We report on a technique that enables to fabricate three-dimensional (3D) metallic microstructures by means of two photon-
induced metal-ion reduction. A femtosecond near-infrared laser is focused by a high-NA objective lens into a
metal-ion aqueous solution. Due to the nonlinear nature of the two-photon absorption (TPA) process, metal-ions are
directly reduced only at the focused spot. By scanning the laser beam spot in three dimensions, we can directly obtain
arbitrary 3D metallic structures. To fabricate silver and gold structures, we use a 0.2-M aqueous solution of silver
nitrate (AgNO3) and a 0.24-M aqueous solution of tetra chloroauric acid (HAuCl4), respectively. We demonstrate the
fabrication of a continuous and electrically conductive silver wire whose minimum width is 400 nm. Electrical
measurement shows that the resistivity of the fabricated silver wire is 5.30 × 10-8 Ωm, which is only 3.3 times larger
than that of bulk silver (1.62 × 10-8 Ωm). We also discuss the resolution of our technique in terms of ions diffusion
based on the Fick's first law and the mobility of metal-ions in aqueous solution. Moreover, the realization of a selfstanding
3D silver microstructures on the substrates are demonstrated. This method will become a promising
technique for fabricating 3D plasmonic micro/nano structures with arbitrary shape.
|