Surface Acoustic Wave Wavelength Research Articles

Phase-modulated pulsing coherent acoustic tweezers for dynamic adjustable spaced particle patterning

Abstract Acoustic tweezer technique with non-contact micro-object manipulation capability has been widely utilized for particle patterning in the fields of cell arraying, target drug delivery, and functional composite fabrication. However, the manipulation resolution of acoustic tweezer device is decided by the configuration of interdigital transducers (IDTs), which is half the wavelength of surface acoustic waves (SAWs). This limitation severely reduces the manipulation versatility of acoustic tweezer devices. To address this limitation, we propose a novel approach that employs phase-modulated pulsing coherent SAWs. This method can halve the patterning spacing and allows for customization of the spacing based on specific manipulation demands. Our proposed technique increases the flexibility and manipulation resolution of acoustic tweezer device, and thus can be widely applied to reconfigurable assembly and precise manipulation of particles and cells.

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.

Detection of surface defects based on the optimized scanning laser source method

In this paper, the interaction between laser-induced surface acoustic wave (SAW) and the surface micro-defect with triangular cross-section is studied. It is found that the reflected wave is greatly affected by the width of the defect, which is mainly suitable for defects whose depth is equal to or larger than the wavelength of SAW. Scanning laser source (SLS) method can detect small defects whose depth is significantly smaller than the wavelength of SAW. It is found that the detection mechanism of SLS method is that the superposition of reflected wave and direct wave increases the peak-to-peak value. The SLS method based on the change of the center peak-to-peak value at the bottom of the defect proposed in this paper is more capable of detecting small damage than the traditional SLS method, and can even find the defect with a depth of only 1/28 of the center wavelength of SAW. The traditional SLS method can only locate defects. This paper proposes a defect width detection method based on SLS method. The detection efficiency of traditional SLS method is low, which limits the application of this method. This paper optimizes the scanning steps of the SLS method by the bisection method, which greatly improves the detection efficiency. Using the optimized SLS method, the position, width and depth of micro artificial defect on the surface of 7075 aluminum alloy is detected quickly and accurately.

Fracture of single crystal silicon caused by nonlinear evolution of surface acoustic waves

High-amplitude surface acoustic waves (SAWs) undergo nonlinear sharpening during propagation due to the elastic nonlinearity of the material, which leads to material fracture. The degree of nonlinear evolution and material strength are usually characterized by two parameters: the second-order nonlinearity parameter and the critical fracture stress. To study the influence of processing defects, residual stress and temperature on these two parameters, we carry out molecular dynamics modeling of nonlinear propagation of SAWs on non-ideal surfaces. The Stillinger and Weber (SW) and the Lee-modified second nearest-neighbor modified embedded-atom-method (2NN MEAM) potentials are used to study the SAW nonlinear evolution and surface crack nucleation in silicon 1111¯1¯2 geometry, respectively. The results show that processing defects have the greatest influence on the SAW evolution and material strength. For a defined level of defects, the size of initial SAW wavelength directly affects the degree of attenuation of harmonic waves and stress concentration, and then determines the measured nonlinear parameter and critical stress. Temperature can affect the magnitude of the two characterization parameters through thermal dissipation and thermal fluctuations. In the presence of residual stresses only, the relative degree of variation of these two parameters is weak. This study may provide a valuable reference for the establishment of nonlinear SAW atomic-level theory and material characterization experiments.

Generation and detection of 50 GHz surface acoustic waves by extreme ultraviolet pulses

We use femtosecond extreme ultraviolet pulses derived from a free electron laser to excite and probe surface acoustic waves (SAWs) on the (001) surface of single crystal SrTiO3. SAWs are generated by a pair of 39.9 nm pulses crossed at the sample with the crossing angle defining the SAW wavelength at 84 nm. Detection of SAWs is performed via diffraction of a time-delayed 13.3 nm probe pulse by SAW-induced surface ripples. Despite the low reflectivity of the sample in the extreme ultraviolet range, the reflection mode detection is found to be efficient because of an increase in the diffraction efficiency for shorter wavelengths. We describe a methodology for extracting the SAW attenuation in the presence of a thermal grating, which is based on measuring the decay of oscillations at twice the SAW frequency. The proposed approach can be used to study ultrahigh frequency SAWs in a broad range of materials and will bridge the wave vector gap in surface phonon spectroscopy between Brillouin scattering and He atom scattering.

Integrated acousto-optic interaction at ultrahigh frequencies

With the development of nanoscale transducers, surface acoustic wave (SAW) with frequencies above 10 GHz can now be excited on the optical waveguides. However, the fundamental mode is very difficult to be excited for SAW at ultrahigh frequencies. Moreover, as the wavelength of SAW is reduced to below the optical wavelength, the acoustic field across the optical waveguide can no longer be approximated as uniform but is spatially periodic. Here, we develop a discussion about integrated acousto-optic (AO) interaction at ultrahigh frequencies. The electromechanical coupling coefficients of different modes are evaluated for SAW at ultrahigh frequencies. The acousto-optic overlap is analyzed to evaluate integrated AO interaction for each given acoustic wavelength and optical waveguide size. An analytic model is presented for integrated AO interaction at ultrahigh frequencies. The research shows that it would be a trade-off for the design of an integrated AO device with ultrahigh frequencies.

X-ray diffraction by surface acoustic waves

The possibilities are presented of X-ray diffraction methods for studying the propagation of surface acoustic waves (SAWs) in solids, including diffraction under total external reflection conditions and Bragg diffraction, using acoustically modulated X-ray multilayer mirrors and crystals. SAW propagation was studied using both meridional and sagittal diffraction geometries where the SAW wavevectors and X-ray photons are collinear or perpendicular, respectively. SAW propagation in a crystal leads to sinusoidal modulation of the crystal lattice and the appearance of diffraction satellites on the rocking curve. The intensities and angular positions of these diffraction satellites are determined by the SAW wavelength, amplitude and attenuation. Therefore, diffraction methods allow the analysis of the SAW propagation process and determination of SAW parameters. The influence of X-ray energy on diffraction by acoustically modulated crystals is studied for the first time. It is shown that changes in the X-ray energy can change the angular region where diffraction satellites exist under conditions of total external reflection. By contrast, in the Bragg diffraction region changes in the X-ray photon energy lead to changes in the X-ray penetration depth into the crystal and redistribution of the diffracted intensity among diffraction satellites, but do not change the angular divergence between diffraction satellites on the rocking curve. It is also shown that, in X-ray diffraction on acoustically modulated crystals on a number of successive reflections, a decrease in interplanar spacing leads to an increase in the number of diffraction satellites and a redistribution of diffracted radiation between them.

Electroosmotic flow in small-scale channels induced by surface-acoustic waves

Numerical simulations of the Navier-Stokes, Nernst-Planck, and the Poisson equations are employed to describe the transport processes in an aqueous electrolyte in a parallel-plate nanochannel, where surface-acoustic waves (SAWs) are standing or traveling along (piezo-active) channel walls. It is found that -- in addition to the conventional acoustic streaming flow -- a time-averaged electroosmotic flow is induced. Employing the stream function-vorticity formulation, it is shown that the Maxwell stress term causes an electroosmotic propulsion that is qualitatively identical to the one discussed in the context of alternating current (AC) electroosmosis (EOF). Differences arise mainly due to the high actuation frequencies of SAWs, which are in the MHz range rather than in the kHz regime typical for ACEOF. Moreover, the instantaneous spatial periodicity of the EOF in the travel direction of the SAW is intrinsically linked to the dispersion relation of the latter rather than a free geometric parameter. This leads to a specific frequency band where an EOF of sizable magnitude can be found. On the low frequency end, the ratio between the electric double layer (EDL) thickness and the SAW wavelength becomes extremely small so that the net force leading to a non-vanishing time-averaged flow becomes equally small. On the high frequency end, the RC time of the EDL is much larger than the inverse of the SAW frequency leading to a vanishing effective charge density of the EDL. For a parallel-plate channel, the EOF can be maximized by using two SAWs on both channel walls that have the same frequency but are phase-shifted by $180^\circ$. It appears that the SAW-EOF is the dominant pumping mechanism for such a scenario. The proposed actuation might be a viable alternative for driving liquid electrolytes through narrow ducts and channels, without the need for electric interconnects and electrodes.

Towards ultraefficient nanoscale straintronic microwave devices

We investigate the ultrafast magnetization dynamics of individual nanomagnets resonantly excited by surface acoustic waves (SAWs) generated nonlocally by nonmagnetic phononic gratings. Using a time-resolved magneto-optic Kerr effect microscope, we report the dependence of the magnetoelastic resonance on SAW wavelength and nanomagnet size. Our measurements show that the precession amplitude of single nanomagnets increases monotonically and nonlinearly with decreasing sample size. In addition, for nanomagnets below a critical size we find that the oscillation amplitude increases with SAW frequency. Field-swept measurements of the magnetization dynamics reveal that the damping of the magnetoelastic resonance also decreases with the size of the magnet, ultimately reaching a minimum value determined by the Gilbert damping. This work shows that acoustically driven spin dynamics possess favorable scaling characteristics, which supports the notion that efficient, high-frequency (10--100 GHz) nanoscale straintronic devices limited only by $\ensuremath{\alpha}$ are feasible.

Enhancement of Photoemission on p-Type GaAs Using Surface Acoustic Waves.

We demonstrate that photoemission properties of p-type GaAs can be altered by surface acoustic waves (SAWs) generated on the GaAs surface due to dynamical piezoelectric fields of SAWs. Multiphysics simulations indicate that charge-carrier recombination is greatly reduced, and electron effective lifetime in p-doped GaAs may increase by a factor of 10× to 20×. It implies a significant increase, by a factor of 2× to 3×, of quantum efficiency (QE) for GaAs photoemission applications, like GaAs photocathodes. Conditions of different SAW wavelengths, swept SAW intensities, and varied incident photon energies were investigated. Essential steps in SAW device fabrication on a GaAs substrate are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of current–voltage (I–V) characteristics of the SAW device, and ability to survive high-temperature annealing. Results obtained and reported in this study provide the potential and basis for future studies on building SAW-enhanced photocathodes, as well as other GaAs photoelectric applications.

Propagations of Rayleigh and Love waves in ZnO films/glass substrates analyzed by three-dimensional finite element method**Project supported by the National Natural Science Foundation of China (Grant No. 11304160), the Natural Science Foundation of Jiangsu Provincial Higher Education Institutions, China (Grant No. 13KJB140008), and the Foundation of Nanjing University of Posts and Telecommunications, China (Grant No. NY213018).

Propagation characteristics of surface acoustic waves (SAWs) in ZnO films/glass substrates are theoretically investigated by the three-dimensional (3D) finite element method. At first, for ZnO films/glass substrates, the simulation results confirm that the Rayleigh waves along the [0001] direction and Love waves along the direction are successfully excited in the multilayered structures. Next, the crystal orientations of the ZnO films are rotated, and the influences of ZnO films with different crystal orientations on SAW characterizations, including the phase velocity, electromechanical coupling coefficient, and temperature coefficient of frequency, are investigated. The results show that at appropriate , Rayleigh wave has a maximum k2 of 2.4% in (90°, 56.5°, 0°) ZnO film/glass substrate structure; Love wave has a maximum k2 of 3.81% in (56°, 90°, 0°) ZnO film/glass substrate structure. Meantime, for Rayleigh wave and Love wave devices, zero temperature coefficient of frequency (TCF) can be achieved at appropriate ratio of film thickness to SAW wavelength. These results show that SAW devices with higher k2 or lower TCF can be fabricated by flexibly selecting the crystal orientations of ZnO films on glass substrates.

Slippery Liquid-Infused Porous Surfaces and Droplet Transportation by Surface Acoustic Waves

On a solid surface a droplet of liquid will stick due to the capillary adhesion and this causes low droplet mobility. To reduce contact line pinning, surface chemistry can be coupled to micro- and/or nano-structures to create superhydrophobic surfaces on which a droplet balls-up into an almost spherical shape thus minimising contact area. Recent progress in soft matter has now led to alternative lubricant impregnated surfaces capable of almost zero contact line pinning and high droplet mobility without causing droplets to ball-up and minimize contact area. Here we report a new approach to Surface Acoustic Wave (SAW) actuated droplet transportation enabled using such a surface. These surfaces maintain the contact area required for efficient energy and momentum transfer of the wave energy into the droplet, whilst achieving high droplet mobility and large footprint, therefore reducing the threshold power required to induce droplet motion. In our approach we used a slippery layer of lubricating oil infused into a self-assembled porous hydrophobic layer, which is significantly thinner than the SAW wavelength and so avoided damping of the wave. A significant reduction (up to 85%) in the threshold power for droplet transportation was found compared to that using a conventional surface treatment method. Moreover, unlike droplets on superhydrophobic surfaces, where interaction with the SAW induced a transition from a Cassie–Baxter state to a Wenzel state, the droplets on our liquid impregnated surfaces remained in a mobile state after interaction with the SAW.

Micro load theory of surface acoustic wave gravimetric sensors

Surface acoustic wave (SAW) gravimetric sensors have been widely applied in the field of chemical and biological detections for decades. However, it is noticed that the operating mechanism of this type of sensors is not understood well. A perturbation theory commonly adopted has been used to predict the mass sensitivity of the SAW sensors, but, as indicated also, “there is a significant and systematic quantitative discrepancy” between the theoretical and experimental results [Wohltjen, et al. IEEE UFFC, 1987]. In this paper, a micro load theory is presented. The coating film with biochemical material adsorbed is not regarded as a perturbation source, but a micro mechanical load. Then, it is revealed that the fractional frequency shift caused by the load is minus and half of the fractional area density change, where the unloaded area density is a product of the substrate material’s density and SAW wavelength. Also, it is interesting to note that the expression is the same with that for Lamb wave sensors if...

Thin film interface stresses produced by high amplitude laser generated surface acoustic waves

Surface acoustic waves (SAWs) have been explored for nondestructive metrology of thin film elastic properties and thickness due to confinement of their energy within a shallow depth from a material surface. In this paper, we study the dynamic interfacial stresses produced by high amplitude SAWs generated by a strongly ablative source in a thin film-substrate system, with the goal of investigating the possibility of inducing thin film delamination at high loading rates. For modeling purposes, we represent the mechanical loading resulting from the pulsed laser-sample interaction in the ablative regime by an equivalent compressive surface load, and the resulting stresses and particle velocities induced by the generated SAWs are calculated using a linear finite element model. We explore the numerical model to study the dependence of the film-substrate interface tractions on the ratio of the film thickness and SAW wavelength for a soft film on a stiff substrate. Furthermore, by matching the numerical results obtained from the finite element model with experimental results, we are able to predict the dynamic interfacial stresses for a copper film on a fused silica substrate produced by SAWs excited by a high power pulsed laser line source. This study has implications for exploring SAWs in the characterization of interfacial failure in thin-film substrate systems.

High quality broadband spatial reflections of slow Rayleigh surface acoustic waves modulated by a graded grooved surface

We report high quality broadband spatial reflections of Rayleigh surface acoustic waves (SAWs) through a graded grooved surface. High quality means that no wave is allowed to transmit and the incident wave is nearly all reflected to the input side. The graded grooved surface is structured by drilling one dimensional array of graded grooves with increased depths on a flat surface. We investigate SAW dispersion relations, wave field distribution at several typical SAW wavelengths, and time evolution of a Gaussian pulse through the graded grooved surface. Results show that the input broadband Rayleigh SAWs can be slowed, spatially enhanced and stopped, and finally reflected to the input side. The study suggests that engraving the flat surface can be used as an efficient and economical way to manipulate Rayleigh SAWs, which has potential application in novel SAW devices such as filters, reflectors, sensors, energy harvesters, and diodes.

An experimental installation for contactless measurements of the velocity and displacement amplitudes of Rayleigh waves on a small surface area

A laboratory installation for recording acoustic signals from a small surface area is described. The installation contains a contactless electromagnetoacoustic receiver in which the inductor has the form of two straight thin parallel conductors that are separated by a short distance. The Rayleigh-wave velocity was determined within a frequency range of 2–10 MHz. At these frequencies, the wavelength of surface acoustic waves varied from 600 to 240 μm, and the minimum base for measuring the Rayleigh-wave velocity at a frequency of 10 MHz was 500 μm. A new technique for measuring the time interval between two acoustic pulses is proposed.

Double flow reversal in thin liquid films driven by megahertz-order surface vibration

Arising from an interplay between capillary, acoustic and intermolecular forces, surface acoustic waves (SAWs) are observed to drive a unique and curious double flow reversal in the spreading of thin films. With a thickness at or less than the submicrometre viscous penetration depth, the film is seen to advance along the SAW propagation direction, and self-similarly over time t 1/4 in the inertial limit. At intermediate film thicknesses, beyond one-fourth the sound wavelength λ ℓ in the liquid, the spreading direction reverses, and the film propagates against the direction of the SAW propagation. The film reverses yet again, once its depth is further increased beyond one SAW wavelength. An unstable thickness region, between λ ℓ /8 and λ ℓ /4, exists from which regions of the film either rapidly grow in thickness to exceed λ ℓ /4 and move against the SAW propagation, consistent with the intermediate thickness films, whereas other regions decrease in thickness below λ ℓ /8 to conserve mass and move along the SAW propagation direction, consistent with the thin submicrometre films.

Microfluidic pumping through miniaturized channels driven by ultra-high frequency surface acoustic waves

Surface acoustic waves (SAWs) are an effective means to pump fluids through microchannel arrays within fully portable systems. The SAW-driven acoustic counterflow pumping process relies on a cascade phenomenon consisting of SAW transmission through the microchannel, SAW-driven fluid atomization, and subsequent coalescence. Here, we investigate miniaturization of device design, and study both SAW transmission through microchannels and the onset of SAW-driven atomization up to the ultra-high-frequency regime. Within the frequency range from 47.8 MHz to 754 MHz, we show that the acoustic power required to initiate SAW atomization remains constant, while transmission through microchannels is most effective when the channel widths w ≳ 10 λ, where λ is the SAW wavelength. By exploiting the enhanced SAW transmission through narrower channels at ultra-high frequencies, we discuss the relevant frequency-dependent length scales and demonstrate the scaling down of internal flow patterns and discuss their impact on device miniaturization strategies.

Elemental theory of a SAW gas sensor based on electrical conductivity changes in bi-layer nanostructures

An elemental theory for a surface acoustic wave (SAW) gas sensor is reported within this work. The sensor mechanism is generally based on changes in electrical conductivity of thin bi-layer sensor nanostructures under the influence of gas molecules. In turn, the interaction between the electrical surface potential (associated with SAW propagating on a piezoelectric substrate) and the mobile electric charges of bi-layer sensor nanostructures (characterized by different electrical conductivities σ1, σ2 and thicknesses h1, h2), cause an acoustoelectric perturbation of the SAW propagation. Both films are very thin in comparison to the SAW wavelength and the Debye screening length. This allows for a one-dimensional treatment of the problem. As a result the relative changes in SAW velocity and attenuation is highly dependent on the electrical surface conductivities ratio of the thin film bi-layer nanostructures (x=σs2/σs1). A proper fitting of the construction parameter x and the initial “work point” (σs1 or σs2 vs. voCs) for such structures allows for obtaining considerable sensitivities in SAW gas sensors.

Theory of SAW Gas Sensor based on Bi-layer Conductivity Changes

The elemental theory of surface acoustic wave (SAW) gas sensor principles is reported within this work. The sensor mechanism is generally based on changes in conductivity of a bi-layer thin sensor structures. In turn, the interaction between electrical surface potential is associated with SAW propagating on piezoelectric and mobile electric charges in bi-layer sensor structures of different electrical conductivities and thicknesses. Both films are very thin in comparison to SAW wavelength and Debye screening length. This allows on one-dimensional treatment of the problem by perturbation theory.