Localized Measurement of a Sub-Nanosecond Shockwave Pressure Rise Time

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.

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.

Interferometric Fiber Optic Probe for Measurements of Cavitation Bubble Expansion Velocity and Bubble Oscillation Time

Cavitation bubbles are used in medicine as a mechanism to generate shock waves. The study of cavitation bubble dynamics plays a crucial role in understanding and utilizing such phenomena for practical applications and purposes. Since the lifetime of cavitation bubbles is in the range of hundreds of microseconds and the radii are in the millimeter range, the observation of bubble dynamics requires complicated and expensive equipment. High-speed cameras or other optical techniques require transparent containers or at least a transparent optical window to access the region. Fiber optic probe tips are commonly used to monitor water pressure, density, and temperature, but no study has used a fiber tip sensor in an interferometric setup to measure cavitation bubble dynamics. We present how a fiber tip sensor system, originally intended as a hydrophone, can be used to track the expansion and contraction of cavitation bubbles. The measurement is based on interference between light reflected from the fiber tip surface and light reflected from the cavitation bubble itself. We used a continuous-wave laser to generate cavitation bubbles and a high-speed camera to validate our measurements. The shock wave resulting from the collapse of a bubble can also be measured with a delay in the order of 1 µs since the probe tip can be placed less than 1 mm away from the origin of the cavitation bubble. By combining the information on the bubble expansion velocity and the time of bubble collapse, the lifetime of a bubble can be estimated. The bubble expansion velocity is measured with a spatial resolution of 488 nm, half the wavelength of the measuring laser. Our results demonstrate an alternative method for monitoring bubble dynamics without the need for expensive equipment. The method is flexible and can be adapted to different environmental conditions, opening up new perspectives in many application areas.

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.