Janus acoustic metascreen with nonreciprocal and reconfigurable phase modulations

Integrating different reliable functionalities in metastructures and metasurfaces has become of remarkable importance to create innovative multifunctional compact acoustic, optic or mechanical metadevices. In particular, implementing different wave manipulations in one unique material platform opens an appealing route for developing integrated metamaterials. Here, the concept of Janus acoustic metascreen is proposed and demonstrated, producing two-faced and independent wavefront manipulations for two opposite incidences. The feature of two-faced sound modulations requires nonreciprocal phase modulating elements. An acoustic resonant unit cell with rotating inner core, which produces a bias by a circulating fluid, is designed to achieve high nonreciprocity, leading to decoupled phase modulations for both forward and backward directions. In addition, the designed unit cell consisting of tunable phase modulators is reconfigurable. A series of Janus acoustic metascreens including optional combinations of extraordinary refraction, acoustic focusing, sound absorption, acoustic diffusion, and beam splitting are demonstrated through numerical simulations and experiments, showing their great potential for acoustic wavefront manipulation.

Acoustic Focusing with Intensity Modulation Based on Sub-Wavelength Waveguide Array

Acoustic focusing with intensity modulation plays an important role in biomedical and life sciences. In this work, we propose a new approach for simultaneous phase and amplitude manipulation in sub-wavelength coupled resonant units, which has not been reported so far. Based on the equivalent impedance and refractive index modulation induced by the change of geometry, arbitrary amplitude response from 0 to 1 and phase shift from 0 to 2π is realized. Thus, the acoustic focusing with intensity modulation can be achieved via waveguide array. Herein, the focal length can be adjusted by alternating the length of supercell, and the whole system can work in a broadband of 0.872f0–1.075f0. By introducing the coding method, the thermal viscosity loss is reduced, and the wavefront modulation can be more accurate. Compared with previous works, our approach has the advantages of simple design and broadband response, which may have promising applications in acoustic communication, non-destructive testing, and acoustic holography.

Research on the Processing Method of Acoustic Focusing Cavities Based on the Temperature Gradient

Aiming at the key factors affecting the quality and efficiency of high-energy in-beam machining, this paper studies the broadband acoustic focusing effect based on a discrete temperature gradient. Firstly, the basic theory and mathematical model of temperature-controlled acoustic focusing are established. Secondly, the acoustic focusing effect is achieved by combining the design of metasurfaces and discrete temperature. Then, the acoustic pressure and intensity distribution of acoustic focusing under a discrete temperature gradient are simulated and experimentally studied. The results show that the phase delay of transmission and reflection of acoustic wave covers the 2π interval by changing the temperature in different transmission units, which provides a theoretical basis for the processing of the acoustic focusing cavity.

High-Sensitivity Optical-Resolution Photoacoustic Microscopy with an Optical-Acoustic Combiner Based on an Off-Axis Parabolic Acoustic Mirror

Optical-resolution photoacoustic microscopy (OR-PAM) is a promising noninvasive biomedical imaging technology with label-free optical absorption contrasts. Performance of OR-PAM is usually closely related to the optical-acoustic combiner. In this study, we propose an optical-acoustic combiner based on a flat acoustic reflector and an off-axis parabolic acoustic mirror with a conical bore. Quantitative simulation and experiments demonstrated that this combiner can provide better acoustic focusing performance and detection sensitivity. Moreover, OR-PAM is based on the combiner suffer low optical disorders, which guarantees the good resolution. In vivo experiments of the mouse brain and the iris were also conducted to show the practicability of the combiner in biomedicine. This proposed optical-acoustic combiner realizes a high-quality optical-acoustic confocal alignment with minimal optical disorders and acoustic insertion loss, strong acoustic focusing, and easy implementation. These characteristics might be useful for improving the performance of OR-PAM.

Acoustic focusing of beads and cells in hydrogel droplets

The generation of hydrogel droplets using droplet microfluidics has emerged as a powerful tool with many applications in biology and medicine. Here, a microfluidic system to control the position of particles (beads or astrocyte cells) in hydrogel droplets using bulk acoustic standing waves is presented. The chip consisted of a droplet generator and a 380 µm wide acoustic focusing channel. Droplets comprising hydrogel precursor solution (polyethylene glycol tetraacrylate or a combination of polyethylene glycol tetraacrylate and gelatine methacrylate), photoinitiator and particles were generated. The droplets passed along the acoustic focusing channel where a half wavelength acoustic standing wave field was generated, and the particles were focused to the centre line of the droplets (i.e. the pressure nodal line) by the acoustic force. The droplets were cross-linked by exposure to UV-light, freezing the particles in their positions. With the acoustics applied, 89 ± 19% of the particles (polystyrene beads, 10 µm diameter) were positioned in an area ± 10% from the centre line. As proof-of-principle for biological particles, astrocytes were focused in hydrogel droplets using the same principle. The viability of the astrocytes after 7 days in culture was 72 ± 22% when exposed to the acoustic focusing compared with 70 ± 19% for samples not exposed to the acoustic focusing. This technology provides a platform to control the spatial position of bioparticles in hydrogel droplets, and opens up for the generation of more complex biological hydrogel structures.