Plasmonic Nanocavity Research Articles

Plasmonic nanocavity-modulated electrochemiluminescence sensor for gastric cancer exosomal miRNA detection

Plasmonic nanocavity possessing highly light field confinement and electromagnetic field enhancement can concentrate and enhance the luminescence signal. The plasmonic nanocavity has the great potential value in biosensing research and improve analytical sensitivity. In this work, we constructed a plasmonic nanocavity between circular Au nanoplate-film and spherical Au nanoparticle with tetrahedral DNA nanostructures. The nanocavity structure can regulate the local density of optical states and provide the field restriction to enhance the spontaneous ECL radiation of PEDOT-S dots. Additionally, Au nanoparticle acted as nanoantenna which localized and modulated ECL to directional emission. Because the plasmonic nanocavity effectively collected and redistributed ECL signal, the emission was enhanced by 5.9 times with polarized characteristics. The proposed plasmonic nanocavity-based ECL sensor was further used to detect exosomal miRNA-223-3p in ascites. The detection results indicated the novel sensing strategy can assist early diagnosis of peritoneal metastasis of gastric cancer.

The Mechanism and Fine-Tuning of Chiral Plexcitons in the Strong Coupling Regime.

Chiral plexcitons, produced by the strong interaction between plasmonic nanocavities and chiral molecules, can provide a promising direction for controlling chiroptical responses on the nanoscale. Here, we reveal the chiral origin and electromagnetic hybridization process in chiral strongly coupled systems. The mechanism and unique advantages of chiral plexcitons for fine-tuning circular dichroism (CD) responses are demonstrated, providing a rule for controlling chiral light-matter interactions in complex chiral nanosystems. Furthermore, we experimentally demonstrate the fine-tuning of chiral plexcitons in hybrid systems consisting of plasmonic nanoparticles and chiral J-aggregates. Continuous and precise tuning of the CD resonance positions was successfully achieved in a given structure. Compared with the previous work, the CD spectral tuning accuracy has been improved by an order of magnitude, which can reach the level of 1 nm. Our findings provide a feasible strategy and theoretical basis for accurately controlling chirality in multiple dimensions.

Multi-faceted plasmonic nanocavities

Abstract Plasmonic nanocavities form very robust sub-nanometer gaps between nanometallic structures and confine light within deep subwavelength volumes to enable unprecedented control of light–matter interactions. However, spherical nanoparticles acquire various polyhedral shapes during their synthesis, which has a significant impact in controlling many light–matter interactions, such as photocatalytic reactions. Here, we focus on nanoparticle-on-mirror nanocavities built from three polyhedral nanoparticles (cuboctahedron, rhombicuboctahedron, decahedron) that commonly occur during the synthesis. Their photonic modes have a very intricate and rich optical behaviour, both in the near- and far-field. Through a recombination technique, we obtain the total far-field produced by a molecule placed within these nanocavities, to reveal how energy couples in and out of the system. This work paves the way towards understanding and controlling light–matter interactions, such as photocatalytic reactions and non-linear vibrational pumping, in such extreme environments.

Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.

Revealing the ultrafast spontaneous emission in plasmon-enhanced monolayer semiconductor nano-light sources

Nanoscale spontaneous light sources are promising alternatives to lasers for high-speed optical communications and interconnections through energy-efficient integrated circuits. Yet, developing the spontaneous light sources faster than lasers is hampered by the detection means (e.g., time-resolved fluorescence spectroscopy). Here, by coupling monolayer WSe2 to individual plasmonic nanocavities, we achieved an efficient spontaneous light source with potential ultrafast modulation bandwidth and superior brightness. The ultrafast radiative decay rates can be determined and derived solely from the experimental parameters by combining the coupling strength and the photoluminescence enhancement in a single nanocavity-WSe2 hybrid. As a result, the hybrid light source has a radiative lifetime down to 350 fs, indicating a potential modulation bandwidth up to 440 GHz, which is 10 times of the traditional semiconductor lasers. Furthermore, the quantum yield is enhanced by a factor of over 300-folds up to 20.8% through making full use of the highly confined nanocavity mode. The nanocavity-WSe2 hybrid we built provides a promising approach for constructing high-speed light-emitting devices.

Disordered plasmonic nanocavity enhanced quantum dot emission

In this paper, a large-scale compatible plasmonic nanocavity design platform is utilized to achieve a nearly order of magnitude photoluminescence (PL) enhancement. The proposed design is made of multi-sized/multi-spacing gold (Au) nanounits that are uniformly wrapped with a thin aluminum oxide (Al2O3) layer, as a foreign host to form a metal–insulator–semiconductor cavity, as they are coated with semiconductor quantum dots (QDs). Our numerical and experimental data demonstrate that, in an optimal insulator layer thickness, the simultaneous formation of broadband Fabry–Perot resonances and plasmonic hot spots leads to enhanced light absorption within the QD unit. This improvement in absorption response leads to the PL enhancement of QDs. This work demonstrates the potential and effectiveness of a random plasmonic nanocavities host in the realization of lithography-free efficient emitters.

Single-molecule mid-infrared spectroscopy and detection through vibrationally assisted luminescence

Room-temperature detection of molecular vibrations in the mid-infrared (MIR, λ = 3–30 µm) has numerous applications, including real-time gas sensing, medical imaging and quantum communication. However, existing technologies rely on cooled semiconductor detectors because of thermal noise limitations. One way to overcome this challenge is to upconvert the low-energy MIR photons into high-energy visible wavelengths (λ = 500–800 nm) where detection of single photons is easily achieved using silicon technologies. This process suffers from weak cross-sections and the MIR-to-visible wavelength mismatch, limiting its efficiency. Here we exploit molecular emitters possessing both MIR and visible transitions from molecular vibrations and electronic states, coupled through Franck–Condon factors. By assembling molecules into a plasmonic nanocavity resonant at both MIR and visible wavelengths, and optically pumping them below the electronic absorption band, we show transduction of MIR light. The upconverted signal is observed as enhanced visible luminescence. Combining Purcell-enhanced visible luminescence with enhanced rates of vibrational pumping gives transduction efficiencies of >10%. MIR frequency-dependent upconversion gives the vibrational signatures of molecules assembled in the nanocavity. Transient picocavity formation further confines MIR light down to the single-molecule level. This allows us to demonstrate single-molecule MIR detection and spectroscopy that is inaccessible to any previous detector.

Molecule–plasmon–photon hybridization and applications

We study a potential hybrid quantum system with a plasmonic nanocavity coupled to a vibrating mode of a single molecule and another optical cavity mode. To explore some important and valuable applications in quantum physics, we discuss and evaluate several different applications with respect to the plasmon-mediated quantum interface, the plasmon-assisted engineering of two-mode continuous-variable entanglement, and pursuing an indirect and ultrastrong molecule–photon cooperativity. In addition, governed by the relation of symmetry breaking and quantum phase transitions (QPTs),single-molecule-induced QPTs are also studied in this tripartite hybrid quantum system. This theoretical study strongly supports potential applications of this hybrid system in the field of quantum information processing. It is believed that our investigation of molecule–plasmon–photon hybridization can not only open a new avenue toward quantum manipulation, but also provide a fresh and reliable platform to carry out many applications with high efficiency.

Terahertz phononic crystal in plasmonic nanocavity

Interaction between photons and phonons in cavity optomechanical systems provides a new toolbox for quantum information technologies. A GaAs/AlAs pillar multi-optical mode microcavity optomechanical structure can obtain phonons with ultra-high frequency (~THz). However, the optical field cannot be effectively restricted when the diameter of the GaAs/AlAs pillar microcavity decreases below the diffraction limit of light. Here, we design a system that combines Ag nanocavity with GaAs/AlAs phononic superlattices, where phonons with the frequency of 4.2 THz can be confined in a pillar with ~4 nm diameter. The Q c/V reaches 0.22 nm−3, which is ~80 times that of the photonic crystal (PhC) nanobeam and ~100 times that of the hybrid point-defect PhC bowtie plasmonic nanocavity, where Q c is optical quality factor and V is mode volume. The optomechanical single-photon coupling strength can reach 12 MHz, which is an order of magnitude larger than that of the PhC nanobeam. In addition, the mechanical zero-point fluctuation amplitude is 85 fm and the efficient mass is 0.27 zg, which is much smaller than the PhC nanobeam. The phononic superlattice-Ag nanocavity optomechanical devices hold great potential for applications in the field of integrated quantum optomechanics, quantum information, and terahertz-light transducer.

Fine-tuning biexcitons-plasmon coherent states in a single nanocavity

Abstract A tunable plexcitonic material that sustains multimode hybridization is highly desirable, which is vital for advanced quantum devices. However, the research about regulations of biexcitons-plasmon coherent states has rarely been reported. Here we apply single-nanoparticle scattering spectroscopy correlative with SEM imaging to identify biexcitons-plasmon interaction in a metal-semiconductor hybrid structure composed of a single Au@Ag nanoparticle, J-aggregates molecules and tungsten disulfide (WS2) monolayer. The mode competition within the localized plasmonic hotspots (∼240 nm3) is revealed by continuously regulating the J-aggregates spacer. Two distinct anticrossings are observed at both excitons resonances, and large double Rabi splittings (137 meV and 124 meV) are obtained successfully. We establish experimentally that J-aggregates and WS2 monolayer are responsible for the middle polariton states, while plasmon rarely contributes. Further calculations show that plasmonic nanocavity enables coherent energy exchange with different excitons by providing a highly enhanced localized E-field. In addition, we find that the multimode coupling strengths can be efficiently tuned by changing the cavity morphology and environment temperature, where the tuning spectral accuracy can reach up to 1 nm. Our findings uncover the distinctive properties of biexcitons-plasmon polaritons, suggest an easily obtainable multiqubit states platform, and open up a new way to construct nanoscale photonic devices.

Amplified Plasmonic Forces from DNA Origami-Scaffolded Single Dyes in Nanogaps.

Developing highly enhanced plasmonic nanocavities allows direct observation of light-matter interactions at the nanoscale. With DNA origami, the ability to precisely nanoposition single-quantum emitters in ultranarrow plasmonic gaps enables detailed study of their modified light emission. By developing protocols for creating nanoparticle-on-mirror constructs in which DNA nanostructures act as reliable and customizable spacers for nanoparticle binding, we reveal that the simple picture of Purcell-enhanced molecular dye emission is misleading. Instead, we show that the enhanced dipolar dye polarizability greatly amplifies optical forces acting on the facet Au atoms, leading to their rapid destabilization. Using different dyes, we find that emission spectra are dominated by inelastic (Raman) scattering from molecules and metals, instead of fluorescence, with molecular bleaching also not evident despite the large structural rearrangements. This implies that the competition between recombination pathways demands a rethink of routes to quantum optics using plasmonics.

Optical Spectra of Plasmon–Exciton Core–Shell Nanoparticles: A Heuristic Quantum Approach

Light–matter coupling in plasmonic nanocavities has been widely studied in the past years. Yet, for core–shell particles, popular electromagnetic models that use the classical Lorentz oscillator to describe the shell predict extinction spectra with three maxima, if the plasmon and the shell absorption are in resonance. In contrast, experiments exhibit only two peaks, as also expected from simple quantum models of hybrid states. In order to reconcile the convenient and widely used classical electromagnetic description with experimental data, we connect it to the quantum world by conceiving a heuristic quantum model. Our model is based on the permittivity of a two-level system in a classical electric field derived from the optical Bloch equations. The light–matter coupling is included via the collective vacuum Rabi frequency Ω0. Using our model, we obtain excellent agreement with a series of experimental extinction spectra of particles with various coupling strengths due to a systematic size variation. The suppression of the third maximum, which mainly stems from the absorption in the shell, can be interpreted as a vacuum induced power broadening, which may occur in lossy (plasmonic) cavities below the strong-coupling regime.

Plasmonic Nanodiamonds.

While nitrogen-vacancy (NV) centers in diamonds have emerged as promising solid-state quantum emitters for sensing applications, the tantalizing possibility of coupling them with photonic or broadband plasmonic nanostructures to create ultrasensitive biolabels has not been fully realized. Indeed, it remains technologically challenging to create free-standing hybrid diamond-based imaging nanoprobes with enhanced brightness and high temporal resolution. Herein, we leverage the bottom-up DNA self-assembly to develop hybrid free-standing plasmonic nanodiamonds, which feature a closed plasmonic nanocavity completely encapsulating a single nanodiamond. Correlated single nanoparticle spectroscopical characterizations suggest that the plasmonic nanodiamond displays dramatically and simultaneously enhanced brightness and emission rate. We believe that they hold huge potential to serve as a stable solid-state single-photon source and could serve as a versatile platform to study nontrivial quantum effects in biological systems with enhanced spatial and temporal resolution.

Highly polarized light emission from hybrid of quantum dots and plasmons in the intermediate coupling regime.

Colloidal semiconductor quantum dots (QDs), with a size tunable bandgap and remarkably high quantum efficiency, have been recognized as ideal light sources in quantum information and light emitting devices. For light sources, besides the emission intensity and spectral profile, the degree of polarization (DoP) is an essential parameter. Here, by embedding a monolayer of QDs inside the nanogap between a bottom Au mirror and a top Ag nanowire, we have demonstrated highly polarized light emission from the QDs with an average DoP of 0.89. In addition to the anisotropic photoluminescence (PL) intensity, the PL spectra are distinct at different polarizations, with an asymmetric spectral shape or even two-peak features. Such an anisotropic emission behavior arises from the coupling between the QDs and the largely confined and polarization-dependent gap-plasmons in the Au/QD/Ag nanocavities in the intermediate coupling regime. Our results demonstrate the possibility of achieving highly polarized light sources by coupling spherical QDs to single anisotropic plasmonic nanocavities, to provide new opportunities in the future design of polarized QD-based display devices.

Molecular Optomechanics Induced Hybrid Properties in Soft Materials Filled Plasmonic Nanocavities.

The optomechanical interaction between nanocavity plasmons and molecular vibrations can result in interfacial phenomena that can be tailored for sensing and photocatalytic applications. Here, we report for the first time that plasmon-vibration interaction can induce laser-plasmon detuning dependent plasmon resonance linewidth broadening, indicating energy transfer from the plasmon field to collective vibrational modes. The linewidth broadening accompanied by the large enhancement of the Raman scattering signal is observed as the laser-plasmon blue-detuning approaches the CH vibrational frequency of the molecular systems integrated in gold nanorod-on-mirror nanocavities. The experimental observations can be explained based on the molecular optomechanics theory that predicts dynamical backaction amplification of the vibrational modes and high sensitivity of Raman scattering when the plasmon resonance overlaps with the Raman emission frequency. The results presented here suggest that molecular optomechanics coupling may be manipulated for creating hybrid properties based on interactions between molecular oscillators and nanocavity electromagnetic optical modes.

Effective Single-Mode Methodology for Strongly Coupled Multimode Molecular-Plasmon Nanosystems.

Strong coupling between molecules and quantized fields has emerged as an effective methodology to engineer molecular properties. New hybrid states are formed when molecules interact with quantized fields. Since the properties of these states can be modulated by fine-tuning the field features, an exciting and new side of chemistry can be explored. In particular, significant modifications of the molecular properties can be achieved in plasmonic nanocavities, where the field quantization volume is reduced to subnanometric volumes, thus leading to intriguing applications such as single-molecule imaging and high-resolution spectroscopy. In this work, we focus on phenomena where the simultaneous effects of multiple plasmonic modes are critical. We propose a theoretical methodology to account for many plasmonic modes simultaneously while retaining computational feasibility. Our approach is conceptually simple and allows us to accurately account for the multimode effects and rationalize the nature of the interaction between multiple plasmonic excitations and molecules.

Extraordinary upconversion enhancement in mono-dispersed plasmonic resonant nanocavity coupling with lanthanide doped upconversion nanoparticles

In this work, we reported a monodispersed MDM (metal–dielectric-metal) type upconversion composite material with ultrahigh emission enhancement for the first time. Different from the planar configuration of traditional MDM systems, the proposed materials here consisting of silver nanoparticles as the metallic core, upconversion nanoparticles(UCNPs) as the intermediate dielectric layer, and silver layer as the metallic shell. The structural characteristics were investigated to explore the spectral band match between the plasmonics and UCNPs. The sandwich structure can create high density hot carriers at the nearfield region around the UCNPs, leading to the remarkable excitation enhancement. According to the proper structure design, the upconversion intensity can be enhanced by over five orders of magnitude. The outer metallic layer thickness, the diameter of attached UCNPs and the ambient refractive index were found responsible for the intensity enhancement. Such upconversion materials combined the merit of monodispersity and the high emission efficiency, which can improve their performance in bioimaging and photovoltaic device.

Enhanced Strong Coupling in the Hybrid Dielectric-Metallic Nanoresonator and WS2 Monolayer

Exciton polaritons in metallic nanocavities and transition-metal dichalcogenide monolayers has led to striking discoveries, ranging from Bose-Einstein condensation to slowing light. Although plasmonic nanocavities offer small mode volumes, the intrinsic losses of plasmonic nanocavities remain an open challenge in exciton-polariton devices. Consequently, dielectric nanocavities are used as an alternative candidate due to their low intrinsic losses. However, large mode volumes are a central bottleneck in dielectric nanocavities. Here, we theoretically propose to use a hybrid dielectric-metallic nanocavity to enhance light-matter interactions between the excitons of the ${\mathrm{WS}}_{2}$ monolayer and the hybrid nanocavity. Such a hybrid nanoresonator inherits the advantages of both dielectric and metallic nanocavities, including ultrasmall mode volume, ultralow losses, and reasonably-high-Q factor. It is demonstrated that the thickness and material of the central gap film together with the thickness of the metallic substrate play vital roles in governing the coupling strength between 1L-${\mathrm{WS}}_{2}$ excitons and the cavity. After optimizing the geometry and material parameters, the Rabi splitting is increased to 113 meV, almost twice that in dielectric metasystems. The significant improvement can be attributed to the greatly enhanced near field and the ultrasmall mode volume. Furthermore, we show that Rabi splitting can be further boosted to 151 meV by increasing the number of layers of ${\mathrm{WS}}_{2}$ and h-$\mathrm{BN}$ film in the nanocavity.

Dual-Modal Nanoplasmonic Light Upconversion through Anti-Stokes Photoluminescence and Second-Harmonic Generation from Broadband Multiresonant Metal Nanocavities.

Metal nanocavities can generate plasmon-enhanced light upconversion signals under ultrashort pulse excitations through anti-Stokes photoluminescence (ASPL) or nonlinear harmonic generation processes, offering various applications in bioimaging, sensing, interfacial science, nanothermometry, and integrated photonics. However, achieving broadband multiresonant enhancement of both ASPL and harmonic generation processes within the same metal nanocavities remains challenging, impeding applications based on dual-modal or wavelength-multiplexed operations. Here, we report a combined experimental and theoretical study on dual-modal plasmon-enhanced light upconversion through both ASPL and second-harmonic generation (SHG) from broadband multiresonant metal nanocavities in two-tier Ag/SiO2/Ag nanolaminate plasmonic crystals (NLPCs) that can support multiple hybridized plasmons with high spatial mode overlaps. Our measurements reveal the distinctions and correlations between the plasmon-enhanced ASPL and SHG processes under different modal and ultrashort pulsed laser excitation conditions, including incident fluence, wavelength, and polarization. To analyze the observed effects of the excitation and modal conditions on the ASPL and SHG emissions, we developed a time-domain modeling framework that simultaneously captures the mode coupling-enhancement characteristics, quantum excitation-emission transitions, and hot carrier population statistical mechanics. Notably, ASPL and SHG from the same metal nanocavities exhibit distinct plasmon-enhanced emission behaviors due to the intrinsic differences between the incoherent hot carrier-mediated ASPL sources with temporally evolving energy and spatial distributions and instantaneous SHG emitters. Mechanistic understanding of ASPL and SHG emissions from broadband multiresonant plasmonic nanocavities marks a milestone toward creating multimodal or wavelength-multiplexed upconversion nanoplasmonic devices for bioimaging, sensing, interfacial monitoring, and integrated photonics applications.

Stimulated tip-enhanced Raman scattering on single molecule in plasmonic nanocavity

Plasmonic nanocavities formed by a metal nanoparticle located close to a thin metal film or other nanoparticle are of great interest for nanophotonics. Their properties can be used for optical superresolution, optoelectronics, and the creation of new materials with increased non-linearity. However, the interpretation of Raman spectra enhanced by a nanoresonator is complicated by depolarization effects. In this work, the analysis of tip-enhanced Raman scattering (TERS) on a single organic molecule in the plasmonic nanocavity formed by a metal tip and a gold substrate is carried out. The linear and nonlinear regimes of TERS are considered. The reasons for the selective enhancement of Raman spectral bands are analysed using anisotropic molecule of the free base porphyrin (FBP) dye as an example. The observed vibrational symmetry of the molecule changes even greater in nonlinear regime of spectroscopic TERS measurements. We believe that our results are beneficial for the development of the tools to control vibrations of molecules at the nanoscale.