Graphene plasmons provide a suitable alternative to noble-metal plasmons because they exhibit much larger confinement and relatively long propagation distances, with the advantage of being highly tunable via electrostatic gating. We report strong lightmatter interaction assisted by graphene plasmons, and in particular, we predict unprecedented high decay rates of quantum emitters in the proximity of a carbon sheet, large vacuum Rabi splitting and Purcell factors, and extinction cross sections exceeding the geometrical area in graphene ribbons and nanometer-sized disks. Our results provide the basis for the emerging and potentially far-reaching field of graphene plasmonics, offering an ideal platform for cavity quantum electrodynamics and supporting the possibility of single-molecule, single-plasmon devices. * To whom correspondence should be addressed Surfaces plasmons (SPs), the electromagnetic waves coupled to charge excitations at the surface of a metal, are the pillar stones of applications as varied as ultrasensitive optical biosensing, 1-3 photonic metamaterials, 4 light harvesting, 5,6 optical nano-antennas, 7 and quantum information processing. [8][9][10][11] However, even noble metals, which are widely regarded as the best available plasmonic materials, 12 are hardly tunable and exhibit large ohmic losses that limit their applicability to optical processing devices.In this context, doped graphene emerges as an alternative, unique two-dimensional plasmonic material that displays a wide range of extraordinary properties. 13 This atomically thick sheet of carbon is generating tremendous interest due to its superior electronic and mechanical properties, 14-20 which originate in part from its charge carriers of zero effective mass (the so-called Dirac fermions 18 ) that can travel for micrometers without scattering, even at room temperature. 21 Furthermore, rapid progress in growth and transfer techniques have sparked expectations for large-scale production of graphene-based devices and a wide range of potential applications such as high-frequency nanoelectronics, nanomechanics, transparent electrodes, and composite materials. 17 Recently, graphene has also been recognized as a versatile optical material for novel photonic 22 and optoelectronic applications, 23 such as solar cells, photodetectors, 24 light emitting devices, ultrafast lasers, optical sensing, 25 and metamaterials. 26 The outstanding potential of this atomic monolayer is emphasized by its remarkably high absorption 27,28 ≈ πα ≈ 2.3%, where α = e 2 /hc ≈ 1/137 is the fine-structure constant. Moreover, the linear dispersion of the Dirac fermions enables broadband applications, in which electric gating can be used to induce dramatic changes in the optical properties. 29All of these photonic and optoelectronic applications rely on the interaction of propagating far-field photons with graphene. Additionally, SPs bound to the surface of doped graphene exhibit a number of favorable properties that make graphene an attractive alternative to tr...

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.

This review discusses how low-energy valence excitations created by swift electrons can render information on the optical response of structured materials with unmatched spatial resolution. Electron microscopes are capable of focusing electron beams on subnanometer spots and probing the target response either by analyzing electron energy losses or by detecting emitted radiation. Theoretical frameworks suited to calculate the probability of energy loss and light emission ͑cathodoluminescence͒ are reconsidered and compared with experimental results. More precisely, a quantum-mechanical description of the interaction between the electrons and the sample is discussed, followed by a powerful classical dielectric approach that can be applied in practice to more complex systems. The conditions are assessed under which classical and quantum-mechanical formulations are equivalent. The excitation of collective modes such as plasmons is studied in bulk materials, planar surfaces, and nanoparticles. Light emission induced by the electrons is shown to constitute an excellent probe of plasmons, combining subnanometer resolution in the position of the electron beam with nanometer resolution in the emitted wavelength. Both electron energy-loss and cathodoluminescence spectroscopies performed in a scanning mode of operation yield snapshots of plasmon modes in nanostructures with fine spatial detail as compared to other existing imaging techniques, thus providing an ideal tool for nanophotonics studies.