| Summary: | 碩士 === 國立臺灣大學 === 應用物理研究所 === 106 === In this thesis, we study the correlation between optical second harmonic generation and the curvature of graphene. Optical second harmonic generation (SHG) from pristine graphene is theoretically prohibited because of its centrosymmetric atomic structure. However, SHG was observed from suspended graphene recently, and the signal was comparable to that from polar crystal such as GaAs. It was proposed that SHG resulted from strain or curvature, leading to broken symmetry. In this work, we make use of femtosecond laser and two different methods to study SHG from graphene with various curvatures.
The first parts of this thesis, we use Leica microscope, a vertical microscope, a three dimensional motion stage, and piezo stage, with gold tip made by chemical etching attached on, and we let it approach suspended graphene slowly. Once we make sure they contact with each other, we slowly let it go up further and scan SHG images. To have comparison group, we also use fiber tip to eliminate the effect to SHG due to the interaction of electrons when metal contact with graphene. We observed that both of them have strong enhancement of signals of SHG, but it has less SHG signals when using fiber tip.
To achieve three dimensional images, we transfer our system into inverted microscope. In this way, we could use movable objective lens to change the focal plane when scanning. When the gold or fiber tip contact with suspended graphene, we can access different SHG images in different focal planes. We could further use them to construct three dimensional SHG images and realize the fact that how the curvature caused by tip influences on the distribution of SHG. We find that when we use gold tip and fiber tip, tip shape would both appear in three dimensional SHG images and it would have strong SHG at the position of the tip.
The second parts of this thesis, we make use of technique of transferring graphene to transfer graphene onto PDMS. Because PDMS is a flexible material, therefore, as we stretch PDMS before transferring graphene, then release it, PDMS would revert to initial length, which makes ripple graphene. We scanned three dimensional SHG images for ripple graphene, and observed that it would have higher SHG signals at the position of wave crest and trough than in the purely oblique region. We analyze the curvatures and SHG intensities for all data points. According to statistics, they would have positive correlation.
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