Acoustic Confinement Research Articles

Pressure Gain Combustion for Gas Turbines: Analysis of a Fully Coupled Engine Model

Abstract The ‘Shockless Explosion Combustion’ (SEC) concept for gas turbine combustors, introduced in 2014, approximates constant volume combustion (CVC) by harnessing acoustic confinement of autoigniting gas packets. The resulting pressure waves simultaneously transmit combustion energy to a turbine plenum and facilitate the combustor's recharging against an average pressure gain. Challenges in actualizing an SEC-driven gas turbine include i) the creation of charge stratifications for nearly homogeneous autoignition, ii) protecting the turbo components from combustion-induced pressure fluctuations, iii) providing evidence that efficiency gains comparable to those of CVC over deflagrative combustion can be realized, and iv) designing an effective one-way intake valve. This work addresses challenges i)-iii) utilizing computational engine models incorporating a quasi-one-dimensional combustor, zero- and two-dimensional compressor and turbine plena, and quasi-stationary turbo components. Two SEC operational modes are identified which fire at roughly one and two times the combustor's acoustic frequencies. Results for SEC-driven gas turbines with compressor pressure ratios of 6:1 and 20:1 reveal 1.5-fold mean pressure gains across the combustors. Assuming ideally efficient compressors and turbines, efficiency gains over engines with deflagration-based combustors of 30% and 18% are realized, respectively. With absolute values of 52% and 66%, the obtained efficiencies are close to the theoretical Humphrey cycle efficiencies of 54% and 65% for the mentioned pre-compression ratios. Detailed thermodynamic cycle analyses for individual gas parcels suggest that there is room for further efficiency gains through optimized plenum and combustor designs.

Metastructure “Trap” Coating by Acoustic Confinement Effect for Antibacterial Sonothermal Therapy

AbstractAntibacterial sonothermal therapy (ASTT) is attractive for treating implant‐associated infection, which addresses the limitations of poor penetration of photothermal therapy. However, achieving implant surface‐specific ultrasound heating is incredibly challenging. Inspired by the porous acoustic metastructure in noise reduction, the acoustic metastructure would be a new strategy for attaining ASTT. Herein, an “acoustic confinement effect” is proposed to “trap” ultrasound for the metastructure TiO2 porous coating on titanium implants. The metastructure coating accomplishes specific acoustic absorption and efficient acoustic‐to‐thermal conversion, surpassing the non‐specific heating of conventional coatings. The theoretical calculation and experiment of metastructure porous coating prove the acoustic‐to‐thermal conversion mechanism, which is also validated in metastructure porous coatings on medical polyetheretherketone, indicating it is universal. Excitingly, this acoustic metastructure “trap” can capture bacteria and local sonothermal enhancement, exhibiting superior ASTT antibacterial rates (S. aureus, 99.69%; E. coil, 99.58%) at low‐power ultrasound. In addition, the metastructure TiO2 coating mimics human bone and demonstrates outstanding osseointegration. This study sheds substantial light on developing the acoustic responsive surface, broadening a new direction of acoustic metamaterials in medical applications.

Control of liquid crystals combining surface acoustic waves, nematic flows, and microfluidic confinement.

The optical properties of liquid crystals serve as the basis for display, diagnostic, and sensing technologies. Such properties are generally controlled by relying on electric fields. In this work, we investigate the effects of microfluidic flows and acoustic fields on the molecular orientation and the corresponding optical response of nematic liquid crystals. Several previously unknown structures are identified, which are rationalized in terms of a state diagram as a function of the strengths of the flow and the acoustic field. The new structures are interpreted by relying on calculations with a free energy functional expressed in terms of the tensorial order parameter, using continuum theory simulations in the Landau-de Gennes framework. Taken together, the findings presented here offer promise for the development of new systems based on combinations of sound, flow, and confinement.

Surface acoustic wave confinement inside uncorrelated distributions of subwavelength scatterers

We report an experimental study of surface acoustic wave (SAW) localization and propagation in random metasurfaces composed of Al scatters using pump–probe spectroscopy. Thanks to this technique, wideband high frequency acoustic modes are generated, and their dynamical propagation directly from inside of the media with a high (micrometric) spatial resolution is enabled. During SAW propagation, part of the acoustic wavefront energy is trapped within free areas between the scatterers, acting as cavities. The spectral content of the localized modes of a few GHz is found to depend on the shape and size of the cavities but also on the landscape seen by the wave during its propagation before arriving inside them. The experimental results are supported by numerical simulations using the finite element method. This study is the phononic part of a more global research on the co-localization of elastic and optical waves on random metasurfaces, with the main objective of enhancing the photon–phonon interaction. Applications could range from the design of acousto-optic modulators to ultrasensitive sensors.

Radiation mode-based personal audio system: Robustness against wall reflection

Confining sounds in the near field of a loudspeaker array is discussed for realizing personal audio systems. Active acoustic power propagates to the far field, whereas reactive acoustic power does not. Therefore, increasing the ratio of the reactive acoustic power to the active acoustic power leads to acoustic confinement in the near field. The reactive-to-active ratio is maximized by optimally driving loudspeakers. The optimal driving signals are derived based on the theory of generalized radiation modes. This method was proposed in earlier studies by the authors. In the literatures, point acoustic sources on an infinite baffle were assumed to be in a free space, and the radiation mode theory was applied to obtain the optimal driving signals. However, in practice, walls may exist nearby the loudspeaker array. Acoustic waves radiating from the loudspeakers are reflected by the walls. Such a wall reflection may degrade the acoustic confinement. In this study, the robustness against a wall reflection is investigated.

Robust acoustic trapping and perturbation of single-cell microswimmers illuminate three-dimensional swimming and ciliary coordination

We report a label-free acoustic microfluidic method to confine single, cilia-driven swimming cells in space without limiting their rotational degrees of freedom. Our platform integrates a surface acoustic wave (SAW) actuator and bulk acoustic wave (BAW) trapping array to enable multiplexed analysis with high spatial resolution and trapping forces that are strong enough to hold individual microswimmers. The hybrid BAW/SAW acoustic tweezers employ high-efficiency mode conversion to achieve submicron image resolution while compensating for parasitic system losses to immersion oil in contact with the microfluidic chip. We use the platform to quantify cilia and cell body motion for wildtype biciliate cells, investigating effects of environmental variables like temperature and viscosity on ciliary beating, synchronization, and three-dimensional helical swimming. We confirm and expand upon the existing understanding of these phenomena, for example determining that increasing viscosity promotes asynchronous beating. Motile cilia are subcellular organelles that propel microorganisms or direct fluid and particulate flow. Thus, cilia are critical to cell survival and human health. The unicellular alga Chlamydomonas reinhardtii is widely used to investigate the mechanisms underlying ciliary beating and coordination. However, freely swimming cells are difficult to image with sufficient resolution to capture cilia motion, necessitating that the cell body be held during experiments. Acoustic confinement is a compelling alternative to use of a micropipette, or to magnetic, electrical, and optical trapping that may modify the cells and affect their behavior. Beyond establishing our approach to studying microswimmers, we demonstrate a unique ability to mechanically perturb cells via rapid acoustic positioning.

Optimizing performance for an on-chip stimulated Brillouin scattering-based isolator

Non-reciprocal optical components such as isolators and circulators are crucial for preventing catastrophic backreflection and controlling optical cross talk in photonic systems. While non-reciprocal devices based on Brillouin intermodal transitions have been experimentally demonstrated in chip-scale platforms, harnessing such interactions has required a suspended waveguide structure, which is challenging to fabricate and is potentially less robust than a non-suspended structure, thereby limiting the design flexibility. In this paper, we numerically investigate the performance of a Brillouin-based isolation scheme in which a dual-pump-driven optoacoustic interaction is used to excite confined acoustic waves in a traditional ridge waveguide. We find that acoustic confinement, and therefore the amount of Brillouin-driven mode conversion, can be enhanced by selecting an appropriate optical mode pair and waveguide geometry of two arsenic-based chalcogenide platforms. Further, we optimize the isolator design in its entirety, including the input couplers, mode filters, the Brillouin-active waveguide as well as the device fabrication tolerances. We predict such a device can achieve 30 dB isolation over a 38 nm bandwidth when 500 mW pump power is used; in the presence of a ± 10 n m fabrication-induced width error, such isolation can be maintained over a 5–10 nm bandwidth.

Surface-Acoustic-Wave Devices Based on Lithium Niobate and Amorphous Silicon Thin Films on a Silicon Substrate

This work presents an acoustic platform using solidly mounted thin-oriented lithium niobate (LiNbO3) film on silicon (Si). A thin layer of amorphous Si eliminates a conductive layer generated in the thin-film bonding processes and contributes to acoustic energy confinement and thermal frequency stability. The gigahertz operating frequency is achieved by resorting to the guided acoustic wave shear-horizontal wave (SH0) in a 400-nm-thick X-cut LiNbO3 thin film. Due to the elimination of the parasitic surface conduction (PSC) effect, the electromechanical coupling of the guide acoustic wave is maximized in the Si-based heterogeneous wafer. Due to the minimized acoustic impedance mismatch between surface material and substrate, longitudinal and higher order spurious modes are suppressed. Due to the stiff substrate with a small temperature expansion coefficient (TEC), the solidly mounted structure features a reduced temperature coefficient of frequency (TCF) and improved power handling. The fabricated resonator shows an extracted electromechanical coupling coefficient of 22.8%, a high loaded <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> of 1208 at 1.6 GHz, and a TCF of −36 p/min/K. The compressed filter is demonstrated with a minimum insertion loss (IL) of 0.6 dB, a fractional bandwidth (FBW) of 8%, an out-of-band rejection of 30 dB, a TCF of −38.5 p/min/K at roll-off, and a miniaturized footprint of 0.4 mm2. The performance has shown the strong potential of the LiNbO3–Si platform for front-end applications in 5G.

Integrated acoustic resonators in commercial fin field-effect transistor technology

In radio communication, the growth of beamforming and multiple-input–multiple-output technologies, which increase transceiver complexity, have led to a drive to reduce the size, weight and power of radio components by integrating them into a single system on chip. One approach is to integrate the frequency references of acoustic microelectromechanical systems (MEMS) with complementary metal–oxide–semiconductor processes, typically through a MEMS-first or MEMS-last approach that requires process customization. Here we report unreleased acoustic resonators that are fabricated in 14 nm fin field-effect transistor technology and operate in the X-band frequency range (8–12 GHz). The devices use phononic waveguides for acoustic confinement and exploit metal–oxide–semiconductor capacitors and transistors to electromechanically drive and sense acoustic vibrations. Fifteen device variations are analysed across 30 bias points, quantifying the importance of phononic confinement on resonator performance and demonstrating the velocity-saturated piezoresistive effect in active resonant transistors. Our results illustrate the feasibility of integrating acoustic devices directly into standard complementary metal–oxide–semiconductor processes.

Role of Confined Optical Phonons in Exciton Generation in Spherical Quantum Dot.

Optical control of excitonic states in semiconducting quantum dots has enabled it to be deployed as a qubit for quantum information processing. For self-assembled quantum dots, these excitonic states couple with phonons in the barrier material, for which the previous studies have shown that such exciton—phonon coupling can also lead to the generation of exciton, paving the way for their deployment in qubit-state preparation. Previous studies on self-assembled quantum dots comprising polar materials have considered exciton—phonon coupling by treating phonon modes as bulk acoustic modes only, owing to nearly the same acoustic property of the dot and barrier material. However, the dimensional confinement leads to significant modification phonon modes, even though acoustic confinement is weak but optical confinement cannot be overlooked. In this paper, we investigate for the first time the exciton—optical phonon coupling using dielectric continuum model duly accounting for the dimensional confinement leading to exciton generation. We report that at low temperatures (below 10 K), the exciton creation rate attributed to confined optical phonon is approximately 5.7 times (~6) slower than bulk acoustic phonons, which cannot be ignored, and it should be accounted for in determining the effective phonon assisted exciton creation rate.

Design and Applications of Integrated Transducers in Commercial CMOS Technology

Monolithic integration of Microelectromechanical Systems (MEMS) directly within CMOS technology offers enhanced functionality for integrated circuits (IC) and the potential improvement of system-level performance for MEMS devices in close proximity to biasing and sense circuits. While the bulk of CMOS-MEMS solutions involve post-processing of CMOS chips to define freely-suspended MEMS structures, there are key applications and conditions under which a solid, unreleased acoustic structure composed of the CMOS stack is preferred. Unreleased CMOS-MEMS devices benefit from lower barrier-to-entry with no post-processing of the CMOS chip, simplified packaging, robustness under acceleration and shock, stress gradient insensitivity, and opportunities for frequency scaling. This paper provides a review of advances in unreleased CMOS-MEMS devices over the past decade, with focus on dispersion engineering of guided waves in CMOS, acoustic confinement, CMOS-MEMS transducers, and large signal modeling. We discuss performance limits with standard capacitive transduction, with emphasis on performance boost with emerging CMOS materials including ferroelectrics under development for memory.

Topological imbalanced phononic crystal with semi-enclosed defect for high-performance acoustic energy confinement and harvesting

Phononic crystals (PnCs) attract attentions in acoustic energy conversion and harvesting due to their excellent properties in regulating elastic and acoustic waves. Topological imbalance and semi-enclosed defect has been proposed in this work to systematically broaden the band gap of PnCs and improve the localization of elastic wave in defect. Parametric equations are introduced to generate nine square lattice unit cells with four-fold rotationally symmetric shapes. The imbalance between the long and short branches is utilized to describe the topological evolution of the perfect PnCs. With the increase of topological imbalance, the jump of the Bloch mode appears and thus leads to the band gap opening and expansion. The defect supercell with the highest topological imbalance has the best energy confinement effect. The 2D and 3D transient numerical simulations for the PnC of the highest topological imbalance indicate that the energy confinement effect of the semi-enclosed line defect is more intensive than the semi-enclosed arrow or bottle defect, which is confirmed by piezoelectric energy harvesting experiments. Under the excitation of 50 kHz, the period-2 nonlinear phenomena of the output voltage by piezoelectric disk are experimentally noticed in the PnC with the semi-enclosed line defect. The peak-to-peak power is 3.08 mW at the optimal resistance. Compared with the traditional point defect case, the voltage of the semi-enclosed line defect case is increased by 12.0 times and its power is increased by 75.1 times due to combination of perfect mirror effect and nonlinear defect state mechanism. This study provides a new avenue in design of high-frequency nonlinear acoustic devices and self-powered acoustic sensors.

Ultrafast laser-induced guided elastic waves in a freestanding aluminum membrane

Ultrafast laser-induced guided acoustic waves in thin, freely suspended films are important for many applications adopting the laser-ultrasonics technique. These waves show unique dispersion relations, leading to minimal propagation losses and acoustic energy confinement. While this principle has been known, the separation of various physical effects in the formation of measured signals involving these guided acoustic waves has not been clearly elaborated. Here, we present a combined experimental and theoretical study on all-optical excitation and detection of these waves in a thin, freestanding aluminum membrane. The acoustic dynamics is excited and measured by using a femtosecond time-resolved pump-probe technique with controlled probing position, enabling spatially resolved detection. The measured signals are compared with an advanced numerical model that we developed earlier [H. Zhang et al., Phys. Rev. Appl. 13, 014010 (2020)], showing excellent agreement. The combination of experiment and simulation allows us to decode various physical effects in the signal formation, including different acoustic field components. Unknown material properties, such as acoustic attenuation coefficients, and the two complex photoelastic constants are quantitatively retrieved by fitting the measured signals. Furthermore, we provide evidence of nonlinear propagation of the excited guided acoustic waves.

An Acoustic Waveguide With Tight Field Confinement for High Temperature Sensing

A fused quartz acoustic waveguide structure was reported as the suspended core waveguide. The waveguide featured good acoustic wave confinement within the core and a relatively small diameter. Consistent high-temperature sensing was also demonstrated with the waveguide after a temperature sensor was fabricated inside. The waveguide was fabricated by suspending its core rod in the middle of the hollow cladding tube by discrete thin supporting disks. It was found that the characteristics of acoustic propagation in the waveguide was similar to that in a bare core rod and that the waveguide was largely insensitive to lossy surface perturbations. The diameter of the waveguide was relatively small, little more than the bulk longitudinal acoustic wavelength. For sensing, the periodic structure, the acoustic fiber Bragg grating, was fabricated on the core of the suspended core waveguide. A temperature test up to 1000 °C was performed. The central frequency of the reflected acoustic wave was found to change unidirectionally and consistently with changes in temperature. With its good field confinement and consistent sensing capabilities, the suspended core waveguide could find vast potential in many applications.

High-acoustic-index-contrast phononic circuits: Numerical modeling

We numerically model key building blocks of a phononic integrated circuit that enable phonon routing in high-acoustic-index waveguides. Our particular focus is on the gallium nitride-on-sapphire phononic platform which has recently demonstrated high acoustic confinement in its top layer without the use of suspended structures. We start with the systematic simulation of various transverse phonon modes supported in strip waveguides and ring resonators with sub-wavelength cross section. Mode confinement and quality factors of phonon modes are numerically investigated with respect to geometric parameters. A quality factor of up to 108 is predicted in optimized ring resonators. Next, we study the design of the phononic directional couplers and present key design parameters for achieving strong evanescent couplings between modes propagating in parallel waveguides. Last, interdigitated transducer electrodes are included in the simulation for direct excitation of a ring resonator and critical coupling between microwave input and phononic dissipation. Our work provides a comprehensive numerical characterization of phonon modes and functional phononic components in high-acoustic-index phononic circuits, which supplements previous theories and contributes to the emerging field of phononic integrated circuits.

Hybrid integrated photonics using bulk acoustic resonators

Integrated photonic devices based on Si3N4 waveguides allow for the exploitation of nonlinear frequency conversion, exhibit low propagation loss, and have led to advances in compact atomic clocks, ultrafast ranging, and spectroscopy. Yet, the lack of Pockels effect presents a major challenge to achieve high-speed modulation of Si3N4. Here, microwave-frequency acousto-optic modulation is realized by exciting high-overtone bulk acoustic wave resonances (HBAR) in the photonic stack. Although HBAR is ubiquitously used in modern communication and superconducting circuits, this is the first time it has been incorporated on a photonic integrated chip. The tight vertical acoustic confinement releases the lateral design of freedom, and enables negligible cross-talk and preserving low optical loss. This hybrid HBAR nanophotonic platform can find immediate applications in topological photonics with synthetic dimensions, compact opto-electronic oscillators, and microwave-to-optical converters. As an application, a Si3N4-based optical isolator is demonstrated by spatiotemporal modulation, with over 17 dB isolation achieved.

Acoustic wave confinement in two-dimensional phononic crystal with multiple nested resonators

In this research, a novel phononic crystal (PC) is investigated theoretically to enhance acoustic pressure confinement. It consists of multiple nested resonators based on a tapered configuration. Nested phononic crystal resonator (NPCR) can enhance the acoustic pressure amplification at resonant cavity to a great degree better than the traditional one with same dimensions. The resonant frequency of NPCR is mainly located within outermost phononic crystal resonator’s (PCR) band gap. Meanwhile, it does not move significantly to high frequency with the addition of inner tapered resonators. The enhanced acoustic pressure resonant amplification is attributed to the improvement of the confinement mode owing to the nested structure working as a taper. Then effects of geometrical dimensions of inner PCRs on acoustic confinement are studied. It shows that resonant frequency and resonant acoustic pressure can be affected by the geometric parameters. NPCR has stronger acoustic confinement effects, which are conducive to improve acoustic sensing sensitivity, acoustic signal frequency resolution and acoustic energy harvesting efficiency.

An 860 MHz Wireless Surface Acoustic Wave Sensor With a Metal-Organic Framework Sensing Layer for CO2 and CH4

Wireless and passive surface acoustic wave (SAW) devices with nanoporous metal-organic framework (MOF) sensing layers are attractive gas sensors for applications in many fields such as energy industries and air pollution control. Here, we report on enhancing the sensitivity and detection limit of zeolitic imidazolate framework-8 (ZIF-8) MOF-coated SAW reflective delay line mass sensors by increasing the operating frequency for sensitive detection of carbon dioxide (CO2) and methane (CH4) at ambient conditions. In particular, we show at least four times higher sensitivity of an 860 MHz (4-{\mu}m periodicity) SAW reflective delay line coated with a 240 nm thick ZIF-8 compared to the sensitivity of a 430 MHz (8-{\mu}m periodicity) otherwise identical sensor device to the targeted gases. The detection limits of the higher frequency wireless devices for CO2 and CH4 were estimated to be 0.91 vol-% and 7.01 vol-%, respectively. The enhanced sensitivity for higher frequency devices is explained in terms of the frequency dependent acoustic wave energy confinement.

Acoustic wave confinement by chiral waveguide made of Helmholtz resonators

Confining sound and controlling acoustic wave propagation is of significant importance for efficient energy harvesting. We propose an acoustic waveguide that exists at the interface between two metamaterials consist of unit cells based on Helmholtz resonators arranged in opposite chirality, which gives good confinement of sound wave in the air for frequencies within the bulk band gap of the metamaterial. The amplitude of pressure attenuates by 3 dB within 1/3 of the lattice constant. This waveguide is shown to be robust to frequency and spatial disorders in the system, as long as the edge mode is situated in the bandgap. An acoustic circuit was formed by introducing disorders at sharp corners. Our simulations demonstrate that the acoustic impedance at the interface, as defined by the ratio of the local pressure to the sound velocity, is 3 orders of magnitude smaller than the bulk impedance of the metamaterial for the frequency of interest, giving rise to confinement of the sound wave.

Brillouin integrated photonics

A recent renaissance in Brillouin scattering research has been driven by the increasing maturity of photonic integration platforms and nanophotonics. The result is a new breed of chip-based devices that exploit acousto-optic interactions to create lasers, amplifiers, filters, delay lines and isolators. Here, we provide a detailed overview of Brillouin scattering in integrated waveguides and resonators, covering key concepts such as the stimulation of the Brillouin process, in which the optical field itself induces acoustic vibrations, the importance of acoustic confinement, methods for calculating and measuring Brillouin gain, and the diversity of materials platforms and geometries. Our Review emphasizes emerging applications in microwave photonics, signal processing and sensing, and concludes with a perspective for future directions. Acousto-optical interactions within integrated optics platforms are reviewed with a discussion of the useful chip-based devices such as lasers, amplifiers, filters, isolators and more besides that can result.