Antenna Tip Research Articles

Surface-enhanced optical absorption and induced heating in tapered silicon nanoprobes

Subject of study. This study investigates the correlation between the heating temperature and the mesoscopic shape of the Si probe of an atomic force microscope subjected to medium-intensity laser irradiation (5mW/cm2) in the presence of a rough metal substrate. Aim of study. This study aims to quantitatively assess the dependence of optical-field enhancement and induced heating of the tip of a tapered Si nanoprobe, under laser irradiation, on the radius of curvature and cone angle of the probe tip, the distance between the tip and substrate, and the surface roughness of an Au substrate. Method. The localization of the electromagnetic field in the gap between a Si nanoantenna and a defect on the surface of an Au substrate was simulated using the finite-difference time-domain method. A thin Au coating (thickness up to 50 nm) on a glass substrate was used as a plasmonic surface. Owing to the excitation of surface plasmon resonance, such a coating enhances the absorption of light and increases the heating temperature of the Si optical antenna. Main results. The influence of the polarization angle of the incident laser radiation on the distribution of the electric field near the tip of the probe was examined. Only the component of the incident light field along the direction of the probe axis was enhanced near the tip of the Si cantilever. The influence of various parameters, including the radius of curvature, cone angle of the tip of the Si nanoantenna, distance between the probe and substrate, and surface roughness of the Au substrate, on the maximum temperature in the tip region of the Si probe was investigated. The probe temperature was found to decrease with decreasing cone angle of the probe, whereas the temperature of the cantilever tip decreased as the cone angle of the probe tip increased. The dependence of temperature on the radius of curvature of the Si nanoantenna tip in the presence of an Au substrate was evaluated. With an increase in the surface roughness of the Au film, the temperature of the Si antenna tip increased, gradually approaching a limit value. Practical significance. The results of this study are useful for selecting the optimal parameters of an experiment in which a heated probe is used. Controlled heating of a Si probe can facilitate the study of phase transitions in various nanomaterials and that of local thermochemical nanocatalysis for creating new structural materials with specific properties.

Terahertz non-destructive testing of power generator bars with a dielectric waveguide antenna

AbstractTerahertz technologies for non-destructive testing (NDT) are continuing to find their way into the industrial sector in the context of very specific inspection tasks. Part of this development is the capability to adapt terahertz systems in such a way that they can meet the sometimes harsh challenges and requirements of real-world industrial scenarios. One such scenario is the inspection of components with limited available measurement space. In particular, we show here the terahertz NDT inspection of the mica insulation of generator bars of turbogenerators at power plants, where an early on-site detection of defects and cracks in the insulation can be crucial, but where only few centimeters of space between adjacent bars are available. To address this problem, we have developed a measurement system combining a 100 GHz all-electronic terahertz transceiver with a low-loss dielectric waveguide antenna with 90 degree tip. We achieve sub-wavelength image resolution by scanning the waveguide antenna's tip over the surface of the generator bars in a near-field measurement setup. Employing a frequency-modulated continuous wave technique, we obtain depth-resolved, volumetric terahertz images of the objects under test. We discuss here the implementation and performance of the implemented measurement system for terahertz NDT inspection. keywords: terahertz, non-destructive testing, dielectric waveguides, frequency-modulated continuous wave, millimeter waves, power generators

Mechanism of Capacitive Coupling of Proximal Electromagnetic Sources With Biological Bodies.

This study investigates the absorption of the induced E-field in homogeneous biological tissue exposed to highly localized field sources in proximity of the body, such as the charged tips of antennas, where E-field coupling dominates. These conditions are relevant for compliance testing of modern mobile phones where exposure is evaluated at small separation between radiators and the body. We derive an approximation that characterizes the decay of the induced E-field in the tissue as a function of distance. The absorption is quantified in terms of the local specific absorption rate (SAR) at the tissue surface as a function of the charge at the antenna tip. The approximation is based on the analytical evaluation of the E-fields of a charged disk under quasi-static conditions. We validate this approximation using full-wave simulations of dipoles. We demonstrate that the coupling mechanism of the E-field is dominated by the perpendicular field component and that wave propagation need not be considered for the characterization of the exposure. The surface SAR decreases approximately with the fourth power of the distance and with the square of the ratio of the permittivities of the tissue and free-space. The approximation predicts the induced maximum E-field with an accuracy of better than 1.5 dB. © 2022 Bioelectromagnetics Society.

Turkestan Cockroaches Avoid Entering a Static Electric Field upon Perceiving an Attractive Force Applied to Antennae Inserted into the Field.

Simple SummaryElectrostatic devices that generate a static electric field (S-EF) are used as barriers to repel insect pests because insects habitually avoid entering a S-EF. Understanding the avoidance mechanism would provide an experimental basis for developing electrostatic-based pest control methods. An apparatus generating a S-EF was constructed by placing a polyvinyl chloride-insulated iron plate (N-PIP) that was negatively charged using a voltage generator parallel to a grounded metal net (G-MN) that was positively polarised via electrostatic induction due to the negative charge of the N-PIP. The S-EF formed in the space between the N-PIP and G-MN, where the negative charge of the N-PIP generated a repulsive force to push electrons in the field toward the ground via the G-MN. A test insect (an adult Turkestan cockroach, Shelfordella lateralis Walker) released in the space surrounded by the apparatus inserted its antenna into the S-EF and reflexively moved backward. Free electrons were pushed out of the antenna tip toward the ground, and the antenna became positive. The positively polarised antenna was attracted to the N-PIP, and in response to this force, the insect pulled its antennae back reflexively and moved backward. This insect action was recognised as avoidance behaviour.This study analysed the mechanism of avoidance behaviour by adult Turkestan cockroaches (Shelfordella lateralis Walker) in response to a static electric field (S-EF) formed in the space between a negatively charged polyvinyl chloride-insulated iron plate (N-PIP) and a grounded metal net (G-MN). The negative surface charge supplied to the iron plate by a voltage generator caused the G-MN to polarise positively via electrostatic induction. In the S-EF, the negative charge of the N-PIP created a repulsive force that pushed free electrons in the field toward the ground via the G-MN. When insects released in the space surrounded by the S-EF inserted their antennae into the S-EF, they pulled them back reflexively and moved backward. The analysis indicated that an electric current flowed transiently toward the ground when an insect inserted its antennae into the S-EF. The insect became positively charged via this discharge and was attracted to the opposite pole (N-PIP). In response to this attractive force, the insect pulled its antennae back quickly. The positive electrification caused by the removal of free electrons from the antenna tip triggered the avoidance behaviour.

Using a High-Speed Plasma as a Conducting Channel to Enable a Novel Antenna Approach

We present a modeling technique to describe the behavior of an antenna with time-varying conductivity. The conducting channel of the antenna will consist of a series of individually-controlled plasma cells. This antenna will be fed with pulse-amplitude modulated signals. The pulses in these signals will travel down the length of the antenna but will be attenuated near the antenna tip. This time-domain matching technique allows for efficient matching at all frequencies. The time-varying antenna scheme is modeled using a modified version of Telegrapher's equations, which includes the effects of plasma dispersion. Theoretical plasma parameters are incorporated to develop requirements for plasma density and switching speed. Simulated results suggest that the time-varying antenna scheme will radiate more efficiently at low frequencies than existing electrically short antenna designs.

650 MHz elliptical superconducting RF cavities for CiADS Project

650 MHz multicell superconducting radio frequency (SRF) elliptical cavities are proposed for effective acceleration of proton beam in the Chinese initiative Accelerator Driven Subcritical System (CiADS). Two families of such cavities will be used in the driver superconducting linac to accelerate the H+ beam from 175 MeV to 500 MeV, with the possibility to upgrade the energy to 1 GeV and higher. Detail designs of a six-cell βopt = 0.62 elliptical cavity and a five-cell βopt = 0.82 elliptical cavity will be presented in this paper, including the multi-parameter electromagnetic design and optimization, high order modes analyses, multipacting simulations, mechanical and engineering analyses. Sub-systems of the cavity such as the fundamental power coupler antenna tip design and mechanical design of cavity-helium vessel-tuner system will also be discussed in detail. Three single cell prototype cavities were fabricated and vertical tested to verify the electromagnetic properties, the fabrication and post-processing technologies. Four multi-cell prototype cavities are under manufacturing and expected to be ready in 2021.

Field Uniformity Measurements of Flat Broadband Dipoles Used for Near-Field Immunity Testing

Near-field immunity tests have become very popular in the last years. Especially in automotive EMC-applications (ISO 11452-9 and many automotive manufacturer standards), new test methods for immunity tests against portable transmitters have been introduced since 2009. In contrast to widely applied radiated immunity tests according to EN 61000-4-3 or MIL-STD-461G, which are typically performed at distances of 1 m to 3 m between antenna tip and equipment under test, the near-field immunity test methods are done at very close spacings of 5 to 50 mm and on component test level only. The small separation between radiating elements and the surface of the equipment under test leads to a very efficient power to fieldstrength conversion. The characterisation of the generated near-field was done using separate miniaturized scanning probes for E-field and H-field. Measurements of the field uniformity are presented in cartesian coordinates with contour lines as “heatmaps”. The frequency range for these near-field tests is 360 MHz to 2.7 GHz, currently (2020) being extended up to 6 GHz.

Direct detection of gap mode plasmon resonances using attenuated total reflection-based tip-enhanced near-field optical microscopy

Resonant excitation of gap mode plasmons between an optical antenna tip and a thin metal film was studied over different tip-surface distance regimes using simultaneous detection of surface reflectance via attenuated total reflection, elastic (Rayleigh) scattering by the antenna tip, and Raman scattering from analytes in the tip-surface gap region. Finite difference time domain optical simulations were used to elucidate the primary mechanisms that influence the optical behavior of this gap plasmonic mode, and spectroscopic measurements were used to demonstrate the effect of the gap mode on the enhancement of Raman and fluorescence signals. In particular, the presence of a gap mode, and the spectral overlap of its resonance with Raman emissions at specific tip-surface distances, can contribute to significant enhancements in Raman scattering signals.

Tip-enhanced strong coupling spectroscopy, imaging, and control of a single quantum emitter.

Optical cavities can enhance and control light-matter interactions. This level of control has recently been extended to the nanoscale with single emitter strong coupling even at room temperature using plasmonic nanostructures. However, emitters in static geometries, limit the ability to tune the coupling strength or to couple different emitters to the same cavity. Here, we present tip-enhanced strong coupling (TESC) with a nanocavity formed between a scanning plasmonic antenna tip and the substrate. By reversibly and dynamically addressing single quantum dots, we observe mode splitting up to 160 meV and anticrossing over a detuning range of ~100 meV, and with subnanometer precision over the deep subdiffraction-limited mode volume. Thus, TESC enables previously inaccessible control over emitter-nanocavity coupling and mode volume based on near-field microscopy. This opens pathways to induce, probe, and control single-emitter plasmon hybrid quantum states for applications from optoelectronics to quantum information science at room temperature.

The value of iterative metal artifact reduction algorithms during antenna positioning for CT-guided microwave ablation

Objectives: To compare image quality between filtered back projection (FBP) and iterative reconstruction algorithm and dedicated metal artifact reduction (iMAR) algorithms during antenna positioning for computed tomography-guided microwave ablation (MWA).Materials and methods: An MWA antenna was positioned in the liver of five pigs under CT guidance. Different exposure settings (120kVp/200mAs-120kVp/50mAs) and image reconstruction techniques (FBP, iterative reconstruction with and without iMAR) were applied. Quantitative image analysis included density measurements in six positions (e.g., liver in extension of the antenna [ANTENNA] and liver >3 cm away from the antenna [LIVER-1]). Qualitative image analysis included assessment of overall quality, image noise, artifacts at the antenna tip, artifacts in liver parenchyma bordering antenna tip and newly generated artifacts. Two independent observers performed the analyses twice and interreader agreement was compared with Bland–Altman analysis.Results: For all exposure and reconstruction settings, density measurements for ANTENNA were significantly higher for the I30-1 iMAR compared with FBP and I30-1 (e.g., 8.3−17.2HU vs. −104.5 to 155.1HU; p ≤ 0.01, respectively). In contrast, for all exposure settings, density measurements for LIVER-1 were comparable between FBP and I30-1 iMAR (e.g., 49.4−50.4HU vs. 50.1−52.5U, respectively). For all exposure and reconstruction settings, subjective image quality for LIVER-1 was better for the I30-1 iMAR algorithm compared with FBP and I30-1. Bland–Altman interobserver agreement was from −0.2 to 0.2 for FBP and iMAR, and Cohen’s kappa was 0.74.Conclusion: Iterative algorithms I30-1 with iMAR algorithm improves image quality during antenna positioning and placement for CT-guided MWA and is applicable over a range of exposure settings.

SURFACE WAVE-BASED RADIO COMMUNICATION THROUGH CONDUCTIVE ENCLOSURES

A surface wave antenna operating in the 2.4 GHz band and efficient for launching surface electromagnetic waves at metal/dielectric interfaces is presented. The antenna operation is based on the strong field enhancement at the antenna tip, which results in efficient excitation of surface waves propagating along nearby metal surfaces. Since surface electromagnetic waves may efficiently tunnel through deeply subwavelength channels from inner to outer metal/dielectric interface of a metal enclosure, this antenna is useful for broadband radio communication through various conductive enclosures, such as typical commercial Faraday cages.

Surface Wave Based Underwater Radio Communication

An underwater portable radio antenna operating in the 50 MHz band and efficient for launching surface electromagnetic waves at the seawater/air interface is presented. The antenna operation is based on the field enhancement at the antenna tip and on an impedance matching antenna enclosure, which is filled with de-ionized water. This enclosure allows us to reduce antenna dimensions and improve the coupling of electromagnetic energy to the surrounding salt water medium. Since surface wave propagation length far exceeds the skin depth of conventional radio waves at the same frequency, this technique is useful for broadband underwater wireless communication over several meters distances.

Strong Light-Matter Coupling between Plasmons in Individual Gold Bi-pyramids and Excitons in Mono- and Multilayer WSe2.

Monolayer transition-metal dichalcogenides (TMDCs) have attracted a lot of research attention recently, motivated by their remarkable optical properties and potential for strong light-matter interactions. Realization of strong plasmon-exciton coupling is especially desirable in this context because it holds promise for the enabling of room-temperature quantum and nonlinear optical applications. These efforts naturally require investigations at a single-nanoantenna level, which, in turn, should possess a compact optical mode interacting with a small amount of excitonic material. However, standard plasmonic nanoantenna designs such as nanoparticle dimers or particle-on-film suffer from misalignment of the local electric field in the gap with the in-plane transition dipole moment of monolayer TMDCs. Here, we circumvent this problem by utilizing gold bi-pyramids (BPs) as very efficient plasmonic nanoantennas. We demonstrate strong coupling between individual BPs and tungsten diselenide (WSe2) monolayers at room temperature. We further study the coupling between multilayers of WSe2 and BPs to elucidate the effect of the number of layers on the coupling strength. Importantly, BPs adopt a reduced-symmetry configuration when deposited on WSe2, such that only one sharp antenna tip efficiently interacts with excitons. Despite the small interaction area, we manage to achieve strong coupling, with Rabi splitting exceeding ∼100 meV. Our results suggest a feasible way toward realizing plasmon-exciton polaritons involving nanoscopic areas of TMDCs, thus pointing toward quantum and nonlinear optics applications at ambient conditions.

Antenna Gain Impact on UWB Wind Turbine Blade Deflection Sensing

Antenna gain impact on ultra-wideband wind turbine blade deflection sensing is studied in this paper. Simulations are applied with a 4.5-m blade tip. The antennas with high gain (HG) and low gain (LG) in free space are simulated inside a blade. It is interesting to find that tip antennas with HG and LG in free space have similar realized gain when allocated inside blades, so that the emission power for the HG and LG antennas in blades can be the same. The antenna gain impacts on time-domain pulse waveforms and power distributions around a blade are carefully investigated (with the tip antenna inside a blade). Higher antenna gain enlarges both direct pulse and multipath but in different levels. To verify the simulations, time-domain measurements are performed with a full 37-m blade. Pulse waveforms and power delay profiles are measured. From all the studies, it follows that, with the similar effective (or equivalent) isotropic radiated power, an HG tip antenna inside a blade gives stronger direct pulse amplitudes and better pulse waveforms for accurate and reliable distance estimations than the LG. Moreover, the direct pulse with the HG antenna is also closer to the blade surface, which is important for the deflection sensing of a blade over 37 m.

Synthesis of core-shell fishbone-like Cu@Ni composites and their electromagnetic wave absorption properties

In this work, core-shell fishbone-like Cu@Ni composites have been successfully prepared via a facile solution reduction method. The morphology, structure and microwave absorption (MA) properties of Cu@Ni paraffin composites were investigated. The results demonstrate that the fishbone-like Cu@Ni composites have core-shell heterostructure with Cu as the core and Ni as the shell. And the Cu@Ni paraffin-composite containing 50wt% Cu@Ni exhibits excellent electromagnetic wave absorption properties. The minimum reflection loss (RLmin) of −32.2dB can be observed at 8.2GHz and the bandwidth with RL less than −10dB reaches 13.2GHz (4.8–18.0GHz) with a thin absorber thickness between 1.0 and 3.0mm. The prominent microwave absorption properties mainly come from its synergistic advantage of magnetic loss and dielectric loss. Besides, this novel fishbone-like Cu@Ni structure is also beneficial to enhance electromagnetic-wave-absorbing properties due to the multiple reflections and scattering that occurs in the branch of fishbone-like structure. Furthermore, the one-dimensional fishbone is supposed to be as antenna tip, which can receive more waves and consume more electromagnetic energy.

Multipath Suppression With an Absorber for UWB Wind Turbine Blade Deflection Sensing Systems

The deflection of a wind turbine blade can be monitored with an ultra-wideband (UWB) deflection sensing system, which consists of one transmitting antenna at the blade tip and two receiving antennas at the blade root. The blade deflection is calculated by two estimated tip-root antenna distances. Applying the tip antenna inside a blade is highly preferred in practice, but it leads to strong multipath. The multipath may cause interference and cancellation to the direct pulse and affect the accuracy of tip-root distance estimations. In this paper, a method of utilizing an absorber inside the blade to suppress the multipath is proposed for the in-blade tip antenna. The physical mechanism of the proposed idea is described with a simplified multipath model. Simulations are performed with a 2.5-m-long blade tip section to investigate the tendencies of how the absorber affects pulse waveforms. More accurate verifications of the proposed method are carried out with different full-blade measurements. From all the results, it is found that the proposed technique can efficiently suppress multipath for the in-blade tip antenna, and improve the pulse wave front fidelity, so that the UWB sensing system can also be utilized in a much longer blade. Finally, some other conclusions are also addressed.

Apertureless near-field optical microscopy

We discuss the operating principles of the apertureless scanning near-field optical microscope (ASNOM), in which the probe acts as a rod antenna and its electromagnetic radiation plays the role of the registered signal. The phase and amplitude of the emitted wave vary depending on the ‘grounding conditions’ of the antenna tip at the sample point under study. Weak radiation from a tiny (2–15 μm long) tip is detected using optical homo- and heterodyning and the nonlinear dependence of the tip polarizability on the tip–surface distance. The lateral resolution of ASNOMs is determined by the tip curvature radius (1– 20 nm), regardless of the wavelength (500 nm–100 μm). ASNOMs are shown to be capable of providing a surface optical map with nanometer resolution and carrying out spectral- and time-resolved measurements at a selected point on the surface.

Improving the position control of a two degrees of freedom robotic sensing antenna using fractional-order controllers

ABSTRACTFlexible links combined with force and torque sensors can be used to detect obstacles in mobile robotics, as well as for surface and object recognition. These devices, called sensing antennae, perform an active sensing strategy in which a servomotor system moves the link back and forth until it hits an object. At this instant, information of the motor angles combined with force and torque measurements allow calculating the positions of the hitting points, which are valuable information about the object surface. In order to move the antenna fast and accurately, this article proposes a new closed-loop control for driving this flexible link-based sensor. The control strategy is based on combining a feedforward term and a feedback phase-lag compensator of fractional order. We demonstrate that some drawbacks of the control of these sensing devices like the apparition of spillover effects when a very fast positioning of the antenna tip is desired, and actuator saturation caused by high-frequency sensor noise, can be significantly reduced by using our newly proposed fractional-order controllers. We have applied these controllers to the position control of a prototype of sensing antenna and experiments have shown the improvements attained with this technique in the accurate and vibration free motion of its tip (the fractional-order controller reduced ten times the residual vibration obtained with the integer-order controller).

Investigation of a UWB Wind Turbine Blade Deflection Sensing System With a Tip Antenna Inside a Blade

An ultra-wideband (UWB) blade deflection sensing system with a tip antenna inside a blade is investigated in this paper. The lower UWB band of 3.1–5.3 GHz is utilized. This system composes of two UWB radio links between one antenna inside the blade tip and two antennas outside the blade root. Blade deflections are tracked via two radio links using delay-based distance estimation and triangulation. In order to build reliable radio links, time-domain pulse field distributions are simulated to optimize the in-blade tip antenna polarization and the locations of the two root antennas around the root surface. Full-blade time-domain measurements are proposed to verify the simulations and realize the blade deflection sensing with an in-blade tip antenna. With the optimized in-blade tip antenna polarization and two root antenna locations, an accuracy of 2 cm is achieved for the tip-root antenna distance estimation, and the sensing system can realize the deflection tracking with a maximum deviation of 0.21 m and a root mean squared error of 0.11 m.

Tip-enhanced near-field optical microscope with side-on and ATR-mode sample excitation for super-resolution Raman imaging of surfaces

A tip-enhanced near-field optical microscope with side-on and attenuated total reflectance (ATR) excitation and collection is described and used to demonstrate sub-diffraction-limited (super-resolution) optical and chemical characterization of surfaces. ATR illumination is combined with an Au optical antenna tip to show that (i) the tip can quantitatively transduce the optical near-field (evanescent waves) above the surface by scattering photons into the far-field, (ii) the ATR geometry enables excitation and characterization of surface plasmon polaritons (SPPs), whose associated optical fields are shown to enhance Raman scattering from a thin layer of copper phthalocyanine (CuPc), and (iii) SPPs can be used to plasmonically excite the tip for super-resolution chemical imaging of patterned CuPc via tip-enhanced Raman spectroscopy (TERS). ATR-illumination TERS is also quantitatively compared with the more conventional side-on illumination scheme. In both cases, spatial resolution was better than 40 nm and tip on/tip off Raman enhancement factors were >6500. Furthermore, ATR illumination was shown to provide similar Raman signal levels at lower “effective” pump powers due to additional optical energy delivered by SPPs to the active region in the tip-surface gap.