Fabrication of Cu-based microstructures by reduction of Cu₂O nanoparticles using green femtosecond laser pulses

Three-dimensional microfabrication technique using multiphoton absorption has attracted attention because threedimensional microstructures enable to be directly written inside raw materials. To date, three-dimensional microstructures of photosensitive polymers and noble metals such as Au and Ag have been achieved using near-infrared (NIR) femtosecond laser pulses. However, it is difficult to apply this technique to the fabrication of Cu-based microstructures because Cu ions exhibit intense absorption in a range of NIR. In this study, we demonstrated Cu-based microfabrication using reduction of Cu₂O nanoparticles induced by green femtosecond laser pulses. Cu₂O nanoparticle solution consisting of Cu₂O nanoparticles synthesized by polyol method, polyvinylpyrrolidone, and 2-propanol, were prepared. Then, the solution was spin-coated and baked on a glass substrate. After that, direct writing using green femtosecond laser pulses was performed in air. Wavelength, pulse duration, and repetition rate of the laser were 515 nm, 100 fs, and 40 MHz, respectively. The laser pulses were focused onto the samples using an objective lens with a numerical aperture of 0.90. The minimum line width formed was approximately 0.7 µm which was almost the same as the spot diameter, indicating that thermal diffusion was negligible in thermochemical reduction of Cu₂O nanoparticles. A square pattern was also written by raster scanning of the laser spot. X-ray diffraction spectrum shows high-intensity peaks corresponding to Cu. These results suggest that Cu₂O nanoparticles were sufficiently reduced to Cu. Furthermore, microstructures with different heights were successfully formed using the internal writing into the Cu₂O nanoparticles. More complex three-dimensional microstructures will be formed by preparing the thick Cu₂O nanoparticle films.