The Research Of Femtosecond Laser Micro-nanofabrication

The development of micro-optics and integrated optics requires the miniaturization and integratability of optical elements. This is a challenge for traditional fabrication technologies. For instance, photolithography has a disadvantage of long production cycle and high cost. As for electron/ion beam lithography, there might be dead zones (e. g. interior of a cavity), which an electron or ion beam cannot reach, in a complicated structure, as thus reveals the weakness of this technology. Fortunately, femtosecond laser micro-nano-fabrication technology shows a conspicuous advantage in the fabrication of complicated structures in virtue of its high accuracy, high flexibility, high efficiency and high tolence to the complixity of objects. By this technology, we carried out researches into micro multileveled Fresnel zone plates and "hot site" that has something to do with enhancement of Raman scattering.According to the characteristics of this micro-nano-fabrication technology, we set up a fabrication system, which realized the three-dimensional moving of the laser focus relative to the photoresist by moving the photoresist with a piezo-electric stage and by reflecting the laser beam with a galvano-mirror for the other two dimensions scanning. The galvano-mirror and the piezo-electric stage both ensure a motion of the laser fucus as accurate as lnm, which makes it possible to fabricate complicated and delicate structures. Had debugged and optimized the fabrication system, we fabricated some planery and three-dimensional structures, such as Chinese characters, the emblem and mascot of Beijing Olympic Games, a ring hitching a tressel and three-dimensional photonic crystals. All of the experiment results have indicated that our fabrication system is practicable.In virtue of high accuracy, high flexibility and high tolerance to the complexity of the object tobe fabricated, this fabrication technology also becomes efficient means for research on micro-optical phenomena. This embodies in our work of studying the enhancement of the Raman scattering of nanodot dimers. As a laser spectrum method that can supply sufficient structural information, Raman has a disadvantage of extremely small cross section of scattering, which is 12-14 orders of magnitude smaller than that of fluorecence. Though surface enhancement of Raman scattering, which depends on surface roughness, has been discovered, the quantization and regeneration of its enhancement factor is a prominent problem, In 1997, the enhancement at a special location-"hot site"-of a silver nanoparticle was found to be 104 times that of common Raman enhancement effects. In 1999, the "hot spot" was located at the junction of a silver nanoparticle. For higher sensitivity and reproducibility, design and shape-controllable fabrication of "hot site" became hot. In our research, the finite difference time domain calculation were introduced to calculate the enhancement at the gap of silver nanoparticles with different internal partitions, and predicted an enormous enhancement at the junction of silver-coated nanodot dimers. Then, we fabricated nanodot dimers with different sizes and internal partitions in polymer by the micro-nano-fabrication technology and the result indicated that the controllability of size and internal partition by this technology as well as the repetition rate was high. Next, we coated the polymeric nanodot dimers with silver by electroless plating and vacuum evaporation, respectively, to find the surface roughness of the silver coating by electroless plating was much higher than by vacuum evaporation. High surface roughness benefits the enhancement of Raman scattering. We confirmed, that, the enhancement of Raman scattering at the silver-coated nanodot dimers was higher than that of common surface enhancement of Raman scattering. At last, we measured the Raman scattering of the adenine molecules absorbed on the silver coating of nanodot dimers, and calculated the electric field enhancement. Adenine molecules were chosen because of their Raman scattering stronger than that of other DNA bases. We observed strong Raman signals, which permitted Raman imaging.Besides the research into the Raman scattering enhancement of "hot site", we fabricated successfully micro-optical elements-Fresnel zone plates and Fresnel lens-by the 3D micro-nano-fabrication technology. Miniaturization is always the development trend of optical elements, hower, the fabrication technology for macro-elements is no longer suitable for the micro counterparts, especially those with complicated surfaces. Fortunately, in virtue of high accuracy and high tolerance to the complexity of arbitary 3D structures, the femtosecond laser micro-nano-fabrication technology is good at fabricating micro-optical elements with complicated surfaces. The slight changes between steps in a 4-leveled Fresnel lens and between ones in an 8-leveled Fresnel lens are clear, which contributes to their higher diffraction efficiency of 67% and 73.9%, respectively, than literatures reported. Moreover, we examined the imaging ability of the Fresnel zone plate, observed not only the real image, but also the virtual one. All the above results indicate the practical value and potential of the Fresnel zone plates and Fresnel lenses by the micro-nano-fabrication technology as focusing and imaging elements applied to micro-optical systems. The significance of fabricating micro-optical elements resides in the research into the performance of the micro counterparts of macro-optical elements and in proving the the feasibility of their application, both of which benefit the development of micro-optics and the research into micro-optical systems.In conclusion, the 3D micro-nano-fabrication technology based on the principle of femtosecond laser induced two-photon absorption photo-polymerization offers efficient means for research into and application of micro-optics in virtue of the advantages of high accuracy, high flexibility and high tolerance to the complexity of the object to be fabricated. The work shown in this dissertation is both the result of research into a specified direction of micro-optics and a good demonstration of the power of this fabrication technology.