Femtosecond laser direct writing in transparent materials based on nonlinear absorption
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Introduction Laser micromachining of transparent materials is difficult, since energy transfer from light to matter is not easily mediated by linear absorption. Recently, two-photon polymerization (TPP) has been widely used as a powerful tool for femtosecond laser direct writing (FsLDW) for ultrahigh-precision, three-dimensional (3D) printing, due to its ability to produce complex 3D models with submicrometer resolution using a wide variety of materials. TPP is based on the two-photon absorption process, followed by polymerization. Two photons from the incident light are simultaneously absorbed by the species, which then reaches an excited state, thereby inducing electron transition and subsequent chemical bond breaking. This is followed by the polymerization step. After the first demonstration of 3D microfabrication using TPP,1 fabrication capabilities, such as resolution, accuracy, writing speed, and modeling size, have steadily improved.2,3 The resolution of two-photon microfabrication has been improved by techniques such as stimulated emission depletion (STED) microscopy.2–4 The idea of STED is to bring the
photoinitiator molecules from the intermediate state back to the ground state via stimulated emission induced by a second depletion laser, leading to the fabrication of structures beyond the diffraction limit.4 Here, a doughnut-like laser beam, used to deactivate TPP, overlaps a focused femtosecond (fs) pulsed laser beam, such that the resultant voxel size can be reduced to sub-100 nm. The highest lateral resolution of two-dimensional (2D) patterning obtained by a STED-like approach was 9 nm.5 In another study, a fs pulsed laser beam with a wavelength of 520 nm formed 3D woodpile structures with a 65-nm linewidth.6 The writing speed was also increased from several hundred µm/s to a few mm/s using a highly sensitive photoinitiator.7 In addition, the depth of the fabrication area was expanded over the working distance of an objective lens by using a setup with a low numerical aperture lens that moved along its optical axis.8 Various materials have been developed to produce functional microdevices by TPP, including nanocomposite resins containing magnetic nanoparticles or carbon nanotubes,9,10 biopolymers,11 and smart gels.12 Postprocessing has attracted
Li Jia Jiang, Department of Electrical and Computer Engineering, University of Nebraska–Lincoln, USA; [email protected] Shoji Maruo, Department of Mechanical Engineering and Materials Science, Graduate School of Engineering, Yokohama National University, Japan; [email protected] Roberto Osellame, Institute for Photonics and Nanotechnologies, Italian National Research Council; and Department of Physics, Polytechnic University of Milan, Italy; [email protected] Wei Xiong, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, China; [email protected] John H. Campbell, Material Science Solutions, Inc., USA; [email protected] Yong Feng Lu, Department of Electrical and Computer Engineering, University of
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