LETTERS PUBLISHED ONLINE: 17 MAY 2009 | DOI: 10.1038/NPHYS1278 Wave–particle duality of single surface plasmon polaritons Roman Kolesov1, Bernhard Grotz1, Gopalakrishnan Balasubramanian1, Rainer J. Stöhr1, Aurélien A. L. Nicolet1, Philip R. Hemmer2, Fedor Jelezko1* and Jörg Wrachtrup1* When light interacts with metal surfaces, it excites electrons, applications include single-shot spin readout for room-temperature which can form propagating excitation waves called surface quantum computers and nano-magnetometers. plasmon polaritons. These collective electronic excitations can To generate non-classical states of surface plasmon polaritons, produce strong electric fields localized to subwavelength dis- single colour centres in diamond nanocrystals were coupled to tance scales1, which makes surface plasmon polaritons in- metallic waveguides. Single nitrogen-vacancy colour centres in teresting for several applications. Many of these potential diamond were chosen as the emitters because of their absolute uses, and in particular those related to quantum networks2, photostability at room temperature. Crystalline, chemically grown require a deep understanding of the fundamental quantum silver wires were used as waveguides because of the low loss in silver properties of surface plasmon polaritons. Remarkably, these in the visible spectral range. Nitrogen-vacancy emitters were placed collective electron states preserve many key quantum mechan- in the near-field of the nanowires by mixing a diamond nanocrystal ical properties of the photons used to excite them, including solution with the silver wires, and spin-coating the resulting entanglement3,4 and sub-Poissonian statistics5. Here, we show nanodiamond-coated wires onto a glass substrate (for details of the that a single-photon source coupled to a silver nanowire ex- sample preparation, see the Methods section). Figure 1a shows an cites single surface plasmon polaritons that exhibit both wave atomic force microscopy (AFM) image of diamond nanocrystals and particle properties, similar to those of single photons. attached to the surface of a nanowire. Surprisingly, an efficient Furthermore, the detailed analysis of the spectral interference self-assembly process leads to sticking of single nanodiamond pattern provides a new method to characterize the dimensions particles to wires (see the AFM picture). The inset shows the of metallic waveguides with nanometre accuracy. fluorescence image of the same wire with bright spots originating One of the most intriguing experiments of contemporary physics from emission of single nitrogen-vacancy defects. The size of the is the double-slit self-interference of single particles. Among the diamond nanocrystals was about 50 nm and the average distance possible quantum systems, photons seem to be ideal for such between the emitter and the metal is expected to be of the order of demonstrations because of their ability to propagate long dis- 25 nm. As a result of being placed in close proximity to the metallic tances in ambient environment, yet still be efficiently detected. A surface, the nitrogen-vacancy defects show a significant reduction key requirement for photon self-interference experiments is the of their fluorescence lifetime owing to efficient coupling of the availability of a true single-photon source. Light sources such as excited nitrogen-vacancy emission to surface plasmon polaritons. lasers show intrinsic fluctuation of photon numbers related to Enhanced decay rates of the emitter due to excitation of surface Poissonian statistics. Hence, the outcome of double-slit experi- plasmons are the most important effect for distances between the ments with such light sources can be described classically without metal surface and the nitrogen-vacancy centre within the range of introducing the concept of photons (a quantized electromag- 10–100 nm (ref. 10). As the coupling is a near-field effect, it depends netic field)6. In the benchmark study realized two decades ago on the distance between the surface of the wire and each nitrogen- by Grangier and co-workers, true single-photon states emitted vacancy defect, and on the orientation of the nitrogen-vacancy from an atomic cascade revealed a clear interference pattern7,8. emission dipole with respect to the axis of the wire. The enhanced Like photons, surface plasmon polaritons can be used for Young coupling is apparent from the analysis of the Purcell factor for double-slit experiments9, and the recent generation of single plas- a collection of different nanocrystals (see Fig. 1). On average, we mons by single quantum emitters opens the door for studying observe a Purcell factor of 2.5, which is in good agreement with their fundamental quantum properties5. Here, both antibunching the theoretically predicted value5 for an emitter–wire separation and self-interference are observed using single plasmons excited of 30 nm. In general, the coupling efficiency depends on the wire by a single-photon emitter, and this unambiguously shows that radius R as 1=R3; that is, stronger coupling is expected for thinner the concept of single-particle self-interference can be applied to wires11. However, the plasmon to photon conversion efficiency at surface plasmon polaritons. As this interference arises from an the wire ends and the plasmon propagation length increase with in situ interferometer wherein one beam splitter is the bi-directional increasing R. Hence, for the present experiment, an average wire emission into the nanowire, and the other beam splitter is the diameter of 70 nm was found to be a good compromise, enabling partially transmitting wire output end, it also provides a sensitive efficient emitter coupling while keeping losses low and photon diagnostic method to determine nanowire properties. In addition, outcoupling high. Coupling of single-photon sources to plasmonic by choosing spin-selective nitrogen-vacancy colour centres in dia- waveguides was reported previously for quantum dots5,12. mond as the single-photon emitters, we open the door to eventually A single-photon emitter coupled to the metal wire is an ideal test achieving strong coupling between spins and plasmons, for which system that provides experimental access to the statistical properties 13. Physikalisches Institut, Universität Stuttgart, 70550 Stuttgart, Germany, 2Department of Electrical and Computer Engineering, Texas A&M University, 77843 College Station, USA. *e-mail: [email protected]; [email protected]. 470 NATURE PHYSICS j VOL 5 j JULY 2009 j www.nature.com/naturephysics © 2009 Macmillan Publishers Limited. All rights reserved. NATURE PHYSICS DOI: 10.1038/NPHYS1278 LETTERS a The quantum mechanical description predicts PC D P1P2 for a AFM Fluor. Poissonian source, PC D 2P1P2 for a thermal source and PC D 0 for a source of single photons6. The detection of a single photon projects the optical emitter into the ground state because of energy conservation. Hence, it is impossible to detect a second photon 500 nm simultaneously. As the PC D 0 case corresponds to a photon at one detector or the other, but never both, it demonstrates the particle-like propagation of single photons. Similar experiments can be realized for surface plasmon polaritons, as shown in the lower panel of Fig. 2a. When the plasmon excitation reaches the end of the wire, it can be coupled to an optical photon that propagates to a detector in the far-field. Hence, coincidence analysis 300 nm Nanodiamonds for photons coupled out at both ends of the wire enables us to measure the second-order intensity autocorrelation function of surface plasmon polaritons. Figure 2b shows the wide-field image of a silver plasmonic b waveguide when the laser beam is focused on a single nitrogen- 1,000 vacancy defect coupled to the wire. Both ends of the wire show de- tectable fluorescence, indicating the propagation of plasmons along Uncoupled NV the wire. To check that single surface plasmon polaritons are excited by the single-photon source, measurements of the second-order correlation function are carried out between both ends of the wire 100 (see Fig. 2c). The absence of the peak at zero delay time provides ex- Coupled NV perimental proof that the plasmons originate from the fluorescence Detected photons of a single quantum system. It is remarkable that even though a sur- face plasmon involves a large number of electrons, it behaves like a single quantum particle. As a control experiment, coincidence mea- 10 surements were also carried out between nitrogen-vacancy defect 0 50 100 150 200 emission to free space and the end of the wire, which corresponds Delay after laser pulse (ns) to the joint detection of a photon and a plasmon. Near-perfect anticorrelation was observed, indicating that the contribution of c 14 background light coupled out at the wire end is negligible. Although On the wire this photon–plasmon anticorrelation does not demonstrate the 12 Off the wire particle-like properties of the plasmon, it is important to mention 10 that it indicates that the silver waveguide conserves the non-classical statistics of the diamond single-photon source, in agreement with a 5 8 previous observation for a quantum dot emitter . Finally, photon correlation measurements were carried out on the emission of the 6 nitrogen-vacancy itself. The absence of the peak at zero delay verifies Number of NVs that the emitter is in fact a single nitrogen vacancy. 4 The next step is to observe the wave-like properties of a single- plasmon polariton, which provides evidence for ‘wave–particle 2 duality'. Quantum mechanical interference of single photons occurs when there are different trajectories for the photons—for example 0 0 5 10 15 20 25 in a Mach–Zehnder interferometer—that contribute to the photon Lifetime (ns) detection probability at the output. The detectors at the two outputs of the interferometer show a modulation in count rate, when the Figure 1 j Single-photon emitter coupled to a silver wire. a, AFM image path difference of the interferometer arms is varied (see Fig. 3a). of diamond nanocrystals attached to a silver nanowire.