Plasmonic Nanocavity Research Articles

Second harmonic generation from aluminum plasmonic nanocavities: from scanning to imaging.

Metamaterials and plasmonic structures made from aluminum (Al) have attracted significant interest due to their low cost, long-term stability, and the relative abundance of aluminum compared to the rare metals. Also, aluminum displays distinct dielectric properties allowing for the excitation of surface plasmons in the ultraviolet region with minimal non-radiative losses. Despite these clear advantages, most of the research has been focused on either gold or silver, probably due to difficulties in forming smooth thin films of aluminum. In the present work, we detect and characterize second harmonic generation (SHG) in the optical regime, emanating from triangular hole arrays milled into thin aluminum films in reflection mode, at normal incidence. We report intense nonlinear responses, year-long stability, and overall superior performances with respect to gold. The robustness of the Al structures and high reproducibility of the measured SHG responses allowed us to investigate changes in the directional emission upon tiny modifications in the structure's symmetry. We also demonstrate large-field instantaneous SHG imaging over areas containing several hole arrays using a recent, non-linear single-spinning disk microscope. Such high spatio-temporal resolution imaging has important applications, e.g., when studying chemical transformations occurring at electrode surfaces during charging and discharging cycles, as well as ageing.

Oblique-Incidence-Excited Localized Hot Spots in Plasmonic Particle-on-Film Nanocavities

Regarding plasmonic nanocavities formed by particle-on-film, the nanogap hot spots play an essential role in wide applications including plasmon-enhanced spectroscopy, ultrasensitive sensing, photovoltaics, and photocatalysis. However, hot spots at the nanogap are symmetrically distributed on both sides of the junction under normal incidence. Herein, under the condition of oblique incidence, properties of plasmonic hot spots in the Au@SiO2 particle-on-film have been investigated theoretically. We find that when the plane wave is incident obliquely, the hot spots in the gap are not only symmetrically distributed on both sides of the contact point but also circularly distributed around the junction at specific excitation wavelengths. With the adjustment of the incidence angle, the maximum surface-enhanced Raman spectroscopy enhancement factor from this circularly distributed hot spot reaches 11.3 orders in magnitude. Moreover, the resonant mode at 785 nm has been readily matched by optimizing the size of the Au core, and the optimal thickness of the silica shell with 1 nm for achieving the maximum electromagnetic (EM) field enhancement was further demonstrated. These findings offer a new strategy for manipulating locations of localized hot spots with strong near-field enhancement in plasmonic nanocavities.

Fate of Molecular Photon Emitters in Plasmonic Hot Spots in Air and a Vacuum

We report on the physicochemical behavior of molecular photon emitters in plasmonic hot spots by comparing the temporal evolution of the scattering signal in a vacuum and air of three chromophores, namely Methylene Blue (MB), Azure A (AzA), and Toluidine Blue O (TBO). The differences in behavior are studied by monitoring the surface-enhanced Raman scattering spectra of the adsorbates as a function of illumination time at 632.8 nm excitation wavelength that overlaps with the electronic transition energy of the chromophores. It is shown that the adsorbates are relatively stable in a vacuum, while they undergo N-demethylation when excited at ambient conditions. Comparison of the vacuum and ambient data suggests that the photochemical transformation is preceded by fluorescence quenching that involves intersystem crossing, energy transfer, and singlet oxygen generation. Generally, the plasmon-induced physicochemical behaviors of the three dyes are similar with some notable exceptions. The strongest signal fluctuation is observed for the N-demethylation product of MB due to most significant disruption of the adsorption geometry as four methyl groups are eliminated, leaving the product in the metastable state. On the other hand, TBO appears to be the most effective photosensitizer as evidenced by the most significant extent of N-demethylation within 0.5 s of exposure as well as through comparison of the N-demethylation signature in a vacuum due to a trace amount of oxygen. These results have important implications in the study of molecular polaritons in plasmonic nanocavities, where interpretation of strong coupling can be ambiguous due to competing excited state processes that lead to photochemical conversion of the emitters.

Construction of a novel hybrid plasmonic metal-2D MXene catalyst for plasmon-driven photoreduction of nitroaromatics

Plasmon-driven photocatalysis offers a green and promising strategy for solar-to-chemical energy conversion under mild reaction conditions. Herein, we developed an efficient strategy to significantly enhance the electromagnetic (EM) field in trimetallic Au@Ag@Pd NPs via engineering nanocavity as plasmonic hotspot. More importantly, a novel hybrid system of plasmonic metal-two-dimensional (2D) materials was constructed via loading of Au@Ag@Pd NPs with nanocavities onto the surface of 2D MXene (Ti3C2). The obtained Au@Ag@Pd/Ti3C2 photocatalyst showed superior catalytic performance on plasmon-driven hydrogenation of nitroaromatics even at 0 °C under visible light. Furthermore, it exhibited excellent stability and reusability in photocatalytic process. The EM field was dramatically enhanced via plasmonic component coupling and nanocavities engineered as plasmonic hotspot in the Au@Ag@Pd NPs, which can boost the generation of hot electrons. The presence of Pd and Ti3C2 facilitated the transfer of hot electrons from Au@Ag to Pd and then to MXene, which would prolong lifetimes of hot electrons and improve the utilization of hot electrons. Our present work paves a way to design more efficient hybrid plasmonic catalysts.

Fast and hydrosensitive switching of plasmonic nanocavities via photothermal effect

Metallic nanoplasmonics, due to its extremely small size and ultrafast speed, has been one of the key components for next-generation information technology. It is vital that the highly tunable nanoplasmonic system in the solid state can be achieved for optoelectronic devices, which, still remains elusive for the visible region. Here we sandwich vanadyl oxalate ( VOC 2 O 4 ) thin films in-between gold nanoparticles and gold film to establish thermo-responsive nanoantennas. The thickness of the VOC 2 O 4 composite films remains almost unchanged within the temperature cycles between 15°C and 80°C, while the refractive index of the films decreases with the increase of temperature due to the dehydration, which results in blueshift of the plasmon peak up to 60 nm. The plasmon resonances can be fully recovered when the temperature cools down again. This process is reversible within the temperature range of 15°C–80°C, which can be optically modulated with photothermal effect. Such thermo-responsive plasmonic nanoantenna works in the solid state with hundreds of kilohertz switching speed, which is highly compatible with traditional optoelectronic devices. It can be envisioned that this thermo-responsive optical thin film can be a promising candidate for integrated nanoplasmonic and optoelectronic devices.

Quantum Electrodynamic Behavior of Chlorophyll in a Plasmonic Nanocavity.

Plasmonic nanocavities have been used as a novel platform for studying strong light-matter coupling, opening access to quantum chemistry, material science, and enhanced sensing. However, the biomolecular study of cavity quantum electrodynamics (QED) is lacking. Here, we report the quantum electrodynamic behavior of chlorophyll-a in a plasmonic nanocavity. We construct an extreme plasmonic nanocavity using Au nanocages with various linker molecules and Au mirrors to obtain a strong coupling regime. Plasmon resonance energy transfer (PRET)-based hyperspectral imaging is applied to study the electrodynamic behaviors of chlorophyll-a in the nanocavity. Furthermore, we observe the energy level splitting of chlorophyll-a, similar to the cavity QED effects due to the light-matter interactions in the cavity. Our study will provide insight for further studies in quantum biological electron or energy transfer, electrodynamics, the electron transport chain of mitochondria, and energy harvesting, sensing, and conversion in both biological and biophysical systems.

Effect of Mirror Quality on Optical Response of Nanoparticle‐on‐Mirror Plasmonic Nanocavities

AbstractAs an essential part of nanoparticle‐on‐mirror (NPoM) plasmonic nanocavities, metal mirrors play an important role not only in determining the optical response of the nanocavities but also their application performance. Here the effect of mirror quality on the optical response of nanosphere‐on‐mirror (NSoM) and nanocube‐on‐mirror (NCoM) nanocavities is experimentally studied. Polycrystalline sputtered gold films (SGFs), template‐stripped gold films (TSGFs), and single‐crystalline gold microflakes (GMFs) are investigated and compared. Due to the great improvement in the surface roughness that can minimize fluctuations in the gap morphology, NSoM and NCoM nanocavities formed on smooth TSGFs and GMFs have a better cavity‐to‐cavity homogeneity in the scattering spectrum than those formed on comparably rougher SGFs. In addition, there is an obvious change in the spectral positions of the resonance modes of NSoM and NCoM nanocavities formed on SGFs due to the variation in the gap morphology, which is reproduced very well by theoretical calculations based on measured dielectric functions of the gold films. Finally, due to the reduction in electron scattering losses from SGFs to TSGFs and GMFs, an increase in the quality factors and scattering intensities of the resonance modes is observed for nanocavities formed on the corresponding films.

Selectively Addressing Plasmonic Modes and Excitonic States in a Nanocavity Hosting a Quantum Emitter.

Controlling the interaction between the excitonic states of a quantum emitter and the plasmonic modes of a nanocavity is key for the development of quantum information processing devices. In this Letter we demonstrate that the tunnel electroluminescence of electrically insulated C60 nanocrystals enclosed in the plasmonic nanocavity at the junction of a scanning tunneling microscope can be switched from a broad emission spectrum, revealing the plasmonic modes of the cavity, to a narrow band emission, displaying only the excitonic states of the C60 molecules by changing the bias voltage applied to the junction. Interestingly, excitonic emission dominates the spectra in the high-voltage region in which the simultaneously acquired inelastic rate is low, demonstrating that the excitons cannot be created by an inelastic tunnel process. These results point toward new possible mechanisms for tunnel electroluminescence of quantum emitters and offer new avenues to develop electrically tunable nanoscale light sources.

Plasmon‐Enhanced Ultrasensitive Digital Imaging Immunoassay for the Quantification of microRNAs Assisted by Convolutional Neural Network Analysis

AbstractDigital imaging immunoassays for precisely detecting microRNAs have attracted much attention in early cancer diagnosis. Currently, there is an urgent demand for improving the sensitivity and selectivity in digital immunoassays, which is limited by low imaging signal‐to‐noise ratio and pollutant interference. Herein, a plasmon‐enhanced ultrasensitive and selective digital‐readout imaging immunoassay is reported for microRNA quantification. Through DNA hybridization technology, Au nanoparticles are bound to target microRNAs and selectively placed onto an Au film coated slide, locally forming plasmonic nanocavities that dramatically enhance the scattering imaging signal of nanoparticles in dark‐field microscopy. Additionally, a convolutional neural network algorithm not only automatically recognizes and counts the image spots of the nanoparticles, but also excludes the counting errors inevitably induced by environmental pollutants or other noises. Ultimately, a quantitative determination of the microRNA‐375 cancer biomarker is demonstrated with a dynamic range that spans across 4 orders of magnitude (from 1 fm to 10 pm) with a low limit of detection (1.29 fm). This plasmon‐enhanced digital imaging immunoassay is a promising analytical tool toward point‐of‐care testing for various disease biomarkers at very low concentrations.

SPP standing waves within plasmonic nanocavities.

Surface plasmons usually take two forms: surface plasmon polaritons (SPP) and localized surface plasmons (LSP). Recent experiments demonstrate an interesting plasmon mode within plasmonic gaps, showing distinct characters from the two usual forms. In this investigation, by introducing a fundamental concept of SPP standing wave and an analytical model, we reveal the nature of the recently reported plasmon modes. The analytical model includes SPP propagating and SPP reflection within a metal-insulator-metal (MIM) cavity, which is rechecked and supplemented by numerical simulations. We systematically analyze SPP standing waves within various nanocavities. During the discussion, some unusual phenomena have been explained. For example, the hot spot of a nanodimer could be off-tip, depending on the order of standing wave mode; and that a nanocube on metal film can be viewed as a nanocube dimer with the same separation. And many other interesting phenomena have been discussed, such as dark mode of SPP standing wave and extraordinary optical transmission. The study gives a comprehensive understanding of SPP standing waves, and may promote the applications of cavity plasmons in ultrasensitive bio-sensings.

Greatly Enhanced Emission from Spin Defects in Hexagonal Boron Nitride Enabled by a Low-Loss Plasmonic Nanocavity.

The negatively charged boron vacancy (VB-) defect in hexagonal boron nitride (hBN) with optically addressable spin states has emerged due to its potential use in quantum sensing. Remarkably, VB- preserves its spin coherence when it is implanted at nanometer-scale distances from the hBN surface, potentially enabling ultrathin quantum sensors. However, its low quantum efficiency hinders its practical applications. Studies have reported improving the overall quantum efficiency of VB- defects with plasmonics; however, the overall enhancements of up to 17 times reported to date are relatively modest. Here, we demonstrate much higher emission enhancements of VB- with low-loss nanopatch antennas (NPAs). An overall intensity enhancement of up to 250 times is observed, corresponding to an actual emission enhancement of ∼1685 times by the NPA, along with preserved optically detected magnetic resonance contrast. Our results establish NPA-coupled VB- defects as high-resolution magnetic field sensors and provide a promising approach to obtaining single VB- defects.

DNA as grabbers and steerers of quantum emitters

Abstract The chemically synthesizable quantum emitters such as quantum dots (QDs), fluorescent nanodiamonds (FNDs), and organic fluorescent dyes can be integrated with an easy-to-craft quantum nanophotonic device, which would be readily developed by non-lithographic solution process. As a representative example, the solution dipping or casting of such soft quantum emitters on a flat metal layer and subsequent drop-casting of plasmonic nanoparticles can afford the quantum emitter-coupled plasmonic nanocavity (referred to as a nanoparticle-on-mirror (NPoM) cavity), allowing us for exploiting various quantum mechanical behaviors of light–matter interactions such as quantum electrodynamics (QED), strong coupling (e.g., Rabi splitting), and quantum mirage. This versatile, yet effective soft quantum nanophotonics would be further benefitted from a deterministic control over the positions and orientations of each individual quantum emitter, particularly at the molecule level of resolution. In this review, we will argue that DNA nanotechnology can provide a gold vista toward this end. A collective set of exotic characteristics of DNA molecules, including Watson-Crick complementarity and helical morphology, enables reliable grabbing of quantum emitters at the on-demand position and steering of their directors at the single molecular level. More critically, the recent advances in large-scale integration of DNA origami have pushed the reliance on the distinctly well-formed single device to the regime of the ultra-scale device arrays, which is critical for promoting the practically immediate applications of such soft quantum nanophotonics.

Plasmonic nanocavity as a spectroscopic probe for molecules

Plasmonic nanocavities have a promising future in various fields such as plasmon-mediated chemical reactions. Yet unresolved questions remain for the physical process and chemical reaction at the surface of the metal. In a recent study published in <i>Nature Nanotechnology</i>, Oksenberg et al. constructed energy-tunable nanoreactors with different chemical environments, revealing distinct plasmon-driven chemical reactions.

Assembly of Centimeter-Scale Plasmonic Nanocavities for Bright and Ultrafast Emission of Red Carbon Dots

Assembly of Centimeter-Scale Plasmonic Nanocavities for Bright and Ultrafast Emission of Red Carbon Dots

Nanocavity-induced trion emission from atomically thin WSe2

Exciton is a bosonic quasiparticle consisting of a pair of electron and hole, with promising potentials for optoelectronic device applications, such as exciton transistors, photodetectors and light emitting devices. However, the charge-neutral nature of excitons renders them challenging to manipulate using electronics. Here we present the generation of trions, a form of charged excitons, together with enhanced exciton resonance in monolayer WSe2. The excitation of the trion quasiparticles is achieved by the hot carrier transport from the integrated gold plasmonic nanocavity, formed by embedding monolayer WSe2 between gold nanoparticles and a gold film. The nanocavity-induced negatively charged trions provide a promising route for the manipulation of excitons, essential for the construction of all-exciton information processing circuits.

Anti-Kasha emissions of single molecules in a plasmonic nanocavity.

Kasha's rule generally holds true for solid-state molecular systems, where the rates of internal conversion and vibrational relaxation are sufficiently higher than the luminescence rate. In contrast, in systems where plasmons and matter interact strongly, the luminescence rate is significantly enhanced, leading to the emergence of luminescence that does not obey Kasha's rule. In this work, we investigate the anti-Kasha emissions of single molecules, free-base and magnesium naphthalocyanine (H2Nc and MgNc), in a plasmonic nanocavity formed between the tip of a scanning tunneling microscope (STM) and metal substrate. A narrow-line tunable laser was employed to precisely reveal the excited-state levels of a single molecule located under the tip and to selectively excite it into a specific excited state, followed by obtaining a STM-photoluminescence (STM-PL) spectrum to reveal the energy relaxation from the state. The excitation to higher-lying states of H2Nc caused various changes in the emission spectrum, such as broadening and the appearance of new peaks, implying the breakdown of Kasha's rule. These observations indicate emissions from the vibrationally excited states in the first singlet excited state (S1) and second singlet excited state (S2), as well as internal conversion from S2 to S1. Moreover, we obtained direct evidence of electronic and vibronic transitions from the vibrationally excited states, from the STM-PL measurements of MgNc. The results obtained herein shed light on the energy dynamics of molecular systems under a plasmonic field and highlight the possibility of obtaining various energy-converting functions using anti-Kasha processes.

Electric field enhancement by a hybrid dielectric-metal nanoantenna with a toroidal dipole contribution.

Plasmonic nanocavities enable extreme light-matter interactions by pushing light down to the nanoscale. The numerical simulation is carried out systematically on the slotted Φ-shaped Si disk system with the super-dipole mode based on the analysis of the scattering strength of electric and toroidal dipoles. New blocks are introduced to the zero-field strength region of a slotted Si disk system as a function of the field enhancement factors. The far-field scattering characteristics and near-field electromagnetic field distributions are investigated by a multipole decomposition analysis to elucidate the intrinsic causes of the field enhancement. In the hybrid metal-dielectric nanoantenna, the Φ-shaped Si structure is prepared by superimposing Au nanoantennas for further field enhancement. In addition, the effects of the placement of an electric dipole emitter on the Purcell factor are derived. The geometric volume of the system is increased, and the electric field strength is improved, leading to an electric field increase of ∼30. Coupling between the super-dipole mode of the dielectric nanostructure and plasmonic modes of the metallic nanoantenna produces an enhancement as large as 16 times. Our results reveal a greatly enhanced super-dipole mode by electromagnetic coupling in composite structures, which will play a significant role in enhanced nonlinear photonics, near-field enhancement spectroscopy, and strong photon-exciton coupling.

Near-field and photocatalytic properties of mono- and bimetallic nanostructures monitored by nanocavity surface-enhanced Raman scattering

Localized surface plasmon resonances (LSPR) generated in a particle-film nanocavity enhance electric fields within a nanoscale volume. LSPR can also decay into hot carriers, highly energetic species that catalyze photocatalytic reactions in molecular analytes located in close proximity to metal surfaces. In this study, we examined the intensity of the electric field (near-field) and photocatalytic properties of plasmonic nanocavities formed by single nanoparticles (SNP) on single nanoplates (SNL). Using 4-nitrobenzenethiol (4-NBT) as a molecular reporter, we determined the near-field responses, as well as measured rates of 4-NBT dimerization into 4,4-dimercaptoazobenzene (DMAB) in the gold (Au) SNP on AuSNL nanocavity (Au-Au), as well as in AuSNP on AgSNL (Au-Ag), AgSNP on AuSNL (Ag-Au), and AgSNP on AgSNL (Ag-Ag) nanocavities using 532, 660, and 785 nm excitations. We observed the strongest near-field signals of 4-NBT at 660 nm in all examined plasmonic systems that is found to be substantially red-shifted relative to the LSPR of the corresponding nanoparticles. We also found that rates of DMAB formation were significantly greater in heterometal nanocavities (Au-Ag and Ag-Au) compared to their monometallic counterparts (Au-Au and Ag-Ag). These results point to drastic differences in plasmonic and photocatalytic properties of mono and bimetallic nanostructures.

Fingerprinting the Hidden Facets of Plasmonic Nanocavities.

The optical properties of nanogap plasmonic cavities formed by a NanoParticle-on-Mirror (NPoM, or patch antenna) are determined here, across a wide range of geometric parameters including the nanoparticle diameter, gap refractive index, gap thickness, facet size and shape. Full understanding of the confined optical modes allows these nanocavities to be utilized in a wide range of experiments across many fields. We show that the gap thickness t and refractive index n are spectroscopically indistinguishable, accounted for by a single gap parameter G = n/t0.47. Simple tuning of mode resonant frequencies and strength is found for each quasi-normal mode, revealing a spectroscopic “fingerprint” for each facet shape, on both truncated spherical and rhombicuboctahedral nanoparticles. This is applied to determine the most likely nanoscale morphology of facets hidden below each NPoM in experiment, as well as to optimize the constructs for different applications. Simple scaling relations are demonstrated, and an online tool for general use is provided.

Manipulating the light-matter interactions in plasmonic nanocavities at 1 nm spatial resolution

The light-matter interaction between plasmonic nanocavity and exciton at the sub-diffraction limit is a central research field in nanophotonics. Here, we demonstrated the vertical distribution of the light-matter interactions at ~1 nm spatial resolution by coupling A excitons of MoS2 and gap-mode plasmonic nanocavities. Moreover, we observed the significant photoluminescence (PL) enhancement factor reaching up to 2800 times, which is attributed to the Purcell effect and large local density of states in gap-mode plasmonic nanocavities. Meanwhile, the theoretical calculations are well reproduced and support the experimental results.