Probing mesoscopic process of laser ablation in liquid by integrated method of optical beam deflection and time-resolved shadowgraphy

Abstract

For nanomaterial fabrication by laser ablation in liquid (LAL), it is very important to understand the mesoscopic process of laser interaction with materials in liquid. We proposed a method combining time-resolved shadowgraphy and optical beam deflection method to study the LAL process in both pure water and water with nanoparticles (colloids). As the laser was focused on the target in pure water, the laser energy was absorbed by the target and plasma, shockwaves and bubbles were produced, along with the generation of nanoparticles. While in case of colloid, laser beam first passed through the solution, interacted with nanoparticles and induced plenty of sporadic shadows (small bubbles) on the beam path which were captured by shadowgraphy. Then, the laser arrived at the target and induced breakdown accompanied by the emergence of plasma, shockwave and bubbles. Meanwhile, the concentration of nanoparticle increased and the sizes of nanoparticle were modified. The radius and oscillation time of bubbles are much smaller in the colloid than that in pure water, mainly due to laser energy loss by breakdown of the nanoparticles and generation of small bubbles before reaching the target. Moreover, we found the maximum bubble radius and bubble oscillation time decrease quickly with laser irradiation times at the beginning, and then reach a plateau, because of laser energy lost on the way to the target. In addition, we used the ablation process to explain a bimodal size distribution of nanoparticles. This work will deepen our understanding on the mechanism of both laser ablation of bulk targets in liquid and laser irradiation of particles in liquid.