Two-Photon Microbubble Printing: Asymmetric Assembly for Continuous Laser Writing

Direct laser writing (DLW) technique is reported, which "two-photon bubble printing" is termed. By asymmetric assembly, this method enables the continuous formation of nanoparticle aggregates from a wide variety of materials, thus providing a material-independent DLW platform. Femtosecond laser irradiation in an AgNO₃ solution containing target nanoparticles leads to the formation of hierarchical cross-sections, in which thick nanoparticle layers are deposited on Ag nuclei. To uncover the writing mechanism, direct observations of the laser focal region and surrounding environment is performed. At the laser focus, a distorted vapor bubble (≈15 µm) is consistently observed. This bubble is formed as a result of localized photothermal heating by Ag nucleus generated through two-photon reduction. This microbubble induces strong rear-to-front flow by Marangoni forces arising from steep temperature gradients at the rear part of the bubble. Detailed analysis revealed that the convective flow preferentially attracted nanoparticles to the rear part of the bubble from a wide area. Such asymmetric assembly is considered to have enabled continuous laser writing of nanoparticle layers. Understanding the writing mechanism that combines nonlinear photochemistry with thermally driven fluid dynamics provides essential insights for the development and applications of a versatile platform for material-independent direct laser writing.