Electric Near Field Research Articles

Ideal magnetic dipole resonances with metal-dielectric-metal hybridized nanodisks.

Magnetic resonances generated with nonmagnetic nanostructures have been widely used to design various functional nanophotonic devices, and it is important to realize pure magnetic dipole scattering for the unambiguous study of magnetic light-matter interactions. However, the magnetic responses often spectrally overlapping with other multipoles, which is the main obstacle to achieve ideal magnetic dipole resonances. This study proposes and theoretically demonstrates that an ideal magnetic dipole resonance can be excited with metal-dielectric-metal hybridized nanodisks. It is shown that although the generated magnetic dipole scattering around the bonding resonance of the hybridized nanodisk is spectrally overlapping with strong electric dipole and electric quadrupole contributions, an almost perfect current loop can be generated by adjusting the geometry parameters and the refractive index of the dielectric layer, thereby leading to the suppressing of the overlapping multipoles and the formation of an ideal magnetic dipole scattering. What's more important is that both electric and magnetic near-fields are enhanced simultaneously with the increasing of the refractive index of the dielectric layer, which makes the hybridized nanodisk a promising platform for enhanced magnetic light-matter interactions.

Wideband Near-Field Correction of a Fabry–Perot Resonator Antenna

A systematic approach to correcting electric near-field phase and magnitude over a wideband for Fabry–Perot resonator antennas (FPRAs) is presented. Unlike all other unit-cell-based near-field correction techniques for FPRAs, which merely focus on phase correction at a single frequency, this method delivers a compact near-field correcting structure (NFCS) with a wide operational bandwidth of 40%. In this novel approach, a time-average Poynting vector in conjunction with a phase gradient analysis is utilized to suggest the initial configuration of the NFCS for wideband performance. A simulation-driven optimization algorithm is then implemented to find the thickness of each correcting region, defined by the gradient analysis, to complete the NFCS design. According to the predicted and measured results, the phase and magnitude distributions of the electric near field have been greatly improved, resulting in a high aperture efficiency of 70%. The antenna under NFCS loading has a peak measured directivity of 21.6 dB, a 3 dB directivity bandwidth of 41% and a 10 dB return loss bandwidth of 46%, which covers the directivity bandwidth. The diameter of the proposed NFCS is $3.8\lambda _{0c}$ , which is around half that of all the other unit-cell-based phase-correcting structures, where $\lambda _{0c}$ is the free-space wavelength at the central frequency of the NFCS (13.09 GHz).

Effective Optical Properties of Inhomogeneously Distributed Nanoobjects in Strong Field Gradients of Nanoplasmonic Sensors

Accurate and efficient modeling of discontinuous, randomly distributed entities is a computationally challenging task, especially in the presence of large and inhomogeneous electric near-fields of plasmons. Simultaneously, the anisotropy of sensed entities and their overlap with inhomogeneous fields means that typical effective medium approaches may fail at describing their optical properties. Here, we extend the Maxwell Garnett mixing formula to overcome this limitation by introducing a gradient within the effective medium description of inhomogeneous nanoparticle layers. The effective medium layer is divided into slices with a varying volume fraction of the inclusions and, consequently, a spatially varying effective permittivity. This preserves the interplay between an anisotropic particle distribution and an inhomogeneous electric field and enables more accurate predictions than with a single effective layer. We demonstrate the usefulness of the gradient effective medium in FDTD modeling of indirect plasmonic sensing of nanoparticle sintering. First of all, it yields accurate results significantly faster than with explicitly modeled nanoparticles. Moreover, by employing the gradient effective medium approach, we prove that the detected signal is proportional to not only the nanoparticle size but also its size dispersion and potentially shape. This implies that the simple volume fraction parameter is insufficient to properly homogenize these types of nanoparticle layers and that in order to quantify optically the state of the layer more than one independent measurement should be carried out. These findings extend beyond nanoparticle sintering and could be useful in analysis of average signals in both plasmonic and dielectric systems to unveil dynamic changes in exosomes or polymer brushes, phase changes of nanoparticles, or quantifying light absorption in plasmon assisted catalysis.

Investigating the origin of third harmonic generation from diabolo optical antennas

We propose to use diabolo nanoantennas for experimentally investigating the origin of the enhanced third harmonic generation by localized surface plasmon polaritons. In such a geometry, the opposing apexes of bowties are electrically connected by a thin gold nanorod, which has two important functions in discriminating the point of harmonic generation. First, the inserted gold nanorod shifts the field enhancement area to be far away from the dielectric substrate material. Next, the accumulation of free charges at the adjacent bowtie tips produces a strong electric field inside the gold nanorod. The diabolo nanoantennas allow us to examine the contribution of the bare gold susceptibility to the third harmonic conversion. Our results reveal that the bare gold does not significantly enhance the harmonic generation at high pump intensity. From this, we deduce that in regular bowtie antennas, the enhanced harmonic photons mainly arise from the substrate sapphire that is located in the feedgap of the bowtie, where the electric near-field is significantly enhanced by the localized surface plasmons.

Optical Manipulation of nanoparticles by simultaneous electric and magnetic field enhancement within diabolo nanoantenna

In this paper, we propose and numerically simulate a novel optical trapping process based on the enhancement and the confinement of both magnetic and electric near-fields by using gold Diabolo Antenna (DA). The later was recently proposed to generate huge magnetic near-field when illuminated by linearly polarized wave along its axis. Numerical 3D – FDTD simulation results demonstrate the high confinement of the electromagnetic field in the vicinity of the DA. This enhancement is then exploited for the trapping of nano-particles (NP) as small as 30 nm radius. Results show that the trapping process greatly depends on the particle dimensions and that three different regimes of, trapping at contact, trapping without contact, or pushing can be achieved within the same DA. This doubly resonant structure opens the way to the design of a novel generation of efficient optical nano-tweezers that allow manipulation of nano-particles by simply changing the operation wavelength.

Near-Electric-Field Tuned Plasmonic Au@SiO2 and Ag@SiO2 Nanoparticles for Efficient Utilization in Luminescence Enhancement and Surface-Enhanced Spectroscopy

Application of core–shell plasmonic nanostructures in fluorescence enhancement and surface-enhanced Raman scattering (SERS) strongly depends on their near-field electrodynamical environments. A nonradiative energy transfer takes place between fluorescent molecules and surface plasmon when they are too close. However, for a dielectric shell, the SERS intensity of analytes decreases exponentially beyond 2 nm thickness. Although electromagnetic-field enhancement due to surface plasmon still occurs at longer distances from the metal core, it needs a proper design of the composite nanostructure to exploit this advantage, and an optimal distance between the metal-core and analyte/fluorescent molecule still seems necessary. We analyze, both theoretically and experimentally, the near-electric-field (NEF) distributions in the proximity of the core–shell and shell–medium interfaces of Au@SiO2 and Ag@SiO2 core–shell structures immersed in common dispersing media such as air, water, and DMSO to investigate the effect...

A Review of Three-Dimensional Scanning Near-Field Optical Microscopy (3D-SNOM) and Its Applications in Nanoscale Light Management

In this article, we present an overview of aperture and apertureless type scanning near-field optical microscopy (SNOM) techniques that have been developed, with a focus on three-dimensional (3D) SNOM methods. 3D SNOM has been undertaken to image the local distribution (within ~100 nm of the surface) of the electromagnetic radiation scattered by random and deterministic arrays of metal nanostructures or photonic crystal waveguides. Individual metal nanoparticles and metal nanoparticle arrays exhibit unique effects under light illumination, including plasmon resonance and waveguiding properties, which can be directly investigated using 3D-SNOM. In the second part of this article, we will review a few applications in which 3D-SNOM has proven to be useful for designing and understanding specific nano-optoelectronic structures. Examples include the analysis of the nano-optical response phonetic crystal waveguides, aperture antennae and metal nanoparticle arrays, as well as the design of plasmonic solar cells incorporating random arrays of copper nanoparticles as an optical absorption enhancement layer, and the use of 3D-SNOM to probe multiple components of the electric and magnetic near-fields without requiring specially designed probe tips. A common denominator of these examples is the added value provided by 3D-SNOM in predicting the properties-performance relationship of nanostructured systems.

Gapless states in microwave artificial graphene

A microwave analogue of graphene comprised of cylindrical metallic rods arranged in a honeycomb array is fabricated. Dispersion curves of the bound electromagnetic eigenmodes of the system were experimentally determined by measuring the electric near-fields just above the surface. Two linear crossings are evident in these dispersion curves at each of the K and K′ points of the Brillouin zone, mimicking the well-celebrated Dirac cones in graphene.

Miniaturized Transparent Metallodielectric Resonator Antennas Integrated With Amorphous Silicon Solar Cells

Miniaturized transparent metallodielectric resonator antennas are developed and integrated on amorphous silicon solar cells. The horizontal metallic strips attached on the sidewalls of transparent low-profile dielectric resonators are used to manipulate electric near-fields and achieve novel dominant modes that represent miniaturized narrowband, wideband, or multiband antennas with various far-field properties. The radiation properties of each mode can also be adjusted to suit a prospective application by a simple change in position and/or size of the strip. The horizontal strip on the sidewall can be as narrow as one twelfth of the resonator height, and the top wall of the resonator is always kept clear to maximize the solar cell efficiency. A comprehensive comparison including far-field, near-field, and impedance properties of different modes is presented. Several prototypes of the integrated antennas were fabricated and tested. The experimental results are along with the theoretical achievements showing miniaturized antennas with maximum dimension as small as $0.18\lambda $ and multimode operation with realized gain higher than 6 dBi.

Minimization of Antenna Quality Factor

The optimal currents on arbitrarily shaped radiators with respect to the minimum quality factor Q are found using a simple and efficient procedure. The solution starts with a reformulation of the problem of minimizing quality factor Q as an alternative, so-called dual, problem. Taking advantage of modal decomposition and group theory, it is shown that the dual problem can easily be solved and always results in minimal quality factor Q. Moreover, the optimization procedure is generalized to minimize quality factor Q for embedded antennas, with respect to the arbitrarily weighted radiation patterns, or with prescribed magnitude of the electric and magnetic near-fields. The obtained numerical results are compatible with previous results based on composition of modal currents, convex optimization, and quasi-static approximations; however, using the methodology in this paper, the class of solvable problems is significantly extended.

Bridged Bowtie Aperture Antenna for Producing an Electromagnetic Hot Spot

In this work we report a new type of nanostructure, the bridged bowtie aperture (BBA) antenna, for producing a simultaneously enhanced and confined electric and magnetic near field. The optical nanocircuit theory is employed to reveal its underlying mechanism. The electric near-field distribution of the nanostructure is observed using transmission-type s-SNOM at the nanoscale, and the magnetic near-field distribution is then derived from the electric near field of a complementary BBA structure using Babinet’s principle. To our knowledge, the generation of such an electromagnetic hot spot has never been experimentally demonstrated. Relative to the existing nanostructures that can produce an electromagnetic hot spot, the BBA antenna has apparent advantages, which offers a new approach for nonlinear optics, surface-enhanced spectroscopy, biosensing, and metamaterials.

Coherent Control of Gap Plasmons of a Complex Nanosystem by Shaping Driving Femtosecond Pulses

Geometry-based control of local field of coupled plasmonic nanostructures is efficient for optimization of the field intensity. However, it provides weak control over spatial and temporal dynamics of the field and thus unsuitable for experimental studies and practical applications where fixed geometries are needed. In this study, we report on pulsed excitation of strongly coupled plasmonic nanosystem comprised of nanorod and split-ring antenna. The near-field intensities are manipulated by controlling time delay, relative phase, and polarization of the ultrafast excitation pulses. We show that the spectral and spatial intensities of the local fields at the gap regions of the coupled nanosystem can be pronounced by using two identical pulses with least time delay and phase difference. The corresponding temporal intensities of electric near-fields for both parallel and orthogonal polarization of the illumination fields are also briefly discussed. These findings might have implications for controlled excitation of complexly coupled plasmonic nanosystems.

Full vectorial mapping of the complex electric near-fields of THz resonators

Using micro-structured photo-conducting probes, we demonstrate full vectorial mapping of the complex electric fields in the near-field region of a resonant structure at THz frequencies. The investigated structure represents the simplest case of a resonator: a metallic rod. We show field amplitude as well as phase maps for the three field components at the half wavelength (λ/2) resonance of the rod. The field as well as the phase distributions are in excellent agreement with our physical understanding of local electric-field distributions in the vicinity of λ/2 resonant structures and are validated by numerical simulations. These measurements can be a platform for performance optimization of the emerging field of THz photonic and plasmonic devices with complex sub-wavelength structures.

Dimerization of Colloidal Particles through Controlled Aggregation for Enhanced Properties and Applications.

Devising syntheses capable of precisely manipulating matter on the nanoscale is central to many areas of research. The underlying motivation is fueled by the fact that at the nanometer scale, the property has a strong correlation with the structure. One such nanostructure that has accrued much attention is the dimer-a structure composed of two colloidal particles separated by a small gap. This Focus Review discusses how colloidal stability can be strategically manipulated to induce dimerization, together with effective purification steps to further improve yields. We conclude the article by providing representative examples for how dimers composed of plasmonic nanoparticles leads to structures with tunable optical properties and strong electric near-fields, ideal for application in surface-enhanced Raman spectroscopy.

Near-field-assisted localization: effect of size and filling factor of randomly distributed zinc oxide nanoneedles on multiple scattering and localization of light

We investigate the influence of the diameter and the filling factor of randomly arranged ZnO nanoneedles on the multiple scattering and localization of light in disordered dielectrics. Coherent, ultra-broadband second-harmonic (SH) microscopy is used to probe the spatial localization of light in representative nm-sized ZnO arrays of needles. We observe strong fluctuations of the SH intensity inside different ZnO needle geometries. Comparison of the SH intensity distributions with predictions based on a one-parameter scaling model indicate that SH fluctuations can be taken as a quantitative measure for the degree of localization. Interestingly, the strongest localization signatures are found for densely packed arrays of thin needles with diameters in the range of only 30 nm range, despite the small scattering cross section of these needles. FDTD simulations indicate that in this case coupling of electric near-fields between neighbouring needles governs the localization.

Light Localization and Magneto-Optic Enhancement in Ni Antidot Arrays

We reveal an explicit strategy to design the magneto-optic response of a magneto-plasmonic crystal by correlating near- and far-fields effects. We use photoemission electron microscopy to map the spatial distribution of the electric near-field on a nanopatterned magnetic surface that supports plasmon polaritons. By using different photon energies and polarization states of the incident light we reveal that the electric near-field is either concentrated in spots forming a hexagonal lattice with the same symmetry as the Ni nanopattern or in stripes oriented along the Γ-K direction of the lattice and perpendicular to the polarization direction. We show that the polarization-dependent near-field enhancement on the patterned surface is directly correlated to both the excitation of surface plasmon polaritons on the patterned surface as well as the enhancement of the polar magneto-optical Kerr effect. We obtain a relationship between the size of the enhanced magneto-optical behavior and the polarization and wavelength of optical excitation. The engineering of the magneto-optic response based on the plasmon-induced modification of the optical properties introduces the concept of a magneto-plasmonic meta-structure.

Silver nanoparticles on conducting electrode: a simple two-step process for realizing plasmonic solar cell design

Solar light harvesting cannot be efficiently managed unless and until the contribution of plasmonics in solar cells is fully realized. Such an investigation can be initiated and achieved by incorporating plasmonic nanoscatterers that support surface plasmons: excitations of conduction electrons at the interface/surface. The challenge is to fabricate these plasmonic scatterers in cost-effective method as well as without hampering optical, electrical and topographical properties of underneath layers. Here, in this report, we showed a simple two-step method in fabricating silver nanoparticles on zinc oxide followed by topographic and elemental analysis thereof. Numerical calculation and near-electric field distribution of single silver nanoparticles of different sizes was simulated by finite different time domain (FDTD) analysis. Since the distributed electric flux due to individual nanoparticles is crucial for excitons generation in active layer, such simulation predicts that nanoparticles excited by radiation of lower energy contribute wider electric flux sacrificing the intensity of localized electric field. A typical model was considered and FDTD simulation was carried out to understand the trend of absorption depth profile within the active layer involved in plasmonic solar cell.

Silver Nanoparticles on Zinc Oxide: An Approach to Plasmonic PV Solar Cell

Efficient light management in solar cells can be achieved by incorporating plasmonic nanoscatterers that support surface plasmons: excitations of conduction electrons at the interface/surface. As known, light trapping increases the amount of light absorbed by bouncing the light within the cell, giving it a chance to be absorbed thereby increasing the absorption and scattering cross-section. The challenge is to fabricate these plasmonic nanoparticles in cost-effective method as well as without hampering optical, electrical and topographical properties of underneath layers. Here in this report a simple two step method was adopted to fabricate silver nanoparticles on zinc oxide followed by topographic and elemental analysis thereof. Numerical calculation was carried out to elucidate optical scattering of silver nanoparticles of various sizes as well as that of dimer. Near-electric field distribution of single silver nanoparticles and dimer along with the individual component of electric field was simulated by finite different time domain analysis. Using the benefit of increased scattering cross-section and ease of such nanoparticles fabrication, a cell configure is proposed herewith.

Optical properties of silver hollow square embedded disk nanostructures

In this paper, we calculate the extinction spectra and the distribution of electric near-field of the nanoparticles which are embedded a disk in a hollow square structure through using the discrete dipole approximation method, and compare our results with extinction spectra and the distribution of electric near-field of the single hollow square nanostructure. The research results show that a new resonant mode, which is located in the traditional excitation wavelength range of surface enhanced raman scattering, can be produced due to the coupling interaction. Then, we can use this mode to meet the shortage of the hollow square nanostructure fabricated by nanoskiving. In addition, the surface plasmon resonance peak can be tuned by changing the shape parameters of the silver nanoparticle. These results can be used in Raman scattering, molecular biological, and chemical detection.

Absorption spectra and near-electric field enhancement effects of Au- and Ag-Fe3O4 dimers

Absorption efficiencies and near-electric field enhancement factors of Au- and Ag-Fe3O4 dimers have been calculated with the discrete dipole approximation (DDA) and electrostatics approximation (EA). The influences of metal nanoparticle size, incident wavelength and incident polarization angle on the enhancement factor are systematically investigated. It is found that the electric field enhancement effect on magnetite cube is positive above the surface plasmon resonance (SPR) wavelength in the longitudinal mode (LM), while being negative in nonresonant region in the transverse mode (TM). The maximum enhancement factors derived from SPR may be achieved in the LM. Most importantly, when we accurately evaluate the enhancement factors of the dimers, the presence of the magnetite nanocube must be taken into account because the cube significantly alters the amplitude distribution of the enhanced electric field of the metal nanosphere especially in SPR region. This study helps to understand and design the plasmonic nanostructures for the applications with strong photon–particle interactions.