低能量電子點投影顯微鏡全像模擬與影像還原

Abstract

使用以單原子針為電子源的低能量點投影顯微鏡(point projection microscope, PPM in short)可以收取到樣品之高對比投影同軸電子全像圖(in-line hologram)。本篇論文中，我們使用PPM觀察懸空石墨烯表面(suspended graphene)的吸附物。本篇論文的工作，主要是為PPM找尋一套影像重建的方法，重建同軸電子全像圖，以得到高解析的樣品影像。第一部分，我們使用傳統全像術重建實驗全相圖，觀察到懸空石墨烯表面吸附物的結構與動態行為。然而，傳統電子全像術著名的Twin image問題使得重建影像殘留干涉條紋，大幅降低的影像解析度。為了解決Twin image問題，第二部分我們用模擬測試了transport intensity equation(TIE)方法的重建。TIE能夠用兩張或多張在些微不同樣品-屏幕間距記錄的全像圖，重建繞射平面上的相位。再藉由菲涅耳反向傳播(Fresnel backward propagation)，可以計算出物體平面上的樣品穿透函數(transmission function)。模擬結果顯示TIE方法重建影像的解析度有大幅的提升。此外，我們也討論了樣品穿透函數振幅與相位的範圍以及實驗上信噪比可能會對重建造成的問題。

High contrast projection in-line holograms of samples can be acquired by the low energy electron point projection microscope(PPM) based on a single-atom-tip(SAT) emitter. The main purpose of this work is to find an image reconstruction method for PPM, so high resolution images of sample can be obtained from reconstructed in-line holograms. In this thesis, we use PPM to investigate adsorbates on suspended graphene. In the first part, dynamics and structures of the adsorbates were reconstructed by a method of conventional digital holography. However, the resolution was apparently reduced because of a well-known twin-image problem. In the second part, a reconstruction method based on transport intensity equation (TIE) is tested by the simulation to solve the twin-image problem. Using two or more holograms recorded at slightly different sample-screen distances, the phase on the diffraction plane can be reconstructed by TIE. The exit wave in the object plane can then be computed by Fresnel backward propagation. In our simulation, the resolution of the images reconstructed by the TIE method is significantly improved. In additional, problems of reconstruction which caused by amplitude and phase of transmission function and the signal-to-noise ratio are discussed.