacement applies an additional compressive force (0.1 nN per nm of Scattering (a.u.) 2 displacement), pushing the tips into closer proximity. After 11.4 nN of 0.6 C A Intertip separation (nm) force in this run, current flow is detected through the tips, indicating B ABC 0 0.8 metal-to-metal surface contact. Detailed calculations confirm that the 500 700 900 600 800 1,000 1,200 coupled plasmonic modes observed are tightly confined in the nano- Wavelength (nm) Wavelength (nm) cavity (see below, and Supplementary Information). Figure 2 | Onset of quantum tunnelling in sub-nm plasmonic cavities. Simultaneously monitoring the optical and electrical properties a, b, Simultaneously measured electrical conductance (a) and dark-field optical during approach reveals in detail the plasmon evolution through the back-scattering (b) with increasing force applied to the inter-tip cavity after subnanometre regime (Fig. 2a–c). As the applied force increases and d snap-to-contact. Conductive contact (CC) indicates d 5 0, with onset of is reduced, all three modes redshift and modes A and B weaken while quantum regime at dQR. Lines track peak positions. c, Selected experimental mode C intensifies. The line widths of modes A and B decrease in spectra from the last 1 nm to contact in b, shown vertically shifted. concert with this reduced scattering strength while mode C broadens d, Theoretical total scattering intensity from a tip–tip system incorporating owing to increased scattering loss. These spectral changes are well- quantum mechanical tunnelling. The threshold (at dQR) indicates where reproduced by simulations that include quantum tunnelling (Fig. 2d). quantum-tunnelling-induced charge screening overcomes the near-field capacitive interaction between plasmons. e, Theoretical scattering intensity as The calculations employ the quantum corrected model (QCM), a new in d but using purely classical calculations. approach derived from time-dependent density-functional theory that allows incorporation of quantum coherent electron tunnelling into a ~ ~ classical electromagnetic framework to treat large plasmonic systems the onset of the quantum regime,dQR lnðÞ 3qla=2p =2q,whereq is the (Supplementary Information)15.