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The ability of cavitation bubbles to effectively focus energy is made responsible for cavitation erosion, traumatic brain injury, and even for catalyse chemical reactions. Yet, the mechanism through which material is eroded remains vague, and the extremely fast and localized dynamics that lead to material damage has not been resolved. Here, we reveal the decisive mechanism that leads to energy focusing during the non-spherical collapse of cavitation bubbles and eventually results to the erosion of hardened metals. We show that a single cavitation bubble at ambient pressure close to a metal surface causes erosion only if a non-axisymmetric energy self-focusing is at play. The bubble during its collapse emits shockwaves that under certain conditions converge to a single point where the remaining gas phase is driven to a shockwave-intensified collapse. We resolve the conditions under which this self-focusing enhances the collapse and damages the solid. High-speed imaging of bubble and shock wave dynamics at sub-picosecond exposure times is correlated to the shockwaves recorded with large bandwidth hydrophones. The material damage from several metallic materials is detected in situ and quantified ex-situ via scanning electron microscopy and confocal profilometry. With this knowledge, approaches to mitigate cavitation erosion or to even enhance the energy focusing are within reach.

Acoustic cavitation, generated by a piezo-driven transducer, is a commonly used technique in a variety of processes, from homogenization, emulsification, and intensification of chemical reactions to surface cleaning and wastewater treatment. An ultrasonic horn, the most commonly used acoustic cavitation device, creates unique cavitation conditions under the horn tip that depend on various parameters such as the tip diameter, the driving frequency of the horn, its amplitude, and fluid properties. Unlike for hydrodynamic cavitation, the scaling laws for acoustic cavitation are poorly understood. Empirical relationships between cavitation dynamics, ultrasonic horn operating conditions, and fluid properties were found through systematic characterization of cavitation under the tip. Experiments were conducted in distilled water with various sodium chloride salt concentrations under different horn amplitudes, tip geometries, and ambient pressures. Cavitation characteristics were monitored by high-speed (200,000 fps) imaging, and numerous relations were found between operating conditions and cavitation dynamics. The compared results are discussed along with a proposal of a novel acoustic cavitation parameter and its relationship to the size of the cavitation cloud under the horn tip. Similar to the classical hydrodynamic cavitation number, the authors propose for the first time an acoustic cavitation parameter based on experimental results.

The inception of a cavitation bubble in a liquid by focusing a short and intense laser pulse near its free surface develops not only an upwards directed jet, but a second jet of opposite direction into the bulk liquid. When the laser is focused a few microns below the surface, the rapid deposition of energy produces a splash, whose later sealing gives origin to two particularly elongated opposing jets. Interestingly, the evolution of the downward jet flowing into the liquid pool has many similarities to that observed in free water entry experiments, e.g. the creation of a slender and stable cavity in the liquid. The downward jet can reach speeds of up to 40 m s⁻¹ and travels distances of more than 15 times the maximum radius of the laser induced cavity before losing momentum. The longer lifetime of this so-called ‘bullet’ jet as compared with conventional cavitation based jets, the alignment of the jet perpendicular to the free surface and the possibility of scaling the phenomenon opens up potential applications when generated on small droplets or in shallow liquids. In this work, the underlying mechanisms behind the formation of the bullet jets are initially investigated by performing a set of experiments designed to address specific questions about the phenomenon under study. Those were followed by numerical simulations used to give a quantitative and detailed explanation to the experimental observations.

Optical bound states in the continuum (BICs) underpin the existence of strongly localized waves embedded into the radiation spectrum. Here we bring the concept of BICs to the field of high-harmonic generation and employ resonant dielectric metasurfaces to generate efficiently optical harmonics up to the 11th order. We design BIC-resonant metasurfaces with a broken in-plane symmetry for the lower harmonics and then observe a transition to the nonlinear regime for higher harmonics. Our approach bridges the fields of perturbative and nonperturbative nonlinear optics on the subwavelength scale.

The origin of second harmonic generation (SHG) signal in otoconia was investigated. SHG signal intensity from otoconia was compared to pure calcite crystals, given calcite is the primary component of otoconia and is known to emit surface SHG. The SHG intensity from calcite was found to be ∼41× weaker than the SHG intensity from otoconia signifying that the SHG signal from otoconia is likely generated from the organic matrix. Furthermore, the SHG intensity from otoconia increased when treated with a chelating agent known to dissolve calcite which confirms that calcite is not the source of SHG. Additionally, polarization-resolved SHG microscopy imaging revealed that the arrangement of the SHG emitters is radial and can form highly ordered domains.

In this work we present a novel method for depolarization compensation based on spatially variable wave plate. Thermally induced depolarization losses were reduced from 14.3% to 1.3% and bifocusing eliminated in a double-pass Yb:YAG amplifier.

The Rayleigh collapse time is the time it would take to shrink an empty spherical bubble in an infinite liquid domain to zero size, which is a function of ambient pressure and initial bubble radius. If a solid boundary is located in the vicinity of the shrinking or collapsing bubble, then liquid flow is hindered, such that the collapse time is prolonged. This can be quantified with the Rayleigh prolongation factor k. Here, we provide k for intermediate to smallest bubble to wall stand-off distances. It is measured with single laser-induced cavitation bubbles in water close to a solid boundary. Maximum bubble radii are determined from microscopic high-speed imaging at one million frames per second. Collapse times are measured acoustically via the acoustic transients emitted during bubble seeding and collapse. The experimental findings are compared, with good agreement, to numerical simulations based on a volume of fluid method. As a result, a polynomial fit of k versus stand-off distance is given for the near-wall bubble collapse in water. Then the influence of the viscosity on k is studied numerically in the near-wall regime.

We demonstrate that besides gaseous pockets also a gas supersaturated spot on a substrate can be a nucleus for cavitation. The supersaturation is achieved by either a formerly dissolved bubble or by heating locally the surface below the boiling temperature. The experiments are conducted in a thin film of water; one side of the water film is in contact with a gold coated substrate that is heated by a continuous laser through plasmonic heating. For nucleation of a bubble, the pressure at the heated spot is reduced by a transient rarefaction wave. The experimental findings suggest that the local gas supersaturation is responsible to nucleate cavitation and thus connects the phase transitions of cavitation and boiling. Additionally, the pressure waves in the liquid gap are studied numerically.

A subpicosecond laser system featuring a fiber chirped pulse amplification-based seed laser and a double-pass end-pumped Yb:YAG crystal power amplifier was investigated. The key novelty of the system was the application of depolarization compensation using a specially designed spatially variable wave plate. To the best of our knowledge, this method was applied for the first time. The presented laser system produced pulses of 441 fs duration, 116 µJ pulse energy at 116 W average power with a beam quality of M²∼2.1, featured optical-to-optical efficiency of 32% at room temperature (T=20°C), and had residual depolarization level of 2.7%.