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Given the enhanced channel capacity of wave chirality, acoustic communications based on the orbital angular momentum (OAM) of acoustic-vortex (AV) beams are of significant interest for underwater data transmissions. However, the stringent beam alignment is required for the coaxial arrangement of transceiver arrays to ensure the accuracy and reliability of OAM decoding. To avoid the required multiple measurements of the traditional orthogonality based algorithm, the beam alignment algorithm based on the OAM spectrum decomposition is proposed for AV communications by using simplified ring-arrays. Numerical studies of the single-OAM and OAM-multiplexed AV beams show that the error of the OAM spectrum increases with the translation distance and the deflection angle of the transceiver arrays. To achieve an ideal arrangement, two methods of the single-array translation alignment and the dual-array deflection alignment are developed based on the least standard deviation of the OAM spectrum (SD-OAM). By decreasing the SD-OAM towards zero using transceiver arrays of 16 transmitters and 16 receivers, accurate beam alignments are accomplished by multiple adjustments in three dimensions. The proposed method is also demonstrated by experimental measurements of the OAM dispersion and the SD-OAM for misaligned beams. The results demonstrate the feasibility of the rapid beam alignment based on the OAM spectrum decomposition by using simplified transceiver ring-arrays, and suggest more application potentials for acoustic communications.

Contactless manipulation of multi-scale objects using the acoustic vortex (AV) tweezers offers tremendous perspectives in biomedical applications. However, it is still hindered by the weak acoustic radiation force (ARF) and torque (ART) around the vortex center. By introducing the elevation angle to the planar transducers of an N-element ring array, the weak-focused acoustic vortex (WFAV) composed of a main-AV and N paraxial-AVs is constructed to conduct a large-scale object manipulation. Different from the traditional focused AV (FAV) generated by a ring array of concave spherical transducers, a much larger focal region of the WFAV is generated by the main lobes of the planar transducers with the size inversely associated with the elevation angle. With the pressure simulation of the acoustic field, the capability of the rotational object driving in the focal plane for the WFAV is analyzed using the ARF and the ART exerted on an elastic ball based on acoustic scattering. With the experimental system built in water, the generation of the WFAV is verified by the scanning measurements of the acoustic field and the capability of object manipulation is also analyzed by the rotational trapping of floating particles in the focal plane. The favorable results demonstrate the feasibility of large-scale rotational manipulation of objects with a strengthened ART and a reduced acousto-thermal damage to biological tissues, showing a promising prospect for potential applications in clinical practice.

To solve the difficulty of generating an ideal Bessel beam, an simplified annular transducer model is proposed to study the axial acoustic radiation force (ARF) and the corresponding negative ARF (pulling force) exerted on centered elastic spheres for acoustic-vortex (AV) beams of arbitrary orders. Based on the theory of acoustic scattering, the axial distributions of the velocity potential and the ARF for AV beams of different orders generated by the annular transducers with different physical sizes are simulated. It is proved that the pulling force can be generated by AV beams of arbitrary orders with multiple axial regions. The pulling force is more likely to exert on the sphere with a smaller k 0 a (product of the wave number and the radius) for the AV beam with a bigger topological charge due to the strengthened off-axis acoustic scattering. The pulling force decreases with the increase of the axial distance for the sphere with a bigger k 0 a. More pulling force areas with wider axial regions can be formed by AV beams using a bigger-sized annular transducer. The theoretical results demonstrate the feasibility of generating the pulling force along the axes of AV beams using the experimentally applicable circular array of planar transducers, and suggest application potentials for multi-position stable object manipulations in biomedical engineering.

Due to the unique magnetic, mechanical and thermal properties, magnetic nanoparticles (MNPs) have comprehensive applications as the contrast and therapeutic agents in biomedical imaging and magnetic hyperthermia. The linear and nonlinear magnetoacoustic responses determined by the magnetic properties of MNPs have attracted more and more attention in biomedical engineering. By considering the relaxation time of MNPs, we derive the formulae of second harmonic magnetoacoustic responses (2H-MARs) for a cylindrical MNP solution model based on the mechanical oscillations of MNPs in magnetoacoustic tomography with magnetic induction (MAT-MI). It is proved that only the second harmonic magnetoacoustic oscillations can be generated by MNPs under an alternating magnetic excitation. The acoustic pressure of the 2H-MAR is proportional to the square of the magnetic field intensity and exhibits a linear increase with the concentration of MNPs. Numerical simulations of the 2H-MAR are confirmed by the experimental measurements for various magnetic field intensities and solution concentrations using a laser vibrometer. The favorable results demonstrate the feasibility of the harmonic measurements without the fundamental interference of the electromagnetic excitation, and suggest a new harmonic imaging strategy of MAT-MI for MNPs with enhanced spatial resolution and improved signal-to-noise ratio in biomedical applications.

Surface plasmons (SPs) in ultraviolet (UV) have attracted a great deal of attention because of their emerging applications in energy resources, environmental protection, and biotechnology. In this article, the dielectric loaded surface plasmon polariton (DLSPP) properties of the Al–Al nanostructure are investigated theoretically. Sharp SP responses can be obtained in deep UV by setting an insulator grating on the aluminum film. It is found that the height of the grating element, the lattice parameter, and the filling factor can all modulate the DLSPPs of the AlO–Al nanostructure. We further find that this structure is sensitive to the embedding medium and can serve as a refractive index sensor in the UV region. The corresponding sensitivity increases with the decrease of the filling factor. The Al–Al nanostructure may be useful for medical diagnostics and biotechnology in deep UV.

As an effective therapeutic modality, high-intensity focused ultrasound (HIFU) can destroy tumour tissues by thermocoagulation with less metastasis, but it is still limited by inaccurate non-invasive temperature monitoring and efficacy evaluation. A model of electrical impedance measurement during HIFU therapy was established using the temperature-impedance relationship. Based on the simulations of acoustic pressure, temperature, and electrical conductivity, the impedance of the phantom was calculated and experimentally demonstrated for different values of acoustic power values and treatment time. We proved that the relative impedance variation (RIV) increases linearly with the increasing treatment time at a fixed acoustic power, and the relative impedance variation rate shows a linear relationship with the acoustic power. The RIV and treatment time required for HIFU treatment efficacy are inversely proportional to the acoustic power and the square of acoustic power, respectively. The favourable results suggest that RIV can be used as an efficient indicator for noninvasive temperature monitoring and efficacy evaluation and may provide new strategy for accurate dose control of HIFU therapy.