Optic generation and perpetuation of acoustic bubble clusters.

Laser-induced cavitation bubbles offer precise control of the flow in space and time, but they are rarely used for the mechanical and chemical processing of liquids. Instead, strong acoustic fields are commonly used to nucleate and drive cavitation bubbles for liquid process applications. While acoustic field creates many more cavitation events, the resulting chaotic dynamics offers little control on the fluid mechanics, i.e., where and how bubbles deliver their energy. Here we present a method that utilizes a laser to nucleate a single cavitation bubble, which is then driven into violent oscillations by the ultrasound field, resulting in splitting of the bubble followed by formation of a cluster of cavitation bubbles. This combination offers means for cavitation control not available in conventional acoustic cavitation. Here, the cavitation bubble is generated with a custom build pulsed laser that is focused below a sonotrode driven at 20 kHz. In absence of the acoustic driving the bubble reaches a maximum diameter of 130 µm with a lifetime of approximately 10 µs. In the presence of the acoustic field the first few expansions and bubble collapses are strongly affected by the phase of nucleation. Over successive acoustic cycles a small bubble cluster develops that loses its connection with the phase of generation. We study the dynamics in the free field and constrained by a rigid boundary. For both geometries the cluster over many acoustic cycles dies off, yet through repetitive optical bubble seeding the cluster lifetime and its location can be controlled.

Evidence of laser-induced nanobubble formation mechanism in water.

Principles of laser-induced nanobubble formation in water are studied and presented. Nanobubbles were generated by laser light at intensities below threshold for laser-induced breakdown and subsequently expanded by a rarefaction wave to facilitate their observation and analysis. Different methods were used to study nanobubble formation and characteristics. Firstly, probability of nanobubble formation as a function of water sample purity was examined. Secondly, relation between laser fluence at different wavelengths and the number of generated nanobubbles was investigated. Thirdly, measurements of nanobubble lifetime were conducted indicating a contradiction to the Epstein-Plesset equation-based prediction of free bubble dissociation. Accumulated evidence suggests that the presence of physical impurities is a prerequisite for nanobubble formation. Consequently, a lack of impurities results in the absence of nanobubbles in contrast to assumptions by existing studies. The findings presented in this paper provide new insights into the fundamental properties of laser-induced nanobubbles in water.

Pulse-on-Demand Operation for Precise High-Speed UV Laser Microstructuring.

Laser microstructuring has been studied extensively in the last decades due to its versatile, contactless processing and outstanding precision and structure quality on a wide range of materials. A limitation of the approach has been identified in the utilization of high average laser powers, with scanner movement fundamentally limited by laws of inertia. In this work, we apply a nanosecond UV laser working in an intrinsic pulse-on-demand mode, ensuring maximal utilization of the fastest commercially available galvanometric scanners at scanning speeds from 0 to 20 m/s. The effects of high-frequency pulse-on-demand operation were analyzed in terms of processing speeds, ablation efficiency, resulting surface quality, repeatability, and precision of the approach. Additionally, laser pulse duration was varied in single-digit nanosecond pulse durations and applied to high throughput microstructuring. We studied the effects of scanning speed on pulse-on-demand operation, single- and multipass laser percussion drilling performance, surface structuring of sensitive materials, and ablation efficiency for pulse durations in the range of 1-4 ns. We confirmed the pulse-on-demand operation suitability for microstructuring for a range of frequencies from below 1 kHz to 1.0 MHz with 5 ns timing precision and identified the scanners as the limiting factor even at full utilization. The ablation efficiency was improved with longer pulse durations, but structure quality degraded.

Ultrafast measurement of laser-induced shock waves.

We present measurements of laser-induced shockwave pressure rise time in liquids on a sub-nanosecond scale, using custom-designed single-mode fiber optic hydrophone. The measurements are aimed at the study of the shockwave generation process, helping to improve the effectiveness of various applications and decrease possible accidental damage from shockwaves. The developed method allows measurement of the fast shockwave rise time as close as 10 µm from an 8 µm sized laser-induced plasma shockwave source, significantly improving the spatial and temporal resolution of the pressure measurement over other types of hydrophones. The spatial and temporal limitations of the presented hydrophone measurements are investigated theoretically, with actual experimental results agreeing well with the predictions. To demonstrate the capabilities of the fast sensor, we were able to show that the shockwave rise time is linked to liquid viscosity exhibiting logarithmic dependency in the low viscosity regime (from 0.4 cSt to 50 cSt). Additionally, the shockwave rise time dependency on propagation distance close to the source in water was investigated, with shock wave rise times measured down to only 150 ps. It was found that at short propagation distances in water halving the shock wave peak pressure results in the rise time increase by approximately factor of 1.6. These results extend the understanding of shockwave behavior in low viscosity liquids.

Near threshold nucleation and growth of cavitation bubbles generated with a picosecond laser.

The nucleation and growth of cavitation bubbles few micrometers in size in water generated by a 60 ps 515 nm fiber laser is observed and visualized near nucleation threshold. The study is performed by monitoring the plasma size, the cavitation bubble size and the emitted shock waves. The latter two aspects are supported by the Gilmore model using a Noble-Abel-stiffened-gas (NASG) equations of state. For the first time, two types of cavitation events are identified and visualized that exhibit a difference of more than two orders of magnitude in the excitation energy converted to mechanical effects with minimal change in excitation laser pulse energy. The result is localized cavitation and reduced mechanical stress on water-based media with potentially positive implications for laser treatments of biological tissue.

Multi-frame multi-exposure shock wave imaging and pressure measurements.

Shock wave visual detection was traditionally performed using streak cameras, limited to homogeneous shock wave emission, with the corresponding shock wave pressure measurements available at rather large distances or numerically estimated through equation of state for water. We demonstrate a multi-frame multi-exposure shock wave velocity measurement technique for all in-plane directions of propagation, based on custom-built illumination system allowing multiple illumination pulses within each frame at multi-MHz frame rates and at up to 200 MHz illumination pulse repetition frequency at sub-nanosecond pulse durations. The measurements are combined and verified using a fiber-optic probe hydrophone, providing independent shock wave pressure and time-of-flight measurements, creating a novel all-optical measurement setup. The measured pressures at distances around 100 µm from the plasma center exceed 500 MPa, while camera-based measurements at even shorter distances indicate pressures above 1 GPa.

Laser-induced shock-wave-expanded nanobubbles in spherical geometry.

The secondary cavitation generation following laser-induced breakdown in aqueous media in spherical geometry, mimicking the geometry of the frontal part of the human eye, was studied. A numerical simulation of the shock wave propagation was performed, yielding peak-pressure maps, correctly predicting the location of the secondary cavitation onset for different shock wave source positions. The comparison between the simulation results and the experiments, performed with a high-precision, multiple-illumination technique, supports the suggested description of the nature of the secondary cavitation onset. It is shown that large transient negative pressures are created at the location of the acoustic image of the shock wave source, which is different from the optical focus. After the passage of the shock wave, abundant secondary cavitation is generated there. Additionally, the existence of an important contributing factor to the reduction of the secondary cavitation threshold is supported by the experimental results, namely the pre-illumination of the water by the breakdown-generating laser pulse, playing a crucial role in conditioning the medium. There is strong experimental evidence of the existence of another mechanism of pre-conditioning the water for the secondary cavitation onset, namely in the form of repetitive negative pressure pulse passage through the same volume, an indication of a possible two- or multiple-stage process.

Microbubble dynamics and jetting near tissue-phantom biointerfaces.

Precise excitation of cavitation is a promising mechanism for microsurgery procedures and targeted drug delivery enhancement. The underlying phenomenon of interest, jetting behaviour of oscillating cavitation bubbles, occurs due to near-surface interactions between the boundary, liquid, and bubble. Within this study we measured boundary effects on the cavitation bubble dynamics and morphology, with an emphasis on observation and measurement of jetting behaviour near tissue-phantom biointerfaces. An important mechanism of boundary poration has been observed using time-resolved optical microscopy and explained for different tissue-phantom surface densities and Young's modulus. Below a critical distance to the boundary, around γ = 1.0, the resulting jets penetrated the tissue-phantom, resulting in highly localized few micrometer diameter jets.

Localized Measurement of a Sub-Nanosecond Shockwave Pressure Rise Time.

In a growing number of applications, fast and localized pressure measurement in aqueous media is desired. To perform such measurements, a custom-made single-mode fiber-optic probe hydrophone (FOPH) was designed and used to measure the pressure pulse generated by laser-induced breakdown (LIB) in water. The sensor enabled sub-nanosecond pressure rise time measurement. Both the rise time and the duration of the shockwave were found to be shorter in the direction perpendicular to the breakdown generating laser beam, compared to the shockwave observed in the parallel direction. Simultaneous high-frame-rate imaging was used to qualitatively validate the novel hydrophone data and to observe the shockwave evolution. The measurements were performed also on pressure pulses emitted during the generation of miniature ( [Formula: see text] diameter) laser-induced bubbles at very small distances (down to [Formula: see text]), further demonstrating the capabilities of the small-size sensor and the ability to measure locally. The results improve understanding of LIB shockwave characteristics dependence on laser pulse energy and duration.

Laser-based material interactions and ablation processes by bursts of 70 ps pulses.

The intermediate pulse duration regime between typical ultra-short and nanosecond pulses has been investigated using MHz-range bursts of 70 ps pulses emitted from a custom-made fiber laser source. The goal of this study was to observe and understand the processes involved during laser ablation on the timescales from picoseconds to nanoseconds, relevant due to pulses in bursts. We developed material processing approaches that enable similar behaviour as single 70 ps pulse ablation to ultra-short pulses in terms of quality and burst-mode behaviour like nanosecond pulses in terms of efficiency. The variability of the fiber laser operation modes was studied and compared to both ultra-short and nanosecond pulses from standard laser sources.

Method for controlled tissue theranostics using a single tunable laser source.

Tissue diseases and related disorders need to be first recognized using diagnostic methods and then later treated by therapeutic methods-a joint procedure called theranostics. One of the main challenges in the field of retinal therapies remains in the success of the treatment, typically improving the local metabolism, by sparing the surrounding tissue and with the immediate information of the laser effect. In our study, we present a concept for real-time controlled tissue theranostics on a proof-of-concept study capable of using a single tunable ps laser source (in terms of irradiance, fluence, and repetition rate), done on ex-vivo human retinal pigment epithelium. We have found autofluorescence intensity and lifetime imaging diagnostics very promising for the recognition and quantification of laser effects ranging from selective non-destructive molecular tissue modification to complete tissue ablation. The main novelty of our work presents the developed algorithm for optimized theranostics based on the model function used to quantify laser-induced tissue changes through the diagnostics descriptors, fluorescence lifetime and fluorescence intensity parameters. This approach, together with the operation of the single adaptable laser source, can serve as a new theranostics method in personalized medicine in the future not only limited to treat retinal diseases.

Cavitation bubble dynamics in a vicinity of a thin membrane wetted by different fluids.

Understanding and controlling the interaction of cavitation bubbles and nearby material is becoming essential optimization of various processes. We examined the interaction of a single bubble with a membrane with different fluids on each side of it. Significant differences in bubble behavior depending on the fluid properties were observed, while the influence of membrane properties was less pronounced. The study has important implications, such as optimization of sonoporation (targeted drug delivery) where the mechanism, by which the permeability of the membrane is increased, is still not well understood. These results show that the focus of the optimization process should, in the first place, lie on the properties of liquids, rather than the mechanical properties of the membrane itself.

Laser-induced cavitation bubbles and shock waves in water near a concave surface.

The interplay among the cavitation structures and the shock waves following a nanosecond laser breakdown in water in the vicinity of a concave surface was visualized with high-speed shadowgraphy and schlieren cinematography. Unlike the generation of the main cavitation bubble near a flat or a convex surface, the concave surface refocuses the emitted shock waves and causes secondary cavitation near the acoustic focus which is most pronounced when triggered by the shock wave released during the first main bubble collapse. The shock wave propagation, reflection from the concave surface and its scattering on the dominant cavity is clearly resolvable on the shadowgraphs. The schlieren approach revealed the pressure build up in the last stage of the collapse and the first stage of the rebound. A persistent low-density watermark is left behind the first collapse. The observed effects are important wherever cavities collapse near indented surfaces, such as in cavitation peening, cavitation erosion and ophthalmology.

Cavitation bubble collapse in a vicinity of a liquid-liquid interface - Basic research into emulsification process.

The initial motivation for the study was to gain deeper understanding into the background of emulsion preparation by ultrasound (cavitation). In our previous work (Perdih et al., 2019) we observed rich phenomena occurring near the liquid-liquid interface which was exposed to ultrasonic cavitation. Although numerous studies of bubble dynamics in different environments (presence of free surface, solid body, shear flow and even variable gravity field) exist, one can find almost no reports on the interaction of a bubble with a liquid-liquid interface. In the present work we conducted a number of experiments where single cavitation bubble dynamics was observed on each side of the oil-water interface. These were accompanied by corresponding simulations. We investigated the details of bubble interface interaction (deformation, penetration). As predicted, by the anisotropy parameter the bubble always jets toward the interface if it grows in the lighter liquid and correspondingly away from the interface if it is initiated inside the denser liquid. We extended the analysis to the relationships of various bubble characteristics and the anisotropy parameter. Finally, based on the present and our previous study (Perdih et al., 2019), we offer new insights into the physics of ultrasonic emulsification process.

Femtosecond CPA hybrid laser system with pulse-on-demand operation.

In this manuscript we present a true pulse-on-demand concept of a hybrid CPA laser system, consisting of a chirped-pulse fiber amplifier and an additional solid-state amplifier, capable of generating femtosecond pulses on demand without an external optical modulator/shutter. Pulse-on-demand operation is achieved by introducing idler pulses with a few nanoseconds duration and selectively switching between the femtosecond and idler pulses. The idler pulses are used to maintain a constant population inversion in the fiber amplifier as well as in the solid-state amplifier. Second harmonic generation (SHG) unit then effectively filters out the idler pulses due to their low peak power, leaving only a stable femtosecond pulse train. This concept is demonstrated on a CPA hybrid system that can generate pulses with up to 200 µJ at 515 nm with a pulse duration under 450 fs. As there is no optical modulator at the laser output, the presented concept also enables further power scaling.

High-speed photography of shock waves with an adaptive illumination.

An adaptable, laser-diode-based illumination system was developed to simultaneously visualize the dynamics of slow and fast phenomena in optically transparent media. The system can be coupled with still or high-speed cameras and makes it possible to generate an arbitrary train of illumination pulses with a variable pulse duration, pulse energy, and an intrapulse delay with a temporal resolution of 12.5 ns. Its capabilities are presented with selected illustrative visualizations of the dynamics of the shock waves and the cavitation entities generated after the laser-induced breakdown in water.

Cavitation induced by shock wave focusing in eye-like experimental configurations.

During laser-induced, breakdown-based medical procedures in human eyes such as posterior capsulotomy and vitreolysis, shock waves are emitted from the location of the plasma. A part of these spherically expanding transients is reflected from the concave surface of the corneal epithelium and refocused within the eye. Using a simplified experimental model of the eye, the dominant secondary cavitation clusters were detected by high-speed camera shadowgraphy in the refocusing volume, dislocated from the breakdown position and described by an abridged ray theory. Individual microbubbles were detected in the preheated cone of the incoming laser pulse and radially extending cavitation filaments were generated around the location of the breakdown soon after collapse of the initial bubble. The generation of the secondary cavitation structures due to shock wave focusing can be considered an adverse effect, important in ophthalmology.

Author Correction: Isolated detection of elastic waves driven by the momentum of light.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Highly adaptable gain-switched fiber laser with improved efficiency.

A highly adaptable fiber laser with pulse-on-demand and precision pulse-duration tuning is presented. It is based on a compact optical design combining the gain-switching technique with the all-fiber master oscillator and pump-recovery amplifier architecture. The approach of laser-pulse stability control by compensation pumping and pulse-duration control by changing the pump wavelength are introduced. In order to prove the concept, a laser setup capable of producing laser pulses with an average power of up to 30 W and a peak power of approximately 1 kW at an improved efficiency and an arbitrary repetition rate is presented.

Isolated detection of elastic waves driven by the momentum of light.

Electromagnetic momentum carried by light is observable through the mechanical effects radiation pressure exerts on illuminated objects. Momentum conversion from electromagnetic fields to elastic waves within a solid object proceeds through a string of electrodynamic and elastodynamic phenomena, collectively bound by momentum and energy continuity. The details of this conversion predicted by theory have yet to be validated by experiments, as it is difficult to distinguish displacements driven by momentum from those driven by heating due to light absorption. Here, we have measured temporal variations of the surface displacements induced by laser pulses reflected from a solid dielectric mirror. Ab initio modelling of momentum flow describes the transfer of momentum from the electromagnetic field to the dielectric mirror, with subsequent creation/propagation of multicomponent elastic waves. Complete consistency between predictions and absolute measurements of surface displacements offers compelling evidence of elastic transients driven predominantly by the momentum of light.