Volume 8, Issue 4 - Special Issue: Plasmon-Exciton Coupling; Guest Editors: Matthew Pelton, Matthew Sheldon, and Jacob Khurgin

The assembly of inorganic nanoparticles often leads to collective properties that are different from the combined properties of the individual components. In particular, coupling plasmonic and excitonic nanoparticles has been shown to modify their optical properties, including absorption, emission, and scattering. Because of this, these coupled assemblies have potential applications in a wide range of areas, including sensing, light harvesting, and photocatalysis. More recently, unique properties, including Fano interference and Rabi splitting, have been observed by increasing the coupling strength. However, the behavior of coupled nanoparticles is highly dependent on the exact organization of the components, including the number of particles coupled, the distance separating them, and their spatial orientation. This is especially true in the case of strongly coupled particles. Because of this, it is important to achieve synthetic techniques that not only can link particles together but also offer good control over how the particles are connected. In this review, assemblies of plasmonic and excitonic nanoparticles are reviewed, including the various methods that have been used for their construction, the properties that these systems have been predicted to possess as well as the ones that have been observed, and their current applications along with current challenges in the field and potential future applications.

The generation in artificial composites of a magnetic response to light, comparable in magnitude with the natural electric response, may offer an invaluable control parameter for a fine steering of light at the nanoscale. In many experimental realizations, however, the magnetic response of artificial meta-atoms is too weak so that there is a need for new designs with increased magnetic polarizability. Numerical simulations show that geometrical plasmonic nanostructures based on Platonic solids are excellent candidates for the production of strong optical magnetism in visible light. Inspired by these models, we report a bottom-up approach to synthesize plasmonic nanoclusters made of 12 gold patches located at the center of the faces of a dodecahedron. The scattering of the electric and magnetic dipole induced by light is measured across the whole visible range. The ratio of the magnetic to electric response at resonance is found three times higher than its counterpart measured on disordered plasmonic clusters (“plasmonic raspberries”) of the same size. Numerical simulations confirm the experimental measurements of the magnetic response.

The complementary optical properties of surface plasmon excitations of metal nanostructures and long-lived excitations of semiconductor quantum dots (QDs) make them excellent candidates for studies of optical coupling at the nanoscale level. Plasmonic devices confine light to nanometer-sized regions of space, which turns them into effective cavities for quantum emitters. QDs possess large oscillator strengths and high photostability, making them useful for studies down to the single-particle level. Depending on structure and energy scales, QD excitons and surface plasmons (SPs) can couple either weakly or strongly, resulting in different unique optical properties. While in the weak coupling regime plasmonic cavities (PCs) mostly enhance the radiative rate of an emitter, in the strong coupling regime the energy level of the two systems mix together, forming coupled matter-light states. The interaction of QD excitons with PCs has been widely investigated experimentally as well as theoretically, with an eye on potential applications ranging from sensing to quantum information technology. In this review we provide a comprehensive introduction to this exciting field of current research, and an overview of studies of QD-plasmon systems in the weak and strong coupling regimes.

Atomically thin, two-dimensional, transition-metal dichalcogenide (TMD) monolayers have recently emerged as a versatile platform for optoelectronics. Their appeal stems from a tunable direct bandgap in the visible and near-infrared regions, the ability to enable strong coupling to light, and the unique opportunity to address the valley degree of freedom over atomically thin layers. Additionally, monolayer TMDs can host defect-bound localized excitons that behave as single-photon emitters, opening exciting avenues for highly integrated 2D quantum photonic circuitry. By introducing plasmonic nanostructures and metasurfaces, one may effectively enhance light harvesting, direct valley-polarized emission, and route valley index. This review article focuses on these critical aspects to develop integrated photonic and valleytronic applications by exploiting exciton–plasmon coupling over a new hybrid material platform.

One-photon up-conversion, also called anti-Stokes photoluminescence (ASPL), is the process whereby photoexcited carriers scavenge thermal energy and are promoted into a higher energy excited state before emitting a photon of greater energy than initially absorbed. Here, we examine how ASPL from CsPbBr 3 nanoparticles is modified by coupling with plasmonically active gold nanoparticles deposited on a substrate. Two coupling regimes are examined using confocal fluorescence microscopy: three to four Au nanoparticles per diffraction limited region and monolayer Au nanoparticle coverage of the substrate. In both regimes, CsPbBr 3 ASPL is blue-shifted relative to CsPbBr 3 deposited on a bare substrate, corresponding to an increase in the thermal energy scavenged per emitted photon. However, with monolayer Au nanoparticle coverage, ASPL is enhanced relative to the conventional Stokes-shifted PL. Together, these phenomena result in a 6.7-fold increase in the amount of thermal energy extracted from the system during optical absorption and reemission.

The phase matching between the propagating fundamental and nonlinearly generated waves plays an important role in the efficiency of the nonlinear frequency conversion in macroscopic crystals. However, in nanoscale samples, such as nanoplasmonic structures, the phase-matching condition is often ignored due to the sub-wavelength nature of the materials. Here, we first show that the phase matching of the lattice plasmon modes at the fundamental and second-harmonic frequencies in a plasmonic nanoantenna array can effectively enhance the surface-enhanced second-harmonic generation. Additionally, a significant enhancement of the second-harmonic generation is demonstrated using stationary band-edge lattice plasmon modes with zero phase.

Plasmonics is a rapidly developing field at the boundary of fundamental sciences and device engineering, which exploits the ability of metal nanostructures to concentrate electromagnetic radiation. The principal challenge lies in achieving an efficient conversion of the plasmon-concentrated field into some form of useful energy. To date, a substantial progress has been made within the scientific community in identifying the major pathways of the plasmon energy conversion. Strategies based on the hot electron injection and the near-field energy transfer have already shown promise in a number of proof-of-principle plasmonic architectures. Nevertheless, there are several fundamental questions that need to be addressed in the future to facilitate the transition of plasmonics to a variety of applications in both light amplification and optical detection. Of particular interest is a plasmon-induced resonance energy transfer (PIRET) process that couples the plasmon evanescent field to a semiconductor absorber via dipole-dipole interaction. This relatively unexplored mechanism has emerged as a promising light conversion strategy in the areas of photovoltaics and photocatalysis and represents the main focus of the present minireview. Along these lines, we highlight the key advances in this area and review some of the challenges associated with applications of the PIRET mechanism in nanostructured systems.

Plasmonic structures are known to support the modes with sub-wavelength volumes in which the field/matter interactions are greatly enhanced. Coupling between the molecular excitations and plasmons leading to the formation of “plexcitons” has been investigated for a number of organic molecules. However, plasmon-exciton coupling in metal/semiconductor structures has not experienced the same degree of attention. In this work, we show that the “very strong coupling” regime in which the Rabi energy exceeds the exciton binding energy is attainable in semiconductor-cladded plasmonic nanoparticles and leads to the formation of Wannier exciton-plasmon polariton (WEPP), which is bound to the metal nanoparticle and characterized by dramatically smaller (by a factor of a few) excitonic radius and correspondingly higher ionization energy. This higher ionization energy, which exceeding approaches 100 meV for the CdS/Ag structures, may make room-temperature Bose-Einstein condensation and polariton lasing in plasmonic/semiconductor structures possible.

Following the initial success of cavity quantum electrodynamics in atomic systems, strong coupling between light and matter excitations is now achieved in several solid-state set-ups. In those systems, the possibility to engineer quantum emitters and resonators with very different characteristics has allowed access to novel nonlinear and non-perturbative phenomena of both fundamental and applied interest. In this article, we will review some advances in the field of solid-state cavity quantum electrodynamics, focussing on the scaling of the relevant figures of merit in the transition from microcavities to sub-wavelength confinement.