Polarization singularities and topological polarization structures are generic features of inhomogeneous vector wave fields of any nature. However, their experimental studies mostly remain restricted to optical waves. Here, we report the observation of polarization singularities, topological Möbius-strip structures, and skyrmionic textures in 3D polarization fields of inhomogeneous sound waves. Our experiments are made in the ultrasonic domain using nonparaxial propagating fields generated by space-variant 2D acoustic sources. We also retrieve distributions of the 3D spin density in these fields. Our results open the avenue to investigations and applications of topological features and nontrivial 3D vector properties of structured sound waves.

Acoustic vortices have attracted a great deal of attention in recent years due to their numerous applications. We introduce a highly efficient method for the generation of acoustic Bessel vortices in air, using spiral-shaped active diffraction gratings, which can be operated within a broad spectral range of ultrasonic frequencies. Using a single-arm active spiral source, we achieve the simultaneous generation of vortices of different topological charges, well separated among each other along the propagation axis. With a theoretical analysis, numerical simulations, and experiments, we demonstrate some features about spiral diffraction gratings, such as the equivalence between specific diffraction orders of an m-armed spiral and a single-arm spiral, and the annihilation of prescribed diffraction orders by tuning the width to pitch ratio.

We present a simple and efficient method for generating focused acoustic vortices in air over a wide range of ultrasonic frequencies by means of an Active-Spiral Fresnel Zone Plate. An important advantage of this device is that the focal length can be finely and continuously tuned by setting the operation frequency. The role of the different design parameters is analyzed in terms of the minimum beam width and the focal depth of the resulting field, allowing an optimized device according to the application. Experimental results show very good agreement with numerical simulations.

Acoustic vortices with subwavelength dimensions and tunable topological charge are theoretically and experimentally synthesized at distances far beyond the Rayleigh diffraction length of the source, using self-demodulation. A dual helical acoustic source is used to generate two primary confocal vortex beams at different frequencies and different topological charges. As a consequence of the conservation of angular momentum during nonlinear wave mixing, a self-demodulated vortex beam at the difference frequency emerges, keeping the spatial features of the primary vortex beams and a topological charge that is the difference of their topological charges. We report subdiffractive vortices the characteristic size of which is 18 times smaller than its wavelength at a distance 2.8 times the Rayleigh diffraction length. The generation and focusing of subwavelength vortices paves the way for long-range communication, biomedical, and wave-matter interaction applications.

A detailed characterization and analysis of the acoustic field radiated by an active diffraction grating is presented. This is a spiral-shaped source of constant width of the spiral paths and spacing among them, built with flexible ferroelectrets, which is suitable to operate within a broad range of ultrasonic frequencies. The simultaneous airborne generation of multiple Bessel vortex beams with different topological charges along the propagation axis is thoroughly discussed in terms of the structure of the spiral source and the driving frequency, along with some technical details of the prototype. Finally, the quality of the experimentally generated Bessel vortices is discussed, including the phase structure and the amplitude of the acoustic pressure.

Airborne acoustic levitation has seen great advances in recent years largely due to the development of devices utilizing arrays of compact, low-power, piezoelectric transducers. In particular, standing wave acoustic levitators are acoustic cavities, consisting of either a single piezoelectric boundary opposing a reflector or two opposing piezoelectric boundaries operating simultaneously. The multiple intra-cavity reflections can be significant enough to generate voltages and currents at the electrical port due to the direct piezoelectric effect, providing valuable information about the optimal operating conditions of the cavity. In this work, we present a theoretical model that employs two planar piezoelectric boundaries to analyze the electrical response (voltage and current) at the cavity's electrical port and to identify the resonance conditions as a function of two key parameters: the cavity length and the phase configuration of the boundaries. We implemented a numerical approach to estimate the voltage and the current in cavities with pairs of planar and spherical boundaries. Additionally, we conduct a comparative study between numerical simulations and experimental results using cavities with spherical cap boundaries to validate our approach. The results demonstrate that source-reflector (S-R) and source-source (S-S) cavities can be set to resonance by tuning the cavity length from directly monitoring in real time the electric response of both S-R and S-S cavities, with voltage (current) exhibiting direct (inverse) correlation with the acoustic pressure amplitudes.