Electric Near-field Distribution Research Articles

A mm-Wave 5G System Architecture With Enhanced-Gain Antenna Solution

A millimeter-wave (mm-wave) system architecture with enhanced-gain antenna solution is proposed for fifth-generation (5G) wireless communications. To enhance the antenna gain, the surface wave currents are converted to constructive far-field radiations through a novel two-stage metallic and dielectric rings. By applying low-profile dielectric ring, the electric near-field distributions of the grounded quarter-wavelength metallic ring are modified to create radiation apertures with appropriate polarization and orientation. The mutual coupling between transmit/receive antennas and cross polarizations is reduced by 7 and 5 dB, respectively, and the gain is enhanced over 4 dB. Low-loss mm-wave transitions are implemented in a standard FR-4 printed circuit board and in an interposer substrate to integrate the active antenna with the rest of the wireless system on a compact universal serial bus (USB)-interface platform. To evaluate the interposer package and define the output power of the embedded 60-GHz RF IC, an evaluation module with a standard WR-15 waveguide interface is designed and implemented. A prototype of the architecture, which is entirely realized using low-cost organic substrates, is presented and evaluated by experiments. Excellent correlations are achieved between simulations and measurements, demonstrating more than 25 dBm of equivalent isotropically radiated power (EIRP) over the unlicensed 60-GHz frequency band.

3D visualization of microwave electric and magnetic fields by using a metasurface-based indicator

Visualizations of the microwave electric and magnetic near-field distributions of radio-frequency (RF) filters were performed using the technique of thermoelastic optical indicator microscopy (TEOIM). New optical indicators based on periodic dielectric-metal structures were designed for electric field visualization. Depending on the structure orientation, such metasurface-based indicators allow separately visualization of the Ex and Ey components of the in-plane electric field. Numerical simulations were conducted to examine the working principle of the designed indicator structures, and the results were compared to the experimental, showing good agreement. In addition, the 3D visualization of the microwave near-field distribution was built, to show the field intensity and distribution dependencies on the distance from the RF filter.

Excitation and manipulation of both magnetic and electric surface plasmons.

Surface plasmons (SPs) is the cornerstone in terahertz (THz) near-field photonics, which play crucial roles in the miniaturization and integration of functional devices. The excitation and manipulation of SPs, however, is currently restricted to electric SPs paradigm, while magnetic SPs receive less attention despite the importance of magnetic light-matter interactions. Here, a scheme is proposed to simultaneously convert the propagating waves in free space into magnetic and electric SPs using a single ultracompact device. First, a plasmonic structure composed of connected slit rings is designed and demonstrated to support both electric and magnetic SPs, which is ascribed to the two distinct eigenmodes of oscillating electrons and vortex currents, respectively. Second, with the assistance of an anisotropic and gradient metasurface, orthogonal linear polarized components of incident THz beams are coupled into different electric and magnetic SP channels with little crosstalk. Furthermore, by encoding two distinct polarization-dependent phase profile into the metasurface, it is shown that the resulting meta-device can individually tailor the wavefronts of magnetic and electric SPs, thus simultaneously engineering magnetic and electric near-field distributions. This work can pave the road to realize bi-channel and on-chip devices, and inspire more integrated functionalities especially related to near-field manipulations of magnetic SPs.

Optical properties of several ternary nanostructures* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774248 and 11974253).

To investigate the optical properties of the ternary nanostructures, the nanodisk, core–shell, and three-sphere structures are constructed. The extinction coefficients and electric near-field distributions of these structures are calculated by the discrete dipole approximation (DDA) method. The result shows that the nanodisk structure has the best extinction efficiency in the three structures. Furthermore, several three-material combinations of the nanodisk structures are investigated. The ternary nanodisk structure composed of TiO2 and two noble metals (Au, Ag or Pt) has higher extinction coefficient and near-field intensity than the nanodisk consisting of Au, TiO2 and a semiconductor (PbSe, Ge, MoS2, CdSe, CdS or TiO2). Especially, TiO2/Ag/Pt has the best extinction efficiency and the max electric near-field intensity. And the extinction spectra of TiO2/Ag/Pt and TiO2/Ag/Au structures are complementary in the visible range. This work conduces to the further research into ternary nanostructure and provides essential information about its performance in visible range.

Hierarchical Hybridization in Plasmonic Honeycomb Lattices.

This paper reports hierarchical hybridization as a mode-mixing scheme to account for the unique optical properties of non-Bravais lattices of plasmonic nanoparticles (NPs). The formation of surface lattice resonances (SLRs) mediated by localized surface plasmons (LSPs) of different multipolar orders (dipole and quadrupole) can result in asymmetric electric near-field distributions surrounding the NPs. This asymmetry is because of LSP hybridization at the individual NP level from LSPs of different multipole order and at the unit cell level (NP dimer) from LSPs of the same multipole order. Fabricated honeycomb lattices of silver NPs exhibit ultrasharp SLRs at the Γ point that can also facilitate nanolasing. Modeling of the stimulated emission process revealed that the multipolar component of the lattice plasmon mode was responsible for feedback for lasing. By leveraging multipolar LSP responses in Al NP lattices, we achieved two distinct Γ point band-edge modes from a single honeycomb lattice. This work highlights how multipolar LSP coupling in plasmonic lattices with a non-Bravais symmetry has important implications for the design of SLRs and their associated plasmonic near-field distributions. These relatively unexplored degrees of freedom can decrease both ohmic and radiative losses in nanoscale systems and enable SLRs to build unanticipated connections among photonics and nanochemistry.

Broadband coherent hyperspectral near-field imaging of plasmonic nanostructures.

We develop a coherent hyperspectral near-field microscope using a combined nano-Fourier Transform Infra-Red (FTIR) spectroscope and a scattering Scanning Near-field Optical Microscope (s-SNOM) illuminated by an ultra-broadband few-cycle femtosecond source, spanning a spectrum from 660 to 1050 nm. Using this spatio-spectral approach, we resolve hyperspectral near-field response of a single plasmonic nano-antennas over 450 nm bandwidth with a spatial resolution of 40 nm and a spectral resolution of 50 cm-1. In particular, we identify the electric near-field spatial distribution of the dipole resonant mode of various nano-antennas and observe, in accordance with previous theoretical reports, that those are spectrally red-shifted from their far-field response. Moreover, we are able to spectrally and spatially differentiate the near-field distribution of the dipole and quadrupole modes at the single nanoparticle level. Being coherent and short-pulsed, our technique opens the path for optical ultrafast characterization and control of light-matter interaction at the nanoscale.

Compact 60 GHz Phased-Array Antennas With Enhanced Radiation Properties in Flip-Chip BGA Packages

Compact phased-array antennas embedded in flip-chip ball grid array (BGA) packages are proposed to operate over the unlicensed 60 GHz frequency band. Radiation properties are enhanced by designing novel corrugated soft-surface structures and implementing them using the low-temperature co-fired ceramic process with no extra cost and fabrication complexity. This is achieved by adjusting via land diameters of periodic grounded vias in top four layers of the package, which in fact create perfect magnetic boundaries between the phased-array antenna elements to modify the electric near-field distributions and consequently improve far-field characteristics including gain, cross polarization, sidelobe level, and beam steering. Low-loss transitions from flip-chip IC to antenna elements are designed with less than 0.1 dB amplitude and 2° phase imbalance over 50–70 GHz. Prototypes with/without soft-surface structures are fabricated to work with flip-chip ICs with/without switches. A USB-interface adapter and a waveguide-interface module with a compact vertical power combiner are proposed for the first time to experimentally evaluate the phased-array antenna while connected to the flip-chip RF IC. Significant miniaturizations and improvements are achieved over the wide frequency range of 56–67 GHz, demonstrating up to 7 dB reduction in sidelobe level, up to 4 dB reduction in cross polarization, and up to 3 dB enhancement in gain at different directions.

Surface current confinement in circular ring optical antennas and its enhancement effect to the photoresponse of longwave infrared photodetectors

In this paper, we report the analysis of surface current confinement in circular ring optical antennas and its enhancement effect to the photoresponse of quantum dot (QD) longwave infrared (LWIR) photodetectors. Circular ring optical antennas with various inner and outer diameters were simulated to determine the electric near-field (E-field) and surface current distribution. Over 80 times E-field enhancement was obtained when the width of the ring is as narrow as 2 nanometers (nm) where the surface current is strongly confined. In addition, the E-field (|E|) shows a direct linear relationship with the surface current density. The linear relationship is analyzed using an RLC circuit model. The E-field enhancement of the circular ring optical antenna is compared with that of the two-dimensional metallic subwavelength hole arrays structure (2DSHA) and is found to have higher enhancement with a broader spectral band coverage. The circular ring optical antenna enhanced LWIR QD infrared photodetectors were fabricated and measured. The ring optical antenna with stronger surface current confinement shows a higher photocurrent enhancement. The experimental results agree well with the simulation. Broader band photocurrent enhancement than the 2DSHA structure was also verified.

Exclusive Magnetic Excitation Enabled by Structured Light Illumination in a Nanoscale Mie Resonator.

Recent work has shown that optical magnetism, generally considered a challenging light-matter interaction, can be significant at the nanoscale. In particular, the dielectric nanostructures that support magnetic Mie resonances are low-loss and versatile optical magnetic elements that can effectively manipulate the magnetic field of light. However, the narrow magnetic resonance band of dielectric Mie resonators is often overshadowed by the electric response, which prohibits the use of such nanoresonators as efficient magnetic nanoantennas. Here, we design and fabricate a silicon (Si) truncated cone magnetic Mie resonator at visible frequencies and excite the magnetic mode exclusively by a tightly focused azimuthally polarized beam. We use photoinduced force microscopy to experimentally characterize the local electric near-field distribution in the immediate vicinity of the Si truncated cone at the nanoscale and then create an analytical model of such structure that exhibits a matching electric field distribution. We use this model to interpret the PiFM measurement that visualizes the electric near-field profile of the Si truncated cone with a superior signal-to-noise ratio and infer the magnetic response of the Si truncated cone at the beam singularity. Finally, we perform a multipole analysis to quantitatively present the dominance of the magnetic dipole moment contribution compared to other multipole contributions into the total scattered power of the proposed structure. This work demonstrates the excellent efficiency and simplicity of our method of using Si truncated cone structure under APB illumination compared to other approaches to achieve dominant magnetic excitations.

The effect of poly vinyl alcohol matrix on the light absorbance of Ag2S nanoparticles; experimental and discrete dipole approximation results

Well-ordered films of silver sulfide (Ag2S) nanoparticles (NPs) embedded in the matrix of poly vinyl alcohol (PVA) were fabricated using Successive Ionic Layer Adsorption and Reaction (SILAR) technique. Enlarging the size and increasing the concentration of Ag2S NPs by increasing the number of SILAR cycles confirmed by surface scanning electron microscopy (SEM) images of the produced films. The material composition was studied using energy dispersive X-ray (EDX) analysis. UV–visible experimental spectra of samples were compared with discrete dipole approximation (DDA) calculations results and good agreements were achieved. Increasing the number of peaks and valleys and redshift of peaks by increasing the number of SILAR cycles were obviously observed from the results. The most interesting conclusion of this work raised from electric near-field contour distributions investigations and implies that a matrix of insulator material (PVA) can cause an asymmetry in the intensity distribution of electric field around Ag2S NPs and let more dipole oscillations happen and more peaks appear in extinction/absorption spectra.

Ultrabroadband Three-Dimensional Printed Radial Perfectly Symmetric Gradient Honeycomb All-Dielectric Dual-Directional Lightweight Planar Luneburg Lens

An ultrabroadband all-dielectric planar Luneburg lens has been designed and fabricated in this study, which is in the form of a radial gradient lightweight honeycomb column. Because of the novel design of a radial symmetric honeycomb-like microstructure in the subwavelength dimension and the radial gradient configuration according to the refractive index distribution of Luneburg lens, the present lens can focus incident plane waves on the opposite side with high convergence, and its operating frequency range is rather broadband, spanning from 6 to 16 GHz. Besides, the all-dielectric honeycomb-like lens is lightweight with a mass density of 0.23 g/cm3, and its broadband transmittance is higher than the reported cases consisting of metallic metamaterial or gradient photonic crystal structures. A prototype of the lens is fabricated by using 3D printing techniques, on which the electric near-field distribution and far-field radiation pattern measurements have been carried out, and the aforementioned performances were demonstrated experimentally. It was also observed that for two point sources placed at the edge of the lens whose intersection angle with the center of the lens is 90°, the far-field radiation pattern was still kept highly directional, which means that the lens can generate two highly directional beams simultaneously, and is an efficient double input-double output device.

Nonlinear plasmonic behavior of nanohole arrays in thin gold films for imaging lipids

We demonstrate linear and nonlinear plasmonic behaviors of periodic nanohole arrays in thin gold (Au) films with varying periodicities. As expected, the linear optical transmission spectra of the nanohole arrays show a red-shift of the resonance wavelength and Wood's anomaly with increasing hole spacing. The optical transmission and electric near-field intensity distribution of the nanohole arrays are simulated using the finite element method. The nonlinear plasmonic behavior of the nanohole arrays is studied by using picosecond pulsed excitation at near-infrared wavelengths. The characteristic nonlinear signals indicating two-photon excited luminescence (TPEL), sum frequency generation, second harmonic generation, and four-wave mixing (FWM) are observed. A maximum FWM/TPEL signal intensity ratio is achieved for nanohole arrays with a periodicity of 500 nm. Furthermore, the significant FWM signal intensity and contrast compared to the background were harnessed to demonstrate the ability of surface-enhanced coherent anti-Stokes Raman scattering to visualize low concentrations of lipids deposited on the nanohole array with a periodicity of 500 nm.

Impact of Plasmon-Induced Atoms Migration in Harmonic Generation

Under illumination of a Ti:sapphire femtosecond oscillator, amplification of third harmonic generation by subwavelength plasmonic apertures is observed. However, the harmonic yield efficiency decays rapidly over time. In this work we investigate the physical phenomena behind the temporal attenuation of the harmonic signal. From high-resolution scanning electron micrographs and two-dimensional energy dispersive X-ray maps, we conclude that the attenuation of the third harmonic is attributed to trapping of a low-density carbon layer inside the plasmonic apertures. Furthermore, we show that the profile of the carbon deposit follows the enhanced electric near-field distribution, which indicates that the carbon atoms are transported to the field hotspot by the plasmonically enhanced optical tweezer effect. From the measurement of linear transmission spectra, we find that the dielectric constant inside the nanoholes is increased by the carbon deposit. However, numerical simulations demonstrate that the increase...

Near-field marking of gold nanostars by ultrashort pulsed laser irradiation: experiment and simulations

Quantitative measurements of the electric near-field distribution of star-shaped gold nanoparticles have been performed by femtosecond laser ablation. Measurements were carried out on and off the plasmon resonance. A detailed comparison with numerical simulations of the electric fields is presented. Semi-quantitative agreement is found, with slight systematic differences between experimentally observed and simulated near-field patterns close to strong electric-field gradients. The deviations are attributed to carrier transport preceding ablation.

Advances in Tip-Enhanced Near-Field Raman Microscopy Using Nanoantennas.

Tip-enhanced near-field Raman microscopy spectroscopy is a scanning probe technique that is capable of providing vibrational spectroscopic information on single nanoobjects and surfaces at (sub-) nanometer spatial resolution and high detection sensitivity. In this review, we first illustrate the physical principle of optical nanoantennas used in tip-enhanced near-field Raman microscopy and tip-enhanced Raman scattering (TERS) to efficiently couple light to Raman excitations on nanometer length scales. Although the antennas' electric near-field distributions are commonly understood to determine the spatial resolution, recent experiments showing subnanometer-resolved optical images put this understanding into question. This is because such images enter a regime in which classical electrodynamical descriptions might no longer be applicable and quantum plasmonic and atomistic effects could become relevant. After summarizing the current understanding of plasmonic phenomena at extremely short length scales, we discuss the different mechanisms contributing to the signal enhancement. In addition to the known contributions from electric-field and chemical enhancement, several new models have been proposed very recently that could provide important guidelines for the optimization of TERS experiments. We then review recent developments in the areas of antenna design, fabrication, and characterization. Finally, we briefly highlight recent applications to illustrate future directions of tip-enhanced near-field Raman microscopy and TERS.

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.

Experimental demonstration of compact spoof localized surface plasmons.

In recent years, the study of generating and detecting localized surface plasmons (LSPs) has been expanded from the optical regime to microwave regime. In this Letter, the compact spoof LSPs are introduced through both numerical simulations and near-field measurements. It is observed that the compact LSP structure could effectively reduce the resonant frequency with a stronger resonance strength (GdBsm) and a higher Q-factor. Both electric near-field and surface-current distributions are monitored to examine the resonance processes of the LSP particle.

Plasmonic Multibowtie Aperture Antenna with Fano Resonance for Nanoscale Spectral Sorting

In this work we report a new type of nanostructure, the plasmonic multibowtie aperture antenna with Fano resonance for spectral sorting at the nanoscale. Redistribution of surface current in our device plays a critical role in mode coupling to generate Fano resonance, which has never been carefully discussed before. Numerical analyses show that interactions of the electric field, the surface current, and the resulting magnetic field are all important for achieving the desired spectral sorting. Depending on the constructive or destructive interference between the broadband dipole mode and the narrow band multipole mode, electric near-field amplitude and phase distributions switch dramatically across the Fano resonance, which are observed in real space using transmission-type s-SNOM. Based on the Fano interference, photons ranging from visible to infrared spectrum can be sorted through different channels at the nanoscale according to their wavelengths, which shows apparent advantages over other existing nan...

Effects of tuning conditions on near field of MRI transmit birdcage coil at 64 MHz.

This study investigates how the tuning conditions of a 64 MHz / 1.5 T radio frequency (RF) birdcage coil modeled with an RF circuit and 3D electromagnetic co-simulation affect the electric and magnetic near-field distribution. Four models were implemented with different tuning conditions and difference between numerical results and measurements was evaluated. The results show that the simulated near-field depends significantly on coil tuning conditions, especially in volumes close to the ASTM phantom walls. Further extensive evaluations should be conducted to cover proper match between measurement and numerical results.

Deep-subwavelength imaging of both electric and magnetic localized optical fields by plasmonic campanile nanoantenna.

Tailoring the electromagnetic field at the nanoscale has led to artificial materials exhibiting fascinating optical properties unavailable in naturally occurring substances. Besides having fundamental implications for classical and quantum optics, nanoscale metamaterials provide a platform for developing disruptive novel technologies, in which a combination of both the electric and magnetic radiation field components at optical frequencies is relevant to engineer the light-matter interaction. Thus, an experimental investigation of the spatial distribution of the photonic states at the nanoscale for both field components is of crucial importance. Here we experimentally demonstrate a concomitant deep-subwavelength near-field imaging of the electric and magnetic intensities of the optical modes localized in a photonic crystal nanocavity. We take advantage of the “campanile tip”, a plasmonic near-field probe that efficiently combines broadband field enhancement with strong far-field to near-field coupling. By exploiting the electric and magnetic polarizability components of the campanile tip along with the perturbation imaging method, we are able to map in a single measurement both the electric and magnetic localized near-field distributions.