| Summary: | 碩士 === 國立中正大學 === 物理系 === 91 === Two-photon photopolymerization can be used to fabricate three-dimensional objects with sub-diffraction-limit resolution. The feature size can achieve the specifications of two-dimensional photonic crystals. With appropriate pattern-transfer techniques, the patterns made by two-photon photopolymerization in photoresists can be transferred into semiconductor materials and then become real two-dimensional photonic crystals.
In this thesis, we employ a Ti:sapphire laser oscillator with an 808 nm wavelength, 100 fs duration, and 100 MHz repetition rate to perform two-photon photopolymerization in an epoxy-based negative photoresist SU-8. In this work, average power below 10 mW can achieve two-photon exposure. We derive a theoretical model suitable for the epoxy-based photoresist which can predict the exposure size controlled by the exposure time under various laser power. This model is quite consistent with the experimental results. We use two-photon photopolymerization to fabricate arrays of posts with 400-nm diameters and aspect ratio ~ 7. By controlling the two-axis stage to move arbitrarily, we can even write characters of widths smaller than 10 mm. These works demonstrate the versatility of this fabrication technique.
In order to achieve the specifications of two-dimensional photonic crystals, we have tried to transfer the patterns in photoresists onto silicon wafers. Because two-dimensional photonic crystals require high resolution, high aspect ratio, and smooth surface, we decided to use reactive ion etching for pattern transfer. At present we have successfully transferred the post arrays, gratings, and characters onto silicon wafers. Using scanning electron microscopy we confirm that the surface flatness and the sharpness of the edges are acceptable. However, the resolution cannot satisfy the requirement of photonic crystals with bandgaps around the wavelength of 1.55 mm. Using the second harmonic of the Ti:sapphire laser light to perform two-photon photopolymerization, we believe silicon-based two-dimensional photonic crystals working at 1.55 mm is readily achievable with the processing parameters developed in this work.
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