Femtosecond laser 3D nanoprinting using inorganic nano-building-blocks

Since the 1990s, femtosecond laser two-photon absorption (TPA), which enables point-by-point photopolymerization, has been a tool for making three-dimensional (3D) microstructures. Approximately ten years later, Kawata et al. improved the spatial resolution of TPA fabrication to ~120 nm, far beyond the diffraction limit of the 780 nm laser source.1 The essential mechanism of this nanoscale fabrication lies in the nonlinear effect, in which TPA-induced photopolymerization occurs only in the vicinity of the focal spot. As a landmark symbol of this field, “microbull” sculptures, ~10 μm in length and 7 μm in height, have been produced using a commercially available resin that consists mainly of urethane acrylate monomers, oligomers and photoinitiators (Figure 1, left inset). Currently, both spatial resolution and fabrication efficiency have been significantly promoted, and TPA fabrication has emerged as a sophisticated 3D nanoprinting protocol. However, the available materials for TPA fabrication are largely limited to photocurable resins, and femtosecond laser 3D printing of functional nanomaterials beyond polymers remains a challenging task.