Reflectivity of a 5.8 kbar shock front in water

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.

A comprehensive review and outlook on the experimental techniques to investigate the complex dynamics of pulsed laser ablation in liquid for nanoparticle synthesis

Pulsed laser ablation in liquid (PLAL) has been established as one of the most efficient and impactful methods for producing pure and ligand-free nanoparticles (NPs). PLAL has successfully been utilized for the synthesis of metal NPs, semiconductor NPs, ceramic NPs, and even nanocomposites. A variety of NPs, including core-shell, nanocubes, nanorods, and many other complex structures, can be synthesized using PLAL. The versatility associated with PLAL has led to the synthesis of NPs that have found applications in the field of biomedicine, sensing technology, energy harvesting, and various industries. Despite all the aforementioned advantages, there has been an ambiguity in terms of conditions/parameters for the nanoparticle synthesis as reported by various research groups. This has led to a perception that PLAL provides little or no control over the properties of the synthesized NPs. The properties of the NPs are reliant on transient dynamics caused due to a high-intensity laser's interaction with the target material. To understand the process of nanoparticle synthesis and to control the properties of NPs, it is critical to understand the various processes that occur during PLAL. The investigation of PLAL is essential for understanding the dynamical processes involved. However, the investigation techniques employed to probe PLAL present their own set of difficulties, as high temporal as well as spatial resolution is a prerequisite to probe PLAL. Hence, the purpose of this Review is to understand the dynamical processes of PLAL and gain an insight into the various investigation techniques and their data interpretation. In addition to the current challenges, some ways of overcoming these challenges are also presented. The benefits of concurrent investigations with special emphasis on the simultaneous investigation by multiple techniques are summarized, and furthermore, a few examples are also provided to help the readers understand how the simultaneous investigation works.

Dynamics of bond breaking and formation in polyethylene near shock front

In a systematic study of shock wave propagating in crystalline polyethylenes using molecular dynamics method and the electron force field (eFF) potential, we show that microscopic structure of shock front is significantly affected by the anisotropy of long carbon chain and the bond breaking and recombination dynamics. However, macroscopic properties measured in Hugoniot experiments, such as compression ratio and shock velocity, are not sensitive to carbon chain anisotropy and bond dynamics. Our work also display that hydrogen molecules are formed when the piston speed is in the region between 10 km/s and 30 km/s. However, carbon-hydrogen pair distribution function does not display an indication of carbon-hydrogen phase segregation.

Effect of focusing conditions on laser-induced shock waves at titanium-water interface

The spatial and temporal evolution of laser-induced shock waves at a titanium-water interface was analyzed using a beam deflection setup. The focusing conditions of the source laser were varied, and its effect onto the dynamics of shock waves was elucidated. For a tightly focused condition, the speed of the shock wave was ~6.4 Km/s, whereas for a defocused condition the velocities reduced to <3 km/s at the vicinity of the titanium-water interface. When the laser is focused a few millimeters above the target, i.e., within the water, the emission of dual shock waves was observed toward the rear side of the focal volume. These shock waves originate from the titanium-water interface as well as from the pure water breakdown region, respectively. The shock wave pressure is estimated from the shock wave velocity using the Newton's second law across a shock wave discontinuity. The shock wave pressure for a tightly focused condition was 18 GPa, whereas under a defocused condition the pressure experienced was ≤1 GPa in the proximity of target.

Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water

BACKGROUND AND OBJECTIVE

Photodisruption in ocular media with high power pulsed lasers working at non-absorbing frequencies have become a well established surgical tool since the late seventies. Shock waves and cavitation bubbles generated by the optical breakdown may strongly influence the surgical effect of photodisruptive lasers. We have investigated the shock wave and cavitation bubble effects of femtosecond laser pulses generated during photodisruption in corneal tissue and water. The results are compared to those obtained with longer laser pulses.

STUDY DESIGN/MATERIALS AND METHODS

Laser pulses with 150 fs duration at approximately 620 nm wavelength have been focused into corneal tissue and water to create optical breakdown. Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles.

RESULTS

A rapid decay of the shock waves is observed in both materials with similar temporal characteristics and with a spatial range considerably smaller than that of shock waves induced by picosecond (or nanosecond) optical breakdown. Cavitation bubbles are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses. In corneal tissue, single intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses.

CONCLUSIONS

The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage. Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers. This indicates a potential benefit from the applications of femtosecond laser technology to intraocular microsurgery.

Dynamics of shock waves and cavitation bubbles generated by picosecond laser pulses in corneal tissue and water

Time-resolved flash photography was used to investigate the dynamics of shock waves and cavitation bubbles generated by picosecond optical breakdown in bovine corneal tissue and water. A picosecond Nd:YLF laser was employed. A rapid decay of the shock waves was observed in both materials, with similar temporal characteristics, indicating that water serves as a good model for shock wave studies. In contrast, differences in the cavitation bubble dynamics were found between cornea and water, which are related to differences in the mechanical and thermal properties of the two media, suggesting that water should not be used to model cavitation dynamics in cornea. The experimental results also suggest that the efficiency of intrastromal ablation may be increased by using short pulses and moderate pulse energies in order to avoid the creation of large cavitation bubbles. The experiment indicates that the optimum laser repetition rate for intrastromal ablation is between 1 and 5 kHz.