Control over the intensity, shape, direction and phase of coherent light is essential in numerous fields, from gravitational wave astronomy, quantum metrology and ultrafast sciences to semiconductor fabrication. Modern photonics, however, can involve parameter regimes where the wavelength or high optical powers involved restrict control due to absorption, light-induced damage or optical nonlinearity in solid media. Here we propose to circumvent these constraints using gaseous media tailored by high-intensity ultrasound waves. We demonstrate an implementation of this approach by efficiently deflecting ultrashort laser pulses using ultrasound waves in ambient air, without the use of transmissive solid media. At optical peak powers of 20 GW, exceeding previous limits of solid-based acousto-optic modulation by about three orders of magnitude, we reach a deflection efficiency greater than 50% while preserving excellent beam quality. Our approach is not limited to laser pulse deflection; gas-phase photonic schemes controlled by sonic waves could potentially be useful for realizing a new class of optical elements such as lenses or waveguides, which are effectively invulnerable against damage and can operate in new spectral regions.

Sandwich panels, composed of two steel faces and a rigid foam core, are an inexpensive and lightweight option for construction industry. However, voids can form in the foam core during the manufacturing process. This paper uses ultrasonic testing to detect such voids in the foam core of sandwich panels, buried a few millimeters below the surface. The testing setup employs both air-coupled and non-contact ultrasonic testing. Different frequencies are investigated for their influence on the detection capabilities. Two air-coupled experimental setups are constructed, one at 40 kHz and the other one at 200 kHz. Artificial defects are carved into the sandwich panel at different depths. The results are compared to a simulation. We found that detecting buried voids in these sandwich panels is feasible. The 40-kHz setup has a larger penetration depth of 14 mm, while the 200-kHz setup has a smaller penetration depth of 2.5 mm. The 200-kHz setup shows a better contrast, i.e. the amplitude at the defect increases by 27% compared to 6% with the 40-kHz setup. These methods enable air-coupled, non-contact ultrasonic testing of buried defects in sandwich panels. They have the potential to be integrated into production lines, contributing to improved material efficiency and quality control for these sandwich panels.

Acoustic levitation enables contactless manipulation for applications in containerless processing in chemistry and materials science. Single-transducer levitators, consisting of an ultrasonic transducer and a reflector, offer a minimal hardware configuration but require precise control of the transducer–reflector distance H. We propose a distance stabilization system based on a motorized linear stage and a precision balance that measures the acoustic radiation force (ARF) as a time-averaged, surface-integrated force magnitude on the reflector. Because ARF versus H exhibits extremely narrow resonance peaks, micrometer-scale deviations markedly reduce force and destabilize levitation. In practice, high-power transducers are actively kept at resonance using resonance frequency tracking (RFT). As the tracked frequency changes, the wavelength changes as well and the optimal distance H shifts. We introduce a two-stage control strategy in order to robustly stabilize H. First, a physics-informed, one-sided adaptive hill climb algorithm locates the cavity resonance. Second, a physics-informed extremum-seeking controller maintains the experimentally determined resonance distance despite drift and disturbances, using only the absolute ARF feedback. Long-term validation at node index n=5 confirmed that ARF remained within the 95% tolerance bound for two hours, ensuring stable levitation. Performance was limited by the balance, mainly due to its internal averaging time. This occasionally caused convergence to local maxima or temporary loss of track during object insertion. The strategy presented enables robust, long-duration operation of acoustic levitators.

The accurate characterization of air-coupled high-power ultrasonic fields is essential for advancing the development and control of ultrasonic transducer arrays. Conventional characterization methods often fail to provide both contactless visualization and a quantitative sound pressure distribution without the need for time-consuming point-by-point scanning. Acoustic pressure variations modulate the refractive index of the medium, e.g., ambient air, and thereby change the optical path length, which can be measured interferometrically. In this paper, Fizeau interferometry is systematically compared with Schlieren imaging and optical microphone measurements. The results demonstrate strong agreement with the reference methods and confirm the reliability of the interferometric approach. Overall, this method enables contactless two-dimensional field visualization with a spatial resolution of 23 µm in combination with quantitative sound pressure data within a single acquisition (40 ms). Consequently, it provides a rapid tool for characterizing high-power ultrasonic fields in applications such as acoustic levitation or sono-photonic systems.