In 1997, Kawata et al. reported a surprising experimental result that full three-dimensional (3-D) microstructures had been fabricated by photopolymerization [1]. Four year later, the world's smallest 3-D bulls were created by the same research group [2]. The bulls were based on polymeric materials and were approximately 10 (mu)m in size, similar to that of human red blood cells. The spatial resolution of their fabrication technique was as high as approximately 100 nm, which is beyond the diffraction limit. Recently, the much higher resolution of sub-30 nm was achieved using 3-D bridge structures [3]. These 3-D structures were created using femtosecond laser direct writing and subsequent development treatment. As illustrated in Fig. 1, when femtosecond laser pulses are focused tightly into transparent materials, photochemical reactions occur only near the focal volume via nonlinear optical processes such as multiphoton absorption (MPA), because the rate of MPA depends strongly on light intensity. For instance, the two-photon absorption rate is proportional to the square of the light intensity. Because of this characteristic, the MPA technique enables us to expose the internal region directly and to create complex polymeric microano-structures by translating a focal spot inside the resin. The strong dependence of MPA also allows for confinement of laser-modified areas to sub-micrometer volume.
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