High-speed observation of semiconductor microsphere generation by laser ablation in the air

We have succeeded in fabricating zinc oxide (ZnO) microsphere by laser ablation in the air and demonstrated ultraviolet whispering gallery mode lasing from an optically pumped ZnO microsphere. However, fabricated microspheres could not be controlled in size, and ejection direction of microsphere was random, because the generation mechanism was attributed to the droplet ejection by laser ablation. In this report, the effect of pulse duration on microsphere generation was investigated, and we introduced optical vortex to control the size and the ejection direction of droplets. The behavior of ablated target was visualized using a high-speed camera. As a result of time-resolved observation, it was found that many ZnO microspheres can be efficiency generated by the long pulse laser irradiation. In addition, it was observed that droplets were scattered randomly by Gaussian beam irradiation. In the case of optical vortex irradiation, straight flight of the droplet was achieved. Furthermore, the size of droplets changed with pulse energy and ablation spot size with optical vortex. From these results, controlled fabrication of ZnO microspheres is expected with optical vortex laser ablation.