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.

Plasmon-exciton interactions are important for many prominent spectroscopic applications such as surface-enhanced Raman scattering, plasmon-mediated fluorescence, nanoscale lasing, and strong coupling. The case of strong coupling is analogous to quantum optical effects studied in solid state and atomic systems previously. In plasmonics, similar observations have been almost exclusively made in elastic scattering experiments; however, the interpretation of these experiments is often cumbersome. Here, we demonstrate mode splitting not only in scattering, but also in photoluminescence of individual hybrid nanosystems, which manifests a direct proof of strong coupling in plasmon-exciton nanoparticles. We achieved these results due to saturation of the mode volume with molecular J-aggregates, which resulted in splitting up to 400 meV, that is, ∼20% of the resonance energy. We analyzed the correlation between scattering and photoluminescence and found that splitting in photoluminescence is considerably less than that in scattering. Moreover, we found that splitting in both photoluminescence and scattering signals increased upon cooling to cryogenic temperatures. These findings improve our understanding of strong coupling phenomena in plasmonics.

Strong coupling of organic chromophores to plasmonic nanoparticles was shown to markedly improve molecular photostability.

Formation of dressed light-matter states in optical structures, manifested as Rabi splitting of the eigen energies of a coupled system, is one of the key effects in quantum optics. In pursuing this regime with semiconductors, light is usually made to interact with excitons, electrically neutral quasiparticles of semiconductors; meanwhile interactions with charged three-particle states, trions, have received little attention. Here, we report on strong interaction between localized surface plasmons in silver nanoprisms and excitons and trions in monolayer tungsten disulfide (WS). We show that the plasmon-exciton interactions in this system can be efficiently tuned by controlling the charged versus neutral exciton contribution to the coupling process. In particular, we show that a stable trion state emerges and couples efficiently to the plasmon resonance at low temperature by forming three bright intermixed plasmon-exciton-trion polariton states. Our findings open up a possibility to exploit electrically charged polaritons at the single nanoparticle level.