Generation of New Nanomaterials by Interfering Femtosecond Laser Processing
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Generation of New Nanomaterials by Interfering Femtosecond Laser Processing Yoshiki Nakata and Okada Tatsuo and Maeda Mitsuo Graduate School of Information Science and Electrical Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan ABSTRACT New nanomaterials such as nanobump array, nanomesh, nanobelt were generated from thin film processed by interfering femtosecond laser beams. Metallic single- or multi-layered film deposited on a silica substrate was used as a raw thin film. With four interfering femtosecond laser beams, a conical nanobump arrayed in a matrix was generated with single laser shot. As the femtosecond laser fluence increased, the nanobump increased in diameter and height, and a bead was found at the top. Moreover, with three or two interfering femtosecond laser beams, ellipsoidal or linear nanobump array was generated. As an application of a conical nanobump array, field emission from the nanobump array was demonstrated, and the I-V characteristics were measured. On the other hand, with much higher fluence and four interfering femtosecond laser beams, a nanohole array was generated. A nanomesh was also generated from the nanohole array by exfoliating the film. A grating was generated with two interfering femtosecond laser beams, and nanobelts were generated from the grating by exfoliating. Bimetallic nanobelt was also generated from multi-layered thin film. INTRODUCTION Nano-technology based on nano-sized structures is recognized as a future fundamental technology. It has many applications in optical devices, electric devices, MEMS, biology, high strength raw materials, etc. The nanomaterials have been generated by plasma process, evaporation, anodization, etc [1,2]. However, these methods are limited in their adaptability to different materials, and have difficulties in tasks such as size, structure and alignment control. In addition, nano-sized structures have been fabricated by lithography, but it requires a toxic solvent, and is costly and time-consuming. On the other hand, material processing by femtosecond (fs) laser is a topic of interest for non-thermal and fine processing. Recently, interfering fs laser beams were applied to laser ablation and photo-polymerization, and nano-sized periodic structures were generated on and in bulk [3-9]. Among them, we found that characteristic structures such as nanobump array, nanomesh, nanobelt, etc. can be generated from thin films [5,8,9]. In this paper, our recent results of the generation and application of new nanomaterials are reported. EXPERIMENTAL DETAILS Experimental setup can be seen in the past papers [7,9]. An ultrashort pulse laser system, of which the pulse width was about 100 fs and the center wavelength was about 800 nm, was used. Instead of widely-used beam correlator consisting of a mirror beam splitter and mirrors, a coherent optical system was used as a beam correlator. A beam is split by a transmission beam splitter, and then focused by two convex lenses onto a target thin film. The advantages of
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