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 (<inline-formula> <tex-math notation="LaTeX">150~\boldsymbol \mu \text{m} </tex-math></inline-formula> diameter) laser-induced bubbles at very small distances (down to <inline-formula> <tex-math notation="LaTeX">40~\boldsymbol \mu \text{m} </tex-math></inline-formula>), 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.
We experimentally compare a thick BIBO crystal, a walk-off compensating configuration of thin BBO crystals, and a thin PPLN crystal for the use as a nonlinear medium in SHG FROG, aiming for increased ...sensitivity required for complete pulse characterization of low-energy (below pJ) pulses a few picoseconds long at wavelengths near 1030 nm. We investigate the dependence of the spectral bandwidth of the frequency-doubled input ultrashort pulse and the conversion efficiency on focal-spot size in the nonlinear crystal. Both numerical and experimental results confirm an increase of the bandwidth with tighter focusing in a thick BIBO and a more-than-threefold increase of conversion efficiency compared to a thin BIBO crystal. A PPLN crystal is found to be the most efficient among the three, but very thin (< 0.3 mm) crystals would be required to extend the measurement capabilities of SHG FROG to sub-ps pulses. A SHG FROG using a PPLN crystal for characterization of picosecond pulse is demonstrated with a sensitivity of <inline-formula> <tex-math notation="LaTeX">2\times 10^{-3} </tex-math></inline-formula> (mW) 2 .
We introduce a practical self-referenced device for the complete temporal intensity-and-phase measurement of few-femtojoule-level fs and ps pulses based on the simple version of ...frequency-resolved-optical gating, called GRENOUILLE. We replace the usual crossed-beam line-focus geometry with a point-focus near-collinear-beam geometry and a traditional delay line that allows a longer path length in the nonlinear medium, yielding greater sensitivity. As an initial demonstration of the device, we measured moderately complex pulses at energies as weak as 24 fJ with high signal-to-noise ratio. We confirmed the results measured using our technique to that of a standard GRENOUILLE and a high-resolution spectrometer.
Laser operation in bursts of pulses has recently attracted a lot of attention, as high average powers and pulse energies have become available. Bursts of pulses have become a mean to achieve ...different laser-matter interaction regimes with a single laser source. We have used an ultra-short pulse laser source in combination with an external array of birefringent crystals to generate near-THz bursts of single-picosecond pulses. Variability of the setup and high single pulse energy were exploited to generate bursts of up to 16 pulses at different delays between consecutive pulses. The experimental setup was applied for surface processing of different industrially relevant materials, including metals, ceramics, and polyimides.
The goal of this study was to find out if the reduced ablation thresholds using near-THz bursts of pulses as reported in previous studies of other group could be utilized for increased ablation efficiency. Our findings show that near-THz bursts of pulses interact with materials in a similar fashion as a single, albeit longer, pulse would. We observed a clear trend on all the tested materials, showing a declining ablation efficiency with increasing total burst duration at a fixed pulse repetition frequency. Findings contradict the results of other groups concluding on an improved efficiency of laser to material energy transfer using bursts at near-THz intra-burst repetition rates. A simple model describing ablation efficiency decline with pulse prolongation was used to compare the experimental data with theoretical predictions. Detailed analysis of surface ablation effects on metallic materials also revealed a presence of melting processes typically associated with laser pulses above 10 ps in duration as opposed to original 1.5 ps pulses.
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•Generation of near-THz bursts of laser pulses•Insight to the surface ablation mechanism at various burst durations•Near-THz bursts behave similarly to single prolonged pulses•Lower ablation efficiency and more heat-related effects using bursts
One of the remaining open questions in cavitation erosion research is the one on the importance of the microjet and the shock wave on the formation of the pit. Up until now, no successful attempt has ...been made to study this in detail, mainly because the damage could only be detected and evaluated after several successive bubble collapses.
A bubble with a maximum diameter of up to 3.3 mm was created during photoionization using a Nd:YAG laser. The damage was observed on a 9 µm thick aluminum foil attached to a glass substrate. Two high speed cameras were simultaneously used. One captured the dynamics of the bubble, while the other recorded the damage of the foil.
We also observed the collapse of a bubble in the presence of shear flow, where most of the damage is created by the microjet mechanism. Sometimes, the collapse of the bubble rim, at the rebound of the initial bubble causes pits in a well-known circular pattern. From the recordings at the very fastest acquisition rate, we determined that the material deforms and then partially relaxes, while a significant deformation remains. The whole process is only 2–3 µs long.
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•Cavitation damage from a single bubble was observed.•Pit formation during 3 μs was measured.•Shear flow influence on erosion was determined.•Importance of microjet and shock wave was determined.
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•Study of cavitation bubble and shock wave dynamics near a concave surface.•Shock wave interaction with the concave reflector and its scattering on a bubble.•Secondary cavitation ...induced by the refocused shock wave.•Visualization by shadow or schlieren photography with adaptive illumination system.•Theoretical treatment of shock wave propagation with geometrical acoustics.
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.
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.
•Cavitation bubble dynamics in the vicinity of liquid-liquid interface was studied.•Advanced simulations were used to interpret the experimental observations.•Dependence of the bubble jetting to the ...anisotropy parameter was investigated.•The study has a great applied value in the field of ultrasonic emulsification.
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.
Pulsed laser sources facilitate various applications, including efficient material removal in different scientific and industrial applications. Commercially available laser systems in the field ...typically use a focused laser beam of 10–20 μm in diameter. In line with the ongoing trends of miniaturization, we have developed a picosecond fiber laser-based system combining fast beam deflection and tight focusing for material processing and optical applications. We have predicted and verified the system’s precision, resolution, and minimum achievable feature size for material processing applications. The analysis of the laser’s performance requirements for the specific applications of high-precision laser processing is an important aspect for further development of the technique. We have predicted and experimentally verified that maximal edge roughness of single-micrometer-sized features was below 200 nm, including the laser’s energy and positioning stability, beam deflection, the effect of spot spacing, and efficient isolation of mechanical vibrations. We have demonstrated that a novel fiber laser operating regime in bursts of pulses increases the laser energy stability. The results of our research improve the potential of fiber laser sources for material processing applications and facilitate their use through enabling the operation at lower pulse energies in bursts as opposed to single pulse regimes.
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