彎曲石墨烯的光學二倍頻研究

Abstract

本論文主要為探討光學的二倍頻效應與石墨烯曲率的影響關係。理想的石墨烯，具有平整且中心對稱的結構，理應不該產生二倍頻效應，但是我們發現當石墨烯是處於懸浮的狀態下，可產生很強的光學二倍頻，本論文中，我們利用飛秒雷射研究石墨烯所產生的二倍頻，分兩種方法來探討二倍頻受到不同石墨烯曲率的影響。 論文第一個部分，我們使用正立式顯微鏡，並且加上自己所建置的三維平台架構，與步進器，將化學蝕刻方法所製造出的金針（尖端外徑約為兩微米以上）放置在其上，慢慢地逼近上方的石墨烯，確定接觸後，再慢慢地上升一至兩微米的距離，並且進行二倍頻訊號的掃描。為了做為對造組，我們也另外使用玻璃針來排除金屬與石墨烯接觸時，材料電子互動對二倍頻所造成的影響。我們發現在使用金針與玻璃針時，皆有在與石墨烯接觸針尖的點上有強烈的二倍頻訊號變化，但使用玻璃針時所觀察到的二倍頻訊號值較使用金針時少。 為了達到三維影像，我們將系統架構轉移至倒立式顯微鏡，改以可移動的物鏡來改變掃描的焦平面，當金針或玻璃針與石墨烯產生接觸時，在不同焦平面得到二倍頻二維訊號圖形，並進一步建置出三維之二倍頻訊號圖，並了解由針所造成的曲率分布，對石墨烯放出的二倍頻訊號分布的影響。我們發現不論是金針或是玻璃針，皆會在三維的二倍頻圖形上，出現一個類似針狀的突起結構，並且在針尖（曲率最大處）產生強烈的二倍頻訊號。 論文第二部份，我們利用石墨烯的轉移技術，將石墨烯轉移至聚二甲基矽氧烷上，由於聚二甲基矽氧烷是個具有彈性的材質，因此我們在轉移前先將其拉開，再轉移上石墨烯，當我們放開受著拉力的聚二甲基矽氧烷時，聚二甲基矽氧烷會恢復原狀，也使得在其上的石墨烯會產生起伏。我們對有波浪狀結構的石墨烯做二倍頻訊號的三維影像，發現在波峰與波谷的位置，相對純粹傾斜的區域比較，有較大的二倍頻訊號。我們進一步對每個點分析其曲率與二倍頻訊號強度的關係，根據統計資料，發現有著正向成長的關係。

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.