Surface Acoustic Wave Field Research Articles

Growth of high quality Li2B4O7 single crystals: Core formation revisited by using electric force microscopy

Lithium tetraborate is a well known piezoelectric crystal with a wide variety of applications in the field of Surface Acoustic Wave (SAW) devices and also as a strong candidate for the fabrication of high performance personal dosimetry systems. However, it is well known the tendency of this crystal to form a central core which greatly affects to its physical properties. Although this issue has been studied four decades, it seems that some issues regarding to the core formation in lithium tetraborate crystals still remain to be understood. In this work, the effect of the thermal gradient and some other parameters like rotation speed and growth rate on the core formation has been analysed in order to ascertain the best growth conditions to obtain high quality single Li2B4O7 crystals. Several lithium tetraborate crystals have been grown by standard Czochralski technique and the formation and structural and electrical properties of the core region have been analysed by using Electric Force Microscopy (EFM).

The complexity of surface acoustic wave fields used for microfluidic applications

Using surface acoustic waves (SAW) for the agitation and manipulation of fluids and immersed particles or cells in lab-on-a-chip systems has been state of the art for several years. Basic tasks comprise fluid mixing, atomization of liquids as well as sorting and separation (or trapping) of particles and cells, e.g. in so-called acoustic tweezers. Even though the fundamental principles governing SAW excitation and propagation on anisotropic, piezoelectric substrates are well-investigated, the complexity of wave field effects including SAW diffraction, refraction and interference cannot be comprehensively simulated at this point of time with sufficient accuracy. However, the design of microfluidic actuators relies on a profound knowledge of SAW propagation, including superposition of multiple SAWs, to achieve the predestined functionality of the devices. Here, we present extensive experimental results of high-resolution analysis of the lateral distribution of the complex displacement amplitude, i.e. the wave field, alongside with the electrical S-parameters of the generating transducers. These measurements were carried out and are compared in setups utilizing travelling SAW (tSAW) excited by single interdigital transducer (IDT), standing SAW generated between two IDTs (1DsSAW, 1D acoustic tweezers) and between two pairs of IDTs (2DsSAW, 2D acoustic tweezers) with different angular alignment in respect to pure Rayleigh mode propagation directions and other practically relevant orientations. For these basic configurations, typically used to drive SAW-based microfluidics, the influence of common SAW phenomena including beam steering, coupling coefficient dispersion and diffraction on the resultant wave field is investigated. The results show how tailoring of the acoustic conditions, based on profound knowledge of the physical effects, can be achieved to finally realize a desired behavior of a SAW-based microacoustic-fluidic system.

Experimental and numerical investigations of mechanical displacements in surface acoustic wave bounded beams

The present paper studies experimentally and numerically the surface acoustic wave (SAW) field on a piezoelectric substrate generated by interdigital transducers (IDT). On the one hand, mechanical displacements produced by the SAW are measured with the help of a laser Doppler vibrometer. On the other hand, mechanical displacements are computed by the two-dimensional finite element method in frequency domain followed by the spatial Fourier transform. Combining these two steps of computations results in the intended two-dimensional distribution of mechanical displacements on the substrate surface. The comparison of experimental and numerical data obtained for a series of different IDTs reveals that it is possible to estimate the shape of the SAW beam and the absolute value of mechanical displacement amplitude using only the basic parameters of the IDT and its electrical admittance measured by a network analyzer.

Towards phonon routing: controlling propagating acoustic waves in the quantum regime

We explore routing of propagating phonons in analogy with previous experiments on photons. Surface acoustic waves (SAWs) in the microwave regime are scattered by a superconducting transmon qubit. The transmon can be tuned on or off resonance with the incident SAW field using an external magnetic field or the Autler–Townes effect, and thus the reflection and transmission of the SAW field can be controlled in time. We observe 80% extinction in the transmission of the low power continuous signal and a 40 ns rise time of the router. The slow propagation speed of SAWs on solid surfaces allows for in-flight manipulations of the propagating phonons. The ability to route short, 100 ns, pulses enables new functionality, for instance to catch an acoustic phonon between two qubits and then release it in a controlled direction.

Microparticle concentration and separation inside a droplet using phononic-crystal scattered standing surface acoustic waves

Rapid particle and biocell concentration and separation are important for the development of miniaturized physical, biological, and chemical systems. In this study, the scattering of standing surface acoustic waves (SSAWs) in an acoustofluidic phononic crystal (PnC) device is used to actuate spatial concentration and separation of suspended microparticles with two different sizes in a sessile droplet. The simulation design of the device and manipulation mechanisms of the two distinct particles are presented and discussed. A surface acoustic wave (SAW) acoustofluidic device with a nickel pillar-type PnC electroplated on a piezoelectric substrate, 128°YX LiNbO3, is designed and fabricated. A 30-MHz SSAW field is scattered within the directional phononic bandgap of the PnC to generate simultaneously acoustic radiation force and acoustic streaming flow to concentrate and separate 2- and 20-μm polystyrene microparticles. Experiments are conducted to verify the device functions on concentrating and separating microparticles. The results also suggest the possibilities of regarding phononic crystals as an effective element to tailor the SAW field for building other innovative acoustofluidic devices to achieve novel concentration, separation, and acoustic manipulation of biocells and biomolecules in discrete microfluidic systems.

Observational and theoretical validation of longitudinal type surface acoustic wave devices

The authors have proposed longitudinal type surface acoustic wave (SAW) devices comprising a LiNbO3 thin plate, acoustic mirror, handle substrate and interdigital transducers (IDT). This paper describes the observational and theoretical validation of the proposed devices by using a laser probe system and finite element analysis (FEA). It is shown that the laser probe system based on the Sagnac interferometer is effective for direct observation of the devices operated at even 2 GHz range through the observation of SAW field distribution of the existence and lack of acoustic mirror, their comparison with FEA results and changes of the transverse mode pattern. In addition, a phenomenon of SAW radiation normal to the busbars from IDT is found both observationally and theoretically, and a possible radiation mechanism explaining the results is also discussed.

Modulation of terahertz radiation from graphene surface plasmon polaritons via surface acoustic wave.

We present a theoretical study of terahertz (THz) radiation induced by surface plasmon polaritons (SPPs) on a graphene layer under modulation by a surface acoustic wave (SAW). In our gedanken experiment, SPPs are excited by an electron beam moving on a graphene layer situated on a piezoelectric MoS2 flake. Under modulation by the SAW field, charge carriers are periodically distributed over the MoS2 flake, and this causes periodically distributed permittivity. The periodic permittivity structure of the MoS2 flake folds the SPP dispersion curve back into the center of the first Brillouin zone, in a manner analogous to a crystal, leading to THz radiation emission with conservation of the wavevectors between the SPPs and the electromagnetic waves. Both the frequency and the intensity of the THz radiation are tuned by adjusting the chemical potential of the graphene layer, the MoS2 flake doping density, and the wavelength and period of the external SAW field. A maximum energy conversion efficiency as high as ninety percent was obtained from our model calculations. These results indicate an opportunity to develop highly tunable and integratable THz sources based on graphene devices.

Surface acoustic waves enable rotational manipulation of Caenorhabditis elegans.

Controllable, precise, and stable rotational manipulation of model organisms is valuable in many biomedical, bioengineering, and biophysics applications. We present an acoustofluidic chip capable of rotating Caenorhabditis elegans (C. elegans) in both static and continuous flow in a controllable manner. Rotational manipulation was achieved by exposing C. elegans to a surface acoustic wave (SAW) field that generated a vortex distribution inside a microchannel. By selectively activating interdigital transducers, we achieved bidirectional rotation of C. elegans, namely counterclockwise and clockwise, with on-demand switching of rotation direction in a single chip. In addition to continuous rotation, we also rotated C. elegans in a step-wise fashion with a step angle as small as 4° by pulsing the signal duration of SAW from a continuous signal to a pulsed signal down to 1.5 ms. Using this device, we have clearly imaged the dopaminergic neurons of C. elegans with pdat-1:GFP expression, as well as the vulval muscles and muscle fibers of the worm with myo-3::GFP fusion protein expression in different orientations and three dimensions. These achievements are difficult to realize through conventional (i.e., non-confocal) microscopy. The SAW manipulations did not detectably affect the health of the model organisms. With its precision, controllability, and simplicity in fabrication and operation, our acoustofluidic devices will be well-suited for model organism studies.

Electrical detection and analysis of surface acoustic wave in line-defect two-dimensional piezoelectric phononic crystals

Line-defect piezoelectric phononic crystals (PCs) show good potential applications in surface acoustic wave (SAW) MEMS devices for RF communication systems. To analyze the SAW characteristics in line-defect two-dimensional (2D) piezoelectric PCs, optical methods are commonly used. However, the optical instruments are complex and expensive, whereas conventional electrical methods can only measure SAW transmission of the whole device and lack spatial resolution. In this paper, we propose a new electrical experimental method with multiple receiving interdigital transducers (IDTs) to detect the SAW field distribution, in which an array of receiving IDTs of equal aperture was used to receive the SAW. For this new method, SAW delay lines with perfect and line-defect 2D Al/128°YXLiNbO3 piezoelectric PCs on the transmitting path were designed and fabricated. The experimental results showed that the SAW distributed mainly in the line-defect region, which agrees with the theoretical results.

Standing surface acoustic wave enabled acoustofluidics for bioparticle manipulation

Techniques that can noninvasively and dexterously manipulate cells and other bioparticles in a compact system are invaluable for many applications in life sciences and medicine. The past two decades have witnessed the emerging of acoustofluidics: the fusion of acoustics and microfluidics, due to it numerous advantages such as biocompatibility, miniaturization with lost cost, compatibility with other microfluidic system, and many others. Here, we first demonstrate surface acoustic wave (SAW) enabled acoustic tweezers platform for on-chip particle manipulation. Surface acoustic wave is a kind of ultrasound wave that propagates on the surface of a substrate. By tuning the standing SAW field, we demonstrated the functions of: (1) manipulation of nanoparticles, cells, and C elegance; (2) label free cell separation; (3) multichannel cell/droplet sorting, and (4) tunable cell patterning. Cells viability and proliferation assays were also conducted to confirm the non-invasiveness of our technique. The simple structure/setup of these acoustic tweezers can be integrated with a small radio-frequency power supply and basic electronics to function as a fully integrated, portable, and inexpensive cell-manipulation system. We believe that these unique advantages and functions position our acoustic tweezers to be a useful tool in medical diagnostics and biological/chemical studies.

Theoretical study on dynamic acoustic modulation of free carriers, excitons, and trions in 2D MoS2 flake

We present a self-consistent model calculation to study the influence of a radio-frequency surface acoustic wave (SAW) on the dynamic processes of photo-generated free carriers, excitons and trions in a 2D MoS2 flake with an odd number of layers. We found that the concentrations of excitons and trions decreased, while the lifetime of the free carriers increased when an external SAW field was applied. We attribute this phenomenon to the spatial separation of the electrons and holes, which are confined by a SAW-induced periodic type-II band-edge. We further studied the effect of the modulations on the spatiotemporal distributions of free carriers and excited quasiparticles by varying the amplitude and wavelength of the SAW field. We finally calculated the photoluminescent properties of the MoS2 flake under modulation of the SAW field and compared our results with those of recent experiments.

Pre-rolling leaning of microparticles

At nano/micro-scales, the resistance of a particle to rolling is a critical factor in many applications and biological phenomena as it affects particle adhesion, motion and flow as well as particle manipulation (e.g. picking and placing of microparticles). In present work, a non-contact non-invasive experimental method is detailed and applied to determine the pre-rolling critical leaning angle (CLA) of single microparticles. Transient Rayleigh surface acoustic waves (SAW) are utilized as the excitation mechanism and interferometry and image processing as detection/monitoring and analysis techniques for capturing the micro/nano-scale dynamics of the microparticles. The CLA values for a set of 30 PSL (polystyrene latex) microparticles with a diameter of 14.9±0.6μm on a soda-lime glass substrate are reported. The CLA of the studied particles are determined to be between 0.9 and 7.8°. It is also observed that during a SAW field burst cycle, microparticles could change their drifting (rolling and/or sliding) directions, speeds and accelerations. The trajectory of a particle is often found to be non-linear. This nonlinear behavior is attributed to the inhomogeneity of the surface properties of the particles and the substrate as well as possible electric charge density, chemistry variations and potential contamination on surfaces. Moreover, the effect of electric charge (developed due to the triboelectric effect) on the particle drifting motion is investigated. It is found that the percentage of drifting particles is not only a function of the amplitude of the excitation field but also a function of possible electrostatic charges developed on the particles due to the drifting motion on the substrate. In addition to their potential uses in particle manipulation, removal and adhesion characterization, the reported results could be utilized in numerical simulations of microparticle motion and deposition.

3D measurement and simulation of surface acoustic wave driven fluid motion: a comparison.

The characterisation of the fluid motion induced by the acoustic streaming effect is of paramount interest for novel microfluidic devices based on surface acoustic waves (SAWs), e.g. for a detailed description of the achievable mixing efficiency and thus the design of such devices. Here, we present for the first time a quantitative 3D comparison between experimental measurements and numerical simulations of the acoustic streaming induced fluid flow inside a microchannel originating from a SAW. On the one hand, we performed fully three-dimensional velocity measurements using the astigmatism particle tracking velocimetry. On the other hand, we derived a novel streaming force approach solving the damped wave equation, which allows fast and easy 3D simulations of the acoustic streaming induced fluid flow. Furthermore, measurements of the SAW amplitude profile inside the fluid filled microchannel were performed. Based on these results, we obtained a very good agreement between the velocity measurements and the simulations of the fluid flow demonstrating the importance of comprising the actual shape of the SAW amplitude profile for quantitatively reliable simulations. It is shown that the novel streaming force approach is a valid approximation for the simulation of the acoustic streaming induced fluid flow, allowing a rapid and simple estimation of the flow field of SAW based microfluidic devices.

Acoustofluidic bacteria separation

Bacterial separation from human blood samples can help with the identification of pathogenic bacteria for sepsis diagnosis. In this work, we report an acoustofluidic device for label-free bacterial separation from human blood samples. In particular, we exploit the acoustic radiation force generated from a tilted-angle standing surface acoustic wave (taSSAW) field to separate Escherichia coli from human blood cells based on their size difference. Flow cytometry analysis of the E. coli separated from red blood cells shows a purity of more than 96%. Moreover, the label-free electrochemical detection of the separated E. coli displays reduced non-specific signals due to the removal of blood cells. Our acoustofluidic bacterial separation platform has advantages such as label-free separation, high biocompatibility, flexibility, low cost, miniaturization, automation, and ease of in-line integration. The platform can be incorporated with an on-chip sensor to realize a point-of-care sepsis diagnostic device.

Optical phonon modulation in semiconductors by surface acoustic waves

We investigate the modulation of optical phonons in semiconductor crystal by surface acoustic wave (SAW) propagating on the crystal surface. The SAW fields induce changes on the order of 10\textsuperscript{-3} in the average Raman scattering intensity by optical phonons in Si and GaN crystals. The SAW-induced modifications in the Raman cross-section are dominated by the modulation of the optical phonon energy by the SAW strain field. In addition to this local contribution, the experiments give evidence for a weaker and non-local contribution arising from the spatial variation of the SAW strain field. The latter is attributed to the activation of optical modes with large wave vectors and, therefore, lower energies. The experimental results, which are well described by theoretical models for the two contributions, prove that optical phonons can be manipulated by SAWs with $\mu$m wavelengths

Effect of surface temperature on microparticle-surface adhesion

The effect of surface temperature on the adhesion properties of the bond between a substrate and a single micro-particle is investigated in a non-contact/non-invasive manner by monitoring the rolling/rocking motion dynamics of acoustically excited single microparticles. In the current work, a set of experiments were performed to observe the change in the rocking resonance frequency of the particles with the change of surface temperature. At various substrate surface temperature levels, the work-of-adhesion values of the surface-particle bond are evaluated from the resonance frequencies of the rocking motion of a set of microparticles driven by an orthogonal ultrasonic surface acoustic wave field. The dependence of adhesion bonds of a microparticle and the substrate on the surface temperature has been clearly demonstrated by the performed experiments. It was also observed and noted that the relative humidity plays a vital role in the rolling behavior of particles.

Radiation dominated acoustophoresis driven by surface acoustic waves

Acoustophoresis-based particle manipulation in microfluidics has gained increasing attention in recent years. Despite the fact that experimental studies have been extensively performed to demonstrate this technique for various microfluidic applications, numerical simulation of acoustophoresis driven by surface acoustic waves (SAWs) has still been largely unexplored. In this work, a numerical model taking into account the acoustic–piezoelectric interaction was developed to simulate the generation of a standing surface acoustic wave (SSAW) field and predict the acoustic pressure field in the liquid. Acoustic radiation dominated particle tracing was performed to simulate acoustophoresis of particles with different sizes undergoing a SSAW field. A microfluidic device composed of two interdigital transducers (IDTs) for SAW generation and a microfluidic channel was fabricated for experimental validation. Numerical simulations could well capture the focusing phenomenon of particles to the pressure nodes in the experimental observation. Further comparison of particle trajectories demonstrated considerably quantitative agreement between numerical simulations and experimental results with fitting in the applied voltage. Particle switching was also demonstrated using the fabricated device that could be further developed as an active particle sorting device.

Characterization of surface acoustic waves by stroboscopic white-light interferometry.

We present phase-sensitive absolute amplitude measurements of surface acoustic wave fields obtained using a stroboscopic white-light interferometer. The data analysis makes use of the high resolution available in the measured interferometric phase data, enabling the characterization of the out-of-plane surface vibration fields in electrically excited microstructures with better than 100 pm amplitude resolution. The setup uses a supercontinuum light source with tailored spectral properties for obtaining the high amplitude resolution. The duration of the light pulses is less than 300 ps to allow the detection of high frequencies. These capabilities enabled a detailed measurement of the focusing of surface acoustic waves by an annular interdigital transducer structure operating at 74 MHz, featuring a maximum vibration amplitude of 3 nm.

Independent dynamic acousto-mechanical and electrostatic control of individual quantum dots in a LiNbO3-GaAs hybrid

We demonstrate tuning of single quantum dot emission lines by the combined action of the dynamic acoustic field of a radio frequency surface acoustic wave and a static electric field. Both tuning parameters are set all-electrically in a LiNbO3-GaAs hybrid device. The surface acoustic wave is excited directly on the strong piezoelectric LiNbO3 onto which a GaAs-based p-i-n photodiode containing a single layer of quantum dots was epitaxially transferred. We demonstrate dynamic spectral tuning with bandwidths exceeding 3 meV of single quantum dot emission lines due to deformation potential coupling. The center energy of the dynamic spectral oscillation can be independently programmed simply by setting the bias voltage applied to the diode.

The patterning mechanism of carbon nanotubes using surface acoustic waves: the acoustic radiation effect or the dielectrophoretic effect.

In this study, we present a simple technique capable of assembling and patterning suspended CNTs using a standing surface acoustic wave (SSAW) field. Individual CNTs could be assembled into larger CNT bundles and patterned in periodic positions on a substrate surface. The mechanism of the SSAW-based patterning technique has been investigated using both numerical simulation and experimental study. It has been found that the acoustic radiation effect due to the acoustic pressure field and the dielectrophoretic (DEP) effect induced by the electric field co-existing in the patterning process however play different roles depending on the properties of the suspended particles and the suspension medium. In the SSAW-based patterning of highly conductive CNTs with high aspect ratio geometry, the positive DEP effect dominates over the acoustic radiation effect. In contrast, the acoustic radiation effect dominates over the DEP effect when manipulating less conductive, spherical or low aspect ratio particles or biological cells. These results provide a meaningful insight into the mechanism of SSAW-based patterning, which is of great help to guide the effective use of this patterning technique for various applications.