Showing papers on "Near and far field published in 2013"

Near-Field Interference for the Unidirectional Excitation of Electromagnetic Guided Modes

Abstract: Wave interference is a fundamental manifestation of the superposition principle with numerous applications. Although in conventional optics, interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near-field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.

Graphene-based photovoltaic cells for near-field thermal energy conversion.

Abstract: Thermophotovoltaic devices are energy-conversion systems generating an electric current from the thermal photons radiated by a hot body. While their efficiency is limited in far field by the Schockley-Queisser limit, in near field the heat flux transferred to a photovoltaic cell can be largely enhanced because of the contribution of evanescent photons, in particular for a source supporting a surface mode. Unfortunately, in the infrared where these systems operate, the mismatch between the surface-mode frequency and the semiconductor gap reduces drastically the potential of this technology. In this paper we propose a modified thermophotovoltaic device in which the cell is covered by a graphene sheet. By discussing the transmission coefficient and the spectral properties of the flux, we show that both the cell efficiency and the produced current can be enhanced, paving the way to promising developments for the production of electricity from waste heat.

High temperature epsilon-near-zero and epsilon-near-pole metamaterial emitters for thermophotovoltaics

Abstract: We propose a method for engineering thermally excited far field electromagnetic radiation using epsilon-near-zero metamaterials and introduce a new class of artificial media: epsilon-near-pole metamaterials. We also introduce the concept of high temperature plasmonics as conventional metamaterial building blocks have relatively poor thermal stability. Using our approach, the angular nature, spectral position, and width of the thermal emission and optical absorption can be finely tuned for a variety of applications. In particular, we show that these metamaterial emitters near 1500 K can be used as part of thermophotovoltaic devices to surpass the full concentration Shockley-Queisser limit of 41%. Our work paves the way for high temperature thermal engineering applications of metamaterials.

Generation and evolution of the terahertz vortex beam

Abstract: Based on the complementary V-shaped antenna structure, ultrathin vortex phase plates are designed to achieve the terahertz (THz) optical vortices with different topological charges. Utilizing a THz holographic imaging system, the two dimensional complex field information of the generated THz vortex beam with the topological number l=1 is directly obtained. Its far field propagation properties are analyzed in detail, including the rotation, the twist direction, and the Gouy phase shift of the vortex phase. An analytic Laguerre-Gaussian mode is used to simulate and explain the measured phenomena. The experimental and simulation results overlap each other very well.

Radiation force of an arbitrary acoustic beam on an elastic sphere in a fluid

Abstract: A theoretical approach is developed to calculate the radiation force of an arbitrary acoustic beam on an elastic sphere in a liquid or gas medium. First, the incident beam is described as a sum of plane waves by employing conventional angular spectrum decomposition. Then, the classical solution for the scattering of a plane wave from an elastic sphere is applied for each plane-wave component of the incident field. The net scattered field is expressed as a superposition of the scattered fields from all angular spectrum components of the incident beam. With this formulation, the incident and scattered waves are superposed in the far field to derive expressions for components of the radiation stress tensor. These expressions are then integrated over a spherical surface to analytically describe the radiation force on an elastic sphere. Limiting cases for particular types of incident beams are presented and are shown to agree with known results. Finally, the analytical expressions are used to calculate radiation forces associated with two specific focusing transducers.

An Improved Dipole-Moment Model Based on Near-Field Scanning for Characterizing Near-Field Coupling and Far-Field Radiation From an IC

Abstract: In this paper, an improved dipole-moment model for characterizing near-field coupling and far-field radiation from an IC based on near-field scanning is proposed. An array of electric and magnetic dipole moments is used to reproduce the field distributions in a scanning plane above an IC. These dipole moments can then be used as noise sources for the IC. In order to ensure the accurate prediction of the near-field coupling from the IC, the regularization technique and the truncated singular-value decomposition method are investigated in this paper, together with the conventional least-squares method, to reconstruct the dipole moments from the near-field scanning data. A simple example is used to demonstrate the approach. The improved dipole-moment model is particularly useful for addressing radio-frequency interference issues where near-field noise coupling needs to be accurately analyzed.

Blackbody spectrum revisited in the near field.

Abstract: We report local spectra of the near-field thermal emission recorded by a Fourier transform infrared spectrometer, using a tungsten tip as a local scatterer coupling the near-field thermal emission to the far field. Spectra recorded on silicon carbide and silicon dioxide exhibit temporal coherence due to thermally excited surface waves. Finally, we evaluate the ability of this spectroscopy to probe the frequency dependence of the electromagnetic local density of states.

Vibrational near-field mapping of planar and buried three-dimensional plasmonic nanostructures.

Abstract: Nanoantennas confine electromagnetic fields at visible and infrared wavelengths to volumes of only a few cubic nanometres. Assessing their near-field distribution offers fundamental insight into light-matter coupling and is of special interest for applications such as radiation engineering, attomolar sensing and nonlinear optics. Most experimental approaches to measure near-fields employ either diffraction-limited far-field methods or intricate near-field scanning techniques. Here, using diffraction-unlimited far-field spectroscopy in the infrared, we directly map the intensity of the electric field close to plasmonic nanoantennas. We place a patch of probe molecules with 10 nm accuracy at different locations in the near-field of a resonant antenna and extract the molecular vibrational excitation. We map the field intensity along a dipole antenna and gap-type antennas. Moreover, this method is able to assess the near-field intensity of complex buried plasmonic structures. We demonstrate this by measuring for the first time the near-field intensity of a three-dimensional plasmonic electromagnetically induced transparency structure.

High optical efficiency and photon noise limited sensitivity of microwave kinetic inductance detectors using phase readout

Abstract: We demonstrate photon noise limited performance in both phase and amplitude readout in microwave kinetic inductance detectors (MKIDs) consisting of NbTiN and Al, down to 100 fW of optical power. We simulate the far field beam pattern of the lens-antenna system used to couple radiation into the MKID and derive an aperture efficiency of 75%. This is close to the theoretical maximum of 80% for a single-moded detector. The beam patterns are verified by a detailed analysis of the optical coupling within our measurement setup.

An Effective Method of Probe Calibration in Phase-Resolved Near-Field Scanning for EMI Application

Abstract: Near-field scanning can be used to determine the far-field emissions of electronic devices. In general, this requires phase-resolved electric and magnetic near-field data. To capture a broad frequency range relatively quickly, a multichannel oscilloscope can be used for data capture. The phase relationship of the fields between different space points and between the electric and the magnetic field needs to be known. Consequently, it is required to determine the complex-valued probe factor (PF) of the probe, cable, and amplifier chain. This paper presents a fast and efficient calibration method which uses the same setup and instruments during calibration and measurement, and it allows for easy and economical integration of the calibration hardware and software into the scanning system. Known fields are created by a microstrip trace driven with a comb generator. By referencing measured data to this known field, the PF is obtained over a broad frequency range by capturing one time-domain waveform.

Broadband all-dielectric magnifying lens for far-field high-resolution imaging.

Abstract: A transformation-optics magnifying lens is reported in the microwave frequency, which is made of inhomogeneous but isotropic dielectrics to reach impedance matching. The authors demonstrate the broadband subwavelength imaging performance and magnification factor experimentally from the far-field radiation patterns.

Strong near field enhancement in THz nano-antenna arrays

Abstract: A key issue in modern photonics is the ability to concentrate light into very small volumes, thus enhancing its interaction with quantum objects of sizes much smaller than the wavelength In the microwave domain, for many years this task has been successfully performed by antennas, built from metals that can be considered almost perfect at these frequencies Antenna-like concepts have been recently extended into the THz and up to the visible, however metal losses increase and limit their performances In this work we experimentally study the light coupling properties of dense arrays of subwavelength THz antenna microcavities We demonstrate that the combination of array layout with subwavelength electromagnetic confinement allows for 104-fold enhancement of the electromagnetic energy density inside the cavities, despite the low quality factor of a single element This effect is quantitatively described by an analytical model that can be applied for the optimization of any nanoantenna array

Mapping magnetic near-field distributions of plasmonic nanoantennas.

Abstract: We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

Graphene-based tunable broadband hyperlens for far-field subdiffraction imaging at mid-infrared frequencies.

Abstract: Considering the dielectric permittivity of graphene can be tuned to be negative by external electric field, we propose to construct alternating graphene/dielectric multilayer based optical hyperlens for far-field subdiffraction imaging at mid-infrared frequencies. For such a scheme, hyperbolic dispersion curve can be achieved under the condition that the thickness of dielectric layer is made comparable to that of graphene layer, which is capable of supporting the propagation of evanescent wave with large wave vector. Simulation results by finite-element method demonstrate that two point sources with separation far below the diffraction limit can be magnified by the systems to the extent that conventional far-field optical microscopy can further manipulate. Such a hyperlens has the advantage of operating in a wideband region due to the tunability of graphene’s dielectric permittivity as opposed to previous metal based hyperlens, enabling the potential applications in real-time super-resolution imaging, nanolithography, and sensing.

Analysis of the spectral behavior of localized plasmon resonances in the near- and far-field regimes.

Abstract: The angular spectrum representation of electromagnetic fields scattered by metallic particles much smaller than the incident wavelength is used to interpret and analyze the spectral response of localized surface plasmon resonances (LSPs) both in the near-field and far-zone regimes. The previously observed spectral redshift and broadening of the LSP peak, as one moves from the far-zone to the near-field region of the scatterer, is analyzed on studying the role and contribution of the evanescent and propagating plane wave components of the emitted field. For such dipolar particles, it is found that the evanescent waves are responsible for those broadenings and shifts. Further, we prove that the shift is a universal phenomenon, and hence, it constitutes a general law, its value increasing as the imaginary part of the nanostructure permittivity grows. Our results should be of use for the prediction and interpretation of the spectral behavior in applications where the excitation of LSPs yield field enhancements like those assisting surface-enhanced Raman spectroscopy or equivalent processes.

Ultrahigh-contrast and large-bandwidth thermal rectification in near-field electromagnetic thermal transfer between nanoparticles

Abstract: We exploit the unique properties of optical waves in nanophotonic structures to enhance the capabilities for active control of electromagnetic thermal transfer at nanoscale. We show that the near-field thermal transfer between two nanospheres can exhibit a thermal rectification effect with very high contrast and large operating bandwidth. In this system, the scale invariance properties of the resonance modes result in a large difference in the coupling constants between relevant modes in the forward and reverse scenarios. Such a difference provides a mechanism for thermal rectification. The two-sphere system can also exhibit negative differential thermal conductance.

Broadband Equivalent Circuit Models for Antenna Impedances and Fields Using Characteristic Modes

Abstract: An approach for modeling antenna impedances and radiation fields in terms of fundamental eigenmodes is presented. Our method utilizes the simple frequency behavior of the characteristic modes to develop fundamental building blocks that superimpose to create the total response. In this paper, we study the modes of a dipole, but the method may be applied to more complicated structures as the modes retain many of their characteristics. We show that the eigenmode-based approach results in a more accurate model for the same complexity compared to a typical series RLC resonator model. Higher order modes can be more accurately modeled with added circuit complexity, but we show that this may not always be necessary. Because this method is based on the physical behavior of the fundamental modes, it also accurately connects circuit models to radiation patterns and other field behavior. To demonstrate this, we show that far field patterns, gain, and beam width of a dipole can be accurately extrapolated over a decade of bandwidth using data at two frequency points.

Magnetic Near-Field Probes With High-Pass and Notch Filters for Electric Field Suppression

Abstract: Several new types of low-cost and robust magnetic near-field probes manufactured in low-temperature co-fired ceramics (LTCC) are presented in this paper. Parallel C-shaped strips and their variations are inserted into the loop area in the front end of probes to achieve common-mode high-pass and notch filters for electric-field noise suppression. These probes with this kind of filter have excellent wideband electric field suppression. They are called high electric field suppression probes type A ~ D. The size of loop aperture in all probes is 100 μm long and 400 μm wide. The signal received from the loop is routed to a measurement apparatus through a semi-rigid coaxial cable with an outer diameter of 0.047 in. The flip-chip junction with low loss and good shielding is used between the probe head in LTCC and the semi-rigid coaxial cable. We take the probes over a 2000-μm-wide microstrip line as device-under-test to measure the probe characteristics. The isolation between electric and magnetic fields for a reference probe based on an old design using the same LTCC process is better than 30 dB from 0.05 to 12.65 GHz. The type A probe has two parallel C-shaped strips, it has better isolation of 35 dB from 0.1 to 11.05 GHz. Type C has one end of its strip shorted to ground, its 30-dB isolation frequency range can be extended to 0.05 ~ 17.8 GHz. With additional layout variation in type D, isolation can be improved to 40 dB up to 10.9 GHz. The spatial resolution for these probes is 140 μm when the distance between the metal surface of the microstrip line and the nearest edge of the loop is held at 120 μm. The calibration factors of the proposed probes are only slightly increased as compared with reference probe.

Observation of near-field dipolar interactions involved in a metal nanoparticle chain waveguide.

Abstract: We present near-field measurements of transverse plasmonic wave propagation in a chain of gold elliptical nanocylinders fed by a silicon refractive waveguide at optical telecommunication wavelengths. Eigenmode amplitude and phase imaging by apertureless scanning near-field optical microscopy allows us to measure the local out-of-plane electric field components and to reveal the exact nature of the excited localized surface plasmon resonances. Furthermore, the coupling mechanism between subsequent metal nanoparticles along the chain is experimentally analyzed by spatial Fourier transformation on the complex near-field cartography, giving a direct experimental proof of plasmonic Bloch mode propagation along array of localized surface plasmons. Our work demonstrates the possibility to characterize multielement plasmonic nanostructures coupled to a photonic waveguide with a spatial resolution of less than 30 nm. This experimental work constitutes a prerequisite for the development of integrated nanophotonic devices.

A Near-Field Dual Polarized (TE–TM) Microwave Imaging System

Abstract: In this paper, we introduce a novel dual polarized microwave imaging system. The system is comprised of a circular array of multiplexed antennas, distributed evenly around an object-of-interest (OI), along with a novel plurality of probes located at the antennas' apertures. Each probe consists of several p-i-n diodes biased in two different states (open and short). The probes are used to measure field scattered by an OI based on the modulated scatterer technique. Half of the probes are oriented vertically with the second half oriented horizontally. The presence of the two probe-orientations enables the imaging system to collect two orthogonal field polarizations, transverse electric (TE) and transverse magnetic (TM), without the need for mechanical rotation. In order to illuminate the object with all possible polarizations of the electromagnetic field, the transmitting antennas are placed at a slant angle with respect to the longitudinal plane of the imaging chamber. Near-field data are collected using each probe set, then calibrated. We show that the calibrated data for each polarization can be used to reconstruct the dielectric profile of various objects using either two-dimensional TE or TM inversion algorithms.

Electromagnetic radiation by quark-gluon plasma in a magnetic field

Abstract: The electromagnetic radiation by quark-gluon plasma in a strong magnetic field is calculated. The contributing processes are synchrotron radiation and one-photon annihilation. It is shown that in relativistic heavy-ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) synchrotron radiation dominates over the annihilation. Moreover, it constitutes a significant part of all photons produced by the plasma at low transverse momenta; its magnitude depends on the plasma temperature and the magnetic field strength. Electromagnetic radiation in a magnetic field is probably the missing piece that resolves a discrepancy between the theoretical models and the experimental data. It is argued that electromagnetic radiation increases with the magnetic field strength and plasma temperature.

Multi-Port Driven Radiators

Abstract: Integrated multi-port driven (MPD) radiator design is presented as an approach that takes advantage of the increased design space offered by using a hybrid design of an antenna with multiple ports and its driver circuitry integrated together on a single substrate. This reduces costly losses by eliminating independent elements for power combination, output impedance matching networks, and power transfer by engineering current patterns on a chip based on the desired far field pattern. The electromagnetic radiation produced by a circularly polarized MPD antenna is calculated analytically to provide design intuition, with supporting electromagnetic simulations. A single element 160 GHz MPD antenna and the supporting driver circuitry is designed and fabricated in a 0.13 μm SiGe BiCMOS process. A tuned 8 phase ring oscillator generates the signal with each phase feeding class A power amplifiers that drive the antenna. The radiator achieves 4.6 dBm single element effective isotropically radiated power (EIRP) and total radiated power of -2.0 dBm at 161 GHz while consuming 117.5 mA DC current from a 3.3 V source. Measurements of three frequency bands at 145, 154 and 161 GHz show greater than 0 dBm EIRP for each band, demonstrating the wide band nature of the antenna.

Radiators in Time Domain–Part I: Electric, Magnetic, and Radiated Energies

Abstract: Closed form expressions are derived in time domain to calculate the electric and magnetic energy linked to the electromagnetic field surrounding an electromagnetic device. The expressions are rigorous, general, and explicit in terms of the time dependent currents flowing on the device. They are computationally very efficient since they involve integrals solely over the device generating the field. The expressions can also be applied to and interpreted in frequency domain. This will lead to a new interpretation of stored energy in the frequency domain.

Design, Manufacturing and Test of a Dual-Reflectarray Antenna With Improved Bandwidth and Reduced Cross-Polarization

Abstract: A dual-offset reflectarray demonstrator has been designed, manufactured and tested for the first time. In the antenna configuration presented in this paper, the feed, the sub-reflectarray and the main-reflectarray are in the near field one to each other, so that the conventional approximations of far field are not suitable for the analysis of this antenna. The antenna is designed by considering the near-field radiated by the horn and the contributions from all the elements in the sub-reflectarray to compute the required phase-shift on each element of the main reflectarray. Both reflectarrays have been designed using broad-band elements based on variable-size patches in a single layer for the main reflectarray and two layers for the sub-reflectarray, incident field. The measured radiation patterns are in good agreement with the simulated results. It is also demonstrated that a reduction of the cross-polarization in the antenna is achieved by adjusting the patch dimensions. The antenna measurements exhibit a 20% bandwidth (12.2 GHz-15 GHz) (with a reduction of gain less than 2.5 dB) and a cross-polar discrimination better than 30 dB in the working frequency band.

Tuning near field radiation by doped silicon

Abstract: In this letter, we demonstrate theoretically and experimentally that bulk silicon can be employed to overcome the challenge of tuning near field radiation. Theoretical calculation shows that the nanoscale radiation between bulk silicon and silicon dioxide can be tuned by changing the carrier concentration of silicon. Near field radiation measurements are carried out on multiple bulk silicon samples with different doping concentrations. The measured near field conductance agrees well with theoretical predictions, which demonstrates a tuning range from 2 nW/K to 6 nW/K at a gap of ∼60 nm.

Plasmonic Nanoclusters with Rotational Symmetry: Polarization-Invariant Far-Field Response vs Changing Near-Field Distribution

Abstract: Flexible control over the near- and far-field properties of plasmonic nanostructures is important for many potential applications, such as surface-enhanced Raman scattering and biosensing. Generally, any change in the polarization of the incident light leads to a change in the nanoparticle’s near-field distribution and, consequently, in its far-field properties as well. Therefore, producing polarization-invariant optical responses in the far field from a changing near field remains a challenging issue. In this paper, we probe experimentally the optical properties of cruciform pentamer structures—as an example of plasmonic oligomers—and demonstrate that they exhibit such behavior due to their symmetric geometrical arrangement. We demonstrate direct control over hot spot positions in sub-20 nm gaps, between disks of 145 nm diameter at a wavelength of 850 nm, by means of scattering scanning near-field optical microscopy. In addition, we employ the coupled dipole approximation method to define a qualitative m...

Triangular metal nanoprisms of Ag, Au, and Cu: Modeling the influence of size, composition, and excitation wavelength on the optical properties

Abstract: We describe herein a systematic investigation on the optical properties of Ag, Au, and Cu triangular nanoprisms as a function of size and excitation wavelength using the discrete dipole approximation. Specifically, the edge length was varied from 40 to 100 nm while the thickness was kept at 10 nm. In the far field, our results suggest that the in-plane localized surface plasmon resonance (LSPR) peaks red-shifted as the edge length increased. In the near field, the magnitude of the electric fields generated close to the surface of the nanoprisms were calculated considering 514, 633, and 785 nm as the excitation wavelengths. The variation on the magnitude of the electric fields can be understood based on the matching between the excitation wavelength and the position of the in-plane dipole and quadrupole LSPR modes. We believe that these results can have important implications in the design of metal nanoprisms for plasmonic applications.