Surface Acoustic Wave Frequency Research Articles

Resonant amplification of intrinsic magnon modes and generation of new extrinsic modes in a two-dimensional array of interacting multiferroic nanomagnets by surface acoustic waves.

Using time-resolved magneto-optical Kerr effect (TR-MOKE) microscopy, we demonstrate surface-acoustic-wave (SAW) induced resonant amplification of intrinsic spin-wave (SW) modes, as well as generation of new extrinsic or driven modes at the SAW frequency, in a densely packed two-dimensional array of elliptical Co nanomagnets fabricated on a piezoelectric LiNbO3 substrate. This system can efficiently serve as a magnonic crystal (MC), where the intrinsic shape anisotropy and the strong inter-element magnetostatic interaction trigger the incoherent precession of the nanomagnets' magnetization in the absence of any bias magnetic field, giving rise to the 'intrinsic' SW modes. The magnetoelastic coupling leads to a rich variety of SW phenomena when the SAW is launched along the major axis of the nanomagnets, such as 4-7 times amplification of intrinsic modes (at 3, 4, 7 and 10 GHz) when the applied SAW frequencies are resonant with these frequencies, and the generation of new extrinsic modes at non-resonant SAW frequencies. However, when the SAW is launched along the minor axis, a dominant driven mode appears at the applied SAW frequency. This reveals that the magnetoelastic coupling between SW and SAW is anisotropic in nature. Micromagnetic simulation results are in qualitative agreement with the experimental observations and elucidate the underlying dynamics. Our findings lay the groundwork for bias-field free magnonics, where the SW behavior is efficiently tuned by SAWs. It has important applications in the design of energy efficient on-chip microwave devices, SW logic, and extreme sub-wavelength ultra-miniaturized microwave antennas for embedded applications.

Sensitivity Analysis of the Surface Acoustic Wave Sensor towards Size-Distributed Particulate Matter

To study the sensitivity of the surface acoustic wave (SAW) sensor towards particulate matter (PM), an analytic model has been built based on single particle perturbation theory of full size range and the lognormal size distribution of the PM. The sensitivity of the frequency shift to 1 nanogram of PM has been calculated. The model shows that the frequency shift is a result of the competition between the negative perturbation by mass loading and the positive perturbation by elastic coupling, determined by particle size distribution parameters, material, and SAW frequency. To verify the model, the relationship of the frequency shift of a 315 MHz SAW to the concentration of aerosols generated by two kinds of powders of different sizes was measured. The experiment is in agreement with the model: the sensor has shown negative sensitivity towards aerosols generated by the finer particles of 1 μm, 3 μm polytetrafluoroethylene (PTFE), and A1 Arizona dust and positive sensitivity towards aerosols generated by the coarser particles of 10 μm PTFE and A4 Arizona dust; and the negative sensitivity is about 1 order higher than the positive.

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.

Modeling and Comparison with Experiment of SAW Induced Water Droplet Motion

A water droplet motion on a piezoelectric substrate, which is actuated using Surface Acoustic Waves (SAW), is presented toward water desalination purposes. The processes of fluid flow motion under different RF powers and SAW frequencies, induced by internal streaming, has been observed both experimentally and numerically. A free droplet dynamics in the volume of 2 and 5 μl interacted with intensive SAW on the piezoelectric substrate surface for 34 and 58 MHz frequencies, and power output between 0 - 4 dBm observed. In tha paper a three-dimensional (3D) finite element model using computational fluid dynamics approach of the free water droplet motion was developed. The different dependences of microfluidic performance such as locally streaming lines, slip length and droplet motion velocity have been studied.

Surface Acoustic Wave Resonators With Hetero Acoustic Layer (HAL) Structure Using Lithium Tantalate and Quartz.

This article reports a new concept of surface-acoustic-wave (SAW) resonator, which uses shear horizontal (SH) wave confined in a thin LiTaO3 (LT) layer supported by a quartz (Qz) substrate. The LT layer is 35-50°YX LT, and the quartz substrate is 35-60°Y90°X Qz. A negative temperature coefficient of frequency (TCF) of the SH SAW in the LT layer is compensated by the quartz substrate, which shows a wide range of positive TCF depending on the crystalline orientation. Excellent TCFs of 2 and -10 ppm/°C were measured for the series and parallel resonance frequencies, respectively. The strong confinement of the SH SAW in the LT layer results in the best level of resonance characteristics ever reported. The measured impedance ratio reached 84 dB. On the other hand, spurious waves other than the SH SAW are not confined in the LT layer due to the unique properties of quartz, which results in spurious-free characteristics throughout a wide frequency range.

Unraveling the Autonomous Motion of Polymer‐Based Catalytic Micromotors Under Chemical−Acoustic Hybrid Power

Artificial nano‐ and microswimmers are promising as versatile nanorobots for applications in biomedicine, environmental chemistry, and materials science. Herein, a hybrid micromotor containing a conjugated polymer (poly(3,4‐ethylenedioxythiophene) (PEDOT), and a catalytic structure composed of platinum (Pt) synthesized using a template‐supported electrochemical deposition process is reported. The movement of this PEDOT/Pt micromotor is characterized under chemical power generated by hydrogen peroxide catalysis, and acoustic power generated by surface acoustic waves (SAWs). The acoustic radiation force acting between the bubbles, the secondary Bjerknes force, is shown to increase the micromotor speed. The movement of the micromotor is precisely controllable using the acoustic field, providing excellent response time and reproducibility over a wide dynamic range. A theoretical model is developed to understand and predict the micromotor propulsion under the hybrid chemical and acoustic power. Predicted micromotor speeds are in excellent agreement with experiment as a function of peroxide fuel concentration, SAW field strength, and SAW frequency. The model allows for design of micromotor geometries and acoustic field strengths to achieve desired speed with excellent on/off control.

Acoustomagnetoelectric effect in two-dimensional materials: Geometric resonances and Weiss oscillations

We study electron transport in two-dimensional materials with parabolic and linear (graphene) dispersions of the carriers in the presence of surface acoustic waves and an external magnetic field using semiclassical Boltzmann equations approach. We observe an oscillatory behavior of both the longitudinal and Hall electric currents as functions of the surface acoustic wave frequency at a fixed magnetic field and as functions of the inverse magnetic field at a fixed frequency of the acoustic wave. We explain the former by the phenomenon of geometric resonances, while we relate the latter to the Weiss-like oscillations in the presence of the dynamic superlattice created by the acoustic wave. Thus we demonstrate the dual nature of the acoustomagnetoelectric effect in two-dimensional electron gas.

Analysis of surface acoustic wave induced spin resonance of a spin accumulation

It is shown that a surface acoustic wave (SAW) can induce spin resonance of a spin accumulation in a paramagnetic material. The high-frequency mechanical motion of the SAW produces an effective AC magnetic field. If it is oriented orthogonal to the spins, spin precession is induced if an additional DC magnetic field ${\mathbf{B}}_{0}$ is applied along the spin axis and the corresponding Larmor frequency ${\ensuremath{\omega}}_{0}$ matches the SAW frequency. The spin accumulation is then resonantly suppressed. We describe this SAW-induced spin resonance quantitatively and analyze the example of a nonlocal spin-transport device with a silicon channel, in which the spin accumulation is created by electrical spin injection from a ferromagnetic contact. The analysis demonstrates that the (nonlocal) electrical detection of SAW-induced spin resonance is feasible.

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.

Effective Light Beam Modulation by Chirp IDT on a Suspended LiNbO3 Membrane for 3D Holographic Displays.

An acousto-optic (AO) holographic display unit based on a suspended waveguide membrane was developed. The AO unit consists of a wide bandwidth chirp interdigital transducer (IDT) on a 20 µm thick suspended crystalline 128° YX LiNbO3 membrane, a light blocker with a 20 µm hole near the entrance, and an active lens near the exit. The 20 µm thickness of the floating membrane significantly enhanced surface acoustic wave (SAW) confinement. The light blocker was installed in front of the AO unit to enhance the coupling efficiency of the incident light to the waveguide membrane and to remove perturbations to the photodetector during measurement at the exit region. The active lens was vertically attached to the waveguide sidewall to collect the diffracted beam without loss and to modulate the focal length in free space through the applied voltage. As SAWs were radiated from the IDT, a Bragg grating with periodic refractive indexes was formed along the waveguide membrane. The grating diffracted incident light. The deflection angle and phase, and the intensity of the light beam were controlled by the SAW frequency and input power, respectively. The maximum diffraction efficiency achieved was approximately 90% for a 400 MHz SAW. COMSOL simulation and coupling of mode modeling were performed to optimize design parameters and predict device performance.

Acoustothermal heating in surface acoustic wave driven microchannel flow

Surface acoustic wave (SAW) is a well-proven tweezer serving various applications such as particle manipulation, cell trapping and separations, fluid mixing, and biosensing. SAWs can cause significant heat generation due to acoustothermal heating as established experimentally. Theoretical understanding of acoustothermal heating is limited, primarily due to the difficulties presented by multiple time scales inherent in this phenomenon. We present a theoretical model based on a multiple scale perturbation approach to solve the fluid flow and heat transfer equations for SAW-driven acoustothermal heating of a Newtonian fluid in a microchannel. The first order fields are oscillatory with the same frequency as that of the SAW, whereas the second order components are time-averaged to account for the mean flow and temperature fields. We find that the temperature rise depends solely on the acoustic energy density and its conversion into internal energy via pressure work on the fluid and hydrodynamic transportation of heat. For a fixed aspect ratio, an increase in system size essentially increases the conversion of acoustic energy into internal energy, leading to an increase in temperature rise. On the other hand, an increase in SAW frequency for a given system size causes the acoustic energy density to increase and thereby increases the temperature rise. Temperature rise is found to increase linearly with SAW power, in agreement with experimental results reported in the literature. The quantitative model for the temperature field presented in this work will find applications in designing biosensors, microreactors, and in other SAW driven controllable digital microfluidic heating applications.

3D patterning/manipulating microparticles and yeast cells using ZnO/Si thin film surface acoustic waves

Manipulating biological cells or microparticles in three dimensions (3D) is invaluable for many biomedical applications, and recently effective and rapid manipulations of microparticles in 2D and 3D within microchannels or chambers using surface acoustic waves (SAWs) with bulk piezoelectric materials have been reported. However, these are generally expensive, or brittle and cannot be easily integrated into a single lab-on-chip. In this paper, we realized microparticle/cell patterning and 3D manipulation of yeast cells inside a chamber with a height of 1 mm using thin film ZnO/Si SAW devices. Effects of SAW frequency, channel width and thickness on alignment of microparticles were firstly investigated, and positions of the microparticles in the direction of SAW propagation can be controlled precisely by changing the phase angle of the acoustic waves from the ZnO/Si SAW device. A numerical model has been developed to investigate the SAW acoustic field and the resulted 3D motions of microparticles under the acoustic radiation forces within the microchamber. Finally, we realized and observed the 3D patterning of yeast cells within the microchannel. Our work shows a great potential for acoustofluidic, neural network research and biomedical applications using the ZnO/Si SAW devices.

Non-leaky surface acoustic waves in the passbands of one-dimensional phononic crystals

The paper theoretically investigates the occurrence of non-leaky surface acoustic waves (SAWs) in the passbands of the Floquet-Bloch spectra of half-infinite one-dimensional phononic crystals. The phononic crystal is represented by a periodic structure of perfectly bonded anisotropic elastic layers. The traction-free boundary plane truncates the phononic crystal at the edge of a period. For the general case of unrestricted anisotropy of the constitutive layers, it is shown that if the passband allows only two partial bulk modes, then the non-leaky SAW must simultaneously satisfy three real equations imposed on problem parameters such as the SAW frequency and tangential wavenumber, the angle of its propagation direction along the given boundary plane, and the characteristics of the medium. If there are four bulk modes in the passband, then the non-leaky SAW must satisfy at least five real equations. In the case where the layers possess a common plane of crystallographic symmetry which is either parallel to the layer interfaces or perpendicular to the direction of propagation, the number of real equations conditioning the occurrence of non-leaky SAWs in the passbands with two and with four bulk modes reduces to two and three, respectively. If the sagittal plane is a plane of symmetry, then the sagittally polarized non-leaky passband SAW must satisfy three real equations; however, if this sagittal symmetry plane coexists with another plane of symmetry which is either parallel to the layer interfaces or perpendicular to the direction of propagation, then the existence of non-leaky passband SAW requires the fulfillment of two conditions only. In particular, this is always the case when the constitutive layers are elastically isotropic. The general conclusions are illustrated by numerical examples.

Frequency dependence of surface acoustic wave swimming.

Surface acoustic waves (SAWs) are elastic waves that can be excited directly on the surface of piezoelectric crystals using a transducer, leading to their exploitation for numerous technological applications, including for example microfluidics. Recently, the concept of SAW streaming, which underpins SAW microfluidics, was extended to make the first experimental demonstration of ‘SAW swimming’, where instead of moving water droplets on the surface of a device, SAWs are used as a propulsion mechanism. Using theoretical analysis and experiments, we show that the SAW swimming force can be controlled directly by changing the SAW frequency, due to attenuation and changing force distributions within each SAW streaming jet. Additionally, an optimum frequency exists which generates a maximum SAW swimming force. The SAW frequency can therefore be used to control the efficiency and forward force of these SAW swimming devices. The SAW swimming propulsion mechanism also mimics that used by many microorganisms, where propulsion is produced by a cyclic distortion of the body shape. This improved understanding of SAW swimming provides a test-bed for exploring the science of microorganism swimming, and could bring new insight to the evolutionary significance for the length and beating frequency of swimming microbial flagella.

Surface acoustic wave coupled to magnetic resonance on multiferroic CuB2O4

We observed surface acoustic wave (SAW) propagation on a multiferroic material CuB$_2$O$_4$ with use of two interdigital transducers (IDTs). The period of IDT fingers is as short as 1.6 $\mu$m so that the frequency of SAW is 3 GHz, which is comparable with that of magnetic resonance. In antiferromagnetic phase, the SAW excitation intensity varied with the magnitude and direction of the magnetic field, owing to the dynamical coupling between SAWs and antiferromagnetic resonance of CuB$_2$O$_4$. The microscopic mechanism is discussed based on the symmetrically allowed magentoelastic coupling.

Surface acoustic waves in dynamic magnonic crystals for microwave signals processing

Surface acoustic waves (SAW) in dynamic magnonic crystals (DMC) are considered for their use to create tunable spin-wave gadgets for processing of microwave signals. To understand the principles of operation of the proposed gadgets, the necessary basic properties of DMC created by a SAW in the structure of “yttrium iron garnet film on a substrate of gallium gadolinium garnet” are given. The prototype of the unique nonreciprocal notch filter working in the range of 3600–4100 MHz at the biasing magnetic field of 640 Oe is fabricated and its main characteristics such as dependences of the frequency and the depth of rejection pits on the frequency and power of the controlling SAW are measured. At the frequency of SAW of 41 MHz and power of 30 mW, the depth of the rejection pit reached 23 dB, the tunable shift of the notch frequency equaled to 7 MHz for a frequency change of SAW by 1 MHz. DMC with SAW may also be used for frequency shifting and modulation of microwave signals. Two configurations of prototypes of frequency shifting devices have been suggested, fabricated and their workability was shown experimentally.

PE9 レーザー干渉縞の位相速度走査法を用いた弾性表面波発生法の高周波化(SAWデバイス,ポスターセッション1)

PE9 レーザー干渉縞の位相速度走査法を用いた弾性表面波発生法の高周波化(SAWデバイス,ポスターセッション1)

Experimental Investigation of Surface Acoustic Wave Acoustoelectric Effect Using a Graphene Film on Lithium Niobate.

Experimental results of the measured surface acoustic wave (SAW) acoustoelectric effect (AE) using single-layer thin-film graphene on 128°, -cut, and -propagating lithium niobate (128-LN) are presented. The use of an AE amplifier in SAW communication devices, especially in gigahertz bands, could make fundamental advancements in radios, sensors, and other applications. Experimental results at 1350 MHz show the delay line asymmetry of 3 dB, given as the ratio of the forward versus reverse peak SAW frequency response, with an applied dc voltage on the graphene film.

Tunable surface acoustic wave device using semiconducting MgZnO and piezoelectric NiZnO dual-layer structure on glass

Tunable surface acoustic wave (SAW) devices have attracted considerable interests due to its applications in emerging fields, such as secured wireless communication, adaptive signal processing, and smart sensing systems. We report a dual-input-voltage-controlled tunable SAW built on glass, which uses a ZnO-based dual-layer structure consisting of a piezoelectric Ni-doped ZnO (NZO) and semiconducting Mg-doped ZnO (MZO). The interdigital (IDT) electrodes are buried in the piezoelectric NZO layer to form the delay line for SAW propagation, while the semiconductor MZO layer serves as the channel of a thin film transistor (TFT) to modulate the conductivity. Results show the interface between MZO channel and SiO2 gate dielectric layer of the TFT significantly impacts on SAW tuning performances due to Zn diffusion from MZO into SiO2. The TFT-SAW device using an ultra-thin MgO layer as interface modification enables SAW frequency tuning of 0.53% under solely Vgs control with 0–12 V. The required voltage range is significantly reduced compared regular MZO-NZO TFT-SAW counterpart (Δf/fc ∼ 0.25%; Vgs −30 to −14 V) without interface modification. With additional control of Vds, the SAW frequency tunability is further expanded from 0.46% to 0.63%. This dual-input voltage-controlled frequency tuning device on glass is promising for low-voltage, low-cost portable smart sensors and voltage-controlled reconfigurable radio-frequency identification tags.

Magnetoelastic excitation of single nanomagnets for optical measurement of intrinsic Gilbert damping

We report an alternative all-optical technique to drive and probe the spin dynamics of single nanomagnets. Optically generated surface acoustic waves (SAWs) drive the magnetization precession in nanomagnets via magnetoelastic (MEL) coupling. We investigate the field-swept dynamics of isolated Ni nanomagnets at various SAW frequencies and show that this method can be used to accurately determine the intrinsic Gilbert damping of nanostructured magnetic materials. This technique opens a new avenue for studying the spin dynamics of nanoscale devices using nonthermal (``cold'') excitation, enabling direct observation of the MEL driven dynamics.