Rayleigh-type Surface Acoustic Wave Research Articles

Quantitative optical imaging method for surface acoustic waves using optical path modulation

A Rayleigh-type surface acoustic wave (SAW) is used in various fields as classical and quantum information carriers because of its surface localization, high electrical controllability, and low propagation loss. Coupling and hybridization between the SAW and other physical systems such as magnetization, electron charge, and electron spin are the recent focuses in phononics and spintronics. A precise measurement of the surface wave amplitude is often necessary to discuss the coupling strengths. However, there are only a few such measurement techniques and they generally require a rather complex analysis. Here we develop and demonstrate a straightforward measurement technique that can quantitatively characterize the SAW. The technique optically detects the surface waving due to the coherently driven SAW by the optical path modulation. Furthermore, when the measurement system operates in the shot-noise-limited regime, the surface slope and displacement at the optical spot can be deduced from the optical path modulation signal. Our demonstrated technique will be an important tool for SAW-related research.

Spin Elastodynamic Motive Force.

The spin-motive force (SMF) in a simple ferromagnetic monolayer caused by a surface acoustic wave is studied theoretically via spin-vorticity coupling (SVC). The SMF has two mechanisms. The first is the SVC-driven SMF, which produces the first harmonic electromotive force, and the second is the interplay between the SVC and the magnetoelastic coupling, which produces the dc and second harmonic electromotive forces. We show that these electric voltages induced by a Rayleigh-type surface acoustic wave can be detected in polycrystalline nickel. No sophisticated device structures, noncollinear magnetic structures, or strong spin-orbit materials are used in our approach. Consequently, it is intended to broaden the spectrum of SMF applications considerably.

Spatiotemporally controlled room-temperature exciton transport under dynamic strain

Two-dimensional transition metal dichalcogenides (TMDs) provide an attractive platform for studying strain dependent exciton transport at room temperature due to large exciton binding energy and strong bandgap sensitivity to mechanical stimuli. Here, we use Rayleigh type surface acoustic wave (SAW) to demonstrate controlled and directional exciton transport under weak coupling regime at room temperature. We screen the in-plane piezoelectric field using photogenerated carriers to study transport under type-I bandgap modulation and measure a maximum exciton drift velocity of 600 m/s. Furthermore, we demonstrate precise steering of exciton flux by controlling the relative phase between the input RF excitation and exciton photogeneration. The results provide important insight into the weak coupling regime between dynamic strain wave and room temperature excitons in a 2D semiconductor system and pave way to exciting applications of excitonic devices in data communication and processing, sensing and energy conversion.

Full extraction of the COM parameters for Rayleigh type surface acoustic wave

Extraction of Coupling of Modes (COM) parameters is important in the design of modern high-performance surface acoustic wave (SAW) devices. In this study, we established an effective connection between the COM parameters and the results of stationary and eigenfrequency studies based on the finite element method. Based on this connection, we proposed an approach for fast and full extraction of the COM parameters including the phases of coupling reflection and transduction coefficient. The COM parameters of the commonly used electrode width controlled single-phase unidirectional transducer and bidirectional interdigital transducer cells are computed and compared. It shows that this is a fast, accurate, general, and full extraction of the COM parameters for Rayleigh type SAW.

Einstein–de Haas phase shifts in surface acoustic waves

We demonstrate the phase shift of the Rayleigh-type surface acoustic wave (RSAW) in a ferromagnetic ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ film based on the gyromagnetic Einstein--de Haas (EdH) torque that can be attributed to the back action of spin-wave excitation. The EdH torque modulates the transverse velocity of the bulk acoustic wave that causes the bipolar phase shift of RSAW. When compared with a magnetoelastic torque, the phase shift can be more prominently enhanced by increasing the frequency. The gyromagnetic torque in the RSAW paves the way for controlling spin-mechatronics devices without magnetoelastic torque which is specific to a certain material.

Mode Analysis of Pt/LGS Surface Acoustic Wave Devices.

Platinum (Pt) gratings on langasite (LGS) substrates are a widely used structures in high temperature surface acoustic wave (SAW) devices. Multiple modes can be excited in PtLGS SAW devices owing to the heavy weight of the Pt electrode and leaky waves in the LGS substrate. In this work, we report on a detailed mode analysis of PtLGS SAW devices, where three commonly used LGS cuts are considered. A three-dimensional (3D) finite element method (FEM) numerical model was developed, and the simulation and experiment results were compared. The experiment and simulation results showed that there are two modes excited in the PtLGS SAW devices with Euler angle (0, 138.5, 27) and (0, 138.5, 117), which are Rayleigh-type SAW and SH-type leaky wave, respectively. Only the Rayleigh-type mode was observed in the PtLGS SAW devices with Euler angle (0, 138.5, 72). It was found that the acoustic velocities are dependent on the wavelength, which is attributed to the change of wave penetration depth in interdigital transducers (IDTs) and the velocity dispersion can be modulated by the thickness of the Pt electrode. We also demonstrated that addition of an Al2O3 passivation layer has no effect on the wave modes, but can increase the resonant frequencies. This paper provides a better understanding of the acoustic modes of PtLGS SAW devices, as well as useful guidance for device design. It is believed that the Rayleigh-type SAW and SH-type leaky waves are potentially useful for dual-mode sensing applications in harsh environments, to achieve multi-parameter monitoring or temperature-compensation on a single chip.

Observation of Gyromagnetic Spin Wave Resonance in NiFe Films.

This Letter demonstrates spin wave resonance (SWR) owing to the gyromagnetic effect by propagating a Rayleigh-type surface acoustic wave (R-SAW) through ferromagnetic thin films. The SWR amplitude in a NiFe film shows a higher-order frequency variation than in a magnetoelastic Ni film. This frequency dependence is well understood in terms of the presence of a gyromagnetic field attributable to the local lattice rotation in the R-SAW. From the frequency dependence of the SWR amplitude, the gyromagnetic SWR could be separated from another SWR caused by a magnetoelastic effect of the ferromagnet.

Excitation of Ferromagnetic Resonance Using Surface Acoustic Waves

SUMMARYMechanical rotation of a crystal lattice in ferromagnetic materials can be energetically coupled with its magnetization via magnetoelastic coupling or spin rotation coupling. Surface acoustic wave (SAW) in piezoelectric materials is one of the promising candidates to realize the mechanical excitation of magnetization dynamics. In order to understand the mechanical rotation induced magnetization dynamics quantitatively, we examined the ferromagnetic resonance in an Ni film using a SAW in a LiNbO3 substrate. By decreasing a period of interdigital transducer as short as 4 μm which is one‐fifth of the previous work by Weiler [M. Weiler and co‐workers, Elastically driven ferromagnetic resonance in Nickel thin films. Phys Rev Lett 106, 117601 (2011)], the fundamental frequency of SAW could be higher than 800 MHz. From the dependence of microwave absorption on the angle between the magnetization and the wave vector of SAW, it was confirmed that the Rayleigh type SAW, which was significant to obtain a large mechanical coupling with the magnetization, was dominantly excited in the 800‐MHz‐SAW device.

AlN/Pt/LN structure for SAW sensors capable of operating at high temperature

There is a need for wireless sensors able to operate in the intermediate temperature range (ITR) between 300 °C and 600 °C. Surface acoustic wave (SAW) sensors are promising candidates to solve this issue. However, existing SAW sensors most often fail in the ITR, due to the quick degradation of the sensor housing in extreme conditions. A promising way to circumvent the issue is to use “package-less” devices, where the acoustic waves are guided in a multilayered structure where they are intrinsically protected from adverse environmental effects. We present here an innovative multilayered structure that fulfills all the basic requirements, to achieve a wireless and “package-less” SAW Sensor for the ITR. The structure is made of a thin AlN layer deposited on top of a Y + 128°LN substrate and equipped with buried Pt electrodes. Numerical simulations of the acoustic waves propagating in SAW resonators built on this structure reveal the existence of a useful Rayleigh-type SAW that propagates at the AlN/LN interface with a velocity up to 4500 m/s and a high electromechanical coupling k2=5.6%, without leakage into the substrate. The existence of this mode is due to specific properties of the Y + 128°LN cut, which are analyzed in detail in this paper. The performances of an optimized AlN/Pt/LN structure are also compared to the ones of previously suggested “package-less” structures, including AlN/ZnO/Sapphire. It is shown that better device characteristics can be expected from the AlN/Pt/LN structure in the ITR.

Evaluation of piezoelectric Ta2O5 thin films deposited on SrTiO3 substrates

Tantalum pentoxide (Ta2O5) piezoelectric thin films were deposited on (100) and (110) planes of strontium titanate (SrTiO3) substrates using an RF magnetron sputtering system. The crystallinity and Rayleigh-type surface acoustic wave propagation properties of the thin films were evaluated. The possibility that the Ta2O5 thin films were crystallized to orthorhombic Ta2O5 on SrTiO3 substrates by providing a buffer layer deposited without applying O2 radical power was demonstrated. From measured pole figures, it is considered that the crystalline Ta2O5/SrTiO3(100) sample has two lattice arrangements orthogonal to each other, and the crystalline Ta2O5/SrTiO3(110) sample has a single lattice arrangement. For a normalized film thickness h/λ of 0.45, the propagation loss for the deposition on SrTiO3(110) was 0.02 dB/λ, whereas that for the X-axis-oriented Ta2O5 thin film on SiO2 substrate was 0.21 dB/λ. Unfortunately, the coupling factor of the crystalline thin film was less than 0.3% for h/λ ranging from 0.29 to 0.45.

A novel method of studying the micro-contact using surface acoustic wave sensor

Purpose This paper aims to investigate the response behavior of the surface acoustic wave (SAW) sensor under the loading of micro-particles and to evaluate the feasibility of using the SAW sensor to study the micro-contact of the particle–plane interface. Design/methodology/approach An analytical perturbation theory of the coupled system of particle and SAW is presented. It shows that in the weak-coupling regime, the SAW sensor detects the coupling stiffness rather than the additional mass of the particle at the interface. The frequency perturbation formula expressed in parameters of the geometry and mechanical properties of the contact is further derived. The frequency shift of a 262-MHz Rayleigh-type SAW in the oscillation configuration under the loading of multiple starch particles of different sizes has been measured. Findings The experiment results of a linear relationship between the frequency increase and the sum of the radius of particles to the power of 2/3 verified the validity of the theory of linking the SAW response to the geometry and mechanical properties of the contact. Originality/value The SAW sensor could serve as a new candidate for studying the details of mechanical properties of the micro-contact of the interface.

Finite element method analysis of surface acoustic wave devices with microcavities for detection of liquids

This paper introduces the use of finite element method analysis tools to investigate the use of a Rayleigh type surface acoustic wave (SAW) sensor to interrogate minute amounts of liquids trapped in microcavities placed on the delay line. Launched surface waves in the ST-X quartz substrate couple to the liquid and emit compressional waves. These waves form a resonant cavity condition and interfere with the surface waves in the substrate. Simulations show that the platform operates in a different mechanism than the conventional mass loading of SAW devices. Based on the proposed detection mechanism, it is able to distinguish between variations of 40% and 90% glycerin based on phase relations while using liquid volumes smaller than 10 pl. Results from shallow microcavities show high correlation with sound velocity parameter of the liquid whereas deeper microcavities display high sensitivities with respect to glycerin concentration. Simulated devices yield a maximum sensitivity of −0.77°/(% glycerin) for 16 μm wavelength operation with 8 μm deep, 24 μm wide, and 24 μm long microcavities.

Evaluation of Piezoelectric Ta2O5 Thin Films Deposited on Sapphire Substrates

X-axis-oriented tantalum pentoxide (Ta2O5) piezoelectric thin films were deposited on sapphire (Al2O3) substrates, from which single crystallization due to epitaxial growth can be expected, using an RF magnetron sputtering system. The crystallinity and Rayleigh-type surface acoustic wave (R-SAW) propagation properties of the thin films were evaluated. From the measured diffraction (X-ray diffraction) patterns and the spotted pattern in the measured pole figures, in which poles were arranged to form the vertices of a hexagon, the possibility of the crystallization of hexagonal Ta2O5 with a (203)-plane oriented in the c-Al2O3 substrate plane due to epitaxial growth was shown. For the first mode of the R-SAW on the Ta2O5/R-plane Al2O3 sample, a coupling factor of 1.65% and a phase velocity of 5,120 m/s were obtained for a normalized thickness of 0.175. Unfortunately, no increase in coupling factor and no major improvement in propagation loss were observed upon the crystallization of hexagonal Ta2O5.

Removal of living cells from biosensing surfaces in droplet-based microfluidics using surface acoustic waves

Removal of living biological cells from surfaces is a critical process for many applications in the area of biosensing and lab-on-a-chip. Trypsin is one of the most effective biochemical tools used to cleave the cells proteins that are responsible for bonding cells to surfaces [K. A. Walsh, Meth. Enzymol. 19, 41 (1970)]. We propose a method using Rayleigh-type (20 MHz) surface acoustic wave (SAW)-based mixing [Renaudin et al., Lab Chip 10, 111 (2010)] as an accelerator for trypsin-mediated removal of living cells from surfaces. In the experiments, a 10 μL droplet of Hank’s Balanced Salt Solution (HBSS)-trypsin is placed on a piezoelectric substrate covered with human embryonic kidney cells (HEK293). Using phase contrast microscopy, cells removal time for different acoustic power levels and trypsin concentrations is measured. Results from validation experiments show that a minimum of 180 s is necessary to completely release surface-bonded cells covered by the 10 µL droplet without the use of SAW (negative control). By using microstreaming flow in the droplets generated by the SAW, cells are released from the surface in less than 8 s. This work will contribute to improved lab-on-a-chip devices based on living cell biosensing.

Surface Acoustic Wave Properties of Amorphous Ta2O5 and Nb2O5 Thin Films Prepared by Radio Frequency Sputtering

The fundamental acoustic properties of amorphous Ta2O5 and Nb2O5 thin films prepared by RF sputtering were evaluated using a Rayleigh-type surface acoustic wave (SAW) on 128° YX-LiNbO3 and a shear-horizontal-type SAW on 36° YX-LiTaO3. The elastic constants c11 and c44 of the thin films were determined from the measured phase velocities of two SAW modes with mutually perpendicular particle motion. Although the elastic constants of the Ta2O5 thin film were found to be approximately 14–17% larger than those of the Nb2O5 thin film, it was clarified that the Ta2O5 thin film has a stronger SAW trapping effect than the Nb2O5 thin film because the density of the former was measured to be 1.5 times higher than that of the latter. On the other hand, the Nb2O5 thin film has a smaller propagation loss for a Rayleigh-type SAW, a lower temperature coefficient of delay, and a higher refractive index than the Ta2O5 thin film.

Surface Acoustic Wave Properties of Amorphous Ta2O5and Nb2O5Thin Films Prepared by Radio Frequency Sputtering

The fundamental acoustic properties of amorphous Ta2O5 and Nb2O5 thin films prepared by RF sputtering were evaluated using a Rayleigh-type surface acoustic wave (SAW) on 128° YX-LiNbO3 and a shear-horizontal-type SAW on 36° YX-LiTaO3. The elastic constants c11 and c44 of the thin films were determined from the measured phase velocities of two SAW modes with mutually perpendicular particle motion. Although the elastic constants of the Ta2O5 thin film were found to be approximately 14–17% larger than those of the Nb2O5 thin film, it was clarified that the Ta2O5 thin film has a stronger SAW trapping effect than the Nb2O5 thin film because the density of the former was measured to be 1.5 times higher than that of the latter. On the other hand, the Nb2O5 thin film has a smaller propagation loss for a Rayleigh-type SAW, a lower temperature coefficient of delay, and a higher refractive index than the Ta2O5 thin film.

Fabrication and Evaluation of Highly Oriented Ta2O5 Piezoelectric Thin Films Prepared by Radio Frequency Magnetron Sputtering

Highly X-axis-oriented tantalum pentoxide (Ta2O5) piezoelectric thin films were deposited on a SiO2 substrate using an RF-magnetron sputtering system with a metal tantalum target and an O2-radical source. The degree of orientation, Rayleigh-type surface acoustic wave properties, and surface morphology were evaluated. The deposition condition with the substrate temperature TS of 700 °C and O2 flow rate of 10 ccm was found to be optimum for obtaining a strongly piezoelectric property. Under the optimum condition, the coupling factor of the oriented Ta2O5 thin film with a normalized thickness h/λ of 0.21 was determined to be 0.88% and was 75% of the reported value. The diffraction angle of the preferential peak under the optimum condition was equal to that of the (200)-plane spacing d(200) in the unit cell of monoclinic Ta2O5. A larger plane spacing with Δd/d(200)=2.9% exists preferentially at TS=600 °C, and the piezoelectricity is considered to be zero or very weak. When TS was higher than 700 °C and the O2 flow rate was more than 8 ccm, a smooth surface with the rms roughness of approximately 8–10 nm was obtained. A correlation was found in which a strongly piezoelectric property was obtained when the thin film had a smooth surface.

A Full-Wave Analysis of Surface Acoustic Waves Propagating on a SiO2 Overlay/Metal Grating/Rotated Y-Cut X-Propagating LiNbO3 Substrate Structure

In this paper, we describe a full-wave analysis of an SiO2 overlay with a finite thickness/metal grating/rotated Y-cut X-propagating LiNbO3 (YX-LN) substrate structure. The effects of the thickness and surface topology of the SiO2 overlay and the metal grating selection upon the excitation and propagation characteristics of acoustic guided modes are discussed in detail. It is shown that in the structure there propagate three acoustic guided modes. One type of mode concentrates the energy near the metal grating and generates piezoelectric boundary acoustic waves (PBAWs) with a relatively large electromechanical coupling factor when the SiO2 thickness becomes semi-infinite. Another mode concentrates its energy near the top surface of the SiO2 overlay and generates the Rayleigh-type surface acoustic wave (SAW) on SiO2 when the thickness is semi-infinite; with an increase in SiO2 thickness, it rapidly loses its piezoelectric coupling. The remaining mode is a series of guided modes trapped in the SiO2 layer, in which the SiO2 layer behaves as a waveguide because of its low acoustic wave velocities.

Characteristics of Reflection of Resonators Using Lamb Wave on AT-Cut Quartz

Lamb waves propagating on an AT-cut quartz substrate were studied. In this study, we propose a high-frequency resonator using a Lamb wave. A Lamb wave is excited on an extremely thin AT-cut quartz substrate which thickness is comparable to the wavelength of an elastic wave propagating on a substrate. This Lamb wave propagates at a higher phase velocity mode than surface acoustic waves. This realizes an RF resonator operable in a high frequency range in which a conventional quartz resonator hardly operates. A theoretical analysis of the reflection coefficient of a Lamb wave with a grating reflector was conducted. The analysis results and experimental results are compared and discussed in this study. A reflection coefficient which is 10 to 20 times higher than that for a Rayleigh-type surface acoustic wave is obtained.

Effects of Dielectric Overlay upon Surface Acoustic Wave Propagation under Metallic Grating Structure

This paper describes surface acoustic wave (SAW) propagation under a periodic metallic grating structure overlaid with an SiO2 film. An analysis was made by modifying the software FEMSDA, in which the finite element method is applied to both the grating electrodes and SiO2 overlay. For Rayleigh-type SAW propagating on 128°YX-LiNbO3, changes in the reflection coefficient and effective velocity are calculated as a function of the SiO2 film thickness overlaying an Al grating structure. It is shown that the SiO2 film overlaying the top surface of the grating electrodes affects both the SAW reflection coefficient and effective SAW velocity more significantly than the film overlaying the electrode gaps.