Two-Photon-Excited Vapor Microbubble in Nanoparticle-Assembly-Based Laser Writing

Femtosecond laser irradiation in nanoparticle-containing AgNO3 solution enables the continuous writing of hierarchical microstructures with thick nanoparticle layers. This phenomenon has the potential to serve as a fundamental principle of direct laser writing, thereby overcoming its long-standing limitation of being restricted to photosensitive materials. In this study, to elucidate this writing mechanism, direct observations were conducted on both the focal region and its surroundings. At the laser focus, a distorted vapor bubble approximately 15 μm in diameter, which was generated by two-photon reduction, was detected on Ag line. This microbubble induced intense convection that flowed from the rear to the front relative to the scanning direction. This flow, symmetrical with respect to the Ag line, remained stable even during laser scanning. Magnified observation revealed that the flow was ejected from the rear part of the bubble. Such a flow could be explained by the strong temperature gradient on the bubble surface in contact with the Ag line, combined with the Marangoni effect. This analysis showed that the convective flow velocity increased closer to the bubble surface, which was consistent with the measured flow behaviors around the bubble. These findings strongly supported the validity of our proposed mechanism based on two-photon-excited microbubble-based nanoparticle assembly.