Photon emitters placed in an optical cavity experience an environment that changes how they are coupled to the surrounding light field. In the weak-coupling regime, the extraction of light from the emitter is enhanced. But more profound effects emerge when single-emitter strong coupling occurs: mixed states are produced that are part light, part matter1, 2, forming building blocks for quantum information systems and for ultralow-power switches and lasers. Such cavity quantum electrodynamics has until now been the preserve of low temperatures and complicated fabrication methods, compromising its use. Here, by scaling the cavity volume to less than 40 cubic nanometres and using host–guest chemistry to align one to ten protectively isolated methylene-blue molecules, we reach the strong-coupling regime at room temperature and in ambient conditions. Dispersion curves from more than 50 such plasmonic nanocavities display characteristic light–matter mixing, with Rabi frequencies of 300 millielectronvolts for ten methylene-blue molecules, decreasing to 90 millielectronvolts for single molecules—matching quantitative models. Statistical analysis of vibrational spectroscopy time series and dark-field scattering spectra provides evidence of single-molecule strong coupling. This dressing of molecules with light can modify photochemistry, opening up the exploration of complex natural processes such as photosynthesis and the possibility of manipulating chemical bonds.

Single-molecule strong coupling at room temperature in plasmonic nanocavities

Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.

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Janus monolayers of transition metal dichalcogenides

The experimental data taken in the NanoPhotonics Group at the Cavendish Laboratory (University of Cambridge), collected between 01/10/2014 to 01/02/2016. Simulations performed at the Blackett Laboratory, Department of Physics (Imperial College, London) and at the Department of Chemistry (King’s College London). Dataset for accepted journal article "Single-molecule strong coupling at room temperature in plasmonic nanocavities"

Research data supporting "Single-molecule strong coupling at room temperature in plasmonic nanocavities"

Nanophotonic systems, including photonic crystal microcavities and plasmonic metal nanoparticles, that are capable of changing the rate of spontaneous emission are reviewed and compared.

Modified spontaneous emission in nanophotonic structures

The study of light at the nanoscale has become a vibrant field of research, as researchers now master the flow of light at length scales far below the optical wavelength, largely surpassing the classical limits imposed by diffraction. Using metallic and dielectric nanostructures precisely sculpted into two-dimensional (2D) and 3D nanoarchitectures, light can be scattered, refracted, confined, filtered, and processed in fascinating new ways that are impossible to achieve with natural materials and in conventional geometries. This control over light at the nanoscale has not only unveiled a plethora of new phenomena but has also led to a variety of relevant applications, including new venues for integrated circuitry, optical computing, solar, and medical technologies, setting high expectations for many novel discoveries in the years to come.

https://science.sciencemag.org/content/sci/348/6234/516.full.pdf

Nanophotonics: shrinking light-based technology.

Plasmonic nanostructures enable spontaneous emission enhancement factors of greater than 1,000 — the largest observed to date. The orientation of dipole emitters in nanogaps plays a vital role.

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Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas

Typical emitters such as molecules, quantum dots and semiconductor quantum wells have slow spontaneous emission with lifetimes of 1–10 ns, creating a mismatch with high-speed nanoscale optoelectronic devices such as light-emitting diodes, single-photon sources and lasers. Here we experimentally demonstrate an ultrafast (<11 ps) yet efficient source of spontaneous emission, corresponding to an emission rate exceeding 90 GHz, using a hybrid structure of single plasmonic nanopatch antennas coupled to colloidal quantum dots. The antennas consist of silver nanocubes coupled to a gold film separated by a thin polymer spacer layer and colloidal core–shell quantum dots, a stable and technologically relevant emitter. We show an increase in the spontaneous emission rate of a factor of 880 and simultaneously a 2,300-fold enhancement in the total fluorescence intensity, which indicates a high radiative quantum efficiency of ∼50%. The nanopatch antenna geometry can be tuned from the visible to the near infrared, providing a promising approach for nanophotonics based on ultrafast spontaneous emission. Typical emitters such as molecules and quantum dots have slow spontaneous emission with lifetimes of 1–10 ns. Here, Hoang et al. have fabricated a hybrid structure of plasmonic nanopatch antennas coupled to quantum dots, achieving ultrafast spontaneous emission with a lifetime of 11 ps.

http://mikkelsen.pratt.duke.edu/sites/mikkelsen.pratt.duke.edu/files/NatureCommunicationsMikkelsen2015.pdf

Ultrafast spontaneous emission source using plasmonic nanoantennas.

An absorptive metasurface based on film-coupled colloidal silver nanocubes is demonstrated. The metasurfaces are fabricated using simple dip-coating methods and can be deposited over large areas and on arbitrarily shaped objects. The surfaces show nearly complete absorption, good off-angle performance, and the resonance can be tuned from the visible to the near-infrared.

Large-Area Metasurface Perfect Absorbers from Visible to Near-Infrared.

Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications.

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Probing the origin of excitonic states in monolayer WSe2

Tailoring light–matter interactions in monolayer MoS2 is critical for its use in optoelectronic and nanophotonic devices. While significant effort has been devoted to enhancing the photoluminescence intensity in monolayer MoS2, tailoring of the emission spectrum including complex excitonic states remains largely unexplored. Here, we demonstrate that the peak emission wavelengths of the A and B excitons can be tuned up to 40 and 25 nm, respectively, by integrating monolayer MoS2 into a plasmonic nanocavity with tunable plasmon resonances. Contrary to the intrinsic photoluminescence spectrum of monolayer MoS2, we are also able to create a dominant B exciton peak when the nanocavity is resonant with its emission. Additionally, we observe a 1200-fold enhancement of the A exciton emission and a 6100-fold enhancement of the B exciton emission when normalized to the area under a single nanocavity and compared to a control sample on thermal oxide.

Jiani Huang

Papers

Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas

Ultrafast spontaneous emission source using plasmonic nanoantennas.

Large-Area Metasurface Perfect Absorbers from Visible to Near-Infrared.

Probing the origin of excitonic states in monolayer WSe2

Tailored emission spectrum of 2D semiconductors using plasmonic nanocavities