第一原理計算研究二維半導體過渡金屬二硫化物之二階非線性光學性質

Ab intio Calculation on Second Order Nonlinear Optical Effects of 2D Transitional Metal Dichalcogenide Semiconductors

呂昆翰(Kun-Han Lu)

指導教授 ： 郭光宇

國立臺灣大學/理學院/物理學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103262

摘要

二階非線性光學因為其廣泛的應用一直是熱門的題目，其中包含二倍頻效應、線性電光效應以及反常光伏效應。因此我們透過第一原理計算，研究二維過度金屬雙硫族化物MX2 (M=Mo, W; X=S, Se) 的二階非線性光學性質。
透過在Wannier90新增功能，我們開發了非線性光學程式以計算二倍頻效應和線性電光效應。其中，我們以GaAs做為測試材料，我們的計算結果與實驗值極為吻合，驗證了我們程式的正確性。根據原理，二倍頻和線性電光效應的極化率頻譜在頻率為零時，兩者應有相同光極化率強度，我們也在計算結果中確認這個關係。
根據我們的計算結果，我們說明了透過雙光子吸收的概念，我們可以用介電常數來解釋二倍頻極化率頻譜。至於線性電光效應的極化率頻譜，我們只關注頻率在材料的能隙能量之下的頻譜。原因是擁有較大線性電光極化率的材料是應用在光學調變器上，而我們不希望光在傳遞的過程中，光的能量被材料所吸收。我們發現在稍低於能隙能量的頻率上，線性電光極化率總是有一個較大峰值的，這個峰值是由靠近費米能階的電子能帶所主導的。最後，偏移電流為反常光伏效應所誘發的電流，偏移電流跟線性光電導率有高度相關。這可以透過將偏移電流拆分成兩個步驟來理解。電子首先被光激發，然後這些被激發的電子其瓦尼爾波中心產生偏移，進而導致靜電流的產生。

關鍵字

第一原理計算 ； 密度泛函理論 ； 二階非線性光學 ； 二倍頻效應 ； 線性電光效應 ； 偏移電流 ； 反常光伏效應 ； 過度金屬雙硫族化物 ； 二維材料 ； Wannier90

並列摘要

Second order nonlinear optical (NLO) properties, which include second-harmonic generation (SHG), linear electro-optic effect (LEO) and bulk photovoltaic effect (BPVE), have been hot topics for decades due to their wide applications. Thus, we study the NLO properties for two-dimensional transitional metal dichalcogenide (TMD) semiconductors MX2 (M=Mo, W; X=S, Se) via first-principles calculations.
We develop our NLO code through extending the functions in Wannier90 to calculate the LEO and SHG susceptibility. We take GaAs as an example and show our theoretical results are in good agreement with the experimental data, which show the correctness of the code. In principle, the SHG and LEO susceptibilities should follow χSHG(ω = 0) = χLEO(ω = 0), so we also check this relation for our computational results.
Through our computational results, we show that χSHG can be explained with the linear dielectric constant using the concept of two-photon absorption
process. For χLEO, we focus on only the spectrum below band gap energy since the materials with large χLEO are normally used in optical modulator and people are expecting not to have energy lost when the light is propagating in the material. We show that there is always a peak slightly below the band gap energy for χLEO which is dominated by the states near the Fermi level. Last, the shift current conductivity σSC is highly related to the linear optical conductivity. This can be realized through decomposing the shift current into two processes. The electrons are first excited by the photon energy, then the Wannier wave center of the electrons is shifted, thus cause a net current.

並列關鍵字

First-principle calculation ； density functional theory ； second order nonlinear optics ； second-harmonic generatio ； linear electro-optic effect ； shift current ； bulk photovoltaic effect ； transition metal dichalcogenides ； two-dimensional materials ； Wannier90

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