Field Enhancement Research Articles

Photocurrent-induced harmonics in nanostructures

Abstract Photocurrent-induced harmonics appear in gases and solids due to tunnel ionization of electrons in strong fields and subsequent acceleration. In contrast to three-step harmonic emission, no return to the parent ions is necessary. Here we show that the same mechanism produces harmonics in metallic nanostructures in strong fields. Furthermore, we demonstrate how strong local field gradient, appearing as a consequence of the field enhancement, affects photocurrent-induced harmonics. This influence can shed light at the state of electron as it appears in the continuum, in particular, to its initial velocity.

Hierarchically Enhanced Plasmonic Absorber with Abundant Nanomirrors for Effective Photoelectrochemical Performance

The synergistic combination of plasmonic metals and semiconductors demonstrates marvelous potential for nanocomposite design, particularly in enhancing photoelectrochemical performance through rich interfacial modifications and component properties adjustments. In terms of it, nanocomplexes of metal/insulator or semiconductor/metal have emerged heatedly, commonly depicted as nanomirror structures. In this work, a sandwiched nanomirror structure comprising a core Ag nanorods, a TiO2 middle nanogap, and dual types of Au and Ag particles outermost is presented, putting forward a novel design idea of rolling the planar complex up to amplify the potential interaction systems. The TiO2 middle layer enables the intrinsic generation of charge carriers upon light exposure, while the established heterojunctions promote their efficient separation. Additionally, both the inner Ag NRs and outer metal nanoparticles exhibit localized surface plasma resonance effects to realize enhancement of electric fields, where plasmonic coupling between them generates field hotspots within TiO2 layer, thereby expanding light absorption range and enhancing light capture intensity. Beyond, hot electrons from plasmonic metals traverse Schottky barriers into TiO2 to participate in further reactions, all of which contribute to heightened PEC performance of the nanostructure. In this work, the path is paved for innovative nanostructure design integrating plasmonic metals with semiconductors, offering promising implications for energy‐related applications.

Ruthenium Oxide: A Near 0.8 µM Epsilon‐Near‐Zero Medium for Multipurpose Nonlinear Photonics

AbstractThe intriguing optical response associated with the vanishingly small dielectric primitivity in epsilon‐near‐zero (ENZ) materials has offered unprecedented opportunities for photonics. Due to the high‐limit of doping concentration, current homogeneous ENZ materials based on heavily doped conductive oxide including ITO usually exhibit a zero‐permittivity wavelength (λ0) longer than 1200 nm. Here, this is identified with combined theoretical and experimental investigations that stoichiometric rutile‐type RuO2 (r‐RuO2) exhibits the shortest λ0 of near 800 nm among conductive oxides, benefiting from the high free electron concentration with characteristic Drude‐type response. The ENZ effect is manifested by the strong field enhancement and the large nonlinear optical (NLO) response in colloidal processed r‐RuO2 nanoparticles (NPs) and thin films. By ultrafast transient spectroscopy, the strong NLO response in r‐RuO2 NPs is revealed to be the result of rapid thermalization/cooling of free electrons in the strong hybridized Ru‐4d/O‐2p orbitals. The ultrafast optical nonlinearity is exploited further for the development of multi‐purpose all‐optical switches that enable stable pulse laser generation in four mode‐locked and Q‐switched fiber lasers. Our results underscore the potential of stoichiometric metallic metal oxides as alternative ENZ materials for nonlinear photonics applications in the visible and NIR regions.

Low-power electrodeless 2.45 GHz microwave plasma generation in ambient air with dielectric resonators

Abstract Microwave (MW) plasma, as an electrodeless non-thermal plasma technique, holds significant potential for gas processing applications under near-ambient conditions. However, current MW plasma methods suffer from low energy efficiency due to the high power required for molecular breakdown via vibrational excitation. A promising approach to mitigate this challenge involves using a pair of high relative permittivity dielectric resonators (DRs) in close proximity to enhance the electric field within the small gap between them, thus reducing power consumption. In this study, we combined simulations and experiments to investigate the electric field enhancement effect of DRs with varying shapes, geometrical parameters (e.g., radius and height), gap distances, and orientations relative to the electric field polarization and MW propagation directions. Simulations performed using Ansys HFSS demonstrate that edge-to-edge configurations of hexagonal, square, and triangular prism DRs can achieve significant electric field enhancement over a broad range of geometric parameters. DRs fabricated from calcium titanate were tested experimentally using a simple setup with a kitchen microwave (2.45 GHz) at ambient pressure. With input power below 100 W, stable and highly confined plasma was generated using hexagonal and triangular prism DRs. These results suggest a simple, cost-effective, and low-power method for generating MW plasma under ambient conditions, offering particular advantages for chemical processing in rural areas utilizing renewable energy resources.

Chiral and Quantum Plasmonic Sensors: New Frontiers in Selective and Ultra-Sensitive Sensing.

Surface Plasmon Polaritons (SPPs) and Localized Surface Plasmon Resonances (LSPRs) are fundamental phenomena in plasmonics that enable the confinement of electromagnetic waves beyond the diffraction limit. This confinement results in a significant enhancement of the electric field, making this phenomenon particularly beneficial for sensitive detection applications. However, conventional plasmonic sensors face several challenges, notably their difficulty in distinguishing chiral molecules, which are vital in drug development. Furthermore, these sensors exhibit sensitivity issues and energy losses, leading to broader resonance peaks and diminished signal-to-noise ratios. Recent research has concentrated on integrating chirality and quantum effects in plasmonics to overcome these limitations. Particularly, the development of plasmonic sensors with exceptional sensitivity and precision at scales smaller than the diffraction limit. This review assesses the latest advancements in chiral and quantum plasmonic sensing technologies. The first section details the theory and operational principles of conventional sensors based on SPPs and LSPRs. The second section discusses recent developments in chiral plasmonic sensors, while the third section focuses on plasmonic quantum sensing, highlighting contemporary findings. Specifically, this section emphasizes quantum-enhanced sensing techniques that mitigate shot noise, a significant barrier to single-molecule detection. The concluding section summarizes the review and identifies potential future research directions.

In-situ observations of the magnetothermodynamic evolution of electron-only reconnection

Field-particle energy exchange is important to the magnetic reconnection process, but uncertainties regarding the time evolution of this exchange remain. We investigate the temporal dynamics of field-particle energy exchange during magnetic reconnection, using Magnetospheric Multiscale mission observations of an electron-only reconnection event in the magnetosheath. The electron energy is in local minimum at the x-line due to a density depletion, while the magnetic energy is in local maximum due to a guide field enhancement. The electromagnetic energy transport comes almost entirely from guide field contributions and is confined within the reconnection plane, while the most significant contribution to electron energy transport is independent of the drift velocity with additional out-of-plane signatures. Multi-spacecraft analysis suggests that the guide field energy is decreasing while the electron density is increasing, both evolving such that the system is moving toward a more uniform distribution of magnetic and thermal energy.

Designing Fluorescent Interfaces at Hotspots in a Plasmonic Nanopore for Homologous Optoelectronic Sensing.

In this work, a site-selective functionalization strategy is proposed for modifying fluorescent dyes in the plasmonic nanopore, which highlights building optoelectronic dual-signal sensing interfaces at "hotspots" locations to construct multiparameter detection nanosensor. Finite-difference time-domain (FDTD) simulations confirmed the high-intensity electromagnetic field due to plasmonic nanostructure. It is demonstrated that adjusting the distance between the nanopore inner wall and fluorophore prevented the fluorescence quenching, resulting in more than a thirty fold fluorescence enhancement. Upon binding with the target analyte, the sensor produces homologous yet independent optoelectronic dual-signal responses that cross-validate one another, providing highly accurate analysis even in the presence of multiple interferences. The platform demonstrates precise, adaptable detection with linear responses to extracellular pH changes at the single-cell level, making it a versatile tool for a range of biosensing applications. By enabling the functionalization of fluorescent interfaces in the "hotspots" of metal nanopores, this interface design strategy efficiently exploits the enhancement of electromagnetic fields to achieve high-precision dual-signal measurements and greatly improves the sensitivity of biosensing applications.

Localized surface plasmon resonance characteristics of gold dimeric nanobipyramids

Dimer is a new type of nanostructure that has attracted much attention due to its ability to achieve more significant field strength enhancement effects through the interaction of two nanoparticles than monomers. This paper applies the dimer structure to gold nanobipyramids particles with unique tip structures. The localized surface plasmon resonance properties of the gold dimeric nanobipyramids are simulated using the finite difference time domain method. The results show that the gold dimeric nanobipyramids achieves several times higher field enhancement in the nanogap compared to the gold monomer nanobipyramids. At the same time, under the same conditions, its electric field intensity is significantly higher than that of the gold dimeric nanospheres and gold dimeric nanorods. Furthermore, by varying the gap, sharpness, and surrounding medium, the changes in the LSPR effects of the gold dimeric nanobipyramids under different conditions were calculated. The research shows that by tuning the structure, morphology, and surrounding medium of the dimeric nanobipyramids, higher extinction intensity, more intensity field enhancement, and greater sensitivity can be achieved, offering significant application prospects in various optoelectronic device fields. This work has significant reference value for studying the localized surface plasmon resonance characteristics of dimeric metal nanoparticles and their related applications.

Resonant enhancement and confinement of microwave field in coupled conductive rod systems

Abstract We investigated the resonant interaction of incident microwaves with a finite linear array of equidistantly arranged conductive rods of finite height. When strong coupling is established between adjacent rods, which act as open resonators for radial waves, a series of resonances emerges, corresponding to the number of rods in the array. Each resonance exhibits distinct charge oscillation amplitudes and phases along the rods, leading to varying radiative losses. Notably, in a system of two resonantly interacting rods, antiphase charge oscillations result in a nearly 30-fold increase in localized field intensity and a quality factor of approximately 400. This study demonstrates the feasibility of creating high-quality open resonators with sub-wavelength confinement, offering the potential for microwave-to-optical conversion and enabling control over quantum processes in materials, thereby broadening their application scope.

High Q-factor Mie lattice resonance generated from out-of-plane magnetic dipole in all-dielectric defected semi-cylinder metasurface array

Abstract Due to the weak far-field radiation and strong local field enhancement properties of Mie lattice resonances, transmission resonance with significant high Q factor is achieved which could be used in applications such as refractive index sensing, filtering and nano lasing. In this paper, we design an semi-cylinder with cuboid defect metasurface array working in the near-infrared, where a resonance with extremely narrow linewidth in transmission spectrum has been demonstrated at the wavelength of 870.9408 nm. The full width at half-maximum of the resonance is as narrow as 0.0032 nm with a corresponding Q factor of 272169. According to distributions of the electric field and displacement current, it is confirmed that the narrow linewidth resonance is generated from the Mie lattice resonance which is originated from the coupling between the out-of-plane magnetic dipole mode with Rayleigh Anomalous diffraction. It is further verified that the out-of-plane magnetic dipole mode is excited due to the induced defect in the semi-cylinder. When being applied in refractive sensing, a sensitivity and a figure of merit of 333 nm/RIU and 104063 RIU-1 are numerically achieved simultaneously. Our study here provide a new way for achieving resonances with high Q-factors.

Chiral Light-Matter Interactions with Thermal Magnetoplasmons in Graphene Nanodisks.

We investigate the emergence of self-hybridized thermal magnetoplasmons in doped graphene nanodisks at finite temperatures upon being subjected to an external magnetic field. Using a semianalytical approach, which fully describes the eigenmodes and polarizability of the graphene nanodisks, we show that the hybridization originates from the coupling of transitions between thermally populated Landau levels and localized magnetoplasmon resonances of the nanodisks. Owing to their origin, these modes combine the extraordinary magneto-optical response of graphene with the strong field enhancement of plasmons, making them an ideal tool for achieving strong chiral light-matter interactions, with the additional advantage of being tunable through carrier concentration, magnetic field, and temperature. As a demonstration of their capabilities, we show that the thermal magnetoplasmons supported by an array of graphene nanodisks enable chiral perfect absorption and chiral thermal emission.

Vacuum DC breakdown characteristics between grid electrodes with ion sputtering

Abstract In ion thrusters, grid electrodes are biased at high voltage to extract and accelerate ions. However, they are susceptible to vacuum breakdown which considerably undermines the ion thruster performance. To optimize the grid electrode design and improve the ion thruster longevity, the impact of ion sputtering on the vacuum DC breakdown characteristics of grid electrodes are systemically examined. The adopted seven-aperture grid electrodes are made of three materials including molybdenum, graphite, and stainless steel, which are exposed to Ar⁺ and Xe⁺ sputtering at energies of 400 eV and 1000 eV for durations ranging from 0.5 to 8 hours. It is found that the initial breakdown voltage, which means the first breakdown voltage after sputtering, generally decreased regardless of the electrode material as the sputtering time increased. However, the breakdown voltage rapidly increases with more repeated discharges. Each kind of electrode material demonstrates unique responses to ion sputtering and breakdowns. In comparison with metal electrodes, the graphite surface relatively easy to appear defects but exhibites the least insulation degradation. The metal surface remains relatively smooth, but breakdown voltage is significantly affected, with electrode materials such as stainless steel failing to recover to their initial insulation levels. Xe⁺ sputtering leads to greater breakdown voltage fluctuations during repeated discharges due to higher sputtering yield. Dark current is measured, and field enhancement factor is calculated to reveal correlations between local electric field and the breakdown voltage. Furthermore, micro particles are identified as major contributor to the first few times breakdown. Based on above observations, two kinds of distinct discharge mechanisms, including the cascade discharge induced by micro particles and the discharge induced by field emission are proposed to interpret experimental data.

Slotted gap-surface plasmon resonator as an efficient platform for sensing.

Film-coupled plasmonic resonators offer efficient platforms for light enhancement due to the excitation of gap surface plasmons (GSPs) at metal-insulator-metal interfaces, where electromagnetic energy is stored within the spacer. In applications like biosensing and spontaneous emission control, spatial overlap between the target molecule and plasmonic hotspots is essential. Here, we propose utilizing the controllable, efficient light enhancement capabilities of a specifically designed GSP disk resonator for biosensing and spontaneous emission enhancement. To create an external plasmonic hotspot and make the strong field stored in the spacer accessible to nearby molecules, we introduce a nanoslot in the top metallic disk with its long axis oriented perpendicular to the incident field polarization. This orientation ensures significant electric field enhancement due to boundary conditions, while the resonant modes of the GSP and nanoslot are further tailored to optimize the field distribution. Finite element method-based simulations reveal the simultaneous excitation of electric-dipole modes due to the nanoslot alongside GSP modes, resulting in a more than two-order magnitude increase in total electromagnetic energy. Additionally, varying the slot length allows precise control over resonances, revealing different modes of the system. The external hotspot in the nanoslot ensures direct interaction with nearby molecules, enhancing the radiative decay rate by nearly three orders of magnitude. The suggested configuration of a plasmonic disk combined with a rectangular nanoslot extends the degree of freedom for designing external electromagnetic hot spots.

Broadband and large surface enhancements of the local electric field enabled by cross-etched hyperbolic metamaterials.

Hyperbolic metamaterials (HMMs) have recently attracted significant research attention due to their hyperbolic wavevector iso-frequency contour, which leads to substantial local electric field (EF) enhancements that benefit optical processes, such as the nonlinear generation, quantum science, biomedical sensing, and more. However, three main challenges hinder their practical implementation: the difficulty in exciting their resonant modes using free-space incidence, the weak enhancement of surface EF, and the narrow spectral range of EF enhancements. Herein, we proposed cross-etched HMMs (CeHMMs) as a novel type of HMM, addressing these issues. The CeHMMs can be easily fabricated by etching periodic cross-shaped arrays in conventional HMMs, and exhibited two resonant high-k modes within the spectral range of 700-1100 nm under linearly, circularly, or elliptically polarized incident light from free space. It was also calculated that the CeHMMs can provide a large surface EF enhancement across a broad spectral range (over 500 nm). After integrating a single layer of WSe2 onto the top surface, the photoluminescence (PL) enhancements of the CeHMMs and their hot spots, based on the emission resonance, were calculated to be 9.72 and 62 times, respectively. With their ability to provide broadband surface EF enhancement, CeHMMs are expected to offer considerable potential for a variety of nanophotonic applications, including nonlinear optics, integrated optics, and quantum photonics.

Gold nanoparticle-modified single-walled carbon nanotube terahertz metasurface for ultrasensitive sensing of trace proteins.

Research on metasurface sensors with high sensitivity, strong specificity, good biocompatibility and strong integration is the key to promote the application of terahertz waves in the field of biomedical detection. However, traditional metallic terahertz metasurfaces have shortcomings such as poor biocompatibility and large ohmic loss in the terahertz frequency band, impeding their further application and integration in the field of biosensing detection. Here, we overcome this challenge by proposing a high-performance terahertz metasurface based on gold nanoparticles and single-walled carbon nanotubes composite film. Compared with metal materials, carbon nanotubes not only have better biocompatibility, which can reduce the potential adverse reactions between metasurfaces and biological samples, but also have strong tunability in electrical and optical properties. Experimentally, we reveal a method to adjust the dielectric properties of single-walled carbon nanotube films by doping with gold nanoparticles. Leveraging this mechanism, we designed and prepared a single-walled carbon nanotube terahertz metasurface composed of periodically arranged asymmetric open resonant rings. Compared with pure single-walled carbon nanotube films, this device based on a composite single-walled carbon nanotube films have better localized electromagnetic field enhancement characteristics. Through integration with microfluidic channels, this metasurface sensor can achieve direct detection of SAA proteins in solution environments at the fM level. In addition, the device also exhibits a detection sensitivity of 41GHz/fM. This work has not only made significant progress in the design and function of new high-sensitivity carbon-based terahertz metasurfaces, but also laid the foundation for its application in liquid environment detection of trace biological samples.

Tuning of plasmonic surface lattice resonances: on the crucial impact of the excitation efficiency of grazing diffraction orders.

Metallic nanoparticles exhibit remarkable optical properties through localized surface plasmon (LSP) resonances. When arranged in arrays, nanoparticles form surface lattice resonances (SLRs) affected by inter-particle distance. SLRs can offer narrower bandwidths and stronger electric field enhancements than LSP modes, crucial for efficient optical device development. Among important aspects, grazing diffracted orders crucially impact SLR properties, facilitating long-range nanoparticle interactions. This study explores how these photonic modes propagating at the substrate surface influence SLRs, by using experimental and theoretical approaches, including Finite Difference Time Domain simulations on gold disk arrays. Results show SLR properties strongly rely on diffracted mode efficiency controlled by the grating constant along the incident polarization. Notably, large inter-particle spacing along the incident polarization can strongly reduce the SLR red-shift. Understanding and managing long-range interactions in engineered plasmonic structures are highlighted, providing insights for enhanced performance in advanced photonic and optoelectronic devices.

Controlling Raman enhancement in particle-aperture hybrid nanostructures by interlayer spacing.

Here we show how surface-enhanced Raman spectroscopy (SERS) features can be fine-tuned in optically active substrates made of layered materials. To demonstrate this, we used DNA-assisted lithography (DALI) to create substrates with silver bowtie nanoparticle-aperture pairs and then coated the samples with rhodamine 6G (R6G) molecules. By varying the spacing between the aperture and particle layer, we were able to control the strength of the interlayer coupling between the plasmon resonances of the apertures and those of the underlying bowtie particles. The changes in the resulting field enhancements were confirmed by recording the Raman spectra of R6G from the substrates, and the experimental findings were supported with finite difference time domain (FDTD) simulations including reflection/extinction and near-field profiles.

Gold nanorod in silver tetrahedron: Cysteamine mediated synthesis of SERS probes with embedded internal markers for AFP detection.

Plasmonic core-shell nanostructures with embedded internal markers used as Raman probes have attracted great attention in surface-enhanced Raman scattering (SERS) immunoassay for cancer biomarkers due to their excellent uniform enhancement. However, current core-shell nanostructures typically exhibit a spherical shape and are coated with a gold shell, resulting in constrained local field enhancement. In this work, we prepared a core-shell AuNR@BDT@Ag structure by depositing silver on the surface of Raman reporter-modified gold nanorods (AuNR). With cysteamine-driven specific deposition of Ag atoms, the internal standard nanoprobes with an ortho-tetrahedral morphology were obtained. Owing to its tetrahedral morphology and the effective plasmon coupling at the Ag-Au interface, this internal standard Raman probe exhibited excellent Raman enhancement. Also, with embedded Raman reporter, the probes of Au nanorod in Ag tetrahedron avoided the desorption of Raman reporter and competitive adsorption of interfering molecule, which greatly improved the stability and reproducibility of SERS signal and addressed the drawbacks of low reproducibility existing in SERS immunoassay. The feasibility of the AuNR@BDT@Ag probe was demonstrated by the sensitive detection of the liver cancer biomarker alpha-fetoprotein (AFP) in Eppendorf (EP) tubes and microfluidic chips. The results in EP tubes revealed a linear range of 1 fg/mL-1 ng/mL and a detection limit of 0.631fg/mL. When the detection was performed in microfluidic chips, the linear range was 10pg/mL to 0.1μg/mL, with a limit detection of 8.29pg/mL. The performance of AFP detection in EP tubes and microfluidic chips demonstrates that the tetrahedral core-shell AuNR@BDT@Ag nanostructures used as internal standard Raman probes have the potential to detect biomarkers in blood samples for cancer screening.

Rhodium nanospheres for ultraviolet and visible plasmonics.

The development and understanding of alternative plasmonic materials are crucial steps for leveraging new plasmonic technologies. Although gold and silver nanostructures have been intensively studied, the promising plasmonic, chemical and physical attributes of rhodium remain poorly investigated. Here, we report the synthesis and plasmonic response of spherical Rh nanoparticles (NPs) with sizes in the 20-40 nm range. Due to the high cohesive energy of this metal, synthesis and experimental investigations of Rh nanospheres in this size range have not been reported; yet, it becomes possible here using a green and one-step laser ablation in liquid method. The localized surface plasmon (LSP) of Rh NPs falls in the ultraviolet spectral range (195-255 nm), but the absorption tail in the visible region increases significantly upon clustering of the nanospheres. The surface binding ability of Rh NPs towards thiolated molecules is equivalent to that of Au and Ag NPs, while their chemical and physical stability at high temperatures and in the presence of strong acids such as aqua regia is superior to those of Au and Ag NPs. The plasmonic features are well described by classical electrodynamics, and the results are comparable to Au and Ag NPs in terms of extinction cross-section and local field enhancement, although blue shifted. This allowed, for instance, their use as an optical nanosensor for the detection of ions of toxic metals in aqueous solution and for the surface enhanced Raman scattering of various compounds under blue light excitation. This study explores the prospects of Rh NPs in the realms of UV and visible plasmonics, while also envisaging a multitude of opportunities for other underexplored applications related to plasmon-enhanced catalysis and chiroplasmonics.

Electron emission performance analysis and application of carbon nanotube cold cathode prepared by cold pressing process

In this work, a carbon nanotube (CNT) cold cathode electron emitter fabricated by the cold pressing process was developed and studied. The electron emission performance was investigated and the application of pulse x-ray emission and imaging was explored by this cold cathode. The results indicated that the electron emission performance was excellent with electric intensities of turn-on and threshold of 0.47 and 1.17 V/μm@1 mA/cm2, respectively, and the field enhancement factor reached 17 514. The application research results showed that the pulse x-ray waveform has a great corresponding well with the grid voltage, and the imaging of a screw was clear, whose thread and pitch could be seen clearly. This article proposed a cold pressing process prepared for the CNT cold cathode, providing a new technical approach for the development of field emission cold cathode preparation processes.