Large Impedance Mismatch Research Articles

High green index electromagnetic interference shields with semiconducting Bi2S3 fillers in a PEDOT:PSS matrix.

Conventional metallic electromagnetic interference (EMI) shields, as well as the emerging 2D material-based shields, meet the shielding effectiveness (SE) needs of most applications. However, their shielding performance is dominated by the reflection of incoming radiation due to their high electrical conductivity, which leads to secondary pollution. This problem is getting exacerbated with the proliferation of electronics and communication networks in modern society. Thus, EMI shields that function dominantly by the absorption of incoming radiation are highly desirable. Such shields would be characterized by a green index, which is the ratio of absorbance over reflectance, close to or greater than one. For nonmagnetic materials, the best way to reduce the undesirable large impedance mismatch is to reduce the effective permittivity of the shield material. Here, we present a new EMI shield with a semiconductor Bi2S3 filler in a conducting PEDOT:PSS polymer matrix, instead of the conventional conductive fillers, to reduce the effective permittivity and demonstrate that even a light loading of only 10% Bi2S3 provides high SE of over 40 dB with a green index value of 0.75. Increasing the filler content to 15 wt% increases the green index close to unity while dropping the SE to 30 dB. The shielding mechanism is explained through electromagnetic parameter measurements and supplemented by density functional theory calculations. This work lays the foundation for the advancement of lightweight and ultrathin green EMI shields with minimum secondary pollution.

A hybrid WENO5IS-THINC reconstruction scheme for compressible multiphase flows

We present a hybrid reconstruction scheme for compressible multiphase flows governed by the five-equation model with surface tension and molecular viscosity. The Godunov-type method utilizes a fifth-order incremental stencil weighted essential non-oscillatory (WENO5IS) scheme to resolve flow structures in each component, while the Tangent of Hyperbola for INterface Capturing (THINC) maintains sharp fluid interfaces. Supplemented with positivity- and boundedness-preserving limiting procedures, the scheme exhibits a high level of robustness for multiphase flow problems with strong compressibility and large acoustic impedance mismatch. One- and two-dimensional benchmark cases demonstrate the efficiency and accuracy of interface-sharpening, and the high resolution capability for each fluid component. Numerical tests under extreme conditions corroborate the excellent robustness, and simulations with surface tension and viscosity also exhibit the versatility of the proposed approach.

Remote Water-to-Air Eavesdropping with a Phase-Engineered Impedance Matching Metasurface.

Efficiently receiving underwater sound remotely from air is a long-standing challenge in acoustics hindered by the large impedance mismatch at the water-air interface. Here, a phase-engineered water-air impedance matching metasurface is proposed and experimentally demonstrated for remote and efficient water-to-air eavesdropping. The judiciously designed metasurface with near-unity transmission efficiency, long monitoring distance, and high mechanical stiffness is capable of making the water-air interface acoustically transparent and, at the same time, freewheelingly patterning the transmitted wavefront. This enables efficient control over the effective spatial location of a distant airborne sensor such that it can measure underwater signals with large signal-to-noise ratios as if placed close to the physical underwater source. Such airborne eavesdropping of underwater sound is experimentally demonstrated with a measured sensitivity enhancement of nearly 104 at 8kHz, far from achievable with the current state-of-the-art methods. Moreover, the opportunities of using the proposed metasurface for cross-media orbital-angular-momentum-multiplexed communication and underwater acoustic window are also demonstrated. This metasurface opens new avenues for communication and sensing in inhomogeneities with totally reflective interfaces, which may be translated to nano-optics and radio frequencies.

Improving transmittance of long-wave infrared guided-mode resonant filters

The long-wave infrared (LWIR) spectral region spanning from 8 to 12 μm is useful for many scientific and industrial applications. Many of these applications require use of either a bandpass or a bandstop filter that can be realized by the guided-mode resonance (GMR) effect with subwavelength periodic features in layered dielectric materials transparent in the LWIR. The GMR filters operating in the LWIR region are fabricated by depositing an amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on a polished zinc selenide (ZnSe) substrate. In general, the backside of a ZnSe substrate with refractive index 2.41 is uncoated causing a 17% Fresnel-reflection loss in the light transmitted through the filter due to a large impedance mismatch at the ZnSe/air interface. Because we use such filters in the LWIR laser experiments for notch filtering, to improve the filter transmittance we used ZnSe substrates coated on one-side with broadband antireflection coating (ARC) covering the 7 to 12 μm spectral range to fabricate GMRFs with one-dimensional (1D) Ge ZC WGG. We employed high-spatial resolution e-beam lithography and reactive-ion etching nanofabrication techniques to achieve high-performance large-area (12 × 12 mm2) 1D notch filters with subwavelength periods. We characterized polarization dependent spectral performance of the prototype filters with both coherent and incoherent incident light using a tunable quantum cascade laser system that spans the 7 to 12 μm region, and a Fourier transform infrared spectrometer with collimated incident beam to achieve close to 15% improvement in the peak transmittance as well as significant reduction in coherent noise compared to our earlier results with GMRFs without ARC. Here, we present the filter design simulation and measurement results.

Resonant Coherent Acoustic Oscillation in Nanoscale Ruddlesden–Popper Perovskite Films

AbstractTime‐domain study of coherent acoustic phonons in nanomaterials provides dynamic and unparalleled insight into their mechanical and structural features. Ruddlesden–Popper (RP) perovskite shows excellent acoustic behaviors due to the large impedance mismatch between its hard perovskite frameworks and soft organic chains. However, the optical probe‐independent acoustic nano‐mechanical resonance and its real application in this important class of semiconductors have not yet been achieved. Herein, the acoustic breathing mode of resonant coherent phonons (RCP) in nanoscale RP perovskite films is reported. In contrast to the previously reported Brillouin mode in thick materials, such resonant breathing mode is no longer interfered by the optical probe, but as a self‐sustained acoustic oscillation source whose features are directly related to material geometry along the direction of phonon propagation. As a nano‐mechanical resonance, RCP oscillation is applied as a novel and non‐destructive approach for quantitatively evaluating the decomposition of moisture‐exposed RP perovskite. These results reveal the decisive effect of structural geometry on acoustic performances in perovskite nanomaterials. The nanoscale counterparts show evident advantages in acoustic mode modulation and structure detection.

Anti-reflection coatings for epsilon-near-zero materials

Epsilon-near-zero (ENZ) materials have attracted much interest within the photonics community due to the various novel light-matter interactions that can occur in the ENZ regime. These materials display a large impedance mismatch between the ENZ material and free space, making it difficult to couple light into the medium at normal incidence. In this article, we demonstrate that enhanced light coupling into an ENZ metamaterial stack can be achieved via the design and fabrication of anti-reflection coatings, which are simple to fabricate via e-beam evaporation. The coating fabricated has been optimized not only to minimize reflection but also aims to maximize transmission — making these designs applicable to e.g. beam shaping applications. We achieve a transmission enhancement of 20% through our metamaterial over a 150 nm range and reflection minimization of 50% over a 200 nm range.

OsciCheck: A Novel Fluidic Transducer for Air-Coupled Ultrasonic Measurements

Ultrasonic measurement technology has become indispensable in NDT-CE. Air-coupled ultrasonic (ACU) measurement techniques promise to reduce measurement time. However, the signal quality suffers from large specific impedance mismatch at the transducer-air and air-specimen interface. Additionally, large pressure amplitudes are necessary for the penetration depth required in NDT-CE applications. To address the specific requirements of ultrasonic testing in NDT-CE, a robust ACU transducer was developed, that generates ultrasound by quickly switching a pressurized air flow. The simple design of the fluidic transducer makes the device maintenance free and resilient against harsh environmental conditions. Since the signal is generated by aeroacoustics, there is no specific impedance mismatch between the transducer and the surrounding air. The ultrasonic signal exhibits frequencies in the 30-60 kHz range and is therefore well suited to penetrate heterogenous materials such as concrete. This contribution gives an introduction in the working principle and signal characteristics of the fluidic transducer. Its application on different materials is validated. A detailed outlook is given to discuss the future potential of fluidic ultrasonic actuators.

Determining the Responsivity of Air-Coupled Piezoelectric Transducers Using a Comparative Method: Theory and Experiments.

The responsivity of an ultrasonic transducer is an important parameter in evaluating its effective frequency band, the electroacoustic conversion efficiency, and the measurement capability of the system. The determination of the responsivity of a traditional immersion or contact piezoelectric transducer has been well investigated. However, due to the high attenuation of waves propagating in air and the large acoustic impedance mismatch between the active piezoceramic material and the load medium, there are few reports of the calibration of an air-coupled piezoelectric transducer. In this work, we present a comparative method of measuring the responsivity of an air-coupled transducer: the air-coupled transducer is used to receive a broadband pulse signal to evaluate its frequency spectrum, and a toneburst signal with known vibration displacement is measured by the air-coupled transducer in order to calibrate the amplitude of the responsivity. The effects of transmitter responsivity, input pulse characteristics, attenuation, and diffraction are taken into account to improve the accuracy of the responsivity determination. In addition, the measurement of the amplitude of the responsivity by comparing the measured displacements avoids the complicated task of characterizing the effects of electrical equipment. The determined responsivity is checked by comparing the measured displacements using different methods at different frequencies in order to evaluate its frequency spectrum and by measuring the nonlinearity parameters of the material to evaluate its amplitude. The agreement between results obtained using different methods demonstrates that the calibrated responsivity of the air-coupled transducer is valid, and the proposed method is effective.

Sound propagation across the air/water interface by a critically coupled resonant bubble

Sound transmission across an air/water interface is known to be ineffective due to a large acoustic impedance mismatch. Efficient sound transmission across an interface between acoustically different media can improve acoustic systems. Here, we demonstrate efficient sound transmission and radiation across an air/water interface using resonant circular bubbles close to the interface. We find that perfect sound transmission is realized under the critical coupling condition, where radiation leakages into air and water are equal. In addition, without incident waves, a resonant bubble as a radiation source exhibits very high sound radiation to air, exceeding sound radiation to water for shallow depths. Moreover, we observe multiresonance sound transmission based on high-order resonance modes in bubbles with a thin elastic shell. Our results provide a critical understanding of the roles of resonant bubbles for sound transmission across an air/water interface.

An Acoustic Meta-Skin Insulator.

Acoustic metamaterials with artificial microstructures are attractive to realize intriguing functions, including efficient waveguiding, which requires large impedance mismatches to realize total side reflection with negligible transmission and absorption. While large impedance mismatch can be readily realized in an air environment, acoustic waveguiding in an underwater environment remains elusive due to insufficient impedance mismatch of state-of-the-art metamaterials. Here, a superhydrophobic acoustic metasurface of microstructured poly(vinylidene fluoride) membrane, referred to as a "meta-skin" insulator, which is able to confine acoustic waves in an all-angle and wide spectrum range due to tremendous impedance mismatch at stable air/water interfaces, viz., the Cassie-Baxter state is demonstrated. By utilizing the meta-skin insulator with broadband and high throughput, orbital-angular-momentum multiplexing at a high spectral efficiency and binary coding along large-angle bending channels for bit-error-free acoustic data transmission in an underwater environment are demonstrated. Very different from optical and/or electrical cable communications, acoustic waves can be simply and effectively coupled into remote meta-skin acoustic fibers from free space, which is technologically significant for long-haul and anti-interference communication. This work can enlighten many fluidic applications based on efficient waveguiding, such as in vivo ultrasound medical treatment and imaging.

Fabrication process of InGaAs-based nanodiode array using electron-beam lithography technique

Self-switching diode (SSD) is a unipolar two-terminal planar device which has a typical channel in nanoscale dimension. It has shown outstanding properties as a microwave and submillimeter wave rectifier by exploiting its nonlinear current-voltage (I-V) characteristic and intrisically low parasitic capacitance. In detection systems, SSDs are often used with an antenna to form a rectifying antenna (rectenna). The large impedance mismatch between SSD and antenna, due to high resistance of a single SSD, has always hampered the rectenna to achieve good extrinsic rectification performance (e.g., voltage responsivity > 400 V/W). As such, a large array of SSDs connected in parallel is very much desired to reduce the device resistance, and hence minimizing the impedance mismatching issue. In this work, an interdigital structure which can accommodate approximately 2,000 SSDs in parallel has been utilized. The material used was InGaAs/InAlAs heterostructure grown onto an InP substrate. The fabrication of the SSD array has implemented an electron-beam lithography (EBL) technique and the use of polymethyl methacrylate (PMMA) as a masking layer. The fabricated SSD array has shown a typical diode-like I-V characteristic, indicating that EBL method is not only convenient to realize nanoscale electronic devices, but also very practical for large area operations.

Tungsten-doped Ge2Sb2Te5 phase change material for high-speed optical switching devices

The large impedance mismatch between the highly resistive amorphous state and the highly conductive crystalline state of Ge2Sb2Te5 is an impediment for the realization of high-speed electrically switched optical devices. In this paper, we demonstrate that tungsten doping can reduce this resistivity contrast and also results in a lower amorphous state resistivity. Additionally, it lowers the contact resistance, improves the optical contrast, and extends the face-centered-cubic state up to 350 °C, with a minimal impact on thermal conductivity.

Thermal conductance of one-dimensional disordered harmonic chains

We study heat conduction mediated by longitudinal phonons in one dimensional disordered harmonic chains. Using scaling properties of the phonon density of states and localization in disordered systems, we find non-trivial scaling of the thermal conductance with the system size. Our findings are corroborated by extensive numerical analysis. We show that a system with strong disorder, characterized by a `heavy-tailed' probability distribution, and with large impedance mismatch between the bath and the system satisfies Fourier's law. We identify a dimensionless scaling parameter, related to the temperature scale and the localization length of the phonons, through which the thermal conductance for different models of disorder and different temperatures follows a universal behavior.

Analysis of a single-mode waveguide at sub-terahertz frequencies as a communication channel

We study experimentally the transmission of an electromagnetic waveguide in the frequency range from 160 to 300 GHz. Photo-mixing is used to excite and detect the fundamental TE10 mode in a rectangular waveguide with two orders-of-magnitude lower impedance. The large impedance mismatch leads to a strong frequency dependence of the transmission, which we measure with a high-dynamic range of up to 80 dB and with high frequency-resolution. The modified transmission function is directly related to the information rate of the waveguide, which we estimate to be about 1 bit per photon. We suggest that the results are applicable to a Josephson junction employed as a single-photon source and coupled to a superconducting waveguide to achieve a simple on-demand narrow-bandwidth free-space number-state quantum channel.

Application of transfer matrix method, Green's function, and variational technique to predict diffuse absorption with edge effect

Acoustic calculations utilizing the Transfer Matrix Method (TMM) occasionally underestimate the sound absorption performance of multi-layered porous material when compared to experimentally obtained data. This is particularly true when experimental data pertains to reverberant (Diffuse-Sabine) absorption. When there is a large impedance mismatch between the reverberant wall and the porous specimen, experimental data indicates Absorption Coefficients greater than 1, which defies the law of conservation of energy. It is hypothesized that the extra absorption is due to the diffraction of waves occurring at the edge of the tested materials, thus increasing the area where energy is absorbed. This contradiction of the law of energy is often referred to as the “edge effect”. This study aims in creating a tool to predict and further understand the edge effect phenomena within reverberation and anechoic rooms and to determine the true absorption performance of the multi-layered porous material by using the TMM, Green’s function, and the variational technique. A plate-foam system is used as a test bed; computation results are compared to experimental absorption data. The outcome of the research will support automobile, aerospace, and architectural companies in reaching sound and vibration targets required for new vehicle/building models.

A Universal Strategy for Improving the Energy Transmission Efficiency and Load Power of Triboelectric Nanogenerators

AbstractTriboelectric nanogenerators (TENGs) have recently been invented as a potential energy technology for harvesting low‐frequency mechanical energy. The load power acquired from a TENG is far less than the maximum output power of the TENG for the large internal impedance and impedance mismatch, and this difference results in an extremely low energy transmission efficiency. Here, a universal strategy is proposed for improving the energy transmission efficiency and load power through dielectric material design, including a reduction in the effective thickness and the directional alignment of the electric dipole. This strategy reduces the internal impedances of TENGs with different modes and results in the improvement of energy transmission efficiency and load power. According to this strategy, the internal impedance of an as‐fabricated TENG is reduced from 16 to 1.3 MΩ, and the energy transmission efficiency is enhanced from 22.46% to 99.5%. Moreover, the load power under 1 MΩ resistance is improved from 0.014 to 0.251 µW, an increase of 18 times. The strategy not only opens a universal and new road to power management, but also paves the way for the industrial applications of TENGs.

Periodic plates with tunneled Acoustic-Black-Holes for directional band gap generation

Abstract Research in Acoustic Black Holes (ABHs) attracts increasing interests for its potential applications in vibration control. ABH effect features the energy focalization of flexural waves within a confined area inside a structure with a reducing power-law profiled thickness. With conventional design of ABH structures, however, systematic broadband ABH effects can only be achieved at relatively high frequencies while the mid-to-low frequency application can hardly be envisaged without prohibitively large ABH dimensions. We propose a kind of periodic plates carved inside with tunneled ABHs to achieve directional broad band gaps for flexural waves at mid-to-low frequencies. Analyses on the band structures and mode shapes show the generation of the band gaps through locally resonant effects of the ABH cells. With additional strengthening studs connecting the two ABH branches, Bragg scattering is produced due to its large impedance mismatch with the residual thickness of ABH profile. With the two effects combined, wide band gaps are achieved over a large frequency range for flexural waves travelling along the direction perpendicular to the tunneled ABHs. Both numerical and experimental results show significant attenuation gaps in finite plates with only three ABH cells. The proposed periodic plates with 1D tunneled ABHs and strengthening studs point at potential applications in wave filtering and vibration isolation applications.

Simulation of microwave propagation properties in human abdominal tissues on wireless capsule endoscopy by FDTD

Abstract The wireless capsule endoscopy plays an important role in the treatment and detection of gastrointestinal diseases. It is of great significance to research the interaction between microwaves and human abdominal tissues during application of wireless capsule endoscope. In this paper, a model of the application of wireless capsule endoscope in human abdomen is proposed. The propagation of the microwaves in human abdominal tissues at four frequencies, such as 50MHz, 300MHz, 700MHz and 900MHz, is numerically analyzed by 2D-FDTD method. The results show that the large electrical impedance mismatches between the tissue layers result in reflection of microwaves from the interface of layers. The interference of reflected waves is related to the effective conductivity of the tissues and the number of layers of biological tissue which microwave pass through. The interference of reflected waves is positively correlated with the effective conductivity of human tissue, and negatively correlated with the number of layers of human tissue the microwaves pass through. The effect of the dielectric constant in the human tissues on both the distribution of electric field and the phase is higher than that of the effective conductivity. The changes of the effective conductivity in human tissue lead to sharply changes in the SAR, and the changes of the distribution of the electric field cause smoothly changes in the SAR of human tissues. Therefore, in the application of wireless capsule endoscopy for pathological examination, the antenna should be prevented from being wrapped by closed tissues with high dielectric constant as much as possible. When microwave is used to transmit the signals, it is necessary to reduce the number of layers and thickness of biological tissues the microwaves pass through. Effective conductivity should be given priority in assessment the safety of electromagnetic by calculating the specific absorption rate of human tissues.

Improving the performance of Ge2Sb2Te5 materials via nickel doping: Towards RF-compatible phase-change devices

High-speed electrical switching of Ge2Sb2Te5 (GST) remains a challenging task due to the large impedance mismatch between the low-conductivity amorphous state and the high-conductivity crystalline state. In this letter, we demonstrate an effective doping scheme using nickel to reduce the resistivity contrast between the amorphous and crystalline states by nearly three orders of magnitude. Most importantly, our results show that doping produces the desired electrical performance without adversely affecting the film's optical properties. The nickel doping level is approximately 2% and the lattice structure remains nearly unchanged when compared with undoped-GST. The refractive indices in amorphous and crystalline states were obtained using ellipsometry which echoes the results of X-ray diffraction. The material's thermal transport properties are measured using time-domain thermoreflectance, showing no change upon doping. The advantages of this doping system will open up opportunities for designing electrically reconfigurable high speed optical elements in the near-infrared spectrum.

Transurethral ultrasound therapy of the prostate in the presence of calcifications: A simulation study.

Transurethral ultrasound therapy is an investigational treatment modality which could potentially be used for the localized treatment of prostate cancer. One of the limiting factors of this therapy is prostatic calcifications. These attenuate and reflect ultrasound and thus reduce the efficacy of the heating. The aim of this study is to investigate how prostatic calcifications affect therapeutic efficacy, and to identify the best sonication strategy when calcifications are present. Realistic computational models were used on clinical patient data in order to simulate different therapeutic situations with naturally occurring calcifications as well as artificial calcifications of different sizes (1-10mm) and distances (5-15mm). Furthermore, different sonication strategies were tested in order to deliver therapy to the untreated tissue regions behind the calcifications. The presence of calcifications in front of the ultrasound field was found to increase the peak pressure by 100% on average while the maximum temperature only rose by 9% during a 20-s sonication. Losses in ultrasound energy were due to the relatively large acoustic impedance mismatch between the prostate tissue and the calcifications (1.63 vs 3.20 MRayl) and high attenuation coefficient (0.78 vs 2.64dB/MHz1.1 /cm), which together left untreated tissue regions behind the calcifications. In addition, elevated temperatures were seen in the region between the transducer and the calcifications. Lower sonication frequencies (1-4MHz) were not able to penetrate through the calcifications effectively, but longer sonication durations (20-60s) with selective transducer elements were effective in treating the tissue regions behind the calcifications. Prostatic calcifications limit the reach of therapeutic ultrasound treatment due to reflections and attenuation. The tissue regions behind the calcifications can possibly be treated using longer sonication durations combined with proper transducer element selection. However, caution should be taken with calcifications located close to sensitive organs such as the urethra, bladder neck, or rectal wall.