第一原理理論計算過渡金屬雙硫族化物原子厚度薄膜之二階非線性光學性質

Abstract

作為一個具有新穎並顯著的電子和光學性質的二維材料，單層過渡金屬雙硫族化物近期獲得了大量的關注，例如與層相關的性質和具備相當大的能隙的二維材料。在這篇論文中，我使用贗勢能及平面波方法對過渡金屬二硫屬化物二階非線性光學性質進行第一原理計算。晶體結構則是參照實驗數據。我們發現所有的單，三層過渡金屬雙硫族化物包括加垂直平面電場的單層過渡金屬雙硫族化物都具有顯著的二階非線性光學常數。 單層WSe2顯示了最大的二階非線性光學常數並表現出最大的線性電光係數。三層膜的的幾何關係，二階非線性光學係數的質應該降低為單層膜的1/3左右，而由於層與層之間的交互作用，使的二階非線性光學係數的值並非在預期的1/3。在單層和三層膜MX2薄膜的二階非線性光學光譜 在單光子和雙光子共振條件下都與線性光學介電函數ɛ(ω)的計算相吻合。我還研究了在單層MX2上加上電場的光學性質。通過施加垂直於薄膜平面的電場將鏡像對稱打破，可以得到新的二階非線性光學係數非零項。這些新的非線性光學常數非零項的光譜 ，也與介電函數ɛ(ω)單光子與雙光子共振的現象有直接的關聯。計算的結果表明，新的非零項大小與所施加場的強度呈線性關係，並得到新的線性電光係數。因此，單層過渡金屬雙硫族化物MX2不但具有發展二維之二階非線性光學材料的潛力，並具有可藉電場調控的二階非線性光學常數，因而具有更加多元彈性的應用。

Transition metal dichalcogenides monolayers are getting lots of attention as a new type of two dimensional materials for their significant electronic and optical properties such as layer-dependent properties and sizable band gaps. In this thesis, a systematic first principle study of second-order nonlinear optical properties of transition metal dichalcogenides is performed under full-potential projector-augmented wave method. The underlying crystal structures are determined by experimental data. We found that all MX2 monolayers and trilayers including electric field applied MX2 monolayers display significant second-order harmonic generation coefficients. WSe2 monolayer presents the largest second-order harmonic generation susceptibility and it also displays largest linear electro-optical coefficient. Because of the geometric factor, the second harmonic susceptibilities of trilayers are expected to reduce to 1/3 of the monolayers. Due to the interlayer interaction, we found that the does not exactly reduce to 1/3. The prominent features in the spectra of for MX2 monolayers and trilayers are successfully correlated with the features in linear optical dielectric function ɛ(ω) in terms of single- and two-photon resonances. This thesis also investigates the MX2 monolayers under an applied electric field. By applying a vertical electric field to the sheet plane, the mirror symmetry is broken and MX2 monolayers display new nonzero terms of second-order nonlinear optical coefficients. These new nonzero spectra of are also correlated with dielectric function ɛ(ω). Our results show that the new nonzero terms are linearly dependent on the strength of the applied electric field. The new linear electro-optical coefficients are also found.