Showing papers on "Near and far field published in 2019"

Isolation Enhancement for MIMO Patch Antennas Using Near-Field Resonators as Coupling-Mode Transducers

Abstract: In this paper, a novel decoupling technique for closely packed patch antennas using near-field resonator (NFR) above each antenna element is proposed. The decoupling mechanism is illustrated by investigating the electric-field (E-field) and magnetic-field (H-field) distributions. The E-field distributions indicate that the NFRs above the patches serve as coupling-mode transducers to produce an orthogonal coupling mode at the desired resonance, leading to high port isolation between the patches. The H-field distributions demonstrate that the H-fields in the substrate are confined within the excited element, leading to the effective suppression of antenna mutual coupling. The NFR can be easily applied to multiple-input multiple-output (MIMO) antennas having multiple patch elements. Three practical decoupling examples are demonstrated and the simulation and measurement results show that impedance matching for each antenna port and isolation of better than 20 dB are achieved for all these examples using the NFRs. Moreover, for the H-plane and E-plane decoupling of wideband two-port MIMO antennas, wide decoupled impedance bandwidths of 6.1% and 5.8% are obtained, respectively. More results of radiation patterns reveal that good radiation performance is reserved with no reduction in realized gain or front-to-back ratio.

Nanophotonic engineering of far-field thermal emitters

Abstract: Thermal emission is a ubiquitous and fundamental process by which all objects at non-zero temperatures radiate electromagnetic energy. This process is often assumed to be incoherent in both space and time, resulting in broadband, omnidirectional light emission toward the far field, with a spectral density related to the emitter temperature by Planck’s law. Over the past two decades, there has been considerable progress in engineering the spectrum, directionality, polarization and temporal response of thermally emitted light using nanostructured materials. This Review summarizes the basic physics of thermal emission, lays out various nanophotonic approaches to engineer thermal emission in the far field, and highlights several applications, including energy harvesting, lighting and radiative cooling. This Review covers the basic physics of thermal emission, ways to engineer the thermal field radiated by hot objects in the far field and applications, such as thermophotovoltaics, radiative cooling, camouflage and privacy.

Localization of Signals in the Near-Field of an Antenna Array

Abstract: The problem of locating signals transmitted in the proximity of an antenna array has been studied extensively in the signal processing literature. In this paper, we consider the standard array manifold models used in these works and show that they differ, sometimes significantly, from the model based on electromagnetic theory. In particular, the standard models do not correspond to the equations governing an electromagnetic field near an antenna or an array. They also do not take into account the characteristics of the near-field source, such as the type and orientation of the transmitting antenna, which may have a profound impact on the signals received by the array. We use selected numerical examples based on a numerical electromagnetic code to illustrate the various issues raised herein.

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell.

Abstract: Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attractin...

Design and analysis of a hexa-band frequency reconfigurable antenna for wireless communication

Abstract: A compact (33 × 16 × 1.6 mm3) and novel shaped hexa-band frequency-reconfigurable antenna with a very wide tuning band is proposed. The proposed antenna operates at two single band modes (i.e., 3.5 GHz and 4.8 GHz) and two dual band modes (i.e., 2.10 GHz, 4.15 GHz and 2.4 GHz, 5.2 GHz) depending upon the switching states. The lumped elements are used in the simulation environment to achieve tunable capacitance, which is responsible for frequency reconfigurability. The measured tuning capability of the fabricated antenna ranges from 2.1 to 5.2 GHz. The proposed antenna has a VSWR 1.3 for all the resonant bands. The radiation efficiency of the proposed structure ranges from 80.41% to 96% at the corresponding frequencies. The far field and the scattering parameters of the proposed antenna are analyzed using Computer Simulation Technology (CST) Microwave Studio 2014. The designed antenna, due to its compact and affordable geometry, can be easily integrated in the modern communication devices such as smart phones, laptops and other portable electronic devices. A prototype of the designed antenna is fabricated and measured using PIN diode switches to validate the simulation results. The proposed reconfigurable antenna demonstrates a reasonable agreement between the measured and simulated results.

Terahertz wave near-field compressive imaging with a spatial resolution of over λ/100

Abstract: We demonstrate terahertz (THz) wave near-field imaging with a spatial resolution of ∼4.5 μm using single-pixel compressive sensing enabled by femtosecond-laser (fs-laser) driven vanadium dioxide (VO2)-based spatial light modulator. By fs-laser patterning a 180 nm thick VO2 nanofilm with a digital micromirror device, we spatially encode the near-field THz evanescent waves. With single-pixel Hadamard detection of the evanescent waves, we reconstructed the THz wave near-field image of an object from a serial of encoded sequential measurements, yielding improved signal-to-noise ratio by one order of magnitude over a raster-scanning technique. Further, we demonstrate that the acquisition time was compressed by a factor of over four with 90% fidelity using a total variation minimization algorithm. The proposed THz wave near-field imaging technique inspires new and challenging applications such as cellular imaging.

Sound radiation and transonic boundaries of a plate with an acoustic black hole.

Abstract: The acoustic black hole (ABH) phenomenon has shown promise for noise and vibration control applications. In this paper, a two-dimensional (2-D) Daubechies wavelet (DW) model is established for the sound radiation prediction of plates embedded with a circular ABH indentation. ABH plates are shown to exhibit a reduced sound radiation efficiency as compared with their flat counterpart. Below the critical frequency, this is caused by the weakening of the structural stiffness due to the ABH indentation. Above the critical frequency, a subsonic region inside the ABH cell may appear, containing acoustically slow structural waves. This region, confined within a transonic boundary, is due to the ABH-specific phase velocity reduction of the bending waves. Drawing energy away from the supersonic region of the plate, this subsonic region warrants a reduced sound radiation to the far field. Numerical results on the investigated configuration show an increase in the sound radiation efficiency due to the added stiffness effect of damping layers. Sound radiation efficiency alongside the transonic boundary changes is scrutinized and quantified. Visualization of the supersonic acoustic intensity and radiation allows identifying the effective sound radiation regions of ABH plates and their relationship with the transonic boundaries at different frequencies.

A Low Frequency Mechanical Transmitter Based on Magnetoelectric Heterostructures Operated at Their Resonance Frequency

Abstract: Magneto-elasto-electric (ME) coupling heterostructures, consisting of piezoelectric layers bonded to magnetostrictive ones, provide for a new class of electromagnetic emitter materials on which a portable (area ~ 16 cm2) very low frequency (VLF) transmitter technology could be developed. The proposed ME transmitter functions as follows: (a) a piezoelectric layer is first driven by alternating current AC electric voltage at its electromechanical resonance (EMR) frequency, (b) subsequently, this EMR excites the magnetostrictive layers, giving rise to magnetization change, (c) in turn, the magnetization oscillations result in oscillating magnetic fields. By Maxwell’s equations, a corresponding electric field, is also generated, leading to electromagnetic field propagation. Our hybrid piezoelectric-magnetostrictive transformer can take an input electric voltage that may include modulation-signal over a carrier frequency and transmit via oscillating magnetic field or flux change. The prototype measurements reveal a magnetic dipole like near field, demonstrating its transmission capabilities. Furthermore, the developed prototype showed a 104 times higher efficiency over a small-circular loop of the same area, exhibiting its superiority over the class of traditional small antennas.

Experimental demonstration of linear and spinning Janus dipoles for polarisation- and wavelength-selective near-field coupling.

Abstract: The electromagnetic field scattered by nano-objects contains a broad range of wavevectors and can be efficiently coupled to waveguided modes. The dominant contribution to scattering from subwavelength dielectric and plasmonic nanoparticles is determined by electric and magnetic dipolar responses. Here, we experimentally demonstrate spectral and phase selective excitation of Janus dipoles, sources with electric and magnetic dipoles oscillating out of phase, in order to control near-field interference and directional coupling to waveguides. We show that by controlling the polarisation state of the dipolar excitations and the excitation wavelength to adjust their relative contributions, directionality and coupling strength can be fully tuned. Furthermore, we introduce a novel spinning Janus dipole featuring cylindrical symmetry in the near and far field, which results in either omnidirectional coupling or noncoupling. Controlling the propagation of guided light waves via fast and robust near-field interference between polarisation components of a source is required in many applications in nanophotonics and quantum optics.

Proactive Conformal Antenna Array for Near-Field Beam Focusing and Steering Based on Curved Substrate Integrated Waveguide

Abstract: The quadratic phase distribution on the antenna aperture is necessary to produce a near-field-focused (NFF) beam. The generation, as well as the steering of the quadratic phase distribution, is difficult for a series-fed NFF array antenna, especially for a large-aperture application. This is because the phase control capability of the planar series-fed beam-forming network is limited. In this paper, a curved substrate integrated waveguide (SIW) is employed as the feeding carrier to enhance the amplitude and phase control capability. The spatial placement of the SIW slot array antenna can be sufficiently used to synthesize the desired NFF beam. As examples, two kinds of proactive conformal SIW NFF slot array antennas, working in the standing-wave mode and the leaky-wave mode, have been developed. Both of them have the ability to generate the high-quality NFF beam. In addition, the proactive conformal SIW leaky-wave slot array antenna has the stable focal height over a wide steerable range. After careful analysis and design, these two kinds of antenna arrays are fabricated and proactively conformal to 3-D-printed frameworks with the desired shape. The theoretical analysis and simulated results are verified by experimental results.

Antenna Design and Near-Field Characterization for Medical Microwave Imaging Applications

Abstract: In medical microwave imaging (MMWI), the antennas usually operate at close distance from the body and are required to pick up weak echoes from the inside that are masked by high skin reflection. The near-field antenna behavior strongly determines how the received signals affect the effectiveness of the imaging algorithms. However, these usually simply assume the usual antenna far-field radiation characteristics. Here, we discuss three antenna effects that can deteriorate the overall imaging performance but are rarely addressed in the literature: antenna internal reflections, angular dispersion of the “near-field phase center,” and antenna frequency dispersive electric length. We propose dedicated methods to characterize these factors and present mitigation strategies to be integrated into the inversion algorithms. We demonstrate these effects for three frequently used broadband antennas, using signals from an experimental breast imaging lab setup. In fact, the antenna study and design cannot ignore that it operates in the near field of breast and interacts with its boundary. The methods and conclusions can be extended to other MMWI applications. To the best of our knowledge, this is the first systematic study of the above antenna factors in the context of MMWI.

Weak Measurement Enhanced Spin Hall Effect of Light for Particle Displacement Sensing.

Abstract: A spherical nanoparticle can scatter tightly focused optical beams in a spin-segmented manner, meaning that the far field of the scattered light exhibits laterally separated left- and right-handed ...

Total Field Reconstruction in the Near Field Using Pseudo-Vector $E$ -Field Measurements

Abstract: Exposure assessments in the frequency range above 10 GHz typically require knowledge of the power density very close to the radiator (at 2-mm distance), which can be obtained through the total electric and magnetic fields. However, phase measurements are often not feasible in this frequency range, in particular in the reactive near field. We developed a novel phase reconstruction approach based on plane-to-plane reconstruction algorithms. It uses $E$ -field polarization ellipse information, which can be obtained extremely close to the source with probes based on the pseudo-vector sensor design. The algorithm's robustness and accuracy were analyzed and optimized for distances of a fraction of the wavelength $\lambda$ , and a comprehensive set of realistic exposure conditions was simulated to evaluate the algorithm. For distances greater than $\lambda$ /5, the error of the spatially averaged peak incident power density is found to be below 0.5 dB. Measurements in four different antenna prototypes revealed that the simulated deviation of reconstructed averaged power density was consistently below 1.1 dB for distances as close as 2 mm, i.e., smaller than the estimated total experimental assessment uncertainty of 1.4 dB. This demonstrates that the power density can be reliably determined by measurements as close as $\lambda$ /5 from any transmitter.

High-power vortex beam generation enabled by a phased beam array fed at the nonfocal-plane.

Abstract: High-power vortex beams have extensive applications in optical communication, nonlinear frequency conversion, and laser processing. To overcome a single beam’s power limitation, generating vortex beams, based on a phased beam array, is an intuitive idea that requires locking each beamlet’s phase to a specific different value. Conventionally, the intensity profiles of the focal plane (far field) are used for extracting the cost functions in active phase control systems. However, as for generating vortex beams, the cost function extraction method at the focal plane suffers because the same intensity profile of the beam array could correspond to different phase distributions in near field. Thus, the accurate phase control signals are difficult to obtain. In this paper, a new concept of extracting cost functions at the non-focal-plane is firstly presented and analyzed in detail by numerical simulation. This cost function extraction method is an efficient way of generating vortex beams with different topological charges, including second-order Bessel-Gaussian beams. The new concept could provide a valuable reference and contribute to the practical implementation of generating vortex beams by coherent beam combining technology.

Harnessing near-field thermal photons with efficient photovoltaic conversion

Abstract: A huge amount of thermal energy is available close to material surfaces in radiative and non-radiative states, which can be useful for matter characterization or for energy devices. One way to harness this near-field energy is to scatter it to the far field. Another way is to bring absorbers close to thermal emitters, and the advent of a full class of novel photonic devices exploiting thermal photons in the near field has been predicted in the last two decades. However, efficient heat-to-electricity conversion of near-field thermal photons, i.e. the seminal building block, could not be achieved experimentally until now. Here, by approaching a micron-sized infrared photovoltaic cell at nanometric distances from a hot surface, we demonstrate conversion efficiency up to 14% leading to unprecedented electrical power density output (7500 W.m-2), orders of magnitude larger than all previous attempts. This proof of principle is achieved by using hot graphite microsphere emitters (~800 K) and indium antimonide cells, whose low bandgap energy matches the emitter infrared spectrum and which are specially designed for the near field. These results pave the way for efficient photoelectric detectors converting thermal photons directly in the near field. They also highlight that near-field thermophotovoltaic converters, which harvest radiative thermal energy in a contactless manner, are now competing with other energy-harvesting devices, such as thermoelectrics, over a large range of heat source temperatures.

Simultaneous Measurement of Electric and Magnetic Fields With a Dual Probe for Efficient Near-Field Scanning

Abstract: A near-field scanning is usually time-consuming to map the electromagnetic field along the surface of a device. In this communication, a dual probe is proposed with a U-shaped loop to sense radio frequency (RF) magnetic and electric fields simultaneously, so that the one-time scanning by the dual probe can replace the two-time scanning by single-detector probes. Different from the traditional near-field scanning by using a single-detector probe and spectrum analyzer, the proposed scanning can be achieved by the two-output probe and a two-port vector network analyzer connecting to the probe. The efficient near-field scanning measurements of the electric and magnetic field on a conductor-backed coplanar waveguide transmission line and an RF power divider by using the dual probe are in agreement with those by using two single-detector probes.

An Optimization Method for the Synthesis of Flat-Top Radiation Patterns in the Near- and Far-Field Regions

Abstract: This paper presents an optimization method for the design of array antennas with flat-top radiation patterns in the far-field and near-field regions, in which the synthesis of flat-top radiation patterns is formulated as a quadratically constrained quadratic programing (QCQP) problem with equality constraints. It is shown that the notoriously difficult QCQP problem can be linearized and simplified for the synthesis of flat-top radiation patterns in the far-field region and can be extended to the near-field region by introducing a weighted diagonal matrix. To validate the optimization theory, a flat-top sector beam antenna for base station applications and an RFID bookshelf reader antenna are designed and fabricated. Simulation and measurement results show that the base station antenna operating at 5.4 GHz provides a flat-top radiation pattern with less than 1 dB fluctuation in the desired covering range of the main beam. The five-element RFID reader antenna working in the UHF band for library applications is demonstrated to generate uniform electric field intensity with less than 3 dB fluctuation across the whole bookshelf.

Laser-Driven Electron Lensing in Silicon Microstructures

Abstract: We demonstrate a laser-driven, tunable electron lens fabricated in monolithic silicon. The lens consists of an array of silicon pillars pumped symmetrically by two 300 fs, 1.95 μm wavelength, nJ-class laser pulses from an optical parametric amplifier. The optical near field of the pillar structure focuses electrons in the plane perpendicular to the pillar axes. With 100±10 MV/m incident laser fields, the lens focal length is measured to be 50±4 μm, which corresponds to an equivalent quadrupole focusing gradient B^{'} of 1.4±0.1 MT/m. By varying the incident laser field strength, the lens can be tuned from a 21±2 μm focal length (B^{'}>3.3 MT/m) to focal lengths on the centimeter scale.

Realization of Split Beam Antenna Using Transmission-Type Coding Metasurface and Planar Lens

Abstract: In this paper, a novel compact split beam configuration of the transmission-type coding metasurface (MS) in combination with the planar lens and the patch antenna is presented. In the first stage, the spherical electromagnetic waves originating from the patch antenna are converted to plane waves by placing the planar MS lens at a height of $0.55\lambda _{0}$ above the antenna aperture. This lens enhances the radiation along the broadside direction resulting in high-gain operation. Furthermore, a concept of the digital metamaterial is adopted by realizing 1 bit transmission-type coding MSs, capable of splitting the aforementioned high-gain beam pattern. These digital MSs (DMSs) have two different types of metaatoms with phase responses of 0 and $\pi $ , corresponding to two basic digital elements 0 and 1, respectively. The assembly of these metamaterial bits (either 0 or 1) leads to a lattice of size $D \times D$ . The proper spatial mixture of these lattices with different sequences facilitates different elemental metamaterial-pattern resulting in multiple-lobe radiation patterns, which are angularly oriented and symmetrical to the antenna axis. The designed planar lens and the DMS with the individual thickness of $0.057\lambda _{0}$ are placed in the near field of antenna leading to an overall compact configuration with a total height of $1.08\lambda _{0}$ . The aforementioned beam splitting using the proposed configuration is also validated both quantitatively (mathematically) and qualitatively (through the simulation and the measurement) for different types of DMSs.

Pattern Reconfigurable, Vertically Polarized, Low-Profile, Compact, Near-Field Resonant Parasitic Antenna

Abstract: A vertically polarized, low-profile, compact, near-field resonant parasitic antenna with pattern reconfigurability is demonstrated. The antenna has three dynamic end-fire states facilitated with only three p-i-n diodes. The radiation pattern in each state covers more than 120° in its azimuth plane and, hence, it achieves beam scanning that covers the entire azimuth plane. The antenna height and transverse size are, respectively, only $0.048\lambda _{0}$ and $0.1\lambda _{0}^{2}$ . Measured results, in good agreement with their simulated values, demonstrate that the antenna exhibits a ~11% fractional impedance bandwidth, and a ~6.6 dBi peak realized gain in all three of its pattern-reconfigurable states. Stable and high peak realized gain values are realized over its entire operational band surrounding 2.22 GHz.

Near-field terahertz probes with room-temperature nanodetectors for subwavelength resolution imaging

Abstract: Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. Here, we conceive a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.

Far-Field Subwavelength Imaging Using Phase Gradient Metasurfaces

Abstract: Imaging subwavelength features of an object is in close relationship with extracting information included in evanescent waves scattered from the object. These evanescent waves decay exponentially with distance; therefore, they cannot be captured at far field, resulting in a resolution limited image of the object. Here, we propose a far-field imaging technique based on gradiant metasurfaces, with the ability to provide an image of subwavelength features. In this technique, gradient metasurfaces are used to convert evanescent waves scattered by subwavelength features into propagating waves resulting in a resolution beyond the diffraction limit. The performance of the proposed imaging technique is evaluated using full-wave numerical analysis, and the results validate its capability for imaging beyond the diffraction limit.

A Microwave Power Transmission Experiment Based on the Near-Field Focused Transmitter

Abstract: For most microwave power transmission (MPT) applications, the microwave power is received within the near field of a large-scale transmitting antenna from a limited distance. This letter explores the design of a near-field focused transmitting antenna to concentrate the microwave power on the receiving antenna's aperture, and thereby to enhance the transmission efficiency of the MPT. A near-field planar array with a size of 1 m × 1 m working at 5.8 GHz is manufactured. To focus the microwave power at a distance of 10 m, its 8 × 8 radiation elements are fed with an equal amplitude but different phase using coaxial cables of different lengths. An MPT experiment is conducted. The measurement shows that, compared with a traditional equal-phase array, the near-field focused array obtains a much more concentrated microwave beam. The transmission efficiency is significantly increased from 32.98% to 41.85%.

Far-field acoustic subwavelength imaging and edge detection based on spatial filtering and wave vector conversion.

Abstract: The resolution of acoustic imaging suffers from diffraction limit due to the loss of evanescent field that carries subwavelength information. Most of the current methods for overcoming the diffraction limit in acoustics still operate in the near-field of the object. Here we demonstrate the design and experimental realization of an acoustic far-field subwavelength imaging system. Our system is based on wave vector filtering and conversion with a transmitter at the near-field and a spatially symmetrical receiver at the far-field. By tuning geometric parameters of the transmitting/receiving pair, different spatial frequency bands can be separated and projected to the far-field. Furthermore, far-field imaging and edge detection of subwavelength objects are experimentally demonstrated. The proposed system brings new possibilities for far-field subwavelength wave manipulation, which can be further applied to medical imaging, nondestructive testing, and acoustic communication. Plasmonic effects and subwavelength scattering arrays are used in the optical domain to access subwavelength resolution imaging in the far field. Here, the authors develop an analogous strategy for far-field, subwavelength imaging at acoustic wavelengths and demonstrate edge detection of acoustic scattering objects.

Fundamental Limits to Near-Field Optical Response over Any Bandwidth

Abstract: A new mathematical framework for describing the electromagnetic near field provides upper limits for light-matter interactions in this complex region regardless of material shape and composition.

Synthesis Algorithm for Near-Field Power Pattern Control and Its Experimental Verification via Metasurfaces

Abstract: Antennas and metasurfaces have enabled a number of far-field manipulation functions. According to the addition theorem of multipoles, near fields have richer spatial spectra than far fields; hence, in the near-field region, it is easier to achieve complex manipulations on field distributions. In this paper, an algorithm is presented to synthesize the near fields of source arrays. Given target distributions of near-field intensities and also predefined source magnitudes, the algorithm will find the needed source phases, which can then be applied on active antenna arrays and passive metasurfaces to induce the target distribution of fields. The algorithm adopts the dyadic Green’s function as the propagator, and hence, it naturally takes account of the near-field and vector properties of electromagnetic fields. As a typical application of the algorithm, an example is given to obtain the excitation phases of a dipole array, which is then physically imitated by a coding metasurface. Experimental measurement is performed and the result proves the validity of the algorithm. From the perspective of information metasurface, the algorithm finds the phase-coding pattern of metasurface to achieve specific functions.

Huygens' Dipole for Polarization-Controlled Nanoscale Light Routing

Abstract: Structured illumination allows for satisfying the first Kerker condition of in-phase perpendicular electric and magnetic dipole moments in any isotropic scatterer that supports electric and magnetic dipole resonances. The induced Huygens' dipole may be utilized for unidirectional coupling to waveguide modes that propagate transverse to the excitation beam. We study two configurations of a Huygens' dipole, longitudinal electric and transverse magnetic dipole moments or vice versa. We experimentally show that only the radially polarized emission of the first and azimuthally polarized emission of the second configuration are directional in the far field. This polarization selectivity implies that directional excitation of either transverse magnetic (TM) or transverse electric (TE) waveguide modes is possible. Applying this concept to a single dielectric nanoantenna excited with structured light, we are able to experimentally achieve scattering directivities of around 23 and 18 dB in TM and TE modes, respectively. This strong directivity paves the way for tunable polarization-controlled nanoscale light routing and applications in optical metrology, localization microscopy, and on-chip optical devices.

Resolution limits in inverse source problem for strip currents not in Fresnel zone.

Abstract: This paper deals with the classical question of estimating achievable resolution in terms of configuration parameters in inverse source problems. In particular, the study is developed for two-dimensional prototype geometry, where a strip source (magnetic or electric) is to be reconstructed from its radiated field observed over a bounded rectilinear domain parallel to the source. Resolution formulas are well known when the field is collected in the far field or in the Fresnel zone of the source. Here, the plan is to expand those results by removing the geometrical limitations due to the far field or Fresnel approximations. To this end, the involved radiation operators are recast as Fourier-type integral operators upon introducing suitable variable transformations. For magnetic sources, this allows one to find a closed-form approximation of the singular system and hence to estimate achievable resolution, the latter given as the main beam width of the point-spread function. Unfortunately, this does not happen for electric currents. In this case, the radiation operator is inverted by a weighted adjoint inversion method (a back-propagation-like method) that directly allows one to find an analytical expression of the point-spread function and hence of the resolution. The derived resolution formulas are the same for magnetic and electric currents; they clearly point out the role of geometrical parameters and coincide with the one pertaining to the Fresnel zone when the geometry verifies the Fresnel approximation. A few numerical examples are also enclosed to check the theory.

Effect of an Electric Field in the Heat Transfer between Metals in the Extreme Near Field

Abstract: Radiative heat transfer between metals in the extreme near field in the presence of a potential difference between them has been calculated. Because of the coupling between the electric field of radiation and displacements of the surfaces, the radiative heat flux between two gold plates at nanometer distances that is due to p-polarized electromagnetic waves increases by many orders of magnitude at the variation of the potential difference from 0 to 10 V. The radiative mechanism of heat transfer is compared to the phonon mechanism associated with the electrostatic and van der Waals interactions. The phonon mechanism determined by the van der Waals interaction dominates at subnanometer distances and small potential difference. However, the radiative contribution dominates at nanometer distances because the phonon contribution decreases with increasing distance more rapidly than the radiative contribution. The results obtained in this work can be used to control heat fluxes at a nanoscale by means of the potential difference.