Two-Photon Laser Micro-Nano Fabrication; Understanding from Single-Voxel Level
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Two-Photon Laser Micro-Nano Fabrication; Understanding from Single-Voxel Level Satoshi Kawata1 and Hong-Bo Sun2 Department of Applied Physics, Osaka University, Suita, Osaka, 565-0871, Japan 1 The Institute of Physical and Chemical Research (RIKEN), Hirosawa, Wako, Saitama 351-0198, Japan 2 PRESTO, Japan Science and Technology Corporation (JST) ABSTRACT For laser nanofabrication using two-photon photopolymerization, a deep understanding of the nature of focal spots that are related to two-photon excitation is essential for achieving a high spatial resolution in three dimensions. Here we report the use of a technology we call ascending scan for characterizing the three-dimensional size and shape of single polymerization elements (voxels), and introduce several features of voxels that have not been fully noticed before. These findings are important for tailoring nanofeatures according to design. INTRODUCTION Two-photon photopolymerization [1-7] has been recognized as an important technology for nanofabrication. The current research effort is mainly devoted to the synthesis of high efficiency photo initiators and sensitizers [4, 10]. However, as a new technology, a lot of work has been done to establish it as a nanotechnology. Examples include the achievement of sub-diffractionlimit spatial resolution by the radical quenching effect [8, 9], improvement of fabrication efficiency by using 3D vector scanning [9], 3D micro-diagnosis by fluorescent dye labeling and two-photon confocal scanning [11], micro-device functionalization [12], and so forth. Different from conventional laser rapid prototyping, depicting micro-objects with SDL features needs a deep understanding of characteristics of two-photon excitation related focal spots. This work has not been done before. In this paper, we will introduce a 3D focal spot imaging technology, ascending scan [13], and an investigation by using this technology on how basic laser parameters influence the focal spot size and shape, and therefore the nanofabrication. EXPERIMENTAL SYSTEM The experimental system was the same as we used before [12, 13]. A Ti: Sapphire laser that was operated at mode-lock and delivered 100 fs pulses at a repetition rate of 80 MHz was
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employed as an exposure source (MaiTai, Spectra Physics). The laser wavelength was tuned to 780 nm. The laser beam was focused by a high-numerical-aperture (NA~1.4) objective lens into a sample. A Galvano mirror set was used for moving the focal spot in two horizontal dimensions and a piezo stage for up-down scanning, both synchronized and controlled with a computer. A urethane acrylate resin, SCR 500 (from Japan Synthetic Rubber, JSR) was used for two-photon-absorption (TPA) photopolymerization, of which the absorption peaks at UV and extends to the visible range to 530 nm. The entire fabrication process was monitored in-situ with a CCD camera. After fabrication, samples were developed in methanol so that unsolidified liquid was removed. Finally samples were dried and imaged with a scanning electron beam m
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