Surface Acoustic Wave Intensity Research Articles

Study of electronic band structure in a 75-nm-wide n-GaAs/AlGaAs quantum well by surface acoustic waves

From measured magnetic field dependences of attenuation and velocity of surface acoustic waves (SAW), we calculated the real part of electron the conductivity in a 75-nm-wide n-GaAs/AlGaAs quantum well in magnetic fields of up to 18 T at frequencies of 30–300 MHz in the temperature range 20–500 mK. We observed slow magneto-oscillations of the conductivity depended on SAW intensity and occurred due to elastic intersubband scattering between the symmetric (S) and antisymmetric (AS) subbands formed by Coulomb repulsion between the electrons. Our theoretical description of the magneto-oscillations allowed estimating the quantum and the intersubband scattering times. The experimentally obtained subband splitting value 0.42 meV was close to the theoretically found 0.57 meV. Additionally, in tilted magnetic fields of 0.5–1.5, T we observed crossings of the Landau levels between the S and AS subbands. According to the developed theory, the crossings at the in-plane field below 1.5 T occur due to different dependences of cyclotron energies in the subbands on the in-plane field. Namely, the cyclotron energy in the S (AS) subband decreases (increases) with the rising of the in-plane field. Work supported by “BASIS,” NSF DMR-1644779 and DMR-1420541, GBMF4420, and State of Florida.]

Compact acousto-optic multimode interference device in (Al,Ga)As.

Multimode interference (MMI) devices are key components in modern integrated photonic circuits. Here, we present acoustically tuned optical switches on an (Al,Ga)As platform that enable robust, compact and fast response systems improving on recently demonstrated technology. The device consists of a 2 × 2 MMI device fine-tuned in its center region by a focused surface acoustic wave (SAW) beam working in the low GHz range. In this way, we can tune the refractive index profile over a narrow modulation region and thus control the optical switching behaviour via the applied SAW intensity. Direct tuning of the MMI device avoids losses and phase errors inherent to arrayed waveguide based switches, while also reducing the dimensions of the photonic circuit.

Inducing Mild Traumatic Brain Injury in C. elegans via Cavitation-Free Surface Acoustic Wave-Driven Ultrasonic Irradiation

Mild traumatic brain injury is an all-too-common outcome from modern warfare and sport, and lacks a reproducible model for assessment of potential treatments and protection against it. Here we consider the use of surface acoustic wave (SAW) irradiation of C. elegans worms—without cavitation—as a potential, ethically reasonable animal-on-a-chip model for inducing traumatic brain injury in an animal, producing significant effects on memory and learning that could prove useful in a model that progress from youth to old age in but a few weeks. We show a significant effect by SAW on the ability of worms to learn post-exposure through associative learning chemotaxis. At higher SAW intensity, we find immediate, thorough, but temporary paralysis of the worms. We further explore the importance of homogeneous exposure of the worms to the SAW-driven ultrasound, an aspect poorly controlled in past efforts, if at all, and demonstrate the absence of cavitation through a change in fluids from a standard media for the worms to the exceedingly viscous polyvinyl alcohol. Likewise, we demonstrate that acoustic streaming, when present, is not directly responsible for paralysis nor learning disabilities induced in the worm, but is beneficial at low amplitudes to ensuring homogeneous ultrasound exposure.

Acoustoelectric Current in Graphene Nanoribbons

Surface acoustic waves (SAWs) propagating on piezoelectric substrates offer a convenient, contactless approach to probing the electronic properties of low-dimensional charge carrier systems such as graphene nanoribbons (GNRs). SAWs can also be used to transport and manipulate charge for applications such as metrology and quantum information. In this work, we investigate the acoustoelectric effect in GNRs, and show that an acoustoelectric current can be generated in GNRs with physical widths as small as 200 nm at room temperature. The positive current in the direction of the SAWs, which corresponds to the transportation of holes, exhibits a linear dependence on SAW intensity and frequency. This is consistent with the description of the interaction between the charge carriers in the GNRs and the piezoelectric fields associated with the SAWs being described by a relatively simple classical relaxation model. Somewhat counter-intuitively, as the GNR width is decreased, the measured acoustoelectric current increases. This is thought to be caused by an increase of the carrier mobility due to increased doping arising from damage to the GNR edges.

An extended Landau–Levich model for the dragging of a thin liquid film with a propagating surface acoustic wave

In this paper we revisit the Landau and Levich analysis of a coating flow in the case where the flow in the thin liquid film is supported by a Rayleigh surface acoustic wave (SAW), propagating in the solid substrate. Our theoretical analysis reveals that the geometry of the film evolves under the action of the propagating SAW in a manner that is similar to the evolution of films that are being deposited using the dip coating technique. We show that in a steady state the thin-film evolution equation reduces to a generalized Landau–Levich equation with the dragging velocity, imposed by the SAW, depending on the local film thickness. We demonstrate that the generalized Landau–Levich equation has a branch of stable steady state solutions and a branch of unstable solutions. The branches meet at a saddle-node bifurcation point corresponding to the threshold value of the SAW intensity. Below the threshold value no steady states were found and our numerical computations suggest a gradual thinning of the liquid film from its initial geometry.

Acoustoelectric photoresponse in graphene

The acoustoelectric current in graphene has been investigated as a function of illumination, using blue (450 nm) and red (735 nm) light-emitting diodes (LEDs), and surface acoustic wave (SAW) intensity and frequency. The measured acoustoelectric current increases with illumination, more than the measured change in the conductivity of the graphene, whilst retaining a linear dependence on the SAW intensity. The latter is consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model suggesting that the change in the acoustoelectric current is caused by the effect of the illumination on the electronic properties of the graphene. The increase in the acoustoelectric current is greatest under illumination with the blue LED, consistent with the creation of a hot electron distribution.

Temperature dependence of the acoustoelectric current in graphene

The acoustoelectric current in graphene has been investigated as a function of temperature, surface acoustic wave (SAW) intensity, and frequency. At high SAW frequencies, the measured acoustoelectric current decreases with decreasing temperature, but remains positive, which corresponds to the transport of holes, over the whole temperature range studied. The current also exhibits a linear dependence on the SAW intensity, consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model. At low temperatures and SAW frequencies, the measured acoustoelectric current no longer exhibits a simple linear dependence on the SAW intensity, and the direction of the acoustoelectric current is also observed to reverse under certain experimental conditions.

Acoustic modulation of droplet size in a T-junction

We introduce an approach and describe the process of acoustically driven formation of droplets in a microfluidic T-junction. Our system allows for fast and precise control of drop volume over a wide range that is fully electrically triggered. We exploit the interaction of a surface acoustic wave (SAW) excited on a piezoelectric, transparent substrate with the fluid to adjust the size of drops in a continuous microflow in real time and relate SAW intensity and drop size. Our device operates in the squeezing regime at low capillary numbers. We describe the mechanism of SAW modulated formation of a monodisperse microemulsion that forms the basis for the integration of more complex operations useful for droplet fluidic systems.

Macroscopic acoustoelectric charge transport in graphene

We demonstrate macroscopic acoustoelectric transport in graphene, transferred onto piezoelectric lithium niobate substrates, between electrodes up to 500 μm apart. Using double finger interdigital transducers we have characterised the acoustoelectric current as a function of both surface acoustic wave intensity and frequency. The results are consistent with a relatively simple classical relaxation model, in which the acoustoelectric current is proportional to both the surface acoustic wave intensity and the attenuation of the wave caused by the charge transport.

Depth Evaluation of Fretting Fatigue Crack Appearing at Bolt Joints of Aluminum Alloy Plates by Synchronized Surface Acoustic Wave Measurement

The surface acoustic wave (SAW) measurement was carried out for the bolted specimen of aluminum alloy by synchronizing the ultrasonic pulsation with a fatigue cycle in the fatigue testing. The reflection from the crack increased in the loading stage of the fatigue cycle, reached maximum and then decreased in the unloading stage of the fatigue cycle. The sigmoidal intensity change of the reflection from the fatigue crack in a fatigue cycle occurred following opening/closing behavior of the crack, and the intensity took the maximum value when the crack was fully opened. The SAW intensity distribution between the crack and the bolt hole edge was also analyzed to evaluate the depth of the crack: the SAW intensity distribution after the crack root was subjected to the band-pass filtering to recognize the creeping wave (the longitudinal wave) reflected from the crack tip, and then the crack depth was estimated from the propagation time of the creeping wave from the crack root to the crack tip, which was in good agreement with the value measured in the cross section of the specimen and evaluated through the scanning acoustic analysis.

Acoustically Induced Spin-Orbit Interactions Revealed by Two-Dimensional Imaging of Spin Transport in GaAs

Magneto-optic Kerr microscopy was employed to investigate the spin-orbit interactions of electrons traveling in semiconductor quantum wells using surface acoustic waves (SAWs). Two-dimensional images of the spin flow induced by SAWs exhibit anisotropic spin precession behaviors caused by the coexistence of different types of spin-orbit interactions. The dependence of spin-orbit effective magnetic fields on SAW intensity indicates the existence of acoustically controllable spin-orbit interactions resulting from the strain and Rashba contributions induced by the SAWs.

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.

Actuating and detecting of microdroplet using slanted finger interdigital transducers

Using surface acoustic wave (SAW) to drive droplets on a piezoelectric chip is a distinctive microfluidic technique developed recently. In the previous research, uniform interdigital transducers (IDTs) were used to radiate SAW amplitude and move the droplets within the same delay line simultaneously. In this paper, we employ slanted finger interdigital transducers (SFITs) to replace uniform IDTs. The merit is that we can move individual droplets at will by varying the frequency. This is because SAW amplitude profile actuated by a SFIT is not uniform along the aperture. The position of the maximum amplitude can be changed by adjusting the input frequency. In addition, we apply the coupling-of-modes model to simulate the amplitude profile and the frequency response of SFITs. Since the transmitted SAW intensity is strongly attenuated by the liquid when SAW hits the droplet, we can use the measured frequency response to detect the position of the droplet and verify the validity of the proposed method; we fabricated a SFIT on an Y128X-LiNbO3 substrate coated with hydrophobic film. The experimental results show good agreement with those predicted by the simulations. We note that the proposed method can further be used to construct a multichannel microfluidic chip.

High-frequency transport inp-typeSi∕Si0.87Ge0.13heterostructures studied with surface acoustic waves in the quantum Hall regime

The interaction of surface acoustic waves (SAWs) with $p$-type $\mathrm{Si}∕{\mathrm{Si}}_{0.87}{\mathrm{Ge}}_{0.13}$ heterostructures has been studied for SAW frequencies of $30--300\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. For temperatures in the range $0.7<T<1.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and magnetic fields up to $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, the SAW attenuation coefficient $\ensuremath{\Gamma}$ and velocity change $\ensuremath{\Delta}V∕V$ were found to oscillate with filling factor. Both the real ${\ensuremath{\sigma}}_{1}$ and imaginary ${\ensuremath{\sigma}}_{2}$ components of the high-frequency conductivity have been determined and compared with quasi-dc magnetoresistance measurements at temperatures down to $33\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. By analyzing the ratio of ${\ensuremath{\sigma}}_{1}$ to ${\ensuremath{\sigma}}_{2}$, carrier localization can be followed as a function of temperature and magnetic field. At $T=0.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the variations of $\ensuremath{\Gamma}$, $\ensuremath{\Delta}V∕V$, and ${\ensuremath{\sigma}}_{1}$ with SAW intensity have been studied and can be explained by heating of the two-dimensional hole gas by the SAW electric field. Energy relaxation is found to be dominated by acoustic phonon deformation potential scattering with weak screening.

Novel concepts for GaAs/LiNbO/sub 3/ layered systems and their device applications

Thin semiconductor quantum well structures fused onto LiNbO(3 ) substrates using the epitaxial lift-off (ELO) technology offer the possibility of controlling the surface acoustic wave (SAW) velocity via field effect. The tunability of the conductivity in the InGaAs quantum well results in a great change in SAW velocity, in general, accompanied by an attenuation. We show that an additional lateral modulation of the sheet conductivity reduces the SAW attenuation significantly, enhancing device performance. At high SAW intensity the bunching of electrons in the SAW potential also leads to a strong reduction of attenuation. These effects open new possibilities for voltage-controlled SAW devices. We demonstrate a novel, wireless, passive voltage sensor, which can be read out from a remote location.

Interaction of surface acoustic waves with a narrow electron channel in a piezoelectric material

The interaction between a surface acoustic wave (SAW) of wave number $k$ and frequency $\ensuremath{\omega}$ and a two-dimensional electron gas in a piezoelectric semiconductor can be expressed in terms of the longitudinal conductivity ${\ensuremath{\sigma}}_{\mathrm{xx}}(k,\ensuremath{\omega})$ and an effective electromechanical coupling coefficient. The resulting velocity change and the attenuation of the transmitted SAW intensity are well known. In a recent paper, Simon [Phys. Rev. B 54, 13 878 (1996)] calculated the fractional energy change $\ensuremath{\Delta}U/U$ for a SAW interacting with a two-dimensional sheet embedded in a semi-infinite piezoelectric material and obtained a relationship with the results for the attenuation coefficient and the fractional velocity change. In this paper, $\ensuremath{\Delta}U/U$ is calculated for a narrow channel of width ${r}_{\ensuremath{\perp}}$ ${(kr}_{\ensuremath{\perp}}\ensuremath{\ll}1)$ at a distance $d$ below the surface of a slab of piezoelectric material of finite thickness when an elastic wave is launched on the surface. $\ensuremath{\Delta}U/U$ is given as a closed-form expression in terms of the velocity of the elastic wave, the elastic constants, and the piezoelectric tensor. Numerical results are presented for $\ensuremath{\Delta}U/U$ as a function of $\mathrm{kd}$ for several values of the thickness of a slab of ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$.

Nonlinear acoustoelectronic interaction in thin superconducting lead films

The effect of high-power surface acoustic waves (SAW) on the superconducting state of Pb films has been studied in different transverse magnetic fields. It was established that a high-intensity sound wave affects the T c of the film and the character of the superconducting transition, and that the observed changes are the larger, the higher is the SAW intensity. It was found that high-power SAW are capable of inducing vortex depinning in the film and reducing the critical current. Various mechanisms which are responsible for the nature of these effects are discussed.

Sidelobe suppression in an acousto-optic filter with a raised-cosine interaction strength

We have demonstrated a 10-dB reduction in the intensity sidelobes of an apodized-interaction-strength guided-wave acousto-optic filter with a center passband of 1530 nm. Acoustic-intensity weighting was achieved by launching a surface acoustic wave (SAW) beam in a straight acoustic waveguide, and gradually transferring this SAW intensity to the active device, and back out, by evanescent-wave coupling across a 20 μm barrier over a 19-mm interaction length. The intensity sidelobe was −5 dB for an unapodized filter with abrupt onset and cutoff of the interaction, but sidelobes were reduced to at most −15 dB for a SAW intensity with raised-cosine weighting. A conversion efficiency of 97% was achieved with 80 mW input power for the 2.0-nm-wide apodized filter.

Phase-shifted surface-acoustic-wave resonator

The spatial distribution of the surface-acoustic-wave (SAW) intensity across a phase-shifted transducer-resonator is studied. A coupled-mode analysis of this structure shows that a highly confined mode can be excited in a pi/2 shifted resonator. Experimental verification of this conclusion was obtained in a LiNbO(3) pi/2 shifted resonator, in which the SAW intensity was measured. The concentration factor (the ratio of the maximum to average SAW intensity) can be ~10 for reasonably long resonators.