Mr. Hengzhu Bao Profile

Hengzhu Bao

at Nanjing Univ of Science and Technology

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Publications (3)

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Proceedings Article | 29 December 2023 Paper

Dynamics of nanosecond laser-induced shock waves in water using multiple excitation beams

Yang Zhi, Hengzhu Bao, Lou Gao, Hongchao Zhang, Jian Lu

Proceedings Volume 12976, 129760K (2023) https://doi.org/10.1117/12.3007697

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When a high-power laser beam is incident on water, the rapid heat transfer process triggers ionization of the water in the active region, resulting in an explosion as well as outward propagating shock waves. Here, the formation, propagation, and interaction of underwater shock waves induced by nanosecond laser pulses were experimentally investigated. By fitting the theoretical model (Sedov-Taylor expansion model) to the experimental results, we quantified the effective energy carried by the shock wave during propagation. Numerical simulations with an analytic model using the distance between adjacent breakdown locations as input obtain the shock wave emission images at different time delays and provide insights into experimentally not accessible shock wave parameters. An empirical model is used to describe the pressure behind the shock wave. The results show the near-acoustic propagation behavior of the shock waves at longer time delays. On top of that, we compared the effect of the distance between adjacent excitation positions on the shock wave emission process. The shock wave parameters in the far field are more accurate and easier to perform. Furthermore, utilizing multipoint excitation offers a flexible approach to delve deeper into the physical mechanisms that cause optical tissue damage in nanosecond laser surgery, leading to a better comprehension of the subject.

Proceedings Article | 29 March 2019 Paper

Numerical simulation of three laser-induced in-phase bubbles

Hengzhu Bao, Hongchao Zhang, Lou Gao, Chong Zhang, Jian Lu

Proceedings Volume 11046, 110461P (2019) https://doi.org/10.1117/12.2524268

KEYWORDS: Cavitation, Wave propagation interference, Numerical simulations, Pulsed laser operation, Wave propagation, Laser applications

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The potential applications of laser-induced bubbles are gradually revealed. And the performance of multiple - bubbles also attracted considerable attention. This paper is intended to study the complex interaction among three in-line arranged in-phase bubbles. This work illustrates the interactions of three bubbles oscillation and the shockwave motion after the recombination of the plasma nanosecond laser excited. The Volume of Fluid (VOF) model is used to numerically analyze the behaviour of same equilibrium size, same distance and in-phase multiple-bubbles, which enables tracking of the gas-liquid interface and the field. The numerical results indicate that, in the expanding stage, due to the pressure gradient, bubbles at both ends move towards the centre, while the bubble surfaces facing to other bubbles get flattening. The central bubble has longer oscillation period than the side bubbles. In the shrinking stage, the side bubbles get deformation faster, and then collapse with a jet or get adhere to the central bubble. The initial high-pressure bubble radiates a shockwave at the beginning of the expansion. The shock wave propagates outwards from the point source in a circular pattern, resulting in interference in the overlap of the two shock waves. A curved pressure wave, which is parallel to the in-line array, is produced by the interference of multiple shock waves. The pressure wave decays to an acoustic wave after a certain distance according to its initial intensity. In particular, a significant variable γ=D/(R+R) deserves studying in this progress.

Proceedings Article | 29 March 2019 Paper

Laser-induced plasma in a single droplet

Chong Zhang, Hongchao Zhang, Hengzhu Bao, Zhonghua Shen, Xiaowu Ni, Jian Lu

Proceedings Volume 11046, 1104604 (2019) https://doi.org/10.1117/12.2523632

KEYWORDS: Plasma, Ionization, Plasma generation, Laser energy, Femtosecond phenomena, Clouds, Water, Laser optics

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Water clouds are considered as the common dispersion systems suspending in the air. In general, droplets can be treated as a transparent dielectric for the laser field. However, once the intensity of laser exceeds the breakdown threshold, laserinduced plasma will be generated in the droplet. This plasma is able to significant influence the propagation of laser field. Since the water clouds are constituted by the countless droplets, it is reasonable to study the interaction between intense laser field and a single droplet as the starting point. The laser-induced plasma is usually generated instantaneously in a micro spatial volume. It is difficult to reveal the detail structure of plasma. We will provide a transient coupling model to study the time-evolution of the laser-induced plasma propagating in a single droplet. Using the above model, we will bring insight into the breakdown threshold firstly. On the other hand, there are abundant features in the both plasma and its coupling laser field. The plasma is initially generated at the location where laser field propagates away from the droplet, if the intensity of the laser is not very high. Meanwhile, after the plasma generates, the droplet becomes opaque. Large amount of laser energy then is deposited in the droplet. The saturation of the energy deposit can be revealed in our model. We would expect this transient coupling model is helpful to estimate the propagation length of the intense laser pulse while it is passing through the water cloud.

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