Amplification Of Acoustic Waves Research Articles

Electrically driven amplification of terahertz acoustic waves in graphene

In graphene devices, the electronic drift velocity can easily exceed the speed of sound in the material at moderate current biases. Under these conditions, the electronic system can efficiently amplify acoustic phonons, leading to an exponential growth of sound waves in the direction of the carrier flow. Here, we show that such phonon amplification can significantly modify the electrical properties of graphene devices. We observe a superlinear growth of the resistivity in the direction of the carrier flow when the drift velocity exceeds the speed of sound — resulting in a sevenfold increase over a distance of 8 µm. The resistivity growth is observed at carrier densities away from the Dirac point and is enhanced at cryogenic temperatures. We develop a theoretical model for the resistivity growth due to the electrical amplification of acoustic phonons — reaching frequencies up to 2.2 THz — where the wavelength is controlled by gate-tunable transitions across the Fermi surface. These findings provide a route to on-chip high-frequency sound generation and detection in the THz frequency range.

Influence of Homo- and Hetero-Junctions on the Propagation Characteristics of Radially Propagated Cylindrical Surface Acoustic Waves in a Piezoelectric Semiconductor Semi-Infinite Medium

This paper theoretically investigates the influence of homo- and hetero-junctions on the propagation characteristics of radially propagated cylindrical surface acoustic waves in a piezoelectric semiconductor semi-infinite medium. First, the basic equations of the piezoelectric semiconductor semi-infinite medium are mathematically derived. Then, based on these basic equations and the transfer matrix method, two equivalent mathematical models are established concerning the propagation of radially propagated cylindrical surface acoustic waves in this piezoelectric semiconductor semi-infinite medium. Based on the surface and interface effect theory, the homo- or hetero-junction is theoretically treated as a two-dimensional electrically imperfect interface in the first mathematical model. To legitimately confirm the interface characteristic lengths that appear in the electrically imperfect interface conditions, the homo- or hetero-junction is equivalently treated as a functional gradient thin layer in the second mathematical model. Finally, based on these two mathematical models, the dispersion and attenuation curves of radially propagated cylindrical surface acoustic waves are numerically calculated to discuss the influence of the homo- and hetero-junctions on the dispersion and attenuation characteristics of radially propagated cylindrical surface acoustic waves. The interface characteristic lengths are legitimately confirmed through the comparison of dispersion and attenuation curves calculated using the two equivalent mathematical models. As piezoelectric semiconductor energy harvesters usually work under elastic deformation, the establishment of mathematical models and the revelation of physical mechanisms are both fundamental to the analysis and optimization of micro-scale surface acoustic wave resonators, energy harvesters, and acoustic wave amplification based on the propagation of surface acoustic waves.

Topological rainbow trapping and acoustic energy amplification in two-dimensional gradient phononic crystals

In this work, we numerically and experimentally investigate topological rainbow trapping and energy amplification of acoustic waves in a gradient phononic crystal (PC) structure. Thanks to the acoustic valley Hall effect, topological interface states (TISs) are generated along the interface between two PCs with different topological phases. To achieve rainbow trapping, we introduce the gradient into a 3D-printed PC structure by varying the geometrical parameter of scatterers along the interface. The incident acoustic waves at different frequencies split, stop, and, hence, are significantly amplified at different positions. Notably, the rainbow trapping of TISs is immune to random structural disorders. The topological rainbow trapping is promising for the design of broadband energy harvesters with excellent robustness.

Acoustic wave amplification with thin film silicon bonded on lithium niobate

Signal processing with the use of acoustic waves is an important technology for various functions in radio frequency (RF) systems, including matched filtering in congested parts of the frequency spectrum. In order to generate long time delays on chip required for these applications, the acoustoelectric effect offers the ability to counter acoustic propagation losses while also generating inherent non-reciprocity. In this work, we demonstrate an approach to directly bond thin film silicon from 200 mm commercial silicon on insulator wafers on X-cut lithium niobate substrates with the use of plasma surface activation. The resulting delay line devices at 410 MHz demonstrate amplification of Rayleigh waves, with a peak non-reciprocal contrast between forward and reverse traveling waves of over 25 dB mm−1 under continuous direct current bias conditions. The demonstrated process can extend the functionality of traditionally passive piezoelectric RF microsystems.

Copper-Catalyzed Covalent Dimerization of Near-Infrared Fluorescent Cyanine Dyes: Synergistic Enhancement of Photoacoustic Signals for Molecular Imaging of Tumors.

Photoacoustic (PA) imaging relies on the absorption of light by chromophores to generate acoustic waves used to delineate tissue structures and physiology. Here, we demonstrate that Cu(II) efficiently catalyzes the dimerization of diverse near-infrared (NIR) cyanine molecules, including a peptide conjugate. NMR spectroscopy revealed a C-C covalent bond along the heptamethine chains, creating stable molecules under conditions such as a wide range of solvents and pH mediums. Dimerization achieved >90% fluorescence quenching, enhanced photostability, and increased PA signals by a factor of about 4 at equimolar concentrations compared to the monomers. In vivo study with a mouse cancer model revealed that dimerization enhanced tumor retention and PA signal, allowing cancer detection at doses where the monomers are less effective. While the dye dimers highlighted peritumoral blood vessels, the PA signal for dimeric tumor-targeting dye-peptide conjugate, LS301, was diffuse throughout the entire tumor mass. A combination of the ease of synthesis, diversity of molecules that are amenable to Cu(II)-catalyzed dimerization, and the high acoustic wave amplification by these stable dimeric small molecules ushers a new strategy to develop clinically translatable PA molecular amplifiers for the emerging field of molecular photoacoustic imaging.

Multi-tonal subwavelength metamaterial for absorption and amplification of acoustic and ultrasonic waves

In this work, an acousto-ultrasound metamaterial-based concept is proposed to achieve high multi-tonal sound absorption at specific design frequencies and their multiple harmonics, which generally requires large and complex systems. This structure can be deployed to improve the performance of air-coupled nonlinear acoustic/ultrasound imaging by filtering unwanted fundamental ultrasound responses while amplifying high order harmonics, since nonlinear ultrasonic experiments generally necessitate advanced signal processing tools digital and pass-band filters to highlight nonlinear features. The structure proposed is an Archimedean inspired spiral cavity metamaterial with a thickness of 1/62 wavelength to achieve high multi-tonal sound absorption performances at a design frequency and the multiple harmonics. The same geometrical configuration can also be used to filter a fundamental design excitation frequency f0 and amplify second harmonic of the desired excitation frequencies, 2f0. An analytical model was developed to optimise the sound absorption and amplification frequencies of the structure with a design frequency of 690 Hz, by matching the geometrical parameters with the resonance and antiresonance mechanisms of the system. Additionally, a parallel arrangement of two Archimedean-inspired spirals is also analytically and experimentally proposed, in order to achieve harmonic absorptions at the resonant frequency of each subsystem (i.e. f01 = 850 Hz, f02 = 950 Hz). Furthermore, acoustic impedance analyses have been analytically conducted in order to physically explain all the resonance and antiresonance mechanism occurring with the proposed structures. Experimental investigations show that the proposed 3D printed metamaterial-based structures are capable to achieve multi-tonal high absorption peaks (above 90%) at the fundamental frequency f0 and odd harmonics (3f0, 5f0, etc) and sound amplification of the even harmonics (2f0 and 4f0). The results show good correlation between the predicted model and experimental results, and thus the sub-wavelength metamaterial provides promising potential for controlling and achieving high level sound absorption at low frequencies and enhancing accuracy of nonlinear ultrasound imaging applications.

Towards single-chip radiofrequency signal processing via acoustoelectric electron\u2013phonon interactions

The addition of active, nonlinear, and nonreciprocal functionalities to passive piezoelectric acoustic wave technologies could enable all-acoustic and therefore ultra-compact radiofrequency signal processors. Toward this goal, we present a heterogeneously integrated acoustoelectric material platform consisting of a 50 nm indium gallium arsenide epitaxial semiconductor film in direct contact with a 41° YX lithium niobate piezoelectric substrate. We then demonstrate three of the main components of an all-acoustic radiofrequency signal processor: passive delay line filters, amplifiers, and circulators. Heterogeneous integration allows for simultaneous, independent optimization of the piezoelectric-acoustic and electronic properties, leading to the highest performing surface acoustic wave amplifiers ever developed in terms of gain per unit length and DC power dissipation, as well as the first-ever demonstrated acoustoelectric circulator with an isolation of 46 dB with a pulsed DC bias. Finally, we describe how the remaining components of an all-acoustic radiofrequency signal processor are an extension of this work.

Tunable unidirectional compact acoustic amplifier via space-time modulated membranes

Space-time modulation has recently attracted considerable attention as it enables new possibilities in wave manipulation and control. Here we propose a tunable unidirectional acoustic amplifier based on a space-time modulated membrane system. The transfer matrix method is applied to design this system and optimize the modulation parameters for the realization of unidirectional amplification. Efficient one-way amplification of acoustic waves is demonstrated theoretically and numerically within a frequency range from 2260 to 2520 Hz, with overall dimension being less than $1/3$ of the corresponding wavelength. Across this operation band, the amplification factor in the positive direction varies smoothly from 3.0 to 5.8, while the transmitted wave energy in the negative direction does not change significantly. Our work identifies a feasible approach to realize a tunable unidirectional acoustic amplifier via space-time modulated membranes, which offers a design platform for a number of applications in sensing, imaging, and communication.

Comparison of amplification via the acousto-electric effect of Rayleigh and Leaky-SAW modes in a monolithic surface InP:InGaAs/lithium niobate heterostructure

This paper demonstrates a monolithic surface acoustic wave amplifier fabricated by state-of-the-art heterogenous integration of a III-V InGaAs-based epitaxial material stack and LiNbO3. We compare amplification of Raleigh-SAW and leaky-SAW modes on a on Y-cut, X-propagating delay line amplifier. Due to the superior properties of the materials employed, we observe a net terminal gain of 3dB for an LSAW overtone mode This platform enables further advances in active and non-reciprocal piezoelectric acoustic devices.

(Invited) Heterogeneous Integration of Sub-Micron Thickness Epitaxial Ingaas(P) with Lithium Niobate for Active and Nonlinear Surface Acoustic Wave Devices

Radio frequency (RF) devices are becoming more multi-band, increasing the number of filters and other front-end components while simultaneously pushing towards reduced cost, size, weight, and power (CSWaP). One approach to reducing CSWaP is to eliminate high quality factor filtering that relies on acoustic wave technologies, allowing end-to-end solutions in CMOS or compound semiconductor platforms. Yet another way would be to augment the achievable functionalities of electromechanical/acoustic filtering chips to include “active” and nonlinear functionalities, such as gain and mixing. The acoustoelectric (AE) effect could enable such active acoustic wave devices, but, to compete with the performance of today’s multi-chip architectures, any solution to this problem needs to provide high gain, large bandwidth, low noise figure, and sufficient power handling, in addition to the reduced CSWaP. Surface acoustic wave (SAW) amplifiers based on the AE effect were demonstrated as early as the 1960s. In these devices, evanescent fields associated with piezoelectric acoustic waves interact with charge carriers undergoing voltage-induced drift. The interaction causes polarization of the drifting majority charge carriers that leads to a Coulomb drag effect, resulting in acoustic wave attenuation or amplification, analogous to a traveling-wave tube amplifier. The amplifier performance is improved by utilizing a piezoelectric substrate with a large electromechanical coupling coefficient, which alone determines the maximum gain, combined with an acoustically thin semiconductor with low conductivity and high mobility to lower the required voltage for a given gain. Previous demonstrations of SAW amplifiers have resulted in over 100 dB of RF output contrast (a.k.a. electronic gain), but at voltages exceeding 1 kV. Molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) were either in their infancy or not yet invented at the time, making it impossible to seamlessly integrate low defectivity, thin semiconductors with low conductivity and high mobility together with high electromechanical coupling coefficient piezoelectric substrates. Thus, previous AE amplifiers consisted of essentially one of two types: 1) evaporated thin semiconductor layers with large conductivity and high defectivity on piezoelectric substrates or 2) high quality bulk crystalline semiconductors separated from the piezoelectric by a thin air gap to prevent acoustic radiation into the thick semiconductor. The first approach yielded performance limited by the sub-optimal material properties; the second approach significantly reduced the AE interaction strength and made the devices more dispersive, which prevents high frequency operation. A third approach uses charge confinement structures or the high mobility of 2D materials, but performance of these devices has never approached that of the other two. Here, we present a high-gain leaky SAW (LSAW) amplifier based on heterogenous integration of an epitaxial III-V semiconductor and lithium niobate (LiNbO3), providing a solution to the aforementioned problems and enabling compact, high-gain devices with significantly lower power consumption. YX LiNbO3 is chosen due to its exceptionally high electromechanical coupling coefficient (~25%), providing the highest electromechanical coupling for a SAW on bulk LiNbO3. We demonstrate gain of 1.9 dB/Λ in a 338 μm long device, where Λ is the acoustic wavelength, equivalent to an electronic gain of 30.3 dB and a terminal gain of 5 dB. Moreover, on account of the directly bonded semiconductor layer’s low thickness and conductivity and relatively high gain, this large peak gain occurs at only -55 V.

High-gain leaky surface acoustic wave amplifier in epitaxial InGaAs on lithium niobate heterostructure

Active surface acoustic wave components have the potential to transform RF front ends by consolidating functionalities that currently occur across multiple chip technologies, leading to reduced insertion loss from converting back and forth between acoustic and electronic domains in addition to improved size and power efficiency. This letter demonstrates a significant advance in these active devices with a compact, high-gain, and low-power leaky surface acoustic wave amplifier based on the acoustoelectric effect. Devices use an acoustically thin semi-insulating InGaAs surface film on a YX lithium niobate substrate to achieve exceptionally high acoustoelectric interaction strength via an epitaxial In0.53Ga0.47As(P)/InP quaternary layer structure and wafer-scale bonding. We demonstrate 1.9 dB of gain per acoustic wavelength and power consumption of 90 mW for 30 dB of electronic gain. Despite the strong intrinsic leaky propagation loss, 5 dB of terminal gain is obtained for a semiconductor that is only 338 μm long due to state-of-the-art heterogenous integration and an improved material platform.

Optically Controlled Topologically Protected Acoustic Wave Amplification

Acoustic topological insulators that lead to backscattering-immune edge states provide an important way to control and manipulate the behaviors of phonons. To date, most of these acoustic systems are based on single-particle Hamiltonians that conserve particle number. Here, we exploit particle-nonconserving phonon–photon interactions to engineer not only the band structure but also the dissipation of acoustic waves. We find a topologically protected edge state for phonons that can be parametrically amplified while all the bulk modes remain stable. This phenomenon can be exploited to realize topologically protected nonreciprocal traveling-wave amplification for phonons, which may find wide applications in classical and quantum acoustics.

Acoustoelectric amplification of Rayleigh waves in low sheet density AlGaN/GaN heterostructures on sapphire

This letter demonstrates amplification of surface acoustic waves through the application of dc electric fields in low sheet density heterostructures of AlxGa1−xN barrier layers over GaN grown by metal organic chemical vapor deposition on a sapphire substrate. The use of the GaN material offers a convenient platform to study acoustoelectric (AE) effects, due to its piezoelectricity and the ability to support a two-dimensional electron gas at the interface with the barrier. In this work, low Al molar fractions (between 6% and 10%) in the barrier layer are investigated to tune the sheet resistivity, which is a key aspect to realizing appreciable AE gains. Rayleigh waves in GaN at 920 MHz exhibit non-reciprocity (defined as the contrast in the peak amplitude of the amplified forward traveling wave and the attenuated reverse traveling wave) of up to 6.5 dB/mm under the influence of dc bias fields applied to a 24 nm Al0.07Ga0.93N barrier over GaN on sapphire. Gain dependence on the sheet resistance is also observed, consistent with the AE effect. This work demonstrates an approach to counteract acoustic propagation losses in GaN, which can be implemented in long delay lines for RF signal processing, in addition to other applications.

Resonance of Feshbach-type and explosive instability of magnetoelastic waves in solids

In this paper, we theoretically describe a magnetoelastic system with an explosive instability in which the magnetic energy can be converted into the mechanical one via an extremely efficient channel. The principle is based on the amplification of acoustic waves traveling in an antiferromagnetic crystal under the action of transversal electromagnetic pumping. It is known that such parametric interaction can produce an exponential acoustic wave amplification. Our finding consists in the addition of another pumping channel by means of a discrete resonant acoustic mode whose impact is similar to the Feshbach resonance observed in another physical system (ultra-cold gazes). The addition of the second pumping mechanism results in the explosive instability having the dynamics of a singularity at a finite time instead of the exponential growth. In our example the traveling waves are Lamb waves in an antiferromagnetic plate, and the additional pumping is a shear resonance. We establish equations governing the considered three-phonon parametric instability, theoretically analyze the instability conditions, and give a relevant numerical example illustrating the explosive magneto-acoustic dynamics. It is shown that the explosive scenario can occur with a very low signal level i.e. Lamb waves amplitudes comparable to spontaneous noise in the system.

Multiple Quantum Wells for PT-Symmetric Phononic Crystals.

We demonstrate that the parity-time symmetry for sound is realized in laser-pumped multiple-quantum-well structures. Breaking of the parity-time symmetry for the phonons with wave vectors corresponding to the Bragg condition makes the structure a highly selective acoustic wave amplifier. Single-mode distributed feedback phonon lasing is predicted for structures with realistic parameters.

Phonon-plasmon interactions in piezoelectric semiconductor quantum plasmas

The phonon-plasmon interactions and resulted acoustic wave amplification in a piezoelectric semiconductor quantum plasma has been studied in the quantum hydrodynamic regime. The important ingredients of this study are the inclusion of particle degeneracy pressure and Bohm potential (quantum diffraction) through a non-dimensional quantum parameter-H in the momentum transfer equation. Typical values of n-InSb are used to estimate the acoustic gain using the analytical expressions obtained. The study reveals that the quantum parameter-H reduces the gain coefficient in moderately doped semiconductor plasma. It is also found that quantum parameter-H has profound effects on the frequency at which maximum gain occurs. The attenuation to amplification crossover point (V0/Vs=1) is found to be same in both classical as well as quantum regime.

Amplification of acoustic waves in armchair graphene nanoribbon in the presence of external electric and magnetic fields

Amplification of acoustic waves in Armchair Graphene Nanoribbon (AGNR) in the presence of an external electric and magnetic fields was studied using the Boltzmann's kinetic equation. The general expression for the amplification (Γ⊥/Γ0) was obtained in the region ql⪢1 for the energy dispersion ε(p→) near the Fermi point. For various parameters of the quantized wave vector (β), the graphs of Γ⊥/Γ0 against the electric field (Ex→), the acoustic wave-number (q→), the energy gap (Eg) and the dimensionless factor (Ωτ0) were numerically analyzed. The results showed a linear relation for Γ⊥/Γ0 with Ex→ but non-linear for q→ and Ωτ0. The observed amplification can lead to SASER in Armchair Graphene Nanoribbon (AGNR).

Multilayer-graphene-based amplifier of surface acoustic waves

The amplification of surface acoustic waves (SAWs) by a multilayer graphene (MLG)-based amplifier is studied. The conductivity of massless carriers (electrons or holes) in graphene in an external drift electric field is calculated using Boltzmann’s equation. At some carrier drift velocities, the real part of the variable conductivity becomes negative and MLG can be employed in SAW amplifiers. Amplification of Blustein’s and Rayleigh’s SAWs in CdS, a piezoelectric hexagonal crystal of the symmetry group C6v, is considered. The corresponding equations for SAW propagation in the device are derived and can be applied to other substrate crystals of the same symmetry. The results of the paper indicate that MLG can be considered as a perspective material for SAW amplification and related applications.

Amplification and directional emission of surface acoustic waves by a two-dimensional electron gas

Amplification of surface acoustic waves (SAWs) by electron drift in a two-dimensional electron gas (2DEG) is analyzed analytically and confirmed experimentally. Calculations suggest that peak power gain per SAW radian occurs at a more practical carrier density for a 2DEG than for a bulk material. It is also shown that SAW emission with tunable directionality can be achieved by modulating a 2DEG's carrier density (to effect SAW generation) in the presence of an applied DC field that amplifies SAWs propagating in a particular direction while attenuating those propagating in the opposite direction.

Surface acoustic wave amplification by direct current-voltage supplied to graphene film

Using a high-resolution X-Ray diffraction measurement method, the surface acoustic wave (SAW) propagation in a graphene film on the surface of a Ca3TaGa3Si2O14 (CTGS) piezoelectric crystal was investigated, where an external current was driven across the graphene film. Here, we show that the application of the DC field leads to a significant enhancement of the SAW magnitude and, as a result, to amplification of the diffraction satellites. Amplification of 33.2 dB/cm for the satellite +1, and of 13.8 dB/cm for the satellite +2, at 471 MHz has been observed where the external DC voltage of +10 V was applied. Amplification of SAW occurs above a DC field much smaller than that of a system using bulk semiconductor. Theoretical estimates are in reasonable agreement with our measurements and analysis of experimental data for other materials.