Leaky Wave Research Articles

Quasi-guided modes resulting from the band folding effect in a photonic crystal slab for enhanced interactions of matters with free-space radiations.

We elucidate that guided modes supported by a regular photonic crystal slab structure composed of a square lattice of air holes in a silicon slab will transition into quasi-guided (leaky) modes when the radius of every second column of air holes is changed slightly. This intentional geometric perturbation will lead to a doubling of the period in one direction and the corresponding shrinkage of the first Brillouin zone. Because of the translational symmetry in the k-space, leaky waves inheriting the spatial dispersion of the original guided modes, which do not interact with external radiation, will appear with the dispersion curves above the light cone. Our results show that ultrahigh Q-factor resonances with large operating bandwidth can be achieved. Interestingly, the perturbation in only one direction of the photonic lattice will lead to an in-plane wave number-dependent resonance characteristic in both directions. Our numerical results demonstrate a local enhancement of the electric field magnitude by the order of 102, which is even more significant than those in most plasmonic structures. These quasi-guided modes with superior properties will provide a new platform for efficient light-matter interactions.

A Miniaturized Multi‐Frequency Wide‐Band Leaky Wave Button Antenna for ISM/5G Communications and WBAN Applications

AbstractA low‐profile multi‐frequency leaky wave button antenna for body‐centric communications is presented. A leaky wave antenna loaded with U‐slots (a combination of U‐slots and slits) and tapered slot structures to improve the radiation efficiency, broadside radiation pattern, and widen the steering range is designed. Furthermore, it comprises a whole ground to meet the Specific Absorption Rate standard requirements based on the known standards. The antenna's performances are examined for on and off‐body conditions. For demonstration, a prototype is implemented, and the measurement is performed on the chest. The antenna operates at multi bands of 1.7–3.3 GHz (Industrial, Scientific, and Medical and 5G communication) and 4.15–10 GHz (sub‐6 GHz and X‐band communications). The peak gains of 6.9 and 8.2 dBi were obtained for on and off‐ body conditions, respectively. Furthermore, the antenna offers maximum radiation efficiencies of 89.3% and 99.3% for on‐body and free‐space conditions. The specific absorption rate (SAR) values obtained for body‐centric communications meet the regulation requirements (e.g., on body tissue at 3.2 GHz, it is 0.78 (1 g) and 0.44 (10 g) W/kg). With an overall miniaturized size, the proposed button antenna could be integrated with clothes. In addition, a multi‐wide bandwidth, circularly polarized radiation, a small size, high efficiency and gain, and low SAR values prove that the proposed antenna can be a potential candidate for wireless body area network and simultaneous wireless information and power transfer applications.

Seismo-acoustic coupling efficiency at the surface of Venus

The extreme conditions at the surface of Venus pose a challenge for monitoring the planet’s seismic activity using long-duration landed probes. One alternative is using balloon-based sensors to detect venusquakes from the atmosphere. This presentation assesses the efficiency of seismic-to-acoustic energy transfer from Venus' crust to its deep atmosphere. It is, therefore, restricted to immediate neighborhood of the planet's surface. In order to account for supercritical conditions near the surface, the Peng-Robinsonequation of state is used to obtain the acoustic wavenumber in the lower atmosphere. The energy transported across the surface from deep and shallow sources is two orders of magnitude larger than on Earth, pointing to a very strong seismo-acoustic coupling. For a more realistic scenario, we simulated the acoustic field generated in the lower atmosphere by the ground motion arising from a vertical array of subsurface point-force sources. The resulting transmission loss maps show a strong epicentral cone accompanied by contributions from leaky surface waves.

All-optical, non-contact, local measurement of both Young’s modulus and Poisson’s ratio in metals using a combination of Rayleigh and leaky surface acoustic waves

In non-destructive evaluation (NDE), measuring ultrasound (US) longitudinal and shear wave speeds is the main method to determine two independent mechanical moduli (Young’s modulus and Poisson’s ratio). Most US techniques use time-of-flight measurements. However, when the local sample thickness is unknown or the sample geometry is complex, bulk-wave propagation speeds cannot be accurately defined. Here we show that a properly shaped beam of nanosecond laser pulses can be used to efficiently excite (without material ablation) two types of surface acoustic waves. In addition to a conventional surface, or Rayleigh, wave, a leaky surface wave (LSAW) can be launched in the near field of the excitation region. We present the theoretical background, numerical simulations, and experimental results clearly showing that both elastic constants can be reconstructed locally by tracking Rayleigh and LSAW waves. Spatially resolved elastic properties can be obtained using local values of wave speeds obtained by sample scanning. Non-contact optical detection of propagating waves at the sample surface, in which a fiber-optic Sagnac interferometer was used, is a key piece of the method. It does not require acoustic coupling and allows remote measurements in the near field of the laser source with micron-scale resolution.

Seismo-acoustic coupling in the deep atmosphere of Venus.

The extreme conditions at the surface of Venus pose a challenge for monitoring the planet's seismic activity using long-duration landed probes. One alternative is using balloon-based sensors to detect venusquakes from the atmosphere. This study aims to assess the efficiency with which seismic motion is coupled as atmospheric acoustic waves across Venus's surface. It is, therefore, restricted to the immediate neighborhood of the crust-atmosphere interface. In order to account for supercritical conditions near the surface, the Peng-Robinson equation of state is used to obtain the acoustic sound speed and attenuation coefficient in the lower atmosphere. The energy transported across the surface from deep and shallow sources is shown to be a few orders of magnitude larger than on Earth, pointing to a better seismo-acoustic coupling. For a more realistic scenario, simulations were made of the acoustic field generated in the lower atmosphere by the ground motion arising from a vertical array of subsurface point-force sources. The resulting transmission loss maps show a strong epicentral cone accompanied by contributions from leaky surface waves. Results at 0.1 Hz and 1 Hz confirm that the width of the epicentral cone is larger at lower frequencies.

A novel leaky surface acoustic wave biosensor for detection of PKA activity in cell lysates based on peptide biomineralized metal nanoclusters

A novel leaky surface acoustic wave biosensor for detection of PKA activity in cell lysates based on peptide biomineralized metal nanoclusters

Methodologies, technologies, and strategies for acoustic streaming-based acoustofluidics

Acoustofluidics offers contact-free manipulation of particles and fluids, enabling their uses in various life sciences, such as for biological and medical applications. Recently, there have been extensive studies on acoustic streaming-based acoustofluidics, which are formed inside a liquid agitated by leaky surface acoustic waves (SAWs) through applying radio frequency signals to interdigital transducers (IDTs) on a piezoelectric substrate. This paper aims to describe acoustic streaming-based acoustofluidics and provide readers with an unbiased perspective to determine which IDT structural designs and techniques are most suitable for their research. This review, first, qualitatively and quantitatively introduces underlying physics of acoustic streaming. Then, it comprehensively discusses the fundamental designs of IDT technology for generating various types of acoustic streaming phenomena. Acoustic streaming-related methodologies and the corresponding biomedical applications are highlighted and discussed, according to either standing surface acoustic waves or traveling surface acoustic waves generated, and also sessile droplets or continuous fluids used. Traveling SAW-based acoustofluidics generate various physical phenomena including mixing, concentration, rotation, pumping, jetting, nebulization/atomization, and droplet generation, as well as mixing and concentration of liquid in a channel/chamber. Standing SAWs induce streaming for digital and continuous acoustofluidics, which can be used for mixing, sorting, and trapping in a channel/chamber. Key challenges, future developments, and directions for acoustic streaming-based acoustofluidics are finally discussed.

Exact radiation boundary conditions to determine the complex wavenumber of an underwater acoustic leaky wave antenna.

Underwater elastic leaky wave antennas (LWAs) steer acoustic energy as a function of frequency by exploiting fluid-solid coupling. LWAs present a modeling challenge due to complex radiation impedance on the waveguide surface that leads to changes in dynamic response. This work presents an approach to model underwater LWAs that considers an elastic unit cell surrounded by a fluid domain and includes a radiation boundary condition to simulate an open boundary. The model solves an eigenvalue problem for the complex-valued wavenumber given a specified frequency, forming an accurate representation for the free response of an elastic LWA in an underwater environment.

Fully Planar Single/Fixed-Beam Ultrawideband Leaky-Wave Antenna Based on Leaky Grounded Coplanar Waveguide

A fully planar single- and fixed-beam leaky-wave antenna (LWA) is proposed based on the leaky grounded coplanar waveguide (GCPW). This antenna consists of a leaky GCPW, a parallel plate waveguide (PPW) prism, and a metal-strip transition. It generates a fixed-beam leaky wave radiation from the GCPW to the PPW, and then radiates it into the free-space through the metal-strip transition. Because the GCPW is nearly nondispersive, this antenna exhibits a fixed-beam radiation inherently, making it much simpler to design and fabricate compared to other fixed-beam LWAs. Measured results show the proposed antenna achieves a peak gain of 19.4 dBi, 3 dB gain bandwidth of 28.8 GHz to > 40 GHz (> 32.6%), −10 dB impedance bandwidth of larger than 26–40 GHz (> 42.4%), and a small beam shifting range of only 3° (±1.5°) within 26–40 GHz, which make it suitable for applications such as point-to-point communication.

Analysis of surface acoustic wave resonance properties on piezoelectric substrates with periodic voids

The resonance properties of leaky surface acoustic waves (LSAWs) and longitudinal LSAWs (LLSAWs) on bonded structures consisting of a LiNbO3 (LN) thin plate and a support substrate, with periodic voids exhibiting a rectangular cross-section, were simulated using the finite element method. The voids were formed below the electrode at the boundary between the LN thin plate and support substrate and presented the same pitch and period as the electrode. LN, quartz, glass, or Si were used as support substrates. Simulations revealed that by introducing periodic voids, resonance properties similar to those of the SH0-mode plate and S0-mode Lamb waves can be obtained. The values of the simulated fractional bandwidths of approximately 13 and 18% were found for LSAW and LLSAW, respectively, when the ratio of the width of the void to the pitch of the electrode was 0.7.

Research on the 2‐D annular leaky‐wave antenna

AbstractThis paper studies a 2‐D planar periodic leaky‐wave antenna based on a series of concentric annular microstrip rings on a grounded dielectric slab. The leaky wave is generated from the perturbation to the surface wave, which is excited from the fundamental and dominant TM0 mode. The computation method for the propagation constant is first analyzed, and then the radiation property is researched. A set of suitable structural parameters for only −1th spacial harmonic radiation are extracted from the theoretical computation and then verified by the simulation, which shows an agreement. The results indicate that this proposed antenna can radiate two symmetrical flat conical beams with over 90% total efficiency. The theoretical results on radiation performance are then verified by simulation and measurement with good agreement.

Metasurface Antennas Design: Full-Wave Feeder Modeling and Far-Field Optimization

This article addresses the optimization of the radiation pattern of surface-wave (SW)-based metasurface (MTS) antennas. Those antennas are considered a promising alternative to parabolic reflectors and phased arrays due to their extremely low profile and their ability to provide high gain, shaped beams and multibeams. However, pattern synthesis with MTS antennas is very challenging because of the single active control point and the need to control surface and leaky waves through the MTS. An accurate optimization of the radiation pattern, along with the sidelobe level requires full-wave modeling of the feeding structure, including its coupling with the MTS. MTS synthesis methods existing in the literature usually approximate the feeder model, and neglect its coupling with the MTS. Such approximation may lead to more than 1 dB error in the predicted antenna directivity. This article presents a technique for optimization of the far-field pattern, built on a Method of Moments (MoM) analysis tool in which the MTS coupling with the feeder, a coax probe, is fully considered. The MTS is modeled as an arbitrarily shaped, spatially modulated electric sheet impedance in a layered medium. At each optimization iteration, the complexity of the underlying analysis is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {O}(\text {N}\log \text {N})$ </tex-math></inline-formula> owing to the use of a fast Fourier transforms (FFT)-based acceleration. The effectiveness of the method is demonstrated through the optimization of MTSs radiating a pencil beam and a conical beam with orbital angular momentum (OAM).

Multi-Feed Metasurface Antennas: Direct Numerical Design and Experimental Validations

This paper provides new designs and realizations of leaky-wave (LW) metasurface (MTS) antennas fed at multiple points. The design technique is based on the direct integral-equation solution by the Method of Moments (MoM). The unknown is no longer the current distribution, but the impedance profile itself. The method can be used to generate any shaped radiation pattern in amplitude, phase, and polarization, provided that the antenna area and the feeder illumination are consistent with the shape of the desired radiation pattern. The algorithm can also be used to design multi-functional antennas (multibeam, multiband, dual-polarizion, etc). While multiple feeds are traditionally used to implement multiple fonctionnalities, they may also be required for the efficient generation of a single shaped beam, thus leading to a higher surface-wave (SW) to LW conversion efficiency. The present paper shows for the first time, two realizations of MTS design with the integral equation formalism, which require the usage of multiple feeds: a circularly polarized sectoral conical beam and a dual-polarized broadside beam MTS antenna. The good comparison between measurements and numerical predictions shows the effectiveness of the design method.

Базовые конструкции СВЧ балансных фильтров на оттекающих поверхностных акустических волнах

Ladder filters on leaky surface acoustic waves (LSAW) with electrical connection of resonators provide the lowest insertion loss IL=0.6-1.2 dB as compared with other types of LSAW filters, such as longitudinally coupled resonator filters. A drawback of ordinary nonsymmetrical ladder filters on LSAW is their limited selectivity (or minimal attenuation in the stop band) about UR=40-45 dB. In addition some problems arise when matching nonsymmetrical ladder filters to balanced or differential microcircuits. The present paper describes basic symmetric П-type and Т-type sections as well as various structures of ladder filters that allow to achieve a high selectivity thanks of parasitic electromagnetic signal (EMS) suppression. An extra advantage of balanced LSAW filters is their possibility to work with high impedance symmetric loads. The results of our study have been illustrated by few examples of balanced filters constructions on frequency 1217 MHz with selectivity UR=55-7- dB and low insertion loss IL=1,5-2,0 dB. Filters exhibiting high selectivity UR=55-70 dB and small insertion loss IL=1,2-4,0 dB. The described constructions may be used as basic for application on TV tuners, receivers of communication and navigation systems in frequency range 600-2000 MHz.

Slotted Waveguide Circularly Polarized Leaky Wave Antenna With Improved Efficiency at W-Band

Slotted Waveguide Circularly Polarized Leaky Wave Antenna With Improved Efficiency at W-Band

Influence Of Patchs Shape On The Radiation Performance a Microstrip Leaky-Wave Antenna

In this article, we present microstrip leaky wave antennas designed using HFSS software. These microstrip antennas differ in the shape of the patches on the substrate. To this Effect, three shapes have been studied and the analysis of the radiation parameters of these antennas shows that the T-shape patches have low levels of minor's lobes and good the scanning angle in the E- and H-plane compared to the other studied shapes. This study of the influence of the patches shape on the performance of microstrip leaky wave antennas shows the impact on the radiation pattern and directivity of the antenna. Through a comparative study of three patch shapes H, inverted T and L, simulations show that inverted T patches have low the levels of minor’s lobes and good the scanning angle in the E and H planes compared to the other shapes studied. However, the L-shape present a better directivity compared to the other shapes.

3-D-Printed Spiral Leaky Wave Antenna With Circular Polarization

This article presents the design, manufacturing and measurement of a fully dielectric leaky wave antenna operating at 18 GHz. The antenna is composed of a grounded dielectric substrate with a top dielectric Archimedean spiral corrugation, which allows obtaining a directive pencil-beam in broadside direction with circular polarization depending on the turning sense of the spiral. The 3D-printing implementation allows having two new degrees of freedom for the spiral when compared to other previous implementations based on metals, which are the relative permittivity and the height of the spiral. A study about the effect of these two parameters is presented and one proposed antenna is manufactured using low-loss and low-cost ABS filaments. Measurement results show good agreement with simulations, validating the implementation of this kind of antennas using low-cost 3D-printing.

Phased Array With Pattern Shaping and Scan Loss Reduction for Millimeter Waves

In this work, we investigate antenna architectures to implement dual-mode operation in phased array designs. Planar slot antenna elements are used in array configuration, in combination with artificial dielectrics layers (ADLs) located in the close proximity of the array, to achieve pattern shaping. The artificial dielectric superstrate supports the propagation of leaky waves that can be optimized to enhance the gain in a specific angular region or to enlarge the array field of view. By controlling the amplitude and phase of the antenna elements, the radiation patterns can be combined to realize either wide or narrow beams. This concept present advantages for both millimeter-wave (mm-wave) communication and radar applications. A design of a four-element array fabricated in standard printed circuit board (PCB) technology validates the feasibility of the dual-mode operation. The measured results also show good agreement with simulations.

Simultaneous determination of Young's modulus and Poisson's ratio in metals from a single surface using laser-generated Rayleigh and leaky surface acoustic waves

Material elastic moduli are used to assess stiffness, elastic response, strength, and residual life. Ultrasound (US) measurements of propagation wave speeds (for longitudinal and shear waves) are now primary tools for non-destructive evaluation (NDE) of elastic moduli. Most US techniques measure the time-of-flight of through-transmission signals or reflected signals from the back wall. In both cases, an independently determined sample thickness is required. However, US methods are difficult for complex (non-flat) samples. When the local thickness is unknown, the propagation speed cannot be determined. On the other hand, the propagation speed of Rayleigh waves can be calculated without knowledge of sample thickness, but another independent measurement is still required to compute both Young's modulus and Poisson's ratio. We present a comprehensive theoretical background, numerical simulations, and experimental results that clearly show that when the material density is assumed known, both elastic constants of an isotropic metal sample can be determined with laser-ultrasound by tracking two types of surface propagating waves without any sample contact (both signal excitation and detection are performed optically). In addition to a conventional surface, or Rayleigh, acoustic wave, a leaky surface wave can also be launched with nanosecond laser pulses in the thermoelastic regime of excitation (i.e., without material ablation) close to the source that propagates along the sample surface with speed close to that of bulk longitudinal waves. Samples can be of arbitrary shape and their thickness need not be measured.

Complex Mode Dispersion Characteristics of Dielectric Loaded Radially Thick Helix

In this article, a generalized analytical and computational numerical theory, for both the guided and leaky modes, has been developed to investigate the dispersive behavior of dielectric-loaded radially thick helix (DLRTH). The complex root finding Muller iterative algorithm is used to compute the normalized phase and attenuation constants. A unique way has been adopted for the classification of HE and EH hybrid modes present in the proposed structure, and based on that, for the first time, we report that this structure shows physical leaky and trapped surface wave propagation characteristics. Furthermore, in the guided mode, it is seen that both slow wave (SW) and backward wave (BW) exist due to the inherently skewed boundary of the helical geometry. The parametric study of helix radius and pitch angle as well as dielectric constant gives optimized parameter values, which help in designing the proposed structure in the desired mode and at the desired frequencies. Moreover, it is reported that pitch angle aids in controlling and figuring out the leaky mode bandwidth. The analytical cum numerical MATLAB results are successfully verified using CST Microwave Studio computational analysis and are found to be in good agreement. The improved characteristic equations for guided mode find practical applications in the design and performance evaluation of the SW structure (SWS) for the phase delay applications. The leaky mode will find application for beam scanning helical antennas.