Excitation Of Surface Acoustic Waves Research Articles

Surface Acoustic Microwave Photonic Filters on Etchless Lithium Niobate Integrated Platform

AbstractLithium niobate on insulator emerges as a promising platform for integrated microwave photonics because of its capability of ultralow‐loss guidance and high‐efficiency modulation of light. Chip‐level integration of microwave filters is important for signal processing in the 5G/6G wireless communication. Here, by employing bound states in the continuum for low‐loss waveguiding, high‐performance microwave photonic filters are realized on an etchless lithium niobate integrated platform. These microwave photonic filters consist of a high‐quality photonic microcavity modulated by piezoelectrically excited surface acoustic waves. Acoustic time delays from 21 to 106 ns and passbands with bandwidth as narrow as 0.89 MHz are achieved in the fabricated filters operating at gigahertz frequencies. This demonstration may open up new applications on the lithium niobate integrated platform, such as optical communication, signal processing, and beam steering.

Transient grating spectroscopy on a DyCo5 thin film with femtosecond extreme ultraviolet pulses.

Surface acoustic waves (SAWs) are excited by femtosecond extreme ultraviolet (EUV) transient gratings (TGs) in a room-temperature ferrimagnetic DyCo5 alloy. TGs are generated by crossing a pair of EUV pulses from a free electron laser with the wavelength of 20.8 nm matching the Co M-edge, resulting in a SAW wavelength of Λ = 44 nm. Using the pump-probe transient grating scheme in reflection geometry, the excited SAWs could be followed in the time range of -10 to 100 ps in the thin film. Coherent generation of TGs by ultrafast EUV pulses allows to excite SAW in any material and to investigate their couplings to other dynamics, such as spin waves and orbital dynamics. In contrast, we encountered challenges in detecting electronic and magnetic signals, potentially due to the dominance of the larger SAW signal and the weakened reflection signal from underlying layers. A potential solution for the latter challenge involves employing soft x-ray probes, albeit introducing additional complexities associated with the required grazing incidence geometry.

Numerical analysis of on-chip acousto-optic modulators for visible wavelengths.

On-chip acousto-optic modulators that operate at an optical wavelength of 780nm and a microwave frequency of 6.835GHz are proposed. The modulators are based on a lithium-niobate-on-sapphire platform and efficiently excite surface acoustic waves and exhibit strong interactions with tightly confined optical modes in waveguides. In particular, a high-efficiency phase modulator and single-sideband mode converter are designed. We found that for both microwave and optical wavelengths below 1µm, the interactions at the cross-sections of photonic waveguides are sensitive to the waveguide width and are significantly different from those in previous studies. Our designed devices have small footprints and high efficiencies, making them suitable for controlling rubidium atoms and realizing hybrid photonic-atomic chips. Furthermore, our devices have the potential to extend the acousto-optic modulators to other visible wavelengths for other atom transitions and for visible light applications, including imaging and sensing.

Removable and replaceable micro-mixing system with surface acoustic wave actuators

Abstract A micromixer based on a surface acoustic wave (SAW) has attracted a great deal of attention because of their ability to mix liquids and multiphase flows with very high mixing efficiency. However, since microfluidic channels cloud only be formed directly on the piezoelectric substrate in conventional SAW devices, contamination problems have arisen. In this study, by placing a solution that propagates sound waves between the microfluidic channel and the SAW excitation substrate, vibrations caused by SAWs are propagated into the micro-well to form a micro-mixing system. This system can provide a micromixer that is removable, replaceable, and contamination-free.

Influence of surface acoustic wave (SAW) on nanoscale in-plane magnetic tunnel junctions

The use of voltage induced strain to switch magnetic tunnel junctions (MTJs) is a promising solution for reducing the switching energy in MRAM technologies. The MTJ is integrated with a piezoelectric layer to generate the strain. A very thin layer is needed to switch with small voltages and small energy dissipation. It is challenging to synthesize ultrathin piezoelectric layers that retain a high degree of piezoelectricity. An alternate approach is to use time-varying strain generated by a surface acoustic wave (SAW). This approach does not require a thin piezoelectric layer since the SAW is confined to the surface of the layer. In this study, we fabricated in-plane MTJs on piezoelectric LiNbO3 substrates and used IDTs to generate the SAW signal within the substrate. Our results showed that the SAW signal had a significant influence on the resistance and the tunneling magnetoresistance (TMR) ratio of the MTJs. The influence was much less significant in nanometer size MTJs than in micrometer sized ones. Most surprisingly, the SAW signal caused the tunneling magnetoresistance ratio (TMR) to drop below zero for the micrometer size MTJ, meaning that the antiparallel resistance RAP is temporarily less than the parallel resistance RP under SAW excitation. Our results provide insight into the dynamic behavior of MTJs under periodic strain and the dependence of this behavior on the device dimensions as they are scaled down to nanometer sizes.

High speed surface acoustic wave and laterally excited bulk wave resonator based on single-crystal non-polar AlN film

One approach to extend the acoustic applications of aluminum nitride (AlN) in the GHz frequency range is to take advantage of the piezoelectric performance and high acoustic velocity (∼11 350 m/s) along the c-axis of this material. In particular, in the case of high-frequency micro-electromechanical systems, it should be possible to simplify the construction of resonators by using a-plane AlN-based structures. In the work described in this Letter, a single-crystalline a-plane AlN layer on an r-plane sapphire substrate is obtained by combining sputtering and high-temperature annealing. Based on this non-polar AlN, a resonator with only planar interdigital transducer electrodes is fabricated. Experiments on this resonator reveal simultaneous excitation of an anisotropic Rayleigh surface acoustic wave (SAW) at 2.38 GHz and a laterally excited bulk acoustic wave (LBAW) at 4.00 GHz. It is found that the Rayleigh SAW exhibits outstanding performance, with a quality factor as high as 2458 and great stability under variations in temperature. The LBAW at 4.00 GHz is excited by pure planar interdigitated electrodes without the need for any cavity or bottom electrode structure, thus demonstrating a promising approach to the construction of high-frequency resonators with a relatively simple structure.

ГЕНЕРАЦИЯ ПОВЕРХНОСТНЫХ АКУСТИЧЕСКИХ ВОЛН СЕГНЕТОЭЛЕКТРИЧЕСКОЙ ПЛЕНКОЙ BST ПРИ ДЕЙСТВИИ ОДНООСНОЙ НАГРУЗКИ

The electro-mechanical properties of a ferroelectric film of barium strontium titanate (BST) film located on a silicon substrate depend on applied external strain. A significant dependence is observed for concentrations close to values, where a phase transition for the ferroelectric film occurs. A model of single-crystal BST film near the phase transition under uniaxial strain is studied by the thermodynamic theory of phase transitions. The material properties of the film obtained by the model are used for numerical study of the excitation of Rayleigh’ acoustic waves on the surface of the film-substrate heterostructure. Shifting the extrema of S-parameters, characterizing the efficiency of excitation of surface acoustic waves, is shown under the applied strain. The change of S-parameters for the first three resonances determined principally by the geometry of the interdigital electrodes is presented. The largest shift of resonant frequency is observed in a case of the second resonance that corresponds to Sezava wave.

Ordered creation and motion of skyrmions with surface acoustic wave

Magnetic skyrmions with a well-defined spin texture have shown unprecedented potential for various spintronic applications owning to their topologically non-trivial and quasiparticle properties. To put skyrmions into practical technology, efficient manipulation, especially the inhibition of skyrmion Hall effect (SkHE) has been intensively pursued. In spite of the recent progress made on reducing SkHE in several substituted systems, such as ferrimagnets and synthetic antiferromagnets, the organized creation and current driven motion of skyrmions with negligible SkHE in ferromagnets remain challenging. Here, by embedding the [Co/Pd] multilayer into a surface acoustic wave (SAW) delay line where the longitudinal leaky SAW is excited to provide both the strain and thermal effect, we experimentally realized the ordered generation of magnetic skyrmions. The resultant current-induced skyrmions movement with negligible SkHE was observed, which can be attributed to the energy redistribution of the system during the excitation of SAW. Our findings open up an unprecedentedly new perspective for manipulating topological solitons, which could possibly trigger the future discoveries in skyrmionics and spin acousto-electronics.

Enhanced effective diffusion in sub-wavelength, axon-scale microchannels using surface acoustic waves

Excitation using surface acoustic waves (SAW) has demonstrated efficacy in improving microscale particle/chemical transport due to its ability to generate microscale wavelengths. However, the effects of acoustic stimulation on transport processes along the length of sub-wavelength microchannels and their underlying mechanisms, essential for long-range transport, have not been examined in detail. In this work, we investigate diffusion along the length of subwavelength microchannels using experimental and simulation approaches, demonstrating enhanced transport under SAW excitation. The microchannel-based enhanced diffusion mechanisms are further studied by investigating the acoustic pressure and streaming fields, finding that the degree of enhancement is a function of applied power, microchannel dimensions, and viscosity. This microchannel-based diffusion enhancement approach is applicable to microfluidic and biomedical microscale transport enhancement, with the findings here being relevant to acoustic-based micro-mixing and neurodegenerative therapies.

Mechano/acousto-electric coupling between ReS2 and surface acoustic wave

Two-dimensional (2D) materials are promising candidates for developing next generation electronic/optoelectronic devices with programmable multi functions, due to their widely tunable properties by various physical stimuli. Mechanical strain is one of the most promising means to effectively modulate the physical properties of 2D materials. Nevertheless, few studies reported micro/nano scale controllable strain application platforms, limiting the development of novel mechano-electrical/optoelectrical devices based on 2D materials. This work proposes surface acoustic wave (SAW) device as a controllable strain modulation platform for 2D materials with sub-micro scale resolution. The platform uses the piezoelectric material (LiNbO3) as the substrate, which is deposited with interdigitated transducers (IDT) to generate SAW on the surface. The propagation of SAW causes surface deformation, which is then transferred to the 2D materials on the substrate. The period of the surface deformation/strain is related with that of SAW, which is determined by the period of IDT with nano meter scale. It is demonstrated that the photo luminescence spectrum of a 2D ReS2 flake on this platform gradually shifts with the SAW excitation power, which reaches a shift of 3 nm as the SAW excitation power achieves 26 dBm, corresponding to a band gap increase of 5 meV. Meanwhile, the platform is also capable to provide acousto-electric coupling between SAW and 2D materials, which is demonstrated by the shift of the SAW resonant frequency due to the re-distribution of photo-generated carriers in ReS2 upon light illumination.

On‐Chip Tightly Confined Guiding and Splitting of Surface Acoustic Waves Using Line Defects in Phononic Crystals

AbstractPhononic crystals (PnCs) exhibit acoustic properties that are not usually found in natural materials, which leads to the possibility of new devices for the complex manipulation of acoustic waves. In this article, a micron‐scale phononic waveguide constructed by line defects in PnCs to achieve on‐chip, tightly confined guiding, bending, and splitting of surface acoustic waves (SAWs) is reported. The PnC is made of a square lattice of periodic nickel pillars on a piezoelectric substrate. The PnC lattice constant, pillar diameter, and pillar height are set to 10, 7.5, and 3.2 µm, respectively, leading to a complete bandgap centered at 195 MHz. Interdigitated transducers are monolithically integrated on the same substrate for SAW excitation. The guiding, bending, and splitting of SAWs in the phononic waveguide are experimentally observed through measurement of the out‐of‐plane displacement fields using a scanning optical interferometer. The combination of destructive interference due to the Bragg bandgap and the interaction of the propagating wave with the pillars around the channel results in a tight confinement of the displacement field. The proposed phononic waveguides demonstrate the feasibility of precise local manipulation of SAW that is essential for emerging frontier applications, notably for phonon‐based classical and quantum information processing.

Influence of thermocapillary flow induced by a heated substrate on atomization driven by surface acoustic waves

Thermocapillary flow on the mechanism of interfacial destabilization prior to atomization of a sessile Newtonian droplet subjected to surface acoustic waves (SAWs) is analyzed. We assumed that an interfacial temperature distribution is induced on the free surface of the millimeter-sized water droplet since the droplet is on a heated substrate. Given the dependence of surface tension on interfacial temperature, shear stresses combined with SAWs lead to the development of thermocapillary flow. The evolution equation for a small-scale droplet under the combined influence of SAW atomization and thermocapillary flow is derived via an asymptotic approach to the hydrodynamic equations, arising the acoustic capillary and Marangoni numbers. In this limit, our simplified droplet model can predict capillary instability leading to atomization once a critical amplitude is reached for the induced capillary waves at the liquid droplet. In doing so, our model also represents the influence of the thermocapillary effect on the interfacial deformation of the droplet and shows how the Marangoni flow promoted by a heated substrate counteracts the acoustic stress, leading to a virtually uniform droplet aspect ratio and thus larger aerosol diameters compared to the isothermal case. These results are supported by the development of a novel analytical expression that has allowed us to estimate the characteristic aerosol size under thermocapillary flow and SAW excitation, and to postulate thermocapillary flow as a new valuable means of explaining the regulation of the characteristic aerosol size at SAW atomization.

ЗАМЕЧАНИЕ О ВЫЧИСЛЕНИИ СКОРОСТИ ВОЛНЫ РЭЛЕЯ И ПРОИЗВОДНОЙ ОПРЕДЕЛИТЕЛЯ РЭЛЕЯ В УПРУГИХ СРЕДАХ

There are many approximate and exact formulae to calculate surface wave velocity in an elastic medium. An analytical expression for Rayleigh wave velocity in volume wave velocity values has been obtained. A formula which determines the remainder in the excitation and diffraction of surface acoustic waves in a homogeneous isotropic elastic half-space involving solutions for the strain and stress fields in the form of quadratures is worked out. The values of the Rayleigh wave velocity and the derivative of the Rayleigh determinant for different media according to the reference data were obtained. The results can help in obtaining analytic expressions and reducing the calculation time of numerical solutions of the diffraction and excitation of acoustic waves.

Acoustically Stimulated Charge Transport in Graphene Film.

The process of acoustically stimulated charge transport in the graphene film on the surface of the YZ-cut of a LiNbO3 crystal was investigated. It was found that the dependence of the current in the graphene film on the frequency of the surface acoustic wave (SAW) excitation repeats the amplitude-frequency response of the SAW delay time line. It is shown that increasing the SAW amplitude leads to an increase in the current in the graphene film, and the current in the graphene film depends linearly on the amplitude of the high-frequency input signal supplied to the interdigital transducer (IDT, in dB). It is demonstrated that at a positive bias potential on the graphene film, the SAW propagation allows to change the direction of the current in the graphene film by changing the amplitude of the SAW. It is also shown that in the frequency range of the amplitude-frequency response of the SAW delay time line, the current in the graphene film can vary from positive to negative values depending on the frequency. The capability to control the SAW excitation frequency or the SAW amplitude makes it possible to control the value and direction of the current in the graphene film. The SAW propagation lets to collect and transport the photo-stimulated charges in the graphene film on the crystal surface.

A novel progressive wave gyroscope based on acousto-optic effects

We propose and numerically investigate a brand-new, high-sensitivity progressive wave gyroscope based on acousto-optic effects for the measurement of rotational angular velocity. Unlike the traditional surface acoustic wave (SAW) gyroscope, which uses shifts in the SAW frequency to characterize the rotational angular velocity, this study uses acousto-optic effects to detect changes in refractive index caused by mechanical strain, measuring the angular velocity by the output optical power intensity of the optical waveguide. The three-dimensional finite element analysis method is utilized to build an SAW excitation model and optical detection model. We show that the sensitivity of the SAW gyroscope is highly dependent upon geometric parameters of the structure and that the mechanical strain induced by the progressive wave of the SAW can be effectively measured by the optical power intensity under the action of external angular velocity. The superiority of the proposed structure is substantiated by its achievement of a theoretical sensitivity of 1.8647 (mW/m2)/(rad/s) and high impact resistance of 220,000 g. By means of normalization, the sensitivity of the proposed structure can be enhanced by four orders of magnitude compared to the traditional SAW gyroscope. The novel structure combines the advantages of both conventional microscale vibrating gyroscopes and optical gyroscopes, providing a powerful solution for performance enhancement of SAW gyroscopes and, thereby, enabling application in the field of inertial devices.

Numerical study on the vapor bubble removal by acoustic streaming in downward-facing pool boiling

The dynamic mechanism of vapor bubble removal by acoustic streaming on downward-facing heating surface during boiling process was numerically investigated. It is observed that under the excitation of longitudinal surface acoustic wave (SAW), vortices are formed at the root of bubble due to acoustic streaming. The vortices promote the bubble to detach from heating surface, while inhibiting the lateral growth and movement of the bubble. Thus, the bubble can be suspended in the liquid, almost statically. In addition, under the excitation of the transverse SAW, the generated vortices produce a translational thrust on the bubble, causing the bubble to move laterally and eventually leave the heating surface. For the case of superimposed SAWs excitation, the transverse SAW breaks the balance of vortical flow around the bubble which initially excited by the longitudinal SAW. Meanwhile, the unbalanced vortices destroy the boundary layer of heating surface, and weaken the influence of the viscous force as well. The results show that under the superimposed SAWs excitation, comparing with transverse SAW solely, the time for the bubble to detach laterally from the downward-facing heating surface is shortened by nearly 33%, and the removal speed of the bubble is also increased by nearly 45%. It indicates that SAW excitation can serve as an effective approach for active control and enhancement of boiling heat transfer on the downward-facing heating surface.

Removal of Non-Specifically Bound Proteins Using Rayleigh Waves Generated on ST-Quartz Substrates.

Label-free biosensors are plagued by the issue of non-specific protein binding which negatively affects sensing parameters such as sensitivity, selectivity, and limit-of-detection. In the current work, we explore the possibility of using the Rayleigh waves in ST-Quartz devices to efficiently remove non-specifically bound proteins via acoustic streaming. A coupled-field finite element (FE) fluid structure interaction (FSI) model of a surface acoustic wave (SAW) device based on ST-Quartz substrate in contact with a liquid loading was first used to predict trends in forces related to SAW-induced acoustic streaming. Based on model predictions, it is found that the computed SAW body force is sufficient to overcome adhesive forces between particles and a surface while lift and drag forces prevent reattachment for a range of SAW frequencies. We further performed experiments to validate the model predictions and observe that the excitation of Rayleigh SAWs removed non-specifically bound (NSB) antigens and antibodies from sensing and non-sensing regions, while rinsing and blocking agents were ineffective. An amplified RF signal applied to the device input disrupted the specific interactions between antigens and their capture antibody as well. ST-quartz allows propagation of Rayleigh and leaky SH-SAW waves in orthogonal directions. Thus, the results reported here could allow integration of three important biosensor functions on a single chip, i.e., removal of non-specific binding, mixing, and sensing in the liquid phase.

Efficiency of Generation of Surface Acoustic Waves in Barium Strontium Titanate Thin Films.

The excitation of surface acoustic waves (SAWs) on the surface of the ferroelectric film [barium strontium titanate (BST)] located on the dielectric substrate (silicon) was studied theoretically. We found that the most effective SAW excitation takes place when spontaneous polarization occurs in the film plane and the wave propagates along the direction adjacent to the direction of the spontaneous polarization vector. Based on a nonlinear model of phase transitions in solid BST solutions, the dependency of the material constants of piezo-effect equations on the misfit strain with a fixed concentration was obtained numerically. The effect of various misfit strains on SAW characteristics was studied for the film located on single-crystal silicon. It was shown that the effectiveness of SAW excitation increases as the misfit strain nears the boundary of phase transition.

Propellant Mass Gauging via Modal Analysis on the International Space Station

In order for NASA to obtain sustained human presence in space, a higher fuel gauging accuracy is required. Current gauging methods have a sub 5-10% error which requires an additional fuel margin for any rocket or satellite. Doing this increases weight, therefore creating a higher cost. Gauging technologies capable of solving this problem either do not exist or only exist at low Technology Readiness Levels (TRLs). The Modal Propellant Gauging (MPG) project is an effort to develop a higher resolution, light-weight, and cost efficient fuel gauging method for low-gravity environments. MPG uses excited surface acoustic waves on the outside of the tank to measure the tanks resonant frequencies. In microgravity, the liquid will adhere to and coat the inner walls of the tank. The resonant mode frequencies are sensitive to the presence of liquid, meaning the accuracy of MPG is amplified in low-gravity. Modal measurements are taken using sensors adhered to the outer tank walls and a small data acquisition system. A Frequency Response Function (FRF) is then generated to create a reading for a fuel gauge.The newest iteration of MPG is Modal Propellant Gauging - International Space Station (MPG-ISS), which will undergo 30 days of autonomous testing onboard the ISS. This payload will implement MPG onto two tanks separated into two ISS payload lockers. A set amount of liquid will be transferred between both tanks as data is taken on both and stored on the in-flight computer. The MPG-ISS experiment will help validate the use of MPG on future space mission, such as Artemis. This experiment is the next step for MPG to be used on future mission to the Moon and Mars.

Macroscopic Disk Rotation by Means of Surface Acoustic Waves.

It is shown experimentally that a flat piezoelectric disk poled perpendicular to its surface and suspended on a thin thread can be rotated about 50° by two surface acoustic waves (SAWs) intentionally excited with different amplitudes and propagating in the opposite directions. The excitation of such counter-propagating SAWs with different amplitudes is based on the nonsymmetrical interdigital transducer with different angular width electrodes located along the disk radius. The angular width of each of the two electrodes per angular period is not change along the disk radius. Such a ring unidirectional transducer realized on one of the disk surfaces is used for the excitation of SAWs. Hereby, the flat surface of the disk allows one to create any transducer pattern using conventional technology. The other disk surface can be free or covered by the silver layer.