Electrons and photons can coexist as a single entity called a surface plasmon—an elementary excitation found at the interface between a conductor and an insulator. Because of their hybrid electric and photonic nature, plasmons allow photons to be precisely controlled on the nanoscale. Plasmons are evident in the vivid hues of rose windows, which derive their color from small metallic nanoparticles embedded in the glass. They also provide the basis for color-changing biosensors (such as home pregnancy tests), photothermal cancer treatments, improved photovoltaic cell efficiencies, and nanoscale lasers. While surface plasmons were first identified nearly 55 years ago, many of their exciting applications are yet to come. This issue of MRS Bulletin reviews the progress and promise of plasmonics—from the characterization tools that have allowed nanometer-scale probing of plasmons to the new materials that may enable low-loss, active, and quantum plasmonics. Within reach are applications ranging from integrated plasmonic circuits for nanophotonic computation to plasmonic optical tweezers for manipulation of nano-sized particles and proteins.

Emerging telecommunication and data routing applications anticipate a photonic roadmap leading to ultra-compact photonic integrated circuits. Consequently, photonic devices will soon have to meet footprint and efficiency requirements similar to their electronic counterparts calling for extreme capabilities to create, guide, modulate, and detect deep-subwavelength optical fields. For active devices such as modulators, this means fulfilling optical switching operations within light propagation distances of just a few wavelengths. Plasmonics, or metal optics, has emerged as one potential solution for integrated on-chip circuits that can combine both high operational speeds and ultra-compact architectures rivaling electronics in both speed and critical feature sizes. This article describes the current status, challenges, and future directions of the various components required to realize plasmonic integrated circuitry.

Enhanced and confined optical fields near metallic nanostructures, supporting surface plasmon (SP) resonances, make it possible to enhance the interaction of light with tiny amounts of matter, down to the molecular level. Such capability has been extensively exploited in the framework of optical spectroscopy, nonlinear optics, imaging and integrated optics, among others. Here we discuss the use of plasmonics for optical trapping. Plasmon-based trapping addresses key limitations of conventional optical tweezers formed at the focus of a diffraction-limited laser beam. Beyond permitting trapping of smaller objects, down to the true nanometer scale, they enable parallel trapping from a single beam and can be easily integrated on a chip. SP-based trapping opens new perspectives in a wide range of fields from biology to quantum optics.

Plasmonic metal nanoparticles have the ability to act as nanoscale antennas for visible and near-IR (infrared) light, leading to increased electromagnetic fields at their surface. As a result, Raman scattering and/or fluorescence from nearby molecules can be enhanced by many orders of magnitude. However, imaging how these molecules interact with the enhanced fields at the surface of noble metal nanoparticles is a challenge due to the diffraction limit of light. In this article, we review super-resolution imaging of plasmonic hot spots using two all-optical readouts, surface-enhanced Raman scattering and surface-enhanced fluorescence, which are used to locate and track single or a few molecules on the surface of nanoscale-roughened metals. These super-resolution imaging techniques allow localization of the emission centroid of an emitter to better than 5 nm and allow mapping of the electromagnetic field enhancement experienced by molecules at the nanoparticle surface.

When an electron beam passes through or near a metal structure, it will excite surface plasmons, providing a unique way to access surface plasmon behavior at the nanoscale. An electron beam focused to nanometer dimensions thus functions as a point source that is able to probe the local plasmonic mode structure at deep-subwavelength resolution. In this article, we show how well-controlled coupling between an electron beam and surface plasmons, combined with a far-field detection system, allows characterization and manipulation of plasmons on a variety of plasmonic devices. By mapping the spatial profile of inelastic scattering to resonant modes, the dispersion and losses of surface plasmons are resolved. The technique further allows probing of the confinement of plasmons within cavities and measuring the angular emission profile of nanoantennas. The coupling of electrons to surface plasmons allows the use of the electron beam as a dipole emitter that can be positioned at will. The beam position thereby can select between modes with different symmetries. This effect can be used to exert forces on plasmonic structures on the nanometer length scale with great control.

Surface plasmon polaritons, combined excitations of light and free electrons of a metal, have emerged as an alternative information carrier for nanoscale circuitry due to their ability to confine light far below the size of the wavelength. They hold the potential to act as a revolutionary bridge between current diffraction-limited microphotonics and bandwidth-limited nanoelectronics. Interestingly, the nanoscale confinement achievable by plasmons also increases the interaction with quantum emitters, paving the way for quantum applications. Exotic non-classical properties of light such as entanglement and squeezing can be embedded into plasmons and faithfully transmitted and received. Recently, it was also shown that unique coupled plasmonic excitations can be engineered on the nanoscale with artificial media (metamaterials) to enhance and control light-matter interaction. A major departure from the conventional classical description of the plasmon is under development. The aim is to incorporate the “wave” nature of matter manifested in ultra-small metallic nanoparticles and the “particle” nature of light, which can play a role in future integrated circuits with capabilities of quantum information processing. This article reviews developments in the field of quantum nanophotonics, an exciting frontier of plasmonic applications ranging from single photon sources and quantum information transfer to single molecule sensing.

Plasmonics aims at combining features of photonics and electronics by coupling photons with a free-electron gas, whose subwavelength oscillations (surface plasmons) enable manipulation of light at the nanoscale and engender the exciting properties of optical metamaterials. Plasmonics is facing a grand challenge of overcoming metal losses impeding its progress. We reflect on the reasons why subwavelength confinement and loss are intimately intertwined and investigate the physics of loss in conductors beyond the conventional Drude model. We suggest that commonly used noble metals may not be the best materials for plasmonics and describe alternate materials such as transparent conducting oxides and transition metal nitrides. We consider the prospects of compensating the loss with gain materials and conclude that the so-far elusive solution to the loss obstacle lies in finding better materials with lower losses.