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.

Realizing strong light-matter interactions between individual two-level systems and resonating cavities in atomic and solid state systems opens up possibilities to study optical nonlinearities on a single-photon level, which can be useful for future quantum information processing networks. However, these efforts have been hampered by unfavorable experimental conditions, such as cryogenic temperatures and ultrahigh vacuum, required to study such systems and phenomena. Although several attempts to realize strong lightmatter interactions at room temperature using plasmon resonances have been made, successful realizations on the single-nanoparticle level are still lacking. Here, we demonstrate the strong coupling between plasmons confined within a single silver nanoprism and excitons in molecular J aggregates at ambient conditions. Our findings show that deep subwavelength mode volumes V together with quality factors Q that are reasonably high for plasmonic nanostructures result in a strong-coupling figure of merit-Q= ffiffiffi ffi V p as high as ∼6 × 10 3 μm −3=2 , a value comparable to state-of-the-art photonic crystal and microring resonator cavities. This suggests that plasmonic nanocavities, and specifically silver nanoprisms, can be used for room temperature quantum optics. Strong light-matter interactions are not only interesting from a fundamental quantum optics point of view, e.g., for studying entanglement and decoherence, but also because of their relevance for high-end emerging applications such as quantum cryptography [1], quantum networks [2], single-atom lasers [3], ultrafast single-photon switches [4], and quantum information processing [5][6][7]. These phenomena rely on a quantum emitter strongly interacting with a resonant cavity, which leads to cavity and emitter mode hybridization and vacuum Rabi splitting [8]. In the time domain, these strong light-matter interactions manifest themselves as a coherent exchange of energy between the cavity and the emitter occurring on time scales faster than both cavity and emitter dissipative dynamics-a situation that is dramatically different from irreversible spontaneous emission. Traditionally, these quantum optical phenomena have been studied in atomic [9,10] and solid state systems [11][12][13], which are associated with considerable experimental challenges, such as ultrahigh vacuum, cryogenic temperatures, and fabrication issues.A possible solution to these challenges could be to use noble metal nanoparticles instead of photonic crystal and microring resonator cavities [14][15][16][17][18]. This is because metal nanostructures can trap electromagnetic fields on subwavelength scales as so-called surface plasmon excitations. These plasmonic nanocavities possess a number of desirable properties, such as room temperature operation, deep subwavelength mode volumes, and nanoscale dimensions that have been shown to lead to many remarkable phenomena including single-molecule Raman spectroscopy [19][20][21], tip-enhanced imaging [22], ultracompact nanolasers [23], ...

Quantum mechanical interactions between electromagnetic radiation and matter underlie a broad spectrum of optical phenomena. Strong light-matter interactions result in the well-known vacuum Rabi splitting and emergence of new polaritonic eigenmodes of the coupled system. Thanks to recent progress in nanofabrication, observation of strong coupling has become possible in a great variety of optical nanostructures. Here, we review recently studied and emerging materials for realization of strong light–matter interactions. We present general theoretical formalism describing strong coupling and give an overview of various photonic structures and materials allowing for realization of this regime, including plasmonic and dielectric nanoantennas, novel two-dimensional materials, carbon nanotubes, and molecular vibrational transitions. In addition, we discuss practical applications that can benefit from these effects and give an outlook on unsettled questions that remain open for future research.

We present a detailed Mie theory, finite-difference time-domain, and quasi-static study of plasmon–exciton interactions in a spherical core–shell geometry. In particular, we report absorption, scattering, and extinction cross sections of a hybrid core–shell system and identify several important interaction regimes that are determined by the electromagnetic field enhancement and the oscillator strength of electronic excitations. We assign these regimes to enhanced-absorption, exciton-induced transparency and strong coupling, depending on the nature of the observed spectra of the coupled plasmon–exciton resonances. We also show the relevance of performing single-particle absorption or extinction measurements in addition to scattering to validate the interaction regime. Furthermore, at relatively high, yet realistic oscillator strengths we observe emergence of a third mode, which is not predicted by a classical coupled harmonic oscillator model and is attributed to the geometrical resonance of the structure as a whole.

Recent progress in nanophotonics includes demonstrations of meta-materials displaying negative refraction at optical frequencies, directional single photon sources, plasmonic analogies of electromagnetically induced transparency and spectacular Fano resonances. The physics behind these intriguing effects is to a large extent governed by the same single parameter—optical phase. Here we describe a nanophotonic structure built from pairs of closely spaced gold and silver disks that show phase accumulation through material-dependent plasmon resonances. The bimetallic dimers show exotic optical properties, in particular scattering of red and blue light in opposite directions, in spite of being as compact as ∼λ3/100. These spectral and spatial photon-sorting nanodevices can be fabricated on a wafer scale and offer a versatile platform for manipulating optical response through polarization, choice of materials and geometrical parameters, thereby opening possibilities for a wide range of practical applications.