Surface Acoustic Wave Frequency Research Articles

Acousto-electric transport in epitaxial monolayer graphene on SiC

We report on the piezoelectric excitation and acoustic charge transport by gigahertz surface acoustic waves (SAWs) in epitaxial monolayer graphene (EG) on SiC. The GHz SAWs frequencies were generated by interdigital transducers fabricated on a piezoelectric island on the SiC substrate. Acoustic transport studies in a Hall bar geometry show that the propagating SAW field transports carriers in EG, the transport direction being determined by the direction of the acoustic beam. Carrier transport is driven by drift in the piezoelectric field induced by the SAW in EG.

Irreversible magnetization switching using surface acoustic waves

An analytical and numerical approach is developped to pinpoint the optimal experimental conditions to irreversibly switch magnetization using surface acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and two switching mechanisms are considered. In precessional switching, a small in-plane field initially tilts the magnetization and the passage of the SAW modifies the magnetic anisotropy parameters through inverse magneto-striction, which triggers precession, and eventually reversal. Using the micromagnetic parameters of a fully characterized layer of the magnetic semiconductor (Ga,Mn)(As,P), we then show that there is a large window of accessible experimental conditions (SAW amplitude/wave-vector, field amplitude/orientation) allowing irreversible switching. As this is a resonant process, the influence of the detuning of the SAW frequency to the magnetic system's eigenfrequency is also explored. Finally, another - non-resonant - switching mechanism is briefly contemplated, and found to be applicable to (Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small perpendicular field is applied opposite the initial magnetization and the passage of the SAW lowers the domain nucleation barrier.

SURFACE-ACOUSTIC-WAVE FIELD AND ACOUSTO-OPTIC INTERACTION IN AlN/128-DEG-ROTATED Y-CUT X-PROPAGATION LiNbO3

The efficiency of guided-wave acousto-optic (AO) interaction in AlN /128-deg-rotated Y-cut X-propagation lithium niobate (128-deg YX- LiNbO 3) structure is analyzed theoretically for the first time by determining the overlap integral between the optical and the surface acoustic wave (SAW) field distribution. The results show that the use of an AlN film can increase the phase velocity of SAW, the electromechanical coupling coefficient and the diffraction efficiency of AO interaction. A maximum of 9.33% for the electromechanical coupling coefficient is obtained when the normalized thickness of AlN film equals 0.09. The diffraction efficiency has a significant improvement when the normalized thickness of AlN film is increased from 0 to 0.05. And, the improvement for the TM polarization is more evident than that for the TE polarization. However, for a well-concentrated optical waveguide, the use of an AlN film reduces the diffraction efficiency of the TM polarization when the SAW frequency is low.

Atomization off thin water films generated by high-frequency substrate wave vibrations

Generating aerosol droplets via the atomization of thin aqueous films with high frequency surface acoustic waves (SAWs) offers several advantages over existing nebulization methods, particularly for pulmonary drug delivery, offering droplet sizes in the 1-5-μm range ideal for effective pulmonary therapy. Nevertheless, the physics underlying SAW atomization is not well understood, especially in the context of thin liquid film formation and spreading and how this affects the aerosol production. Here, we demonstrate that the film geometry, governed primarily by the applied power and frequency of the SAW, indeed plays a crucial role in the atomization process and, in particular, the size of the atomized droplets. In contrast to the continuous spreading of low surface energy liquids atop similar platforms, high surface energy liquids such as water, in the present case, are found to undergo transient spreading due to the SAW to form a quasisteady film whose height is determined by self-selection of the energy minimum state associated with the acoustic resonance in the film and whose length arises from a competition between acoustic streaming and capillary effects. This is elucidated from a fundamental model for the thin film spreading behavior under SAW excitation, from which we show good agreement between the experimentally measured and theoretically predicted droplet dimension, both of which consistently indicate a linear relationship between the droplet diameter and the mechanical power coupled into the liquid by the SAW (the latter captured by an acoustic Weber number to the two thirds power, and the reciprocal of the SAW frequency).

Surface acoustic wave driven ferromagnetic resonance in nickel thin films: Theory and experiment

We present an extensive experimental and theoretical study of surface acoustic wave-driven ferromagnetic resonance. In a first modeling approach based on the Landau-Lifshitz-Gilbert equation, we derive expressions for the magnetization dynamics upon magnetoelastic driving that are used to calculate the absorbed microwave power upon magnetic resonance as well as the spin current density generated by the precessing magnetization in the vicinity of a ferromagnet/normal metal interface. In a second modeling approach, we deal with the backaction of the magnetization dynamics on the elastic wave by solving the elastic wave equation and the Landau-Lifshitz-Gilbert equation selfconsistently, obtaining analytical solutions for the acoustic wave phase shift and attenuation. We compare both modeling approaches with the complex forward transmission of a LiNbO$_3$/Ni surface acoustic wave hybrid device recorded experimentally as a function of the external magnetic field orientation and magnitude, rotating the field within three different planes and employing three different surface acoustic wave frequencies. We find quantitative agreement of the experimentally observed power absorption and surface acoustic wave phase shift with our modeling predictions using one set of parameters for all field configurations and frequencies.

Experimental methods to suppress random telegraph signal noise in acoustic charge transport nanostructure devices

AbstractWe present an experimental investigation of how the different operating parameters affect the random telegraph signal (RTS) noise in surface acoustic wave (SAW) devices. The altered parameters include: SAW power and frequency, phase difference between two counterpropagating SAW beams, and asymmetrical voltage applied to the two side gates of the quantum point contact. The results indicate that proper tuning of the relevant parameters can reduce the RTS noise, which would be beneficial for metrological applications. Building on the experimental results, physical models have been proposed to explain the reason why the effective reduction of the noise can be achieved.

Surface acoustic wave in yttrium iron garnet as tunable magnonic crystals for sensors and signal processing applications

Experimental results on investigation of magnonic crystals arising at surface acoustic wave (SAW) propagation in yttrium iron garnet as an artificial structure for surface magnetostatic waves propagation are presented. Tuning of magnonic gap frequency and gap depth were obtained by changing frequency and intensity of SAW that was explained in the frame of inelastic scattering of magnetostatic waves by SAW. Such tuning was shown can compensate drift of parameters due to possible change of temperature when using magnonic crystals as very sensitive magnetic field sensors. Suggested magnonic crystals are promising also for signal processing in GHz range.

Detecting surface acoustic wave gyroscopic signal by acousto-optic coupling

A technique for surface acoustic wave (SAW) gyroscopic signal detection by acousto-optic coupling is presented in this article. The principles of the technique and the detection structure were analyzed, and experiments were conducted using the gyroscope. The static results show that the maximum coupling efficiency obtained using this technique is 80.02% when the acousto-optic interaction length is maintained at 15 mm, the SAW electrical power is 99 mW, and the SAW frequency is 168.201 MHz. When the gyroscope was rotating, a linear voltage-rotation response curve was obtained and the sensitivity was approximately 1.2 mV/deg/s.

Propagation of acoustic surface waves on a phononic surface investigated by transient reflecting grating spectroscopy

We present a study of surface acoustic waves (SAW) propagation on a 1D phononic surface (PS) by mean of an heterodyne-detected transient reflecting grating experiment. We excited and detected coherent stationary SAWs characterized by variable wave-vectors. The measured SAW frequencies enables the characterization of the band diagram of this PS sample beyond the first Brillouin zone (BZ). Four different SAW frequencies have been revealed, whose band diagram show articulated dispersion phenomena. In order to address the nature of the investigated SAWs, the experimental results are compared with a numerical simulation of elastic modes based on a finite element model. The observed SAWs are addressed to four Bloch waves characterized by different frequencies and surface energy localization. Moreover, we measured the SAW propagation on a flat non-phononic part of the sample surface and compared it with results from the PS.

Experimental Studies of Quality Factor Deterioration in Shear-Horizontal-Type Surface Acoustic Wave Resonators Caused by Apodization of Interdigital Transducer

We investigate how the apodization applied to interdigital transducers (IDTs) affects the quality factor Q at the antiresonance frequency of surface acoustic wave (SAW) resonators. First, we fabricated SAW resonators with different apodization patterns on a SiO2/Cu/0°YX-LiNbO3 structure, and showed that the Q factor become worse when the apodization becomes steeper and the IDT aperture is narrower. Next, it was shown by optical measurement that the Q degradation is mainly due to the radiation of SH waves from the resonator. The radiation consists of energy leakage as bulk waves, diffraction, and scattering of incident SAWs, and their strength strongly depends on the steepness of the apodization pattern.

Impact of Photocapacitance on Phase Response of GaN/Sapphire SAW UV Sensor

The response of GaN/sapphire SAW delay line to the focused ultraviolet (UV) illumination was investigated at the SAW frequency 306 MHz and optical wavelength 375 nm. The UV irradiation of an interdigital transducer (IDT) led to 4 to 5 times larger phase change of transmitted signal than the irradiation of the surface acoustic wave (SAW) propagation path between the IDTs at the same UV light spot size of 2.4 mm. The UV-induced phase change has been attributed to the variation in IDT impedance due to photo-capacitive effect. Simulation of the SAW sensor phase response on the basis of this model is in good agreement with experimental results.

The Frequency Dependence of Quantized Current Plateaus Induced by Surface Acoustic Waves Through a Quantum Point Contact

The acousto-electric current through a split gate induced by surface acoustic waves (SAW) was studied. The current plateaus were observed when proper frequencies of SAW were applied to the device. The resulting current plateau values corresponded to nef, where n is the number of electrons transferred per cycle and f is the induced SAW frequency. Also, the width of the plateaus was found to change periodically as the SAW frequency changed. Unwanted coupling of the RF signal (applied to inter-digitized transducer to produce SAW) to the gate of QPC were found to cause the changes in plateaus width.

Mass sensitivity analysis and designing of surface acoustic wave resonators for chemical sensors

The sensitivity of surface acoustic wave (SAW) chemical sensors depends on several factors such as the frequency and phase point of SAW device operation, sensitivity of the SAW velocity to surface mass loading, sensitivity of the SAW oscillator resonance to the loop phase shift, film thickness and oscillator electronics. This paper analyzes the influence of the phase point of operation in SAW oscillator sensors based on two-port resonator devices. It is found that the mass sensitivity will be enhanced if the SAW device has a nonlinear dependence on the frequency (delay ∼ frequency−1). This requires the device to generate and operate in a ωτg(ω) = const region in the device passband, where ω denotes the angular frequency of oscillation and τg(ω) denotes the phase slope of the SAW resonator device. A SAW coupled resonator filter (CRF) that take advantage of mode coupling is considered in realizing such a device to help in shaping the phase transfer characteristics of a high mass sensitivity sensor. The device design and simulation results are presented within the coupling-of-modes formalism.

Ultrafast microfluidics using surface acoustic waves.

We demonstrate that surface acoustic waves (SAWs), nanometer amplitude Rayleigh waves driven at megahertz order frequencies propagating on the surface of a piezoelectric substrate, offer a powerful method for driving a host of extremely fast microfluidic actuation and microbioparticle manipulation schemes. We show that sessile drops can be translated rapidly on planar substrates or fluid can be pumped through microchannels at 1-10 cms velocities, which are typically one to two orders quicker than that afforded by current microfluidic technologies. Through symmetry-breaking, azimuthal recirculation can be induced within the drop to drive strong inertial microcentrifugation for micromixing and particle concentration or separation. Similar micromixing strategies can be induced in the same microchannel in which fluid is pumped with the SAW by merely changing the SAW frequency to rapidly switch the uniform through-flow into a chaotic oscillatory flow by exploiting superpositioning of the irradiated sound waves from the sidewalls of the microchannel. If the flow is sufficiently quiescent, the nodes of the transverse standing wave that arises across the microchannel also allow for particle aggregation, and hence, sorting on nodal lines. In addition, the SAW also facilitates other microfluidic capabilities. For example, capillary waves excited at the free surface of a sessile drop by the SAW underneath it can be exploited for micronanoparticle collection and sorting at nodal points or lines at low powers. At higher powers, the large accelerations off the substrate surface as the SAW propagates across drives rapid destabilization of the drop free surface giving rise to inertial liquid jets that persist over 1-2 cm in length or atomization of the entire drop to produce 1-10 mum monodispersed aerosol droplets, which can be exploited for ink-jet printing, mass spectrometry interfacing, or pulmonary drug delivery. The atomization of polymerprotein solutions can also be used for the rapid synthesis of 150-200 nm polymerprotein particles or biodegradable polymeric shells in which proteins, peptides, and other therapeutic molecules are encapsulated within for controlled release drug delivery. The atomization of thin films behind a translating drop containing polymer solutions also gives rise to long-range spatial ordering of regular polymer spots whose size and spacing are dependent on the SAW frequency, thus offering a simple and powerful method for polymer patterning without requiring surface treatment or physicalchemical templating.

Surface Vibration Induced Spatial Ordering of Periodic Polymer Patterns on a Substrate

We demonstrate the possibility of producing regular, long-range, spatially ordered polymer patterns without requiring the use of physical or chemical templating through the interfacial destabilization of a thin polymer film driven by surface acoustic waves (SAWs). The periodicity and spot size of the pattern are observed to be dependent on a single parameter, that is, the SAW frequency (or wavelength), therefore offering a rapid, simple, yet novel method for self-organized regular spatial polymer pattern formation that is far more tunable than conventional polymer patterning procedures.

Effects of inelastic capture, tunneling escape, and quantum confinement on surface acoustic wave-dragged photocurrents in quantum wells

We developed a dual-charged-fluid model for studying the steady-state transport of surface acoustic wave (SAW)-dragged photocurrents of one-dimensional (1D) confined-state carriers. This model includes the effects of quantum confinement and the escape via tunneling of SAW-dragged 1D carriers, as well as the effects of the inelastic capture of two-dimensional continuous-state carriers and the self-consistent space-charge field. Our numerical results revealed a high photocurrent gain due to the suppressed recombination of 1D carriers in a crossover region of the sample between an absorption strip and a surface gate. Based on this model, responsivities for the SAW-dragged photocurrents in a quantum well are calculated as functions of the gate voltage, photon flux, SAW power and frequency, and temperature, respectively. A responsivity as high as 103 A/W was found for high gate voltages and SAW powers, as well as for low photon fluxes and SAW frequencies.

Theoretical, Numerical and Experimental Investigations of Gas Vapour Effects on a ZnO/Quartz SAW Gas Sensor

The sensitivity of a ZnO/Quartz SAW (Surface Acoustic Waves) sensor has been evaluated under alcohol vapour exposure. Ethanol, isopropanol and butanol have been tested. In order to better understand the SAW frequency shifting effect due to the alcohol molecules, a Langmuir/B.E.T analysis has been carried out. Possible shifting mechanisms have been proposed and tested by numerical simulation.

Acoustomagnetic pulse experiments in LiNbO3∕Mn12 hybrids

We report here on the influence of surface acoustic waves (SAWs) on the magnetization of a Mn12-acetate single crystal. The crystal was mounted on the surface of a piezoelectric LiNbO3 substrate containing an interdigital transducer for the excitation of SAWs. The magnetization of the crystal was measured using a rf superconducting quantum interference device with a time resolution of 1μs. The piezoelectric material was excited by SAW pulses of different frequencies produced by applying microwave pulses to the transducer. Our data show that molecular magnets onto the LiNbO3 surface can be used as very sensitive detectors of the SAW frequency and intensity.

UV-LED controlled GaN-based SAW phase shifter

The UV-LED controlled phase shifter of an RF signal based on a GaN-sapphire surface-acoustic-wave (SAW) filter has been implemented. At the optical wavelength 298 nm and SAW frequency 307 MHz, the UV-induced relative change in SAW velocity per unit optical power density is 2×10−6 (µW/mm2)−1 corresponding to 3.7° phase shift. The phase modulation of an RF signal by rectangular UV pulses has been demonstrated. The efficiency of the phase shifter can be considerably improved by proper selection of sheet-resistivity of the GaN layer.

The effects of ZnO films on surface acoustic wave properties of modified lead titanate ceramic substrates

Poly-crystal zinc oxide (ZnO) films with c-axis (002) orientation have been successfully grown on the strontium (Sr) modified lead titanate ceramic substrates with different Sr dopants by r.f. magnetron sputtering technique. Highly oriented ZnO films with c-axis normal to the substrates can be obtained under a total pressure of 10 mTorr containing 50% argon and 50% oxygen and r.f. power of 70 W for 3 hours. Crystalline structures of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of surface acoustic wave (SAW) devices with ZnO/IDT/PT (IDT, inter-digital transducer; PT, PbTiO3 ceramics) structure were investigated. The devices with ZnO/IDT/PT structure shows that the ZnO film effectively raise the electromechanical coupling coefficient (kappa2) from 3.8% to 9.9% of the device with the concentrations of Sr dopants of 0.15. It also improves the temperature coefficient of frequency of SAW devices.