Collective chiroptical activity through the interplay of excitonic and charge-transfer effects in localized plasmonic fields.

The collective light-matter interaction of chiral supramolecular aggregates or molecular ensembles with confined light fields remains a mystery beyond the current theoretical description. Here, we programmably and accurately build models of chiral plasmonic complexes, aiming to uncover the entangled effects of excitonic correlations, intra- and intermolecular charge transfer, and localized surface plasmon resonances. The intricate interplay of multiple chirality origins has proven to be strongly dependent on the site-specificity of chiral molecules on plasmonic nanoparticle surfaces spanning the nanometer to sub-nanometer scale. This dependence is manifested as a distinct circular dichroism response that varies in spectral asymmetry/splitting, signal intensity, and internal ratio of intensity. The inhomogeneity of the surface-localized plasmonic field is revealed to affect excitonic and charge-transfer mixed intermolecular couplings, which are inherent to chirality generation and amplification. Our findings contribute to the development of hybrid classical-quantum theoretical frameworks and the harnessing of spin-charge transport for emergent applications.

Symmetry-Breaking of Nanoparticle Surface Function Via Conformal DNA Design.

Asymmetric surface functionalization of complex nanoparticles to control their directional self-assembly remains a considerable challenge. Here, we demonstrated a conformal DNA design strategy for flexible remodeling of the surface of complex nanoparticles, taking Au nanobipyramids (AuNBPs) as a model. We sheathed one or both tips of AuNBPs into conformal DNA origami with an exceptionally accurate orientation control. Such asymmetrically and symmetrically distributed surface patches possess regioselective, sequence, and site-specific DNA binding capabilities. As a result, we realized a series of prototypical multicomponent "colloidal molecules" made of AuNBPs and Au nanospheres (AuNSs) with defined directionality and number of "bonding valence" as well as 1D and 3D hierarchical assemblies, e.g., inverse core-satellites of AuNBPs and AuNSs, side-by-side and tip-to-tip linear assemblies of AuNBPs, and 3D helical superstructures of AuNBPs with tunable twists. These findings inspire new opportunities for nanoparticle surface engineering and the high-order self-assembly of nanoarchitectures with higher complexity and broadened functionalities.

Enhancing Light Out-coupling in Perovskite Light-Emitting Diodes through Plasmonic Nanostructures.

Perovskite light-emitting diodes (PeLEDs) have emerged as promising candidates for lighting and display technologies owing to their high photoluminescence quantum efficiency and high carrier mobility. However, the performance of planar PeLEDs is limited by the out-coupling efficiency, predominantly governed by photonic losses at device interfaces. Most notably, the plasmonic loss at the metal electrode interfaces can account for up to 60% of the total loss. Here, we investigate the use of plasmonic nanostructures to improve the light out-coupling in PeLEDs. By integrating these nanostructures with PeLEDs, we have demonstrated an effectively reduced plasmonic loss and enhanced light out-coupling. As a result, the nanostructured PeLEDs exhibit an average 1.5-fold increase in external quantum efficiency and an ∼20-fold improvement in device lifetime. This finding offers a generic approach for enhancing light out-coupling, promising great potential to go beyond existing performance limitations.

Quantum plasmonics pushes chiral sensing limit to single molecules: a paradigm for chiral biodetections.

Chiral sensing of single molecules is vital for the understanding of chirality and their applications in biomedicine. However, current technologies face severe limitations in achieving single-molecule sensitivity. Here we overcome these limitations by designing a tunable chiral supramolecular plasmonic system made of helical oligoamide sequences (OS) and nanoparticle-on-mirror (NPoM) resonator, which works across the classical and quantum regimes. Our design enhances the chiral sensitivity in the quantum tunnelling regime despite of the reduced local E-field, which is due to the strong Coulomb interactions between the chiral OSs and the achiral NPoMs and the additional enhancement from tunnelling electrons. A minimum of four molecules per single-Au particle can be detected, which allows for the detection of an enantiomeric excess within a monolayer, manifesting great potential for the chiral sensing of single molecules.

Giant Out-of-Plane Exciton Emission Enhancement in Two-Dimensional Indium Selenide via a Plasmonic Nanocavity.

Out-of-plane (OP) exciton-based emitters in two-dimensional semiconductor materials are attractive candidates for novel photonic applications, such as radially polarized sources, integrated photonic chips, and quantum communications. However, their low quantum efficiency resulting from forbidden transitions limits their practicality. In this work, we achieve a giant enhancement of up to 34000 for OP exciton emission in indium selenide (InSe) via a designed Ag nanocube-over-Au film plasmonic nanocavity. The large photoluminescence enhancement factor (PLEF) is attributed to the induced OP local electric field (Ez) within the nanocavity, which facilitates effective OP exciton-plasmon interaction and subsequent tremendous enhancement. Moreover, the nanoantenna effect resulting from the effective interaction improves the directivity of spontaneous radiation. Our results not only reveal an effective photoluminescence enhancement approach for OP excitons but also present an avenue for designing on-chip photonic devices with an OP dipole orientation.

How to Obtain the Correct Rabi Splitting in a Subwavelength Interacting System.

We unambiguously extract the individual decay channels of a coupled plasmon-exciton system by using correlated single-particle absorption and scattering measurements. A remarkable difference in the two channels is present─clear Rabi splitting in the plasmon channel but no Rabi splitting in the exciton channel. Discordance in the absorption and scattering spectra are mainly originated from the distinct contributions of plasmon and exciton channels in the absorption and scattering process. Our findings provide insights into plasmon-exciton interaction in an open cavity and can impact the design of plexcitonic devices for ultrafast nonlinear nanophotonics.

Multiband tunable exciton-induced transparencies: Exploiting both strong and intermediate coupling in a nanocube-hexagonal-nanoplate heterodimer J-aggregates hybrid.

Understanding and mastering the light-light and light-matter interactions in coupled structures have become significant subjects, as they provide versatile tools for manipulating light in both classical and quantum regimes. Mimicking quantum interference effects in pure photonic nanostructures, from weak Fano dip to intense electromagnetically induced transparency, usually requires strong asymmetries in complex geometries and larger interactions between resonances, i.e., in the intermediate coupling regime. Here, we numerically demonstrate a simple and chemically feasible plasmonic nanocube-hexagonal-nanoplate heterodimer with a strong, tunable self-induced transparency window created by the intermediate coupling between the near-degenerate dark and bright hybridized modes. Further assisted by the strong coupling introduced by the J-aggregate excitons covering the heterodimer, three evident exciton-induced transparency windows were observed. These multiband transparencies in a single-particle-level subwavelength configuration, could on one hand enrich the toolbox of multi-frequency light filtering, slowing and switching beyond the diffraction limit, and on the other hand, work as a fundamental testbed for investigating multiscale light-matter interactions at the nanoscale.

Unraveling the Complex Chirality Evolution in DNA-Assembled High-Order, Hybrid Chiroplasmonic Superstructures from Multi-Scale Chirality Mechanisms.

Hierarchical, chiral hybrid superstructures of chromophores and nanoparticles are expected to give rise to intriguing unveiled chiroptical responses originating from the complex chiral interactions among the components. Herein, DNA origami cavity that could self-assemble into one-dimensional (1D) DNA tubes was employed as a scaffold to accurately organize metal nanoparticles and chromophores. The chiral interactions were studied at the level of individual hybrid particles and their 1D hybrid superstructures. Complex chirality mechanisms involving global structural chirality, plasmon-induced circular dichroism (PICD) and exciton-coupled circular dichroism (ECCD) were disentangled. The multiplexed CD spectrum superposition revealed the chirality evolution at different length scales. These results can offer a model for boosting the theoretical understanding of classical-quantum hybrid systems, and would inspire the future design of optically-active substances across length scales.

Phononic Cavity Optomechanics of Atomically Thin Crystal in Plasmonic Nanocavity.

In the picture of molecular cavity optomechanics, surface-enhanced Raman scattering (SERS) can be understood as molecular oscillators parametrically coupled to plasmonic nanocavities supporting an extremely localized optical field. This enables SERS from conventional fingerprint detection toward quantum nanotechnologies associated with, e.g., frequency upconversion and optomechanically induced transparency. Here, we study a phononic cavity optomechanical system consisting of a monolayer MoS2 placed inside a plasmonic nanogap, where the coherent phonon-plasmon interaction involves the collective oscillation from tens of thousands of unit cells of the MoS2 crystal. We observe the selective nonlinear SERS enhancement of the system as determined by the laser-plasmon detuning, suggesting the dynamic backaction modification of the phonon populations. Anomalous superlinear power dependence of a second-order Raman-inactive phonon mode with respect to the first-order phonons is also observed, indicating the distinctive properties of the phononic nanodevice compared with the molecular system. Our results promote the development of robust phononic optomechanical nanocavities to further explore the related quantum correlation and nonlinear effects including parametric instabilities.

Plasmonic nanobar-on-mirror antenna with giant local chirality: a new platform for ultrafast chiral single-photon emission.

Providing an additional degree of freedom for binary information encoding and nonreciprocal information transmission, chiral single photons have become a new research frontier in quantum optics. Without using complex external conditions (e.g., magnetic field, low temperature), coupling emitters to chiral optical antennas has become a promising strategy to efficiently convert single photons from linear to circular polarization states. For ideal chiral single-photon sources, essential properties such as giant Purcell factor, large degree of circular polarization (DCP), and high collection efficiency are highly demanded. Herein, to meet these combined requirements, we propose an emitter-coupled nanobar-on-mirror antenna platform with significant local chirality acquired from the broken symmetry, as well as the giant Purcell factor owing to its ultrasmall mode volume. An emitter embedded at the corner in the gap exhibits above 3 orders of magnitude enhancement of the chiral spontaneous emission with more than 80% collection efficiency, along with up to 70% DCP. Compatible with a myriad of nanoscale quantum emitters (e.g. transition metal dichalcogenides, color centers, quantum dots, etc.), this platform, not only manifests the potential for realizing ultrafast chiral single-photon generator towards GHz and THz operation speed but also provides versatile testbeds for investigating chiral light-matter interaction at the single-quantum level.

Continuous-wave frequency upconversion with a molecular optomechanical nanocavity.

Coherent upconversion of terahertz and mid-infrared signals into visible light opens new horizons for spectroscopy, imaging, and sensing but represents a challenge for conventional nonlinear optics. Here, we used a plasmonic nanocavity hosting a few hundred molecules to demonstrate optomechanical transduction of submicrowatt continuous-wave signals from the mid-infrared (32 terahertz) onto the visible domain at ambient conditions. The incoming field resonantly drives a collective molecular vibration, which imprints a coherent modulation on a visible pump laser and results in upconverted Raman sidebands with subnatural linewidth. Our dual-band nanocavity offers an estimated 13 orders of magnitude enhancement in upconversion efficiency per molecule. Our results demonstrate that molecular cavity optomechanics is a flexible paradigm for frequency conversion leveraging tailorable molecular and plasmonic properties.

Plasmon-Directed On-Wire Growth of Branched Silver Nanowires with Chiroptic Activity.

Silver nanowires (Ag NWs) present prominent waveguiding properties of subwavelength light due to their nanoconfinement with propagating surface plasmons, which is of great importance for on-chip integration of nanophotonic devices and optical computation. Such propagating plasmons also exert plasmonic forces, which can be utilized to manipulate nanoparticles (NPs) beyond the diffraction limit. However, such controllability is spatially limited to the near fields, whereas a large portion of uncontrolled particles are randomly deposited on the chips, which could be detrimental to the integrated optical devices. Herein we shine continuous wave laser at one end of the Ag NW immersed in AgNO3 solution to launch the propagating surface plasmons. The laser irradiation also induces the photoreduction of Ag+ ions to locally generate tiny Ag NPs, which evolve into large Ag flake branches closer to the other end of the Ag NW. Such a peculiar growth is due to the synergistic effect of plasmonic forces and the thermophoretic/thermo-osmosis forces induced by temperature gradient. These branched Ag NWs with sharp angles are intrinsically chiral, which can be partially controlled by changing the irradiation location, forming plasmonic chiral enantiomers. The circular differential scattering (CDS) response of these branched Ag NWs can be as large as 40%, which can be used for chiral enantiomer sensing with spectral dissymmetric factor up to 4 nm induced by phenylalanine. This plasmon-directed on-wire growth not only offers a facile approach for generating plasmonic chiral nanostructures with remote controllability, but also provides significant insights on the synergistic effect of plasmonic forces and thermal-induced forces, which has great implications for self-assembly and integration of on-chip optics.

Controllable Plexcitonic Coupling in a WS2-Ag Nanocavity with Solvents.

Strong coupling between emitters and cavities underlies many of the current strategies aiming at generating and controlling quantum states at room temperature. Recent experiments reveal strong coupling between two-dimensional transition metal dichalcogenides (TMDCs) and individual plasmonic structures; however, the coupling strength is quite limited (<200 meV), and the active control of the coupling strength is challenging. Here, we demonstrate the active tuning of plexcitonic coupling in monolayer WS2 coupled to a plasmonic nanocavity by immersing into a mixed solution of dichloromethane (DCM) and ethanol. By adjusting the mixture ratio, continuous tuning of the Rabi splitting energy ranged from 183 meV (in ethanol) to 273 meV (in DCM) is achieved. The results are mainly attributed to the remarkable increase of the neutral exciton density in monolayer WS2 as the concentration of DCM is increased. It offers an important stepping stone toward a further study on plexcitonic coupling in layered materials, along with potential applications in quantum information processing and nonlinear optical materials.

Unified treatment of scattering, absorption, and luminescence spectra from a plasmon-exciton hybrid by temporal coupled-mode theory.

Plasmonic nanostructures are capable of tailoring the emission of a nearby emitter by increasing (or reducing) the brightness, shortening (or prolonging) the lifetime, and shaping the spectrum. Experimental characterization of such coupled plasmon-exciton (plexciton) systems usually relies on the acquisition and comparison of scattering, absorption, or luminescence spectra. However, theoretical accounts of these optical spectra, which are key to distinguishing between the coupling regimes and to standardizing the coupling criteria, often scatters in different frameworks, varying from classical to quantum-mechanical. Therefore, developing a unified and simple formalism that can simultaneously compare all these spectral signatures in different coupling regimes is nontrivial. Here, we use a temporal coupled-mode formalism to reproduce the scattering, absorption, and luminescence spectra of a plexciton system and find that its luminescence reaches a maximum at a critical coupling point, featuring a light-emitting plexciton with intense brightness and ultrafast lifetime. This simple approach provides a unified and phenomenological treatment of these spectra by simply including or excluding an external driving term. It therefore allows for a direct comparison of different spectroscopic signatures from the plexciton system and provides an easy-to-use guidance for the design of broadband light-emitting devices.

Intrinsic luminescence blinking from plasmonic nanojunctions.

Plasmonic nanojunctions, consisting of adjacent metal structures with nanometre gaps, can support localised plasmon resonances that boost light matter interactions and concentrate electromagnetic fields at the nanoscale. In this regime, the optical response of the system is governed by poorly understood dynamical phenomena at the frontier between the bulk, molecular and atomic scales. Here, we report ubiquitous spectral fluctuations in the intrinsic light emission from photo-excited gold nanojunctions, which we attribute to the light-induced formation of domain boundaries and quantum-confined emitters inside the noble metal. Our data suggest that photoexcited carriers and gold adatom - molecule interactions play key roles in triggering luminescence blinking. Surprisingly, this internal restructuring of the metal has no measurable impact on the Raman signal and scattering spectrum of the plasmonic cavity. Our findings demonstrate that metal luminescence offers a valuable proxy to investigate atomic fluctuations in plasmonic cavities, complementary to other optical and electrical techniques.

Strong plasmon-exciton coupling in transition metal dichalcogenides and plasmonic nanostructures.

Achieving strong coupling between emitters and cavity photons holds an important position in the light-matter interaction due to its applications such as polariton lasing, all-optical switches, and quantum information processing. However, room-temperature polaritonic devices with subwavelength dimensions based on strong light-matter coupling are difficult to realize using traditional emitter-cavity coupled systems. In recent years, coupled systems constructed from plasmonic nanostructures and transition metal dichalcogenides (TMDs) have shown their potential in achieving room-temperature strong coupling and robustness in the nanofabrication processes. This minireview presents the recent progress in strong plasmon-exciton coupling in such plasmonic-TMD hybrid structures. Differing from a broader scope of strong coupling, we focus on the plasmon-exciton coupling between excitons in TMDs and plasmons in single nanoparticles, nanoparticle-over-mirrors, and plasmonic arrays. In addition, we discuss the future perspectives on the strong plasmon-exciton coupling at few-excitons level and the nonlinear response of these hybrid structures in the strong coupling regime.

Controlling the immobilization process of an optically enhanced protein microarray for highly reproducible immunoassay.

By virtue of its high throughput multiplex detection capability, superior read-out sensitivity, and tiny analyte consumption, an optically enhanced protein microarray assay has been developed as a promising diagnostic tool for various applications, ranging from the field of pharmacology to diagnostics. However, so far, the development of an optically enhanced protein microarray (OEPM) toward widespread commercial availability is mainly hampered by insufficient detection reproducibility. Here, we develop an OEPM platform with an order of magnitude optical enhancement induced by the interference effect. High assay reproducibility of the OEPM is achieved by optimizing the protein immobilization schemes, linking to the surface energy of the substrate, surfactant-tuned wetting ability, and the washing and drying dynamics. As a result, smearing-free and uniform spot arrays with a coefficient of variation less than 7% can be achieved. Furthermore, we demonstrate the assay performance of the OEPM by detecting five biomarkers, showing an order of magnitude higher sensitivity, many-fold higher throughput, and 10 times less analyte consumption than those of the commercial enzyme-linked immunosorbent assay kits. Our results provide new insight for improving the reproducibility of OEPMs toward practical and commercial diagnostic assays.

Selectively Depopulating Valley-Polarized Excitons in Monolayer MoS2 by Local Chirality in Single Plasmonic Nanocavity.

Transition metal dichalcogenides, whose valley degrees of freedom are characterized by the degree of circular polarization (DCP) of the photoluminescence, draw broad interests due to their potential applications in information storage and processing. However, this DCP is usually low at room temperature due to the phonon-assisted intervalley scattering, severely degrading the fidelity of the valley-stored signals. Therefore, achieving high DCP at room temperature is vital for valley-encoded nanophotonic devices. In this work, we demonstrate a high DCP of 48.7% at room temperature by embedding monolayer MoS2 into a compact plasmonic nanocavity. Such a high DCP is proven to originate from the prominent chiral Purcell effect owing to the degeneracy-lifted circularly polarized local density of states in the nanocavity. In addition, the DCP can be further manipulated by an in situ plasmon-scanned technique. This highly compact system provides possibilities for developing versatile valley-encoded light-emitting devices at room temperature.

Duplicating Plasmonic Hotspots by Matched Nanoantenna Pairs for Remote Nanogap Enhanced Spectroscopy.

Plasmonic nanoantennas are capable of reversibly interconverting free-space radiation with localized modes at the nanoscale. However, optical access to a single nanoantenna, through a laser beam, is always accompanied by disruptive background perturbations and heating effects. Remote spectroscopy is one promising route to overcome these effects. Here, we demonstrate excitation-collection-separated enhanced spectroscopy using a matched nanoantenna pair. The receiving and transmitting antennas are geometrically separated but bridged by the propagating surface plasmon polaritons (SPPs) on the metal film. The receiving antenna, consisting of a silver nanowire on a mirror, ensures a high light-to-plasmon conversion efficiency. The transmitting antenna consists of a silver nanocube over a mirror and is impedance matched to free space photons and the propagating SPPs. As a proof-of-principle, we demonstrate remote surface-enhanced Raman scattering with a high signal-to-noise ratio. This matched nanoantenna pair may have applications for remote entanglement of quantum emitters, biochemistry detection, or optical interconnects.

Monolithic Metal Dimer-on-Film Structure: New Plasmonic Properties Introduced by the Underlying Metal.

Dimers, two closely spaced metallic nanostructures, are one of the primary nanoscale geometries in plasmonics, supporting high local field enhancements in their interparticle junction under excitation of their hybridized "bonding" plasmon. However, when a dimer is fabricated on a metallic substrate, its characteristics are changed profoundly. Here we examine the properties of a Au dimer on a Au substrate. This structure supports a bright "bonding" dimer plasmon, screened by the metal, and a lower energy magnetic charge transfer plasmon. Changing the dielectric environment of the dimer-on-film structure reveals a broad family of higher-order hybrid plasmons in the visible region of the spectrum. Both of the localized surface plasmons resonances (LSPR) of the individual dimer-on-film structures as well as their collective surface lattice resonances (SLR) show a highly sensitive refractive index sensing response. Implementation of such all-metal magnetic-resonant nanostructures offers a promising route to achieve higher-performance LSPR- and SLR-based plasmonic sensors.