Laser ablation synthesis in liquid solution (LASiS) is a “green” technique that gives access to the preparation of a library of nanomaterials. Bare noble metal spherical particles, multiphase core–shell oxides, metal–semiconductor heterostructures, layered organometallic compounds and other complex nanostructures can be obtained with the same experimental set up, just by varying a few synthetic parameters. How to govern such versatility is one of the current challenges of LASiS and requires a thorough understanding of the physical and chemical processes involved in the synthesis. In this perspective, the fundamental mechanisms of laser ablation in liquids are summarized, organized according to their temporal sequence and correlated with relevant examples taken from the library of nanomaterials disclosed by LASiS, in order to show how synthesis parameters influence the composition and the structure of products. The resulting framework suggests that, to date, much attention has been devoted to the physical aspects of laser–matter interaction and to the characterization of the final products of the synthesis. Conversely, the clarification of chemical processes active during LASiS deserves more research efforts and requires the synergy among multiple investigation techniques.

What controls the composition and the structure of nanomaterials generated by laser ablation in liquid solution

When a high-intensity laser pulse is focused into a liquid the energy is converted into mechanical energy via an optodynamic process. The conversion starts with plasma formation; this is followed by shock-wave propagation and the expansion of a cavitation bubble. A cavitation bubble developed near boundaries results in an asymmetrical collapse, with the generation of a liquid jet during the bubble’s rebound. In the case of a free surface this liquid jet is directed away from the surface and the oscillation times are prolonged. On the other hand, in the case of a rigid boundary, the liquid jet is directed toward the boundary and the oscillation times are shortened. We present measurements of a cavitation bubble oscillating between a free surface and a rigid boundary using deflections of a laser beam as the optical probe. Shadow photography was used simultaneously as a comparison during the experiments. With the beam-deflection probe we also measured the shortening of the oscillation times near a free surface as well as the prolongation of oscillation times near a rigid boundary. In order to explain this shortening of the cavitation-bubble oscillation times near a free surface, Rayleigh’s model was extended and compared with our experimental results.

Investigation of a cavitation bubble between a rigid boundary and a free surface

Theoretical analysis and extensive numerical simulations are performed on the system of a spherical absorber illuminated by a pulsed laser. Both bubble formation and shock wave generation are investigated. We find that a strong shock front forms just outside the absorber and attenuates quickly into an acoustic wave as it propagates away from the absorber. The formation and growth of the bubble is shown to have a gentle threshold dependence and to be a continuous function of the energy of the laser pulse with no explosive threshold. There is a weak dependence of the threshold fluence for bubble formation on the pulse duration of the laser. The bubble growth dynamics are also pulse-duration dependent for pulse lengths shorter than the thermal conduction time. For ultrashort pulses (shorter than the acoustic transit time across the absorber), the bubble dynamics approach a limiting growth trajectory. The process is a cooperative effect of both thermal and pressure interactions. We also find that variation of...

Bubble dynamics and shock waves generated by laser absorption of a photoacoustic sphere

Numerical simulation on void bubble dynamics using moving particle semi-implicit method

We present an optodynamic measurement of a laser-induced cavitation bubble and its oscillations based on a scanning technique using a laser beam-deflection probe. The deflection of the beam was detected with a fast quadrant photodiode which was built into the optical probe. The applied experimental setup enabled us to carry out one- or two-dimensional scanning of the cavitation bubble, automatic control of the experiment, data acquisition and data processing. Shadow photography was used as a comparative method during the experiments.

Measurements of cavitation bubble dynamics based on a beam-deflection probe

The oscillation of a laser-generated single cavitation bubble near a solid boundary is investigated by a fiber-optic diagnostic technique based on optical beam deflection (OBD). The maximum bubble radii and collapse time for each oscillation cycle are determined from a sequence of bubble oscillations. Furthermore, by combining the revised Rayleigh theory, the prolongation factor κ at different dimensionless parameter γ ( γ = L / R max , where Rmax is the maximum bubble radius and L is the distance of a cavity inception point from a boundary) is obtained. In addition, the prolongation factor of the collapse time versus laser energy is also derived, which are valuable in the fields of hydraulic cavitation, laser lithotripsy and laser ophthalmology.

Experimental investigation of the collapse of laser-generated cavitation bubbles near a solid boundary

The oscillating properties of laser-induced cavitation bubbles in water are investigated by means of a fiber-optic sensor based on optical beam deflection. The experimental results show two important points. One is that the smaller the bubble radius the more quickly the bubble surface moves. Thus, the variations of the temperature and the pressure inside the bubble will be close to those of an adiabatic process. The other is that the high-energy vapor inside the newborn bubble diffuses and coagulates rapidly through violent expansion and thermal conduction. Thus, the gas content of the bubble reduces significantly in the first oscillation. Numerical simulation is made for the bubble model with consideration of liquid viscosity, surface tension, and gas content. Through modification of the polytropic index and the gas content parameter, two parameters of this model, the numerical results fit the experimental results well.

The influence of thermodynamic gas parameters on laser-induced bubble dynamics in water

Dynamics of laser-induced shock wave by optical probe in air

In this paper, an optofluidic phase modulator array based on the piezoelectric effect is designed, fabricated, and characterized. This array is composed of three piezoelectric ceramics arranged on the vertices of an equilateral triangle. A transparent liquid fills the inner cavity of the ceramics. Due to the inverse piezoelectric effect, the length of the transparent liquid is changed at different voltages, which contributes to the optical phase modulation. According to experiment results, it is found that our modulator arrays exert continuous optical phase adjustment ability. When the voltage ranges from 0 to 135 V, the relative length variation reaches up to 9.286 µm, and consequently our proposed modulator arrays perform about 9.685 π optical phase modulation.

Design, fabrication, and characterization of an optofluidic phase modulator array based on the piezoelectric effect.

A simple analytic model is derived for describing the attenuation of a shock wave in a Plexiglas plate. At the same time, experimental measurements are presented with a well-designed optical-fiber sensor based on detection-beam deflection. The amplitude of the shock-wave pressure is measured experimentally and calculated numerically for analytic expressions at different distances from the region of the surface breakdown by the radiation of a single-pulse Nd:YAG laser. Good agreement between the experimental and the calculated values of the shock-wave pressure is established.

Rui Zhao

Papers

Experimental investigation of the collapse of laser-generated cavitation bubbles near a solid boundary

The influence of thermodynamic gas parameters on laser-induced bubble dynamics in water

Dynamics of laser-induced shock wave by optical probe in air

Design, fabrication, and characterization of an optofluidic phase modulator array based on the piezoelectric effect.

Attenuation of laser-generated shock waves in Plexiglas.