Visualization of microparticle explosion and flow field in nanoparticle synthesis by pulsed laser ablation

Laser ablation of microparticles is one of the promising methods for efficient nanoparticle synthesis but the physical mechanisms of nanoparticle formation are not yet clearly understood, particularly in the case of metallic or ceramic particles. In this work, we report, for the first time, the results of in situ visualization of the metal microparticle explosion. Ablation of a Cu microparticle in CuO nanoparticle synthesis by a Q-switched Nd:YAG laser is visualized using laser-flash shadowgraphy. The dynamics of the ablation plume and shock wave is also analyzed by monitoring the photoacoustic probe-beam deflection. Strong shock-wave emission from a single particle has been observed in the velocity range 1000-4000 m/s even at near-threshold fluences. The threshold laser fluence for particle ablation has been found to be substantially lower than that required for bulk metal ablation. Consequently, the results confirm that the photomechanical mechanism associated with the shock-wave generation plays a significant role in the ablation process.