Quarter-wavelength Matching Layer Research Articles

Lossless Matching Layer for Silicon Lens Arrays at 500 GHz Using Laser Ablated Structures

We present the design, fabrication and characterization of a broadband lossless matching layer for silicon lens arrays. The proposed matching layer is based on silicon frusta (truncated pyramids) on top of the lens array fabricated by means of laser ablation. This matching layer is advantageous over quarter-wavelength dielectric matching layers since it covers more than an octave of bandwidth. We compare the performance of this matching layer with the commonly-used parylene-C matching layer at the center of the targetted band (500 GHz) in a lens-antenna integrated system. We measure a 1.6 dB higher transmission of the proposed silicon frusta matching compared to the parylene-C matching layer.

Encapsulation of high specific surface area red blood cell-like mesoporous carbon spheres by magnetic nanoparticles: A new strategy to realize multiple electromagnetic wave loss mechanism

Accompanied by the increasingly serious disordered electromagnetic radiation to trigger electromagnetic pollution become a new generation source which is harmful and difficult to protect, it is still a enormous challenge to develop a novel microwave absorbing materials (MAM) with low-density and wide absorption band. Here, we propose an advanced strategy to encapsulate red blood cell-like mesoporous carbon spheres with magnetic nanoparticles in terms of the problems of poor impedance matching, narrow absorption band of single dielectric loss material or high density of single magnetic loss material. A novel microwave absorber (MRBC), exhibited with the characteristics of ultra-high specific surface area (BET specific surface area: >500 m2/g), uniform mesoporous properties (Pore diameter: 7–8 nm) and special hollow structure and composed of carbon spheres with intact red blood cell morphology and magnetic nanoparticles, was obtained by the combination of double vacuum sintering processes and glycolate pyrolysis process. The unique microstructure and the synergistic effect of multiple dielectric loss and magnetic loss capability make MRBC-3 achieve an effective bandwidth of 5.77 GHz at a low filling rate (15 wt%) and an ultra-thin thickness (1.74 mm). Meanwhile, the minimum reflection loss of MRBC-4 is −64.26 dB. Furthermore, we use normalized input impedance to discuss the special properties of quarter wavelength matching layer, and verify the location of minimum reflection loss.

Parametric Analysis of Negative and Positive Refractive Index Lens Antenna by ANSYS HFSS

Lens antennas with multibeam, high gain, and low sidelobe level are potential candidates for base station antennas in 5G mobile communication. In this paper, the authors perform simulation and parametric analysis of a lens antenna with positive and negative refractive indexes (NRI) using the modern electromagnetic field simulation software ANSYS HFSS. The simulation results of structures and theoretical calculations are analyzed and compared. The simulation results show the effectiveness of using negative refractive index lens antennas to minimize the dimension. The lens thickness with a negative refractive index decreased from 24.5 mm to 6.1 mm compared to the positive refractive index lens’s thickness. The results also indicate the similarities in gain, sidelobe level, amplitude, and electric field distribution on the aperture plane of the negative and positive refractive indexes (PRI) lens antennas compared to the theoretical calculation. In addition, the authors simulate a lens structure with additional quarter wavelength matching layers (MLs) to estimate the antireflection performance.

Multi-layer polymer-metal structures for acoustic impedance matching in high-frequency broadband ultrasonic transducers design

Although high-frequency (≥100 MHz) ultrasound has demonstrated its capability in a variety of applications, the fabrication of high frequency ultrasonic transducers with both high sensitivity and broad bandwidth remains challenging. One main reason is the mismatch of acoustic impedance between piezoelectric materials and the loading medium. Due to reliance on both specific acoustic impedance of matching materials and precise thickness control, the conventional quarter wavelength (1/4 λ) matching layer design is impractical for high-frequency transducers. Based on the transmission line theory and Mason model, our work interfaced polymer-metal-polymer matching layers for transducers over 100 MHz. The modeling result comparison between transducers without matching layer and those with two conventional matching layers demonstrated that the matching performance of polymer-metal-polymer matching layers could be as good as the one of conventional matching layers. Meanwhile, unlike conventional 1/4 λ matching layer design, our design is independent on materials with specific acoustic impedance, while precise thickness control of polymer and metal can be achieved by deposition. The polymer-metal-polymer matching layers scheme paves the way to high frequency ultrasonic transducers.

A Low-Cost Miniature Histotripsy Transducer for Precision Tissue Ablation.

A miniature, 10 mm aperture histotripsy transducer with an f-number of 0.7 was fabricated using an elliptically shaped aluminum lens, which was epoxy-bonded to an air-backed 5.0 MHz, PTZ-5A, 1-3 dice-and-fill piezoelectric composite, and the lens coupled to water using a quarter-wavelength matching layer of Parylene-C. A Krimholtz-Leedom-Matthaei model of the device and curved lens was developed. The epoxy layer resulted in an increased power output at 6.8 MHz compared to the 5 MHz composite design. Cavitation was observed in water by driving the composite with a 173 V single-cycle, unipolar 6.8 MHz pulse at a pulse repetition frequency of 50 Hz, and a bubble cloud 264 long by 124 wide was measured. A coregistered imaging and ablation device was also fabricated and characterized. The coregistered device was modified to include a mm square hole through the center, allowing access for a high-frequency imaging array, and both imaging and ablation are demonstrated in cerebral tissue with this device. Radial -3 dB beam widths were measured as 0.145 and 0.116 mm, and axial -3 dB depths of field were 0.698 and 0.752 mm for the noncoregistered and coregistered transducers, respectively. Total material cost for the transducer and pulser board is below $200 USD.

Magnesium Alloy Matching Layer for PMN-PT Single Crystal Transducer Applications.

In this paper, we propose using magnesium alloy as the matching layer for the ultrasonic transducer made of 0.68 Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-0.32PT) single crystal. The complete sets of elastic constants of AZ31B, GW83, and ZK60 magnesium alloys have been measured, which is practically a homogeneous material. The AZ31B magnesium alloy has an acoustic impedance of 10.36 MRayl, which is suitable for the development of high-performance ultrasonic transducers. A 3.5-MHz PMN-PT single crystal transducer has been designed and fabricated successfully using AZ31B magnesium alloy as the first quarter-wavelength matching layer. The -6-dB bandwidth and two-way insertion loss at the center frequency of the transducer are about 67% and 11.4 dB, respectively, much superior to the transducer fabricated using 0-3 composite matching layer. The high performance of this transducer indicates that the magnesium alloy is indeed an excellent matching layer material for ultrasonic transducer applications.

Development of a shear ultrasonic spectroscopy technique for the evaluation of viscoelastic fluid properties: Theory and experimental validation

In-situ measurement of viscosity advances the field of rheology, and aides the development of sensing systems for condition and performance monitoring of lubricated mechanisms. Many lubricated mechanisms, such as journal bearings or seals, are characterised by three-layer interfaces; an oil separating two solid (usually metallic) bodies. The viscoelastic study of the lubricating oil in layered systems is possible in-situ by means of ultrasonic reflection (Schirru et al. (2015)). General solutions exist for the reflection of longitudinal plane waves from multi-layered solid-fluid systems. Similar solutions can be applied to plane shear waves. The use of a quarter-wavelength intermediate matching layer improves the sensitivity of the ultrasonic measurement and overcomes problems of acoustic mismatch. This opens the possibility of using reflectance methods to measure engineering (metal-oil) bearing applications that are acoustically mismatched. In this paper, a rigorous mathematical model for wave propagation in a three-layer system is solved for the reflection coefficient modulus and validated using a quarter wavelength ultrasonic viscometer. The model was tested against experimental data for two Newtonian reference fluids, water and hexadecane, and for one non-Newtonian reference fluid, squalene plus polyisoprene (SQL + PIP), measured ultrasonically at frequencies between 5 and 15 MHz. The results are in agreement with the expected viscosity values for the reference fluids. Further, the viscosity measurement is not limited to the resonance frequency, but it is performed over a broad band frequency range. This is important to improve measurement confidence and accurate spectroscopy measurement for the determination of viscoelastic properties.

Anodic aluminum oxide–epoxy composite acoustic matching layers for ultrasonic transducer application

The goal of this work is to demonstrate the application of anodic aluminum oxide (AAO) template as matching layer of ultrasonic transducer. Quarter-wavelength acoustic matching layer is known as a vital component in medical ultrasonic transducers to compensate the acoustic impedance mismatch between piezoelectric element and human body. The AAO matching layer is made of anodic aluminum oxide template filled with epoxy resin, i.e. AAO–epoxy 1–3 composite. Using this composite as the first matching layer, a ∼12MHz ultrasonic transducer based on soft lead zirconate titanate piezoelectric ceramic is fabricated, and pulse-echo measurements show that the transducer exhibits very good performance with broad bandwidth of 68% (−6dB) and two-way insertion loss of −22.7dB. Wire phantom ultrasonic image is also used to evaluate the transducer’s performance, and the results confirm the process feasibility and merit of AAO–epoxy composite as a new matching material for ultrasonic transducer application. This matching scheme provides a solution to address the problems existing in the conventional 0–3 composite matching layer and suggests another useful application of AAO template.

Research Papers. An Approach to Design Broadband Air Backed Piezoelectric Sensor

Abstract In this work, an approach to the design of broadband thickness-mode piezoelectric transducer is pre- sented. In this approach, simulation of discrete time model of the impulse response of matched and backed piezoelectric transducer is used to design high sensitivity, broad bandwidth, and short-duration impulse response transducers. The effect of matching the performance of transmitting and receiving air backed PZT-5A transducer working into water load is studied. The optimum acoustical characteristics of the quarter wavelength matching layers are determined by a compromise between sensitivity and pulse duration. The thickness of bonding layers is smaller than that of the quarter wavelength matching layers so that they do not change the resonance peak significantly. Our calculations show that the −3 dB air backed transducer bandwidth can be improved considerably by using quarter wavelength matching layers. The computer model developed in this work to predict the behavior of multilayer structures driven by a transient waveform agrees well with measured results. Furthermore, the advantage of this this model over other approaches is that the time signal for optimum set of matching layers can be predicted rapidly

Mass-spring matching layers for high-frequency ultrasound transducers: a new technique using vacuum deposition.

We have developed a technique of applying multiple matching layers to high-frequency (>30 MHz) imaging transducers, by using carefully controlled vacuum deposition alone. This technique uses a thin mass-spring matching layer approach that was previously described in a low-frequency (1 to 10 MHz) transducer design with epoxied layers. This mass- spring approach is more suitable to vacuum deposition in highfrequency transducers over the conventional quarter-wavelength resonant cavity approach, because thinner layers and more versatile material selection can be used, the difficulty in precisely lapping quarter-wavelength matching layers is avoided, the layers are less attenuating, and the layers can be applied to a curved surface. Two different 3-mm-diameter 45-MHz planar lithium niobate transducers and one geometrically curved 3-mm lithium niobate transducer were designed and fabricated using this matching layer approach with copper as the mass layer and parylene as the spring layer. The first planar lithium niobate transducer used a single mass-spring matching network, and the second planar lithium niobate transducer used a single mass-spring network to approximate the first layer in a dual quarter-wavelength matching layer system in addition to a conventional quarter-wavelength layer as the second matching layer. The curved lithium niobate transducer was press focused and used a similar mass-spring plus quarter-wavelength matching layer network. These transducers were then compared with identical transducers with no matching layers and the performance improvement was quantified. The bandwidth of the lithium niobate transducer with the single mass-spring layer was measured to be 46% and the insertion loss was measured to be -21.9 dB. The bandwidth and insertion loss of the lithium niobate transducer with the mass-spring network plus quarter-wavelength matching were measured to be 59% and -18.2 dB, respectively. These values were compared with the unmatched transducer, which had a bandwidth of 28% and insertion loss of -34.1 dB. The bandwidth and insertion loss of the curved lithium niobate transducer with the mass-spring plus quarter-wavelength matching layer combination were measured to be 68% and -26 dB, respectively; this compared with the measured unmatched bandwidth and insertion loss of 35% and -37 dB. All experimentally measured values were in excellent agreement with theoretical Krimholtz-Leedom-Matthaei (KLM) model predictions.

High-Frequency Annular Array Fabrication Using a Flex Circuit Matching Layer

Fabricating arrays for high-frequency image applications such as ophthalmic imaging, intravascular imaging, and small animal imaging is challenging. For example, an array for intravascular imaging must be small enough to fit within the lumen of a catheter and inexpensive enough to be discarded after a single use. This article presents a new method for fabricating high-frequency annular arrays that is simple and inexpensive. The annular array elements are defined by the electrode pattern on a back surface of a polyimide quarter-wavelength matching layer that is glued to the front face of a ceramic transducer substrate (PZT5H). Electrical losses associated with bonding the matching layer to the transducer substrate are reduced by fabricating a second set of electrodes on the transducer substrate and then bonding the substrates using an anisotropic conductive epoxy. The feasibility of this technique was established by fabricating a seven-element, 20-MHz, 5-mm diameter annular array. The prototype array produced a pulse with a -6-dB factional bandwidth of 50%, an insertion loss of 22 dB, and secondary lobes in the radiation pattern at f/2 that decreased to -65 dB with respect to the main lobe with a peak amplitude of -53 dB.

Focal Array Fed Dielectric Lenses: An Attractive Solution for Beam Reconfiguration at Millimeter Waves

We investigate the radiation capabilities of focal array fed dielectric lens antennas using the FDTD method. Several relevant configurations sharing the same lens architecture (namely an extended hemispherical lens coated with a quarter wavelength matching layer), but illuminated by different sub-array topologies, are compared. In particular, we study the impact of the feeding amplitude coefficients, sub-array size and location upon the radiation patterns and beam scanning performance of the lens. Our numerical results demonstrate that such lens configurations are very attractive and flexible for beam synthesis at millimeter waves (beam shaping, beam scanning and beam reconfiguration). These conclusions are confirmed successfully by several experimental results in Ka-band obtained with a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$8 \times \lambda_{0}$</tex></formula> lens in Rexolite illuminated by linear printed antenna arrays of variable size.

Dielectric Parameters and Wave Propagation Characteristics in Multilayer Media

Abstract In order to understand the plane wave propagation characteristics in multilayer media, selecting the adjacent 3-layer dielectrics, two boundaries surfaces for example, models of plane wave propagation were established within each layer. Simulation results show that both the incident and reflected waves were in the ordinary dielectric, their synthesis wave wasn’t complete stationary wave; Imaginary part of complex permittivity was attenuation factor. Other results also show when the wave struck a dielectric layer normally, the dielectric wave impedance and the thickness of the layer met the condition of a quarter wavelength matching layer, there was no reflected wave. This conclusion is of great value in many engineering and practical applications.

Modelling and optimum design of ultrasonic transducer for therapy application

In ultrasound imaging applications, transducers with high sensitivity, good impulse response, low-ripple, and wide-band performance are desired. Widening the bandwidth of transducers by introducing quarter-wavelength matching layers on the piezoelectric ceramic may result in a reduction in power transmission and sensitivity. For therapy, it is more important to have high-transmitting power than to have wide-bandwidth characteristics. Since a 15% bandwidth is sufficient for therapy applications, a matching-layer thickness that is a little bit shorter or longer than one-quarter wavelength is proposed as a good trade-off between the two criteria. By tuning the thickness and acoustical impedance of the matching layer, transducers with optimum bandwidth and transmitting power can be obtained. An acoustical lossy transmission line model for an ultrasonic transducer is proposed. Then, the developed model is implemented on PSPICE-controlled source programming technology to precisely predict the performance of the transducer's characteristics. Good agreement between the simulation results and experimental results has been achieved.

Novel multi-layer polymer-metal structures for use in ultrasonic transducer impedance matching and backing absorber applications

This paper presents a novel design principle for designing multilayer polymer-metal structures which are well suited for front surface impedance conversion (matching) and for back surface acoustic absorption. It is shown that a polymer layer with an outer metal layer, when loaded by a low impedance propagation medium, acts as an efficient impedance converter. The resulting impedance seen at the inner polymer surface is increased and the structure provides the same performance as a traditional quarter-wavelength matching layer. Experimental evidence is also shown for a double-matching scheme for a lead zirconate titanate (PZT) transducer using an inner polymer-metal multilayer and an outer polymer quarterwavelength layer, resulting in a 55% bandwidth at 2.6 MHz with air backing. Also, it is theoretically shown that multiple layers of a lossy polymer adhesive-metal structure produce low propagation velocity and high absorption. Experimental proof of this ultrasonic multilayer backing absorber is provided. Design theories based on both a simplified mass and spring model and a rigorous one-dimensional wave model have been developed and show fair agreement.

Finite-difference time-domain simulations of the effects of air gaps in double-shell extended hemispherical lenses

The effects of parasitic air gaps on the input impedance and radiation characteristics of dense double-shell integrated lens antennas are studied numerically at millimetre waves using the finite-difference time-domain method. The lens core is made up of Macor or silicon, and is coated with a quarter wavelength matching layer. Two kinds of gaps are compared: they are located either (i) between both shells of the lens, or (ii) between the lens base and the feed substrate. We show that their impact is much more critical in the second case, and that it becomes dramatic for silicon lenses, even with very thin gaps (smaller than λ0/100); the three major observed effects are the following: (i) strong shift of the resonant frequency, (ii) beam broadening and directivity loss, (iii) increase of the side lobe level.

The design of quarter-wavelength impedance matching layers for cylindrical transducers.

Impedance matching layers are commonly used in piezoelectric underwater acoustic projectors. The layer maximizes transmitted power from the ceramic into the water and also increases the bandwidth of the projector. For the design of cylindrical transducers, it is common in practice to utilize the familiar plane wave formulation of the quarter-wavelength impedance matching layer as a starting point for the design of the cylindrical layer. Material properties and thickness are then modified by trial and error, heuristic, or empirical methods to optimize the design. This practice is undertaken because, apparently, the quarter-wavelength impedance matching layer formulation is not readily available in the acoustics literature for the cylindrical coordinate system. To address this deficiency, the reflection and transmission coefficients for the cylindrical three-medium problem were derived. No general zero-reflection, perfect transmission condition was found, but the equations can be used to find the material properties and layer thickness required to maximize transmission at a given frequency. The results of the derivation are shown and used in the design of a layered cylindrical piezoelectric transducer.

Very Broadband Extended Hemispherical Lenses: Role of Matching Layers for Bandwidth Enlargement

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The design and optimization of very broadband integrated lens antennas (ILAs) constitutes one of the future trends in lens antenna field. To this end we investigate numerically the radiation performance of millimeter wave ILAs coated with multiple anti reflection layers. We propose lens structures of moderate size (four wavelengths in diameter at the center frequency) and made from a dense dielectric material (ceramic). They are illuminated by two kinds of on-axis primary sources, namely a dielectric-loaded metallic waveguide and a patch antenna. This enables to assess the role of the lens illumination law on the performance of broadband ILAs. In particular, we demonstrate that ILAs coated with three stacked quarter wavelength matching layers exhibit a very broadband promising features. First their radiation characteristics remain very stable over a large frequency band: a 36% relative bandwidth is achieved using dielectric-loaded waveguide feeds. Secondly very high values of aperture efficiencies (beyond 91% over a 21% bandwidth) are obtained using printed feeds. The truncation effects of the ground plane and substrate of planar feeds upon the beam characteristics are also studied. We conclude that they must be taken into account at the very first stages of the design process of ILAs. </para>

Silicon oxide colloidal∕polymer nanocomposite films

The quarter-wavelength (λ∕4) acoustic matching layer, a vital component in medical ultrasonic transducer, can bridge the large acoustic impedance mismatch between the piezoelectric material and the human body. Composite materials are widely used as matching materials in order to cover the wide acoustic impedance range that cannot be accomplished by using a single-phase material. At high frequencies (&amp;gt;50MHz), the λ∕4 matching layers become extremely thin so that the fabrication of homogeneous composite material matching layers becomes very challenging. A method is reported in this letter to fabricate sol-gel silicon oxide colloidal∕polymer composite film on silicon substrate, in which the particle size of silicon oxide colloidal is between 10 and 40 nm. The acoustic impedance of the nanocomposite films versus aging temperature has been measured at the desired operating frequency.

Characterizing ultra-thin matching layers of high-frequency ultrasonic transducer based on impedance matching principle

The quarter-wavelength (lambda/4) acoustic matching layer is a vital component in medical ultrasonic transducers, which can compensate for the large acoustic impedance mismatch between the piezoelectric material and the human body. At high frequencies (approximately 100 MHz), the lambda/4 matching layers become extremely thin, and the characterization of their properties becomes very challenging. We report a method to measure the phase velocity and attenuation of ultra-thin layers using the lambda/4 matching principle, in which the acoustic impedance of the thin layer is between the substrate and water. The method has been successfully used to characterize epoxy films on glass substrate. The experimental results show good agreement in the phase-velocity measurement between our proposed method and the conventional ultrasonic spectroscopy method, but the attenuation measurement is sensitive to the properties of the substrate and water medium as well as the alignment of the sample.