Strong coupling between a dipole emitter and localized plasmons: enhancement by sharp silver tips

The Rise and Current Status of Polaritonic Photochemistry and Photophysics

The interaction between molecular electronic transitions and electromagnetic fields can be enlarged to the point where distinct hybrid light-matter states, polaritons, emerge. The photonic contribution to these states results in increased complexity as well as an opening to modify the photophysics and photochemistry beyond what normally can be seen in organic molecules. It is today evident that polaritons offer opportunities for molecular photochemistry and photophysics, which has caused an ever-rising interest in the field. Focusing on the experimental landmarks, this review takes its reader from the advent of the field of polaritonic chemistry, over the split into polariton chemistry and photochemistry, to present day status within polaritonic photochemistry and photophysics. To introduce the field, the review starts with a general description of light-matter interactions, how to enhance these, and what characterizes the coupling strength. Then the photochemistry and photophysics of strongly coupled systems using Fabry-Perot and plasmonic cavities are described. This is followed by a description of room-temperature Bose-Einstein condensation/polariton lasing in polaritonic systems. The review ends with a discussion on the benefits, limitations, and future developments of strong exciton-photon coupling using organic molecules.

Quantum surface effects in the electromagnetic coupling between a quantum emitter and a plasmonic nanoantenna: time-dependent density functional theory vs. semiclassical Feibelman approach

We use time-dependent density functional theory (TDDFT) within the jellium model to study the impact of quantum-mechanical effects on the self-interaction Green's function that governs the electromagnetic interaction between quantum emitters and plasmonic metallic nanoantennas. A semiclassical model based on the Feibelman parameters, which incorporates quantum surface-response corrections into an otherwise classical description, confirms surface-enabled Landau damping and the spill out of the induced charges as the dominant quantum mechanisms strongly affecting the nanoantenna-emitter interaction. These quantum effects produce a redshift and broadening of plasmonic resonances not present in classical theories that consider a local dielectric response of the metals. We show that the Feibelman approach correctly reproduces the nonlocal surface response obtained by full quantum TDDFT calculations for most nanoantenna-emitter configurations. However, when the emitter is located in very close proximity to the nanoantenna surface, we show that the standard Feibelman approach fails, requiring an implementation that explicitly accounts for the nonlocality of the surface response in the direction parallel to the surface. Our study thus provides a fundamental description of the electromagnetic coupling between plasmonic nanoantennas and quantum emitters at the nanoscale.

Near-field depolarization of tip-enhanced Raman scattering by single azo-chromophores

The intrinsic symmetry and orientation of single molecules play a crucial role in enhanced optical spectroscopy and nanoscopic imaging. Unlike bulk materials, in which all molecular orientations are unavoidably averaged in the far-field, intensities of vibrational modes in tip-enhanced Raman scattering (TERS) depend greatly on the polarization direction of near-field light. It means that a near-field Raman "dichroism" becomes possible for anisotropic single molecules. Quantitative evaluation of the molecular orientation gets complicated by the depolarization of TERS intensities. Clearly, the depolarization effect is enhanced with an optical antenna and/or a substrate due to their anisotropic origin. In this study, we provide theoretical and experimental insights into Raman tensors of a single azobenzene chromophore, a Disperse Orange 3 (DO3) molecule, supported with a glass base. It is shown that the Raman intensities of the spectral bands corresponding to symmetric and antisymmetric vibrations of the DO3 molecule, for example, -NO₂ and -NH₂ moieties, behave differently on the nanoscale. In particular, three-dimensional far- and near-field Raman diagrams indicate that antisymmetric vibrations become highly depolarized, whereas symmetric vibrations remain unchangeable but intensities of their spectral bands are enhanced. Here, we introduce a near-field depolarization factor defined as a normalized discrepancy of longitudinal and transverse TERS signals. We believe that our first steps will ultimately lead to advanced facilities of TERS spectroscopy and nanoscopy, related to the orientation of anisotropic single molecules and their symmetries.

Strong Light-Matter Interactions in Single Open Plasmonic Nanocavities at the Quantum Optics Limit

Reaching the quantum optics limit of strong light-matter interactions between a single exciton and a plasmon mode is highly desirable, because it opens up possibilities to explore room-temperature quantum devices operating at the single-photon level. However, two challenges severely hinder the realization of this limit: the integration of single-exciton emitters with plasmonic nanostructures and making the coupling strength at the single-exciton level overcome the large damping of the plasmon mode. Here, we demonstrate that these two hindrances can be overcome by attaching individual J aggregates to single cuboid Au@Ag nanorods. In such hybrid nanosystems, both the ultrasmall mode volume of ∼71  nm^{3} and the ultrashort interaction distance of less than 0.9 nm make the coupling coefficient between a single J-aggregate exciton and the cuboid nanorod as high as ∼41.6  meV, enabling strong light-matter interactions to be achieved at the quantum optics limit in single open plasmonic nanocavities.

Sub-nanometre control of the coherent interaction between a single molecule and a plasmonic nanocavity

The coherent interaction between quantum emitters and photonic modes in cavities underlies many of the current strategies aiming at generating and controlling photonic quantum states. A plasmonic nanocavity provides a powerful solution for reducing the effective mode volumes down to nanometre scale, but spatial control at the atomic scale of the coupling with a single molecular emitter is challenging. Here we demonstrate sub-nanometre spatial control over the coherent coupling between a single molecule and a plasmonic nanocavity in close proximity by monitoring the evolution of Fano lineshapes and photonic Lamb shifts in tunnelling electron-induced luminescence spectra. The evolution of the Fano dips allows the determination of the effective interaction distance of ∼1 nm, coupling strengths reaching ∼15 meV and a giant self-interaction induced photonic Lamb shift of up to ∼3 meV. These results open new pathways to control quantum interference and field–matter interaction at the nanoscale.

Assessing the coupling between a plasmonic nanocavity and a single quantum emitter is challenging due to the lack of spatial control at the atomic scale. Here Zhang et al. achieve control with sub-nanometre precision and demonstrate the Fano resonance and Lamb shift at the single-molecule level.

Effect of Grazing Angle Cross-Ion Irradiation on Ag Thin Films

Apart from the spherical shape, control over other shapes is a technical challenge in synthesis approaches of nanostructures. Here, we studied the effect of grazing angle cross-irradiation Ag thin films for the nanostructures evolution from a top-down approach. Ag thin films of different thicknesses were deposited on Si (100) and glass substrates by electron beam evaporation system and subsequently irradiated at grazing angle ions by 80 keV Ar⁺ in two steps (to induce effectively a cross-ion irradiation). Pristine films exhibited dense and uniform distribution of Ag nanoparticles with their characteristic surface plasmon resonance-induced absorption peak around 420 nm. When the film surfaces were treated with cross-grazing angle irradiation of Ar ions with varying effective fluences from 0.5 × 10¹⁷ ions/cm² to 2.0 × 10¹⁷ ions/cm², it was found that fluence values governed the competition of sputtering and sputter re-deposition of Ag. As a result, lower irradiation fluence favoured the formation of cone-like nanostructures, whereas high fluence values demonstrated dominant sputtering. Fluence-dependent modification of surface features was studied through the Fourier transform infrared spectroscopy and the Rutherford backscattering spectroscopy. Theoretical justifications for the underlying mechanisms are presented to justify the experimental results.

Silver Nanoshell Plasmonically Controlled Emission of Semiconductor Quantum Dots in the Strong Coupling Regime

Strong coupling between semiconductor excitons and localized surface plasmons (LSPs) giving rise to hybridized plexciton states in which energy is coherently and reversibly exchanged between the components is vital, especially in the area of quantum information processing from fundamental and practical points of view. Here, in photoluminescence spectra, rather than from common extinction or reflection measurements, we report on the direct observation of Rabi splitting of approximately 160 meV as an indication of strong coupling between excited states of CdSe/ZnS quantum dots (QDs) and LSP modes of silver nanoshells under nonresonant nanosecond pulsed laser excitation at room temperature. The strong coupling manifests itself as an anticrossing-like behavior of the two newly formed polaritons when tuning the silver nanoshell plasmon energies across the exciton line of the QDs. Further analysis substantiates the essentiality of high pump energy and collective strong coupling of many QDs with the radiative dipole mode of the metallic nanoparticles for the realization of strong coupling. Our finding opens up interesting directions for the investigation of strong coupling between LSPs and excitons from the perspective of radiative recombination under easily accessible experimental conditions.

Can electrodynamic interaction between a molecule and metal dominate a continuum background in surface-enhanced Raman scattering?

A continuum background is always coincident with the Raman spectrum enhanced by metallic nanostructures and still remains elusive. Not only does it constitute a stymied mystery in the origin per se, but also it reduces the useful quantifiable range of detection based on surface-enhanced Raman scattering (SERS). We examined theoretically near-field molecule-metal interaction to reveal its contribution to the SERS background. The results show that the spectral broadening of fluorescence and Raman scattering due to a nearby metal object is insignificant compared with experimental findings. This study abnegates the role of near-field interaction in the SERS continuum background and elucidates the microscopic molecule-metal electromagnetic interaction, despite being unable to pinpoint the primary source of the SERS background.

Strong coupling between surface plasmon polaritons and emitters: a review

In this review we look at the concepts and state-of-the-art concerning the strong coupling of surface plasmon-polariton modes to states associated with quantum emitters such as excitons in J-aggregates, dye molecules and quantum dots. We explore the phenomenon of strong coupling with reference to a number of examples involving electromagnetic fields and matter. We then provide a concise description of the relevant background physics of surface plasmon polaritons. An extensive overview of the historical background and a detailed discussion of more recent relevant experimental advances concerning strong coupling between surface plasmon polaritons and quantum emitters is then presented. Three conceptual frameworks are then discussed and compared in depth: classical, semi-classical and fully quantum mechanical; these theoretical frameworks will have relevance to strong coupling beyond that involving surface plasmon polaritons. We conclude our review with a perspective on the future of this rapidly emerging field, one we are sure will grow to encompass more intriguing physics and will develop in scope to be of relevance to other areas of science.

Ultrastrong coupling of plasmons and excitons in a nanoshell

The strong coupling regime of hybrid plasmonic-molecular systems is a subject of great interest for its potential to control and engineer light-matter interactions at the nanoscale. Recently, the so-called ultrastrong coupling regime, which is achieved when the light-matter coupling rate reaches a considerable fraction of the emitter transition frequency, has been realized in semiconductor and superconducting systems and in organic molecules embedded in planar microcavities or coupled to surface plasmons. Here we explore the possibility to achieve this regime of light-matter interaction at nanoscale dimensions. We demonstrate by accurate scattering calculations that this regime can be reached in nanoshells constituted by a core of organic molecules surrounded by a silver or gold shell. These hybrid nanoparticles can be exploited for the design of all-optical ultrafast plasmonic nanocircuits and -devices.