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Plane/polar geometry simplifies measurement of near-field data for this antenna and allows a determination of far-field pattern by Jacobi-Bessel series expansion of data. Measuring probe is an undersized, dielectrically loaded and open-ended waveguide with a far-field pattern similar to that of a small magnetic dipole in its forward directions, making it unnecessary to rotate probe in direction similar to antenna rotation.
Near field antenna measurements as an alternative to far field measurements are discussed. Most of the serious shortcomings of far field measurements can be alleviated using near field techniques. The near field data can be used as a direct diagnostic of antenna performance before the integration by the aperture which can make it difficult to isolate the cause of far field pattern anomalies.
Jet mixing noise data at different measurement distances are compared with values calculated from the Lockheed prediction method. Although the method does not include any acoustic near-field effects, the measured and predicted results agree well where the measured data deviates from the inverse square law. It is therefore suggested that departures from the inverse square law are primarily the result of (1) the non-negligible distance between the nozzle exit plane and the true axial source location and (2) the jet mixing noise directionality, as modeled in the prediction method. Allowing for these effects, jet noise data at 8 and 96 diameters over a wide range of frequencies, angles and jet conditions are shown to collapse with reasonable accuracy.
A large very precise near-field planar scanner was proposed for NASA Lewis Research Center. This scanner would permit near-field measurements over a horizontal scan plane measuring 18.3 m by 18.3 m. Large aperture antennas mounted with antenna boresight vertical could be tested up to 60 GHz. When such a large near field scanner is used for pattern testing, the antenna or antenna system under test does not have to be moved. Hence, such antennas and antenna systems can be positioned and supported to simulate configuration in zero g. Thus, very large and heavy machinery that would be needed to accurately move the antennas are avoided. A preliminary investigation was undertaken to address the mechanical design of such a challenging near-field antenna scanner. The configuration, structural design and results of a parametric NASTRAN structural optimization analysis are contained. Further, the resulting design was dynamically analyzed in order to provide resonant frequency information to the scanner mechanical drive system designers. If other large near field scanners of comparable dimensions are to be constructed, the information can be used for design optimization of these also.
Abstract Subwavelength metallic resonators provide a route to achieving strong light–matter coupling by means of tight confinement of resonant electromagnetic fields. Investigation of such resonators however often presents experimental difficulties, particularly at terahertz (THz) frequencies. A single subwavelength resonator interacts weakly with THz beams, making it difficult to probe it using far‐field methods, whereas arrays of resonators exhibit inter‐resonator coupling, which affect the resonator spectral signature and field confinement. Here, traditional far‐field THz spectroscopy is systematically compared with aperture‐type THz near‐field microscopy for investigating complementary THz resonators. While the far‐field method proves impractical for measuring single resonators, the near‐field technique gives high signal‐to‐noise spectral information, only achievable in the far‐field with resonator arrays. At the same time, the near‐field technique allows to analyze single resonators without significant interaction with the near‐field probe. Furthermore, the near‐field technique allows highly confined fields and surface waves to be mapped in space and time. This information gives invaluable insight into spectral response in resonator arrays. This near‐field microscopy and spectroscopy method enables investigations of strong light–matter coupling at THz frequencies in the single‐resonator regime.
Scanning configuration for near-field antenna measurements uses cylindrical and plane-polar coordinate surfaces to guide motions of electromagnetic-field probes. Near-field measurements needed especially to enable determination of far fields of large, low-sidelobe, multiplebeam antennas, for which application of far-field measurements often not suitable. New cylindrical/plane-polar scanning configuration requires less space, yet expected to give complete measurements with fewer mechanical complications.
In this paper, the issues pertaining to microwave imaging and holographic diagnostic to antennas in cylindrical near-field measurements are addressed. The theoretical approach is based on expanding the work in [1] and [2] where a cylindrical wave expansion of the field on a cylindrical near-field surface is given. The sampling probe is modeled by its equivalent aperture current (idealized circular aperture) and incorporated into the near-field to far-field transformation. The method of steepest decent is applied to obtain the far-field. In its implementation, however, one could specify directly the angular spectrum at which the far-field is desired to be calculated without resorting to interpolation. The microwave imaging and holographic diagnostic is based on back projection where a plane wave expansion of the far-field is obtained. This approach necessitates the knowledge of the far-field at exact angular spectrum resulting from application of 2-D FFT. Hence, we were able to construct simply the near-field on a plane not necessarily on the aperture plane of the test antenna but also on planes perpendicular to the aperture plane [3]. And a 3-D high resolution and high precision antenna imaging of the test antenna is obtained from cylindrical near-field simulated measurements. In addition microwave holographic diagnostic of large NASA scatterometer radar antenna obtained from measured near-field on a cylindrical surface will be given if time permits.
Scanning near-field optical microscopy (SNOM) is an important technique used to study the optical properties of material systems at the nanoscale. In previous work, we reported on the use of nanoimprinting to improve the reproducibility and throughput of near-field probes including complicated optical antenna structures such as the ‘campanile’ probe. However, precise control over the plasmonic gap size, which determines the near-field enhancement and spatial resolution, remains a challenge. Here, we present a novel approach to fabricating a sub-20 nm plasmonic gap in a near-field plasmonic probe through the controlled collapse of imprinted nanostructures using atomic layer deposition (ALD) coatings to define the gap width. The resulting ultranarrow gap at the apex of the probe provides a strong polarization-sensitive near-field optical response, which results in an enhancement of the optical transmission in a broad wavelength range from 620 to 820 nm, enabling tip-enhanced photoluminescence (TEPL) mapping of 2-dimensional (2D) materials. We demonstrate the potential of this near-field probe by mapping a 2D exciton coupled to a linearly polarized plasmonic resonance with below 30 nm spatial resolution. This work proposes a novel approach for integrating a plasmonic antenna at the apex of the near-field probe, paving the way for the fundamental study of light-matter interactions at the nanoscale.
This paper examines the discrepancy between distances to nearby open clusters as determined by parallaxes from Hipparcos compared to traditional main-sequence fitting. The biggest difference is seen for the Pleiades, and our hypothesis is that if the Hipparcos distance to the Pleiades is correct, then similar subluminous zero-age main-sequence (ZAMS) stars should exist elsewhere, including in the immediate solar neighborhood. We examine a color-magnitude diagram of very young and nearby solar-type stars and show that none of them lie below the traditional ZAMS, despite the fact that the Hipparcos Pleiades parallax would place its members 0.3 mag below that ZAMS. We also present analyses and observations of solar-type stars that do lie below the ZAMS, and we show that they are subluminous because of low metallicity and that they have the kinematics of old stars.
Abstract Thermophotovoltaics, devices that convert thermal infrared photons to electricity, offer a key pathway for a variety of critical renewable energy technologies including thermal energy storage, waste heat recovery, and direct solar‐thermal power generation. However, conventional far‐field devices struggle to generate reasonable powers at lower temperatures. Near‐field thermophotovoltaics provide a pathway to substantially higher powers by leveraging photon tunneling effects. Here a large area near‐field thermophotovoltaic device is presented, created with an epitaxial co‐fabrication approach, that consists of a self‐supported 0.28 cm 2 emitter‐cell pair with a 150 nm gap. The device generates 1.22 mW at 460 °C, a 25‐fold increase over the same cell measured in a far‐field configuration. Furthermore, the near‐field device demonstrates short circuit current densities greater than the far‐field photocurrent limit at all the temperatures tested, confirming the role of photon tunneling effects in the performance enhancement. Modeling suggests several practical directions for cell improvements and further increases in power density. These results highlight the promise of near‐field thermophotovoltaics, especially for low temperature applications.
Probe pattern compensation is essential in near-field scanning geometry, where there is a great need to accurately know far-field patterns at wide angular range. This paper focuses on a novel formulation and computer simulation to determine the precise need for and effect of probe compensation in cylindrical near-field scanning. The methodology is applied to a linear test array antenna and the NASA scatterometer radar antenna. The formulation is based on representing the probe by its equivalent tangential magnetic currents. The interaction between the probe equivalent aperture currents and the test antenna fields is obtained with the application of a reciprocity theorem. This allows us to obtain the probe vector output pickup integral which is proportional to the amplitude and phase of the electric field induced in the probe aperture with respect to its position to the test antenna. The integral is evaluated for each probe position on the required sampling point on a cylindrical near-field surface enclosing the antenna. The use of a hypothetical circular-aperture probe with a different radius permits us to derive closed-form expressions for its far-field radiation patterns. These results, together with the probe vector output pickup, allow us to perform computer simulated synthetic measurements. The far-field patterns of the test antenna are formulated based on cylindrical wave expansions of both the probe and test antenna fields. In the limit as the probe radius becomes very small, the probe vector output is the direct response of the near-field at a point, and no probe compensation is needed. Useful results are generated to compare the far-field pattern of the test antenna constructed from the knowledge of the simulated near-field with and without probe pattern compensation and the exact results. These results are important since they clearly illustrate the angular range over which probe compensation is needed. It has been found that a probe with an aperture radius of 0.25(lambda), 0.5(lambda), and 1(lambda) needs a little probe compensation, if any, near the test antenna main beam. In addition, a probe with low directivity may provide a better signal-to-noise ratio than a highly directive one. This is evident in test antenna patterns without probe compensation at wide angles.
The feasibility of acoustic verification of microwave near field to far field transformation algorithms using the Phased Array Sonic Simulation System was studied. Existing electromagnetic near field measurement techniques and transformation algorithms (equations) were investigated. It was analytically determined that acoustic verification is valid. Acoustic simulation of electromagnetic near field to far field transformations is emphasized. The acoustic simulation of electromagnetic near field to far field transformation is verified for the modal expansion method. In the modal expansion method, data from antenna near field measurements are converted to a summation or spectrum of modes corresponding to wave numbers in the measurement coordinate system. Fourier transformation of those measurements preserves the far field information in a spectral form that is then readily extractable.
Terahertz (THz) metasurfaces have become a key platform for engineering light-matter interaction at THz frequencies. They have evolved from simple metallic resonator arrays into tunable and programmable devices, displaying ultrafast modulation rates and incorporating emerging quantum materials. The electrodynamics which govern metasurface operation can only be directly revealed at the scale of subwavelength individual metasurface elements, through sampling their evanescent fields. It requires near-field spectroscopy and imaging techniques to overcome the diffraction limit and provide spatial resolution down to the nanoscale. Through a series of case studies, this review provides an in-depth overview of recently developed THz near-field microscopy capabilities for research on metamaterials.
Gold nanoparticles with complex anisotropic geometries offer unique opportunities for tailoring plasmonic near-fields beyond the limits of conventional nanorods. Here we combine photon-induced near-field electron microscopy (PINEM) and full-wave simulations to resolve plasmon hybridization and near-field localization in gold dog-bone nanoparticles and their dimers with and without conformal SiO 2 shells. We demonstrate that the dog-bone geometry drives unconventional polarization-dependent field confinement, including pronounced tip-localized near-fields under both longitudinal and transverse excitation. Conformal SiO 2 shells red-shift the plasmon resonances through dielectric screening and prevent charge-transfer coupling in adjacent dimers. Depending on the dimer orientation and excitation polarization, coupled structures exhibit distinct bonding- and antibonding-like hybridized modes as well as interparticle near-field channels. However, weakly interacting dimers retain largely independent particle responses with geometry-governed asymmetries. The close agreement between PINEM measurements and simulations establishes a direct picture of how shape, dielectric environment, and nanoscale coupling together define plasmonic modes in complex anisotropic nanoparticles. These findings provide design principles for engineering localized optical fields in plasmonic nanoscale structures.
Enhancement and peaks in near-field radiative heat transfer (NFRHT) typically arise due to surface phonon-polaritons, plasmon-polaritons, and electromagnetic (EM) modes in structured materials. However, the role of material quantum coherence in enhancing near-field radiative heat transfer remains unexplored. Here, we unravel that NFRHT in superconductor-ferromagnetic systems displays a unique peak at the superconducting phase transition that originates from the quantum coherence of Bogoliubov quasiparticles in superconductors. Our theory takes into account evanescent EM radiation emanating from fluctuating currents related to Cooper pairs and Bogoliubov quasiparticles in stark contrast to the current-current correlations induced by free electrons in conventional materials. Our proposed NFRHT configuration exploits ferromagnetic resonance at frequencies deep inside the superconducting band gap to isolate this superconducting coherence peak. Furthermore, we reveal that Cooper pairs and Bogoliubov quasiparticles have opposite effects on near-field thermal radiation and isolate their effects on many-body radiative heat transfer near superconductors. As a result, our proposed phenomenon can have applications for developing thermal isolators and heat sinks in superconducting circuits.
The near-field electromagnetic interaction between nanoscale objects produces enhanced radiative heat transfer that can greatly surpass the limits established by far-field blackbody radiation. Here, we present a theoretical framework to describe the temporal dynamics of the radiative heat transfer in ensembles of nanostructures, which is based on the use of an eigenmode expansion of the equations that govern this process. Using this formalism, we identify the fundamental principles that determine the thermalization of collections of nanostructures, revealing general but often unintuitive dynamics. Overall, our results provide an elegant and precise approach to efficiently analyze the temporal dynamics of the near-field radiative heat transfer in systems containing a large number of nanoparticles.
Work was completed on the near-field range control software. The capabilities of the data processing software were expanded with the addition of probe compensation. In addition, the user can process the measured data from the same computer terminal used for range control. The design of the laser metrology system was completed. It provides precise measruement of probe location during near-field measurements as well as position data for control of the translation beam and probe cart. A near-field range measurement system was designed, fabricated, and tested.