Scattering of surface acoustic waves by a phononic crystal revealed by heterodyne interferometry

Surface acoustic wave propagation within a two-dimensional phononic band gapstructure has been studied using a heterodyne laser interferometer.Acoustic waves are launched by interdigital transducers towards a square lattice of holes etched in a piezoelectric medium. Interferometer measurements performed at frequencies lying below, within, and above the expected band gap frequency range provide direct information of the wave interaction with the phononic crystal, revealing anisotropic scattering into higher diffraction orders depending on the apparent grating pitch at the boundary between the phononic crystal and free surface. Furthermore, the measurements also confirm the existence of an elastic band gap, in accordance with previous electrical measurements and theoretical predictions.Peer reviewe

'Forced to think'—an analysis of how students benefit from online warm-up exercises with feedback

Active learning yields better learning outcomes than traditional, lecture-based teaching. Common approaches in large lecture courses are activating elements during the lectures and warm-up activities using online learning environments. However, implementing warm-up exercises, on which students work on by themselves makes formative feedback increasingly important. We have studied students’ views on warm-up online exercises, focusing on the role of automated feedback. We compared two types of feedback: feedback which guided the student towards the correct answer and feedback that guided the student towards the correct reading materials. We have also examined the correlation between students’ views on the exercises and learning outcomes, as measured by exercise and exam points. The exercises were administered with a Moodle-based system focusing on teaching and assessment using a computer-based algebra kernel. The study was conducted during the first introductory mechanics course in physics at the University of Helsinki. We concluded that the preferred type of feedback was dependent on the student, i.e. different students prefer different kinds of feedback. Importantly, the students who completed the exercises alone reported benefiting more from them, which shows that the warm-up exercises successfully extended support for students outside of classroom hours.Peer reviewe

Nonlinear excitation of a rotational mode in a piezoelectrically excited square-extensional mode resonator

We present an experimental study of the nonlinear behavior of a square-extensional (SE) mode microelectromechanicalresonator, actuated with a piezoelectricAlNthin film. The acoustic vibration fields of the device are characterized using laser probing. A nonlinear vibration behavior of the SE mode is observed above a drive power level of −10 dBm such that the vibration amplitude of the SE mode saturates and a rotational in-plane vibration mode is excited at a significantly lower frequency (0.725 MHz) than the SE mode (16.670 MHz). Interestingly, the measured∼10 nm saturation amplitude of the SE mode is more than a decade below the amplitude value at which mechanical or electromechanical nonlinearities are estimated to become significant.Peer reviewe

Frequency splitting of the main mode in a microelectromechanical resonator due to coupling with an anchor resonance

We present an experimental study of the frequency scaling of the main, square-extensional mode in a piezoelectrically actuated plate resonator. The studied set consists of resonators of different plate sizes with identical anchors. The behavior of the square-extensional mode is analyzed using electrical impedancemeasurements and optical characterization of the mechanical vibration fields. The results reveal a detrimental anchor effect, where for certain plate sizes the square-extensional mode branch is split into two due to a coupled oscillation of the resonator plate and the anchors. Peer reviewe

Scanning heterodyne laser interferometer for phase-sensitive absolute-amplitude measurements of surface vibrations

We describe a scanning heterodyneinterferometer for imaging surface vibrations with a wide frequency range, with current electronics, up to 6GHz. The heterodyne operation facilitates measurement of absolute amplitude and phase of the surface vibration without calibration. Currently, the setup allows detection of vibration amplitudes down to ∼1pm with a lateral resolution of <1μm. The interferometer is designed to accommodate the different sample types, e.g., surface and bulk acoustic wave devices and micromechanical resonators. The absolute-amplitude and phase information allows for a thorough characterization of surface vibrations in such components and provides direct information of the vibration fields not obtainable via electrical measurements.inPeer reviewe

Phase sensitive absolute amplitude detection of surface vibrations using homodyne interferometry without active stabilization

A detection scheme for obtaining phase and absolute amplitude information of surface vibrations on microacoustic components using homodyne laser interferometry is described. The scheme does not require active stabilization of the optical path length of the interferometer. The detection setup is realized in a homodyneMichelson interferometer configuration, and selected measurements on a 374 MHz surface acoustic wave fan-shaped filter and two different piezoelectrically actuated micromechanical resonators are presented to demonstrate the performance of the instrument. With the current detection electronics, the interferometer is capable of detecting out-of-plane surface vibrations up to 2 GHz with a lateral resolution of better than 1 μm and with a minimum detectable vibration amplitude of ∼1 pm.Peer reviewe

Thrombectomy in acute ischemic stroke in the extended time window : Real-life experience in a high-volume center

Objectives: Selected patients with acute ischemic stroke (AIS) caused by proximal middle cerebral artery (MCA) or internal carotid artery occlusion benefit from endovascular thrombectomy (EVT) in extended time window (6-24 h from last seen well) based on two landmark randomized controlled trials (RCTs) DAWN and DEFUSE-3. We evaluated patients' outcome in the real-life with the focus on adherence to protocol of the two RCTs. Materials and methods: We included consecutive patients with AIS (excluding basilar artery occlusions) referred to EVT in our stroke center in the extended time window between January 2018 and December 2019 and compared the outcome of patients who fulfilled criteria of the RCTs with those who did not. Results: Of the total of 100 patients, 23 complied with RCT's criteria and 18 presented with minor non-adherence (lower NIHSS score or longer treatment delay), whereas 22 patients had large baseline ischemia (>1/3 MCA), 28 presented with M2 and more distal occlusions, and 9 patients did not undergo perfusion imaging prior to EVT. Good 3-month outcome (modified Rankin Scale 0-2) was observed in 54% of those who either met the RCT criteria or presented with lower NIHSS score or longer treatment delay, but only in 30% of M2 occlusions, and in none of the patients with large baseline ischemia. Conclusions: Our findings highlight the impact of mostly large baseline ischemia but also vessel status when selecting patients for EVT in the extended time window and emphasize the need for further data in these patient subgroups. (c) 2022 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)Peer reviewe

Material anisotropy unveiled by random scattering of surface acoustic waves

We consider launching a monochromatic surface acoustic wave packet on a large set of random scatterers. The interference of the multiple scatteredwaves creates a random pattern of ripples on the crystal surface that is recorded by optical interferometry. The Fourier transform of the amplitude and phase data of the measured wave field unveils the complete slowness curve, i.e., the wave-vector as a function of the propagation angle. A simple acoustic speckle model is proposed to explain this observation.Peer reviewe