Showing papers on "Near and far field published in 2017"

Real-time near-field terahertz imaging with atomic optical fluorescence

Abstract: A time-averaged intensity distribution of terahertz waves is imaged by converting terahertz waves to optical fluorescence. The conversion becomes possible by exciting Cs atoms to a Rydberg state. The image acquisition time is 40 ms. Terahertz (THz) near-field imaging is a flourishing discipline1,2, with applications from fundamental studies of beam propagation3 to the characterization of metamaterials4,5 and waveguides6,7. Beating the diffraction limit typically involves rastering structures or detectors with length scale shorter than the radiation wavelength; in the THz domain this has been achieved using a number of techniques including scattering tips8,9 and apertures10. Alternatively, mapping THz fields onto an optical wavelength and imaging the visible light removes the requirement for scanning a local probe, speeding up image collection times11,12. Here, we report THz-to-optical conversion using a gas of highly excited Rydberg atoms. By collecting THz-induced optical fluorescence we demonstrate a real-time image of a THz standing wave and use well-known atomic properties to calibrate the THz field strength.

Antenna system with planar antenna and directors and methods for use therewith

Abstract: Aspects of the subject disclosure may include a planar antenna configured to transmit first signals as a first guided electromagnetic wave that is bound to a surface of a transmission medium, wherein the first guided electromagnetic wave propagates along the surface of the transmission medium without requiring an electrical return path, wherein the planar antenna includes an array of patch antennas that generates first near field signals in response to the first signals and a plurality of directors configured to guide a portion of the first near field signals from the array of patch antennas to the surface of the transmission medium, and wherein the portion of the first near field signals combines to induce the first guided electromagnetic wave that is bound to the surface of the transmission medium.

Antenna system with planar antenna and methods for use therewith

Abstract: Aspects of the subject disclosure may include a planar antenna configured to transmit first signals as a first guided electromagnetic wave that is bound to a surface of a transmission medium, wherein the first guided electromagnetic wave propagates along the surface of the transmission medium without requiring an electrical return path, wherein the planar antenna includes an array of patch antennas that generates first near field signals in response to the first signals, and wherein a portion of the first near field signals combines to induce the first guided electromagnetic wave that is bound to the surface of the transmission medium.

Label-Free Single-Molecule Imaging with Numerical-Aperture-Shaped Interferometric Scattering Microscopy

Abstract: Our ability to optically interrogate nanoscopic objects is controlled by the difference between their extinction cross sections and the diffraction-limited area to which light can be confined in the far field. We show that a partially transmissive spatial mask placed near the back focal plane of a high numerical aperture microscope objective enhances the extinction contrast of a scatterer near an interface by approximately T–1/2, where T is the transmissivity of the mask. Numerical-aperture-based differentiation of background from scattered light represents a general approach to increasing extinction contrast and enables routine label-free imaging down to the single-molecule level.

Compressed sensing with near-field THz radiation

Abstract: We demonstrate a form of near-field terahertz (THz) imaging that is compatible with compressed sensing algorithms. By spatially photomodulating THz pulses using a set of shaped binary optical patterns and employing a 6-μm-thick silicon wafer, we are able to reconstruct THz images of an object placed on the exit interface of the wafer. A single-element detector is used to measure the electric field amplitude of transmitted THz radiation for each projected pattern, with the ultra-thin wafer allowing us to access the THz evanescent near fields to achieve a spatial resolution of ∼9 μm (λ/45 at 0.75 THz). We conclude by experimentally improving the image rate by a factor of ∼3 by undersampling the object with adaptive and compressed sensing algorithms.

High‐Quality‐Factor Mid‐Infrared Toroidal Excitation in Folded 3D Metamaterials

Abstract: With unusual electromagnetic radiation properties and great application potentials, optical toroidal moments have received increasing interest in recent years. 3D metamaterials composed of split ring resonators with specific orientations in micro-/nanoscale are a perfect choice for toroidal moment realization in optical frequency considering the excellent magnetic confinement and quality factor, which, unfortunately, are currently beyond the reach of existing micro-/nanofabrication techniques. Here, a 3D toroidal metamaterial operating in mid-infrared region constructed by metal patterns and dielectric frameworks is designed, by which high-quality-factor toroidal resonance is observed experimentally. The toroidal dipole excitation is confirmed numerically and further demonstrated by phase analysis. Furthermore, the far-field radiation intensity of the excited toroidal dipoles can be adjusted to be predominant among other multipoles by just tuning the incident angle. The related processing method expands the capability of focused ion beam folding technologies greatly, especially in 3D metamaterial fabrication, showing great flexibility and nanoscale controllability on structure size, position, and orientation.

Robust Ultraminiature Capsule Antenna for Ingestible and Implantable Applications

Abstract: Progress in implantable and ingestible wireless biotelemetry requires versatile and efficient antennas to communicate reliably from a body. We propose an ultraminiature 434 MHz antenna immune to impedance detuning caused by varying electromagnetic properties of the surrounding biological environment. It is designed for a standard input impedance of 50 $\Omega $ . The antenna is synthesized and miniaturized using a hybrid analytical–numerical approach, and then optimized to conform to the inner surface of a 17 mm long biocompatible encapsulation (7 mm diameter). The substrate is 50 $\mu \text{m}$ thick. The capsule antenna is analyzed both in simplified and anatomically realistic heterogeneous phantoms. It remains matched at common implantation sites and through the whole gastrointestinal tract. Enhanced robustness allows using the antenna for a wide range of in-body applications. Computed reflection coefficients and radiation performance both show good agreement with measurements. The far field is characterized with the direct illumination technique using an analog fiber optic link. The realized gain (measured max. value −19.6 dBi) exceeds the counterparts by about 3 dBi. The proposed antenna contributes to the further development of a new generation of miniature in-body devices that involve complex and dense integration of sensors, logic, and power source.

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

Abstract: 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.

Wavelet decomposition of hydrodynamic and acoustic pressures in the near field of the jet

Abstract: An experimental investigation of pressure fluctuations generated by a single-stream compressible jet is carried out in an anechoic wind tunnel. Measurements are performed using a linear array of microphones installed in the near region of the jet and a polar arc of microphones in the far field. The main focus of the paper is on the analysis of the pressure fluctuations in the near field. Three novel signal processing techniques are presented to provide the decomposition of the near-field pressure into hydrodynamic and acoustic components. The procedures are all based on the application of the wavelet transform to the measured pressure data and possess the distinctive property of requiring a very simple arrangement to obtain the desired results (one or two microphones at most). The hydrodynamic and acoustic pressures are characterized separately in terms of their spectral and statistical quantities and a direct link between the acoustic pressure extracted from the near field and the actual noise in the far field is established. The analysis of the separated pressure components sheds light on the nearly Gaussian nature/intermittent behaviour of the acoustic/hydrodynamic pressure. The higher sensitivity of the acoustic component to the Mach number variation has been highlighted as well as the different propagation velocities of the two pressure components. The achieved outcomes are validated through the application to the same data of existing separation procedures evidencing the advantages and limitations of the new methods.

Design and Analysis of a Reconfigurable Holographic Metasurface Aperture for Dynamic Focusing in the Fresnel Zone

Abstract: We present numerical simulations of the near-field focusing capabilities of a dynamically reconfigurable holographic metasurface aperture. The aperture consists of a parallel-plate waveguide in which the upper plate is patterned with a number of metamaterial irises that can be dynamically switched between radiating (ON) and non-radiating (OFF) states. A cylindrically symmetric waveguide mode, excited by a coaxial probe in the center of the lower plate, serves to excite the radiating irises, forming a focused spot in the radiating near-field (or Fresnel zone). The layout of the metamaterial elements and their tuning states is determined using holographic design principles, in which the interference pattern of the waveguide (or reference) mode and the desired radiated field pattern leads to the required phase distribution over the surface of the aperture. We also develop an analytical model of the aperture to confirm the numerical simulations, and to illustrate the advantage of the guided-mode as the reference wave versus a plane-wave. We further leverage this analytical model to analyze the diffracted order characteristics of the holographic metasurface aperture, showing high-fidelity focusing patterns even for difficult focusing scenarios across the entire investigated field-of-view.

Single mode fiber based delivery of OAM light by 3D direct laser writing

Abstract: We demonstrate orbital-angular momentum (OAM) light up to a topological charge of l=3 behind a single mode fiber. Femtosecond 3D direct laser writing is used to fabricate spiral phase plates of l=1,2 and 3, composed of 10 discrete steps, on the tip of single mode optical fibers. These structures efficiently convert out-coupled light from the fiber at 785 nm wavelength into optical vortex beams carrying an orbital-angular momentum of lℏper photon. Far field intensity patterns and interferograms of the OAM beams are recorded using a CCD camera. The results are in excellent agreement with numerical simulations obtained from the wave propagation method.

Depletion of Intense Fields

Abstract: The interaction of charged particles and photons with intense electromagnetic fields gives rise to multiphoton Compton and Breit-Wheeler processes. These are usually described in the framework of the external field approximation, where the electromagnetic field is assumed to have infinite energy. However, the multiphoton nature of these processes implies the absorption of a significant number of photons, which scales as the external field amplitude cubed. As a result, the interaction of a highly charged electron bunch with an intense laser pulse can lead to significant depletion of the laser pulse energy, thus rendering the external field approximation invalid. We provide relevant estimates for this depletion and find it to become important in the interaction between fields of amplitude a0?103 and electron bunches with charges of the order of 10 nC.

Decoupling absorption and emission processes in super-resolution localization of emitters in a plasmonic hotspot.

Abstract: The absorption process of an emitter close to a plasmonic antenna is enhanced due to strong local electromagnetic (EM) fields. The emission, if resonant with the plasmonic system, re-radiates to the far-field by coupling with the antenna via plasmonic states, whose presence increases the local density of states. Far-field collection of the emission of single molecules close to plasmonic antennas, therefore, provides mixed information of both the local EM field strength and the local density of states. Moreover, super-resolution localizations from these emission-coupled events do not report the real position of the molecules. Here we propose using a fluorescent molecule with a large Stokes shift in order to spectrally decouple the emission from the plasmonic system, leaving the absorption strongly resonant with the antenna’s enhanced EM fields. We demonstrate that this technique provides an effective way of mapping the EM field or the local density of states with nanometre spatial resolution. Reporting the position of molecules and the electromagnetic enhancement in a plasmonic hotspot is difficult. Here Macket al. use a large Stokes-shifted molecule to spectrally decouple the emission process of the dye from the plasmonic system, keeping the absorption on resonance with the plasmon resonance of the antenna.

A Miniature Ultrawideband Electric Field Probe Based on Coax-Thru-Hole via Array for Near-Field Measurement

Abstract: In this paper, a miniature electric field probe with an ultrawideband of 9 kHz–20 GHz is proposed, fabricated, and tested. The electric field probe is fabricated on a four-layer printed circuit board using high-performance and low-loss Rogers material ( $\varepsilon _{\mathrm{r}}= 3.48$ and tan $\delta = 0.0037$ ). Coax-thru-hole via array is used to control the signal via impedance to achieve impedance $50~\Omega $ match over the whole working band, reducing the harmful influence on the probe’s characteristic. The ground vias, called via fence, are utilized to suppress the resonance caused by the parallel-plate mode of conductor-backed coplanar waveguide (CB-CPW), expanding the working frequency band. Experimental result shows $\vert S_{21}\vert $ rather smooth in operation band, demonstrating the working frequency band is up to 9 kHz–20 GHz. The electric field probe has a 2–3 mm spatial resolution, which has a good ability to locate the interference source.

Near-Field Microwave Imaging Using Open-Ended Circular Waveguide Probes

Abstract: Near-field microwave imaging and subsurface sensing have shown significant potential in a wide range of applications, including corrosion mapping and composite structure inspection. Most of the conventional near-field imaging systems developed so far utilize an antenna (probe) scanned over the structure-under-test (SUT). The near-field interaction between the microwave signal and the SUT reveals the sought-after information about the interior of the irradiated structure. Previous systems commonly used open-ended rectangular waveguides as the near-field imaging probe. In this paper, the utility of circular waveguide probes for near-field imaging will be discussed in detail and compared with the conventional rectangular probe. The motivation behind utilizing the circular probe is the fact that its aperture field distribution yields lower side lobes. Herein, the reflection properties, near-field characteristics, sensitivity, and spatial resolution of both probes are contrasted using simulations and measurements. Furthermore, imaging results of various targets, such as flat-bottom holes, notches, as well as corrosion-under-paint samples acquired using circular and rectangular probes are presented. It will be shown that the circular probes offer key advantages in terms of resolution as well as sensitivity compared with the conventional open-ended rectangular waveguides.

Laser-Induced Linear-Field Particle Acceleration in Free Space.

Abstract: Linear-field particle acceleration in free space (which is distinct from geometries like the linac that requires components in the vicinity of the particle) has been studied for over 20 years, and its ability to eventually produce high-quality, high energy multi-particle bunches has remained a subject of great interest. Arguments can certainly be made that linear-field particle acceleration in free space is very doubtful given that first-order electron-photon interactions are forbidden in free space. Nevertheless, we chose to develop an accurate and truly predictive theoretical formalism to explore this remote possibility when intense, few-cycle electromagnetic pulses are used in a computational experiment. The formalism includes exact treatment of Maxwell’s equations and exact treatment of the interaction among the multiple individual particles at near and far field. Several surprising results emerge. We find that electrons interacting with intense laser pulses in free space are capable of gaining substantial amounts of energy that scale linearly with the field amplitude. For example, 30 keV electrons (2.5% energy spread) are accelerated to 61 MeV (0.5% spread) and to 205 MeV (0.25% spread) using 250 mJ and 2.5 J lasers respectively. These findings carry important implications for our understanding of ultrafast electron-photon interactions in strong fields.

Optical Polarization Möbius Strips on All-Dielectric Optical Scatterers

Abstract: In this article, we study the emergence of polarization singularities in the scattering of optical resonators excited by linearly polarized light. First, we prove analytically that spherical all-dielectric nanoparticles described by combinations of electric and magnetic isotropic polarizabilities can sustain L surfaces and C lines that propagate from the near-field to the far field. Based on these analytical results, we are able to derive anomalous scattering Kerker conditions using singular optics arguments. Next, through full-field calculations, we demonstrate that high refractive index spherical resonators present such topologically protected features. We calculate the polarization structure of light around the generated C lines, unveiling a Mobius strip structure in the main axis of the polarization ellipse when calculated on a closed path around the C line. These results prove that high-index nanoparticles are excellent candidates for the generation of polarization singularities and that they may lea...

Color-selective holographic retroreflector array for sensing applications

Abstract: Corner cube retroreflectors (CCRs) have applications in sensors, image processing, free space communication and wireless networks. The ability to construct low-loss wavelength filters embedded in CCRs can enable the development of wavelength multiplexing, tunable lasers and photonic integrated circuits. Here we created an ~10-μm-thick holographic corner cube retroreflector (HCCR) array that acted as a color-selective wavelength filter and diffracted light at broad angles. Angle-resolved spectral measurements showed that the Bragg peak of the diffracted light from the HCCR array could be tuned from 460 to 545 nm by varying the incident angle. The HCCR array also exhibited a wavelength-selective tuning capability based on the rotation angle in the visible spectrum. HCCRs projected holographic images with the rotational property in the far field. The utility of the HCCR was demonstrated as optical temperature and relative humidity sensors that produced a visible colorimetric response for rapid diagnostics. Planar arrays of reflectroreflectors can function as sensitive optical sensors for determing changes in temperature or relative humidity. Haider Butt at the University of Birmingham in the UK and co-workers used holography to write an array of planar holographic corner cube reflectors in a layer of gelatin containing silver bromide nanocrystals. Due to diffraction, the array reflects a narrow-band signal (Bragg peak) whose wavelength is sensitive to external conditions. In particular, the color of reflection changes when variations in temperature or relative humidity cause the internal structure of the holographic medium to swell or shrink. The peak wavelength of reflection of the array changed between 500 and 700 nanometers as the relative humidity was varied between 10 and 80%. The researchers anticipate that such arrays could find application in compact optical systems and sensors.

Source Reconstruction for IC Radiated Emissions Based on Magnitude-Only Near-Field Scanning

Abstract: Near-field scanning with phase measurement is always a challenge in practical experiments because of its complexity and lack of accuracy. Therefore, utilizing the magnitude-only information to achieve phase-retrieval and emissions source reconstruction is preferred. This paper proposes a fast and accurate algorithm that can model the emissions from ICs by generating a set of equivalent dipole elements. The process of this algorithm only requires the information of the field magnitudes on two near-field scanning planes of different heights. Unlike other conventional source reconstruction techniques (such as genetic algorithm, gradient optimization), the proposed approach iteratively performs the back-and-forth transformations among the fields and equivalent dipole elements. By optimizing the locations of the equivalent dipole array and the initial values of these dipole elements, the iteration process is capable of quickly converging to the correct electromagnetic field values, including both magnitude and phase. The fields recovered from the calculated equivalent dipole sources have been validated by comparing to the fields emitted from a practical IC source.

Analytic Optimization of Near-Field Optical Chirality Enhancement.

Abstract: We present an analytic derivation for the enhancement of local optical chirality in the near field of plasmonic nanostructures by tuning the far-field polarization of external light. We illustrate the results by means of simulations with an achiral and a chiral nanostructure assembly and demonstrate that local optical chirality is significantly enhanced with respect to circular polarization in free space. The optimal external far-field polarizations are different from both circular and linear. Symmetry properties of the nanostructure can be exploited to determine whether the optimal far-field polarization is circular. Furthermore, the optimal far-field polarization depends on the frequency, which results in complex-shaped laser pulses for broadband optimization.

A Meander Line UHF RFID Reader Antenna for Near-field Applications

Abstract: A novel ultrahigh frequency radio frequency identification reader antenna based on electromagnetic coupling between two open-ended microstrip (MS) meander lines for near-field applications is investigated in this paper. The corresponding currents flowing along the two MS meander lines are reversed in phase with approximately identical amplitudes. Meander-line units are introduced to achieve a uniform distribution of strong magnetic and electric fields. The performance of an antenna prototype comprised of six pairs of meander lines is analyzed. The proposed antenna simultaneously exhibits a uniform magnetic field distribution with a reading region of 480 mm $\times200$ mm $\times20$ mm and a uniform linear electric field distribution with a reading region of 480 mm $\times420$ mm $\times300$ mm. The proposed antenna exhibits a low far-field gain, and has a bandwidth from 914 to 929 MHz. Both simulated and measured results have shown a good performance of the antenna.

Exciting Vorticity Through Higher Order Bessel Beams With a Radial-Line Slot-Array Antenna

Abstract: In this communication, it is shown that a nondiffracting vortex beam (i.e., a higher order Bessel beam with azimuthal phase variation) can be generated in the near field by synthesizing an inward cylindrical traveling-wave distribution over a finite aperture antenna. A radial line slot array (RLSA) is then designed to prove the concept. The collimated vortex beam is excited in the proximity of the RLSA, within a region properly defined by the nondiffracting range of the generated beam. The radial dependence of the longitudinal electric field of the vortex-beam magnitude follows a first-order Bessel function, and its phase presents a linear azimuthal variation. Full-wave results validate the generation of the nondiffractive higher order Bessel beam within the radiative near field of the launcher.

Fast Antenna Far-Field Characterization via Sparse Spherical Harmonic Expansion

Abstract: A procedure is proposed to significantly reduce the amount of time to characterize 3-D antenna far-field patterns. The measured far field is expanded into spherical harmonics, and a sparse recovery algorithm is used to recover the spherical wave coefficients giving access to the field radiated by the antenna everywhere. A small number of measurement points are required, since the relevant information of the most antenna patterns is concentrated in only a few spherical wave coefficients. Sampling strategies enabling fast spherical scans are discussed, which makes the approach both efficient and easy to implement in existing far-field measurement facilities. Simulations are first provided to show the potentialities of this compressive sensing-based approach. The proposed strategy is then applied to characterize 3-D far-field patterns radiated by several antennas operating in different frequency bands measured in far field in direct line of sight configuration and in a compact antenna test range. Experimental results show that a saving in the number of measurement points up to 70% can be achieved compared with standard approaches. These results pave the way to a more efficient use of far-field measurement facilities.

A Factorization Method for Multifrequency Inverse Source Problems with Sparse Far Field Measurements

Abstract: We consider a multifrequency inverse source problem for time-harmonic acoustic or electromagnetic waves with a limited set of far field data. Assuming that far field measurements of the wave radiat...

Near-Field Shielding Performances of EMI Noise Suppression Absorbers

Abstract: This paper describes a systematic study on the near-field behavior of an electromagnetic interference (EMI) noise suppression absorber materials considering two different types of commercial absorbers and different types of excitation source: at first, elementary electric dipole and elementary magnetic loop, and then using real dimensions of a coaxial cable for designing a dipole and loop. Virtual versions of the material are created and simulated with the purpose of understanding the mechanisms of high values of power loss when the absorber is in the near field of the source The focus of this study is the evaluation of the electromagnetic power balance in near field for each absorbing material and considered source to understand their near-field shielding properties. Full-wave simulations and measurement are performed and compared to verify the performance of the EMI noise suppression absorber materials in the near field.

Bandwidth broadening of a linear polarization converter by near-field metasurface coupling.

Abstract: We experimentally demonstrate a highly efficient, broadband linear polarization converter functioning at terahertz frequencies. The linear polarization converter is composed of three metasurfaces and two dielectric layers interlaced with each other. The neighboring unit cells of the central metasurface layer of the linear polarization converter exhibit strong electromagnetic coupling, which increases the number of resonances and results in significant bandwidth broadening. The simulation and experimental results show that in the frequency range of 0.2 to 0.4 THz, the proposed polarization converter has a flat transmission curve and exhibits a transmission efficiency that is higher than 80%. High performance terahertz polarization conversion is desirable in many fields, such as terahertz spectroscopy, imaging, and communications.

High-performance near-field electromagnetic wave attenuation in ultra-thin and transparent graphene films

Abstract: Ultra-thin and transparent electromagnetic interference (EMI) shielding and absorbing materials are in increasing demand for near-field electromagnetic wave attenuation in transparent electronic devices that get thinner and lighter. Here, we report chemical-doped and undoped graphene as the thinnest and transparent shield for high-performance near-field electromagnetic wave attenuation. The electromagnetic loss characterization demonstrate that a single layer graphene film exhibits a giant magnetic field transmission loss normalized to the film thickness that is at least two orders of magnitude higher than those of conventional EMI shielding and absorbing materials, which is attributed to the outstanding magnetic field mirroring in graphene. The doped and double-layer graphene films exhibit superior power and transmission losses than the commercial transparent indium tin oxide shield over the frequency range from 0.1 GHz to 6 GHz. The high-performance near-field electromagnetic wave attenuation in graphene enables broad range applications such as futuristic transparent display devices.