Direct-Write Micro- and Nanostructuring with Femtosecond Lasers
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Direct-Write Micro- and Nanostructuring with Femtosecond Lasers B. N. Chichkov, J. Koch, A. Ovsianikov, S. Passinger, C. Reinhardt, and J. Serbin Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hannover, Germany Abstract Our recent progress in the direct-write femtosecond laser material processing technologies, nanostructuring, and fabrication of photonic devices is reviewed. Special attention is given to the sub-wavelength microstructuring of metals and two-photon polymerization (2PP) technique. Formation of microbumps and nanojets on thin metal films under single pulse laser irradiation is discussed. Microstructuring of different photosensitive materials by 2PP technique is demonstrated. Numerous applications of this technology for the fabrication of 3d structures, waveguides, photonic crystals, etc. are studied. Introduction Applications of ultrashort laser sources for high-resolution material processing are very rapidly growing. Using tightly focused femtosecond laser pulses one can produce micrometersize holes and periodic structures in almost all solid materials. But when the structure sizes become smaller than one micron, femtosecond laser structuring forfeits its universal character. In transition metals (like Cr, Mo, W, Fe), high-quality holes and structures with a resolution (structure size) down to several 100 nm can be fabricated by direct ablation with tightly focused femtosecond laser pulses [1]. These materials are characterized by rapid electron-phonon relaxation which is responsible for the fast energy transfer to the lattice and material removal. One can think about different applications of this technique like e.g. fabrication of lithographic masks, fabrication of prototypes (when lithography is too expensive), or structuring of nonplanar surfaces. In noble metals (like Au, Ag), the electron-phonon relaxation is much slower. For example, the parameter γ (in 1016 Wm-3K-1 units) characterizing the electron-phonon coupling is equal to 42 for Cr, whereas for Au γ =2.3 [2]. Due to the slower energy transfer from the electron subsystem, the lattice becomes melted and the molten phase in noble metals exists much longer. This effect drastically changes the ablation dynamics of noble metals and results in the formation of micro-bubbles and nanojets [3]. This technology can be used for tribological applications and can be applied for the fabrication of metallic band-gap nanostructures utilizing surface plasmon polaritons as information carriers. Another very promising and rapidly developing high-resolution laser technology is based on two-photon polymerization (2PP) technique. Several groups have demonstrated that 2PP of photosensitive resins allows the fabrication of true 3D nanostructures and photonic crystals [413]. The main idea is to trigger a chemical reaction in a highly localized focal region inducing cross-linking or dissociation of a photosensitive material. Nonlinear character of this process allows to overcome the diffraction limit for resolution, by applying laser powers slightly
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