Femtosecond laser-induced fabrication of gold nanorod/polymer composite with two-photon photopolymerization

We demonstrated a microstructure fabrication method of gold nanorod/polymer composite, which was caused by the cooperation of aggregation and adhesion of gold nanorods induced by femtosecond laser light [1]. Near infrared femtosecond laser light (780 nm) was tightly focused into gold nanorods dispersing photo-polymerizable resin and excited two-photon polymerization (TPP) onto the surface of gold nanorods dominantly. Concurrently occurring accumulation of the gold nanorods into focus spot, gold nanorods were aggregated and stuck each other, resulting in creation of gold nanorods/polymer composite. Gold nanorods were synthesized by seed mediated growth method [2], and their longitudinal local surface plasmon resonance (LSPR) mode was tuned close to the laser light by changing aspect ratio. In order to disperse into photo-polymerizable resin which contained methyl methacrylate (MMA) monomer and photo-initiator, the surface of gold nanorods was modified by thiol-termincated polyethylene glycol. Near infrared femtosecond laser light was focused into the gold nanorods dispersing photo-polymerizable resin by a microscope objective lens. LSPR mode was excited onto gold nanorods by laser light irradiation, resulting in the formation of polymer layer only at their surface through TPP. Simultaneously occurring accumulation of gold nanorods by continuous irradiation of laser light, the TPP layer worked as a glue to form aggregation of gold nanorods. By scanning the focus spot of laser light according to the trace of pre-programmed structure, we fabricated arbitrary microstructures formed by aggregation of gold nanorods. By controlling the intensity and the exposure time of the laser light, we found optimal conditions to induce dominant polymerization without causing thermal damage. This method has a potential to create plasmonic optical materials into desired microstructures by controlling the alignment of gold nanorods.