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Effects of Interlayer Coupling and Band Offset on Second Harmonic Generation in Vertical MoS2/MoS2(1-x)Se2x Structures.
ACS Nano ( IF 15.8 ) Pub Date : 2020-03-26 , DOI: 10.1021/acsnano.9b09901 Chinh Tam Le 1 , Jungcheol Kim 2 , Farman Ullah 1 , Anh Duc Nguyen 1 , Thao Nhi Nguyen Tran 1 , Tuan-Em Le 3 , Koo-Hyun Chung 3 , Hyeonsik Cheong 2 , Joon I Jang 2 , Yong Soo Kim 1
ACS Nano ( IF 15.8 ) Pub Date : 2020-03-26 , DOI: 10.1021/acsnano.9b09901 Chinh Tam Le 1 , Jungcheol Kim 2 , Farman Ullah 1 , Anh Duc Nguyen 1 , Thao Nhi Nguyen Tran 1 , Tuan-Em Le 3 , Koo-Hyun Chung 3 , Hyeonsik Cheong 2 , Joon I Jang 2 , Yong Soo Kim 1
Affiliation
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Noncentrosymmetric monolayers (MLs) of transition metal dichalcogenides (TMDCs) and their 3R-type vertical stacks provide an ideal platform for studying atomic-scale nonlinear light-matter interaction in terms of second harmonic generation (SHG). Unlike the case of MLs, SHG from artificial stacks can be nontrivially affected by interlayer coupling and band offset between the constituent MLs, where the latter occurs for band-gap-engineered vertical heterostructures (VHs). In order to study these effects, we produced different sets of 3R-type homobilayers (homo-BLs) and heterobilayers (hetero-BLs) composed of MoS2 and its ternary alloy MoS2(1-x)Se2x. We first investigated the impact of interlayer coupling on the SHG response across the A- and B-exciton resonances in the MoS2 homo-BLs. The coupling strength was varied by preparing (i) decoupled BLs (SiO2 intercalated), (ii) weakly coupled BLs (dry transferred), and (iii) strongly coupled BLs (postannealed) and monitored by photoluminescence, Raman, and reflectance difference spectroscopy, and atomic force microscopy. Unlike the decoupled BL, SHG in the coupled BLs cannot be explained by the simple square law in thickness due to coupling-induced band modification. The impact of exciton-resonance offset on SHG was also investigated in the hetero-BLs by controlling the Se concentration in MoS2xSe2(x-1). Although these VHs can significantly broaden the spectral range for efficient SHG by vertically superposing distinct resonances of the constituent MLs, coherent reinforcement of SHG cannot be achieved basically because of the π/2 phase difference between the on-resonance SHG field in one ML and the off-resonance SHG field in the other ML. Upon postannealing, however, the overlapping resonance regime exhibited unexpectedly high SHG enhancement. This may arise from the formation of the strong resonance when the VHs approach ideal 3R-type hetero-BLs. Our approach may be utilized for fully exploiting the TMDC VHs for highly efficient broadband SHG applications.
中文翻译：
垂直MoS2 / MoS2（1-x）Se2x结构中层间耦合和能带偏移对第二谐波产生的影响。
过渡金属二硫化氢（TMDC）的非中心对称单分子层（MLs）及其3R型垂直叠层为研究基于二次谐波（SHG）的原子级非线性光物质相互作用提供了理想的平台。与ML的情况不同，来自人工堆叠的SHG会受到层间耦合和组成ML之间的带偏移的影响，而后者在带隙工程垂直异质结构（VH）中会发生。为了研究这些影响，我们生产了由MoS2及其三元合金MoS2（1-x）Se2x组成的3R型同质双分子层（homo-BLs）和异质双分子层（hetero-BLs）。我们首先研究了MoS2 homo-BLs中跨A和B激子共振的层间耦合对SHG响应的影响。通过准备（i）解耦的BL（嵌入SiO2），（ii）弱耦合的BL（干转移）和（iii）强耦合的BL（后退火）并通过光致发光，拉曼光谱和反射率差光谱法进行监测来改变偶联强度和原子力显微镜。与解耦的BL不同，由于耦合引起的能带修改，耦合的BL中的SHG无法用简单的平方律来解释。还通过控制MoS2xSe2（x-1）中的Se浓度，研究了杂质BLs中激子共振偏移对SHG的影响。尽管这些VH通过垂直叠加组成ML的不同共振可以显着拓宽有效SHG的频谱范围，由于一个ML的谐振SHG场和另一个ML的失谐SHG场之间存在π/ 2相位差，因此基本上无法实现SHG的相干增强。然而，在退火后，重叠的共振方式表现出出乎意料的高SHG增强。当VH接近理想的3R型异质BL时，这可能是由于形成强共振而引起的。我们的方法可用于为高效宽带SHG应用充分利用TMDC VH。
更新日期：2020-03-26
中文翻译：
垂直MoS2 / MoS2（1-x）Se2x结构中层间耦合和能带偏移对第二谐波产生的影响。
过渡金属二硫化氢（TMDC）的非中心对称单分子层（MLs）及其3R型垂直叠层为研究基于二次谐波（SHG）的原子级非线性光物质相互作用提供了理想的平台。与ML的情况不同，来自人工堆叠的SHG会受到层间耦合和组成ML之间的带偏移的影响，而后者在带隙工程垂直异质结构（VH）中会发生。为了研究这些影响，我们生产了由MoS2及其三元合金MoS2（1-x）Se2x组成的3R型同质双分子层（homo-BLs）和异质双分子层（hetero-BLs）。我们首先研究了MoS2 homo-BLs中跨A和B激子共振的层间耦合对SHG响应的影响。通过准备（i）解耦的BL（嵌入SiO2），（ii）弱耦合的BL（干转移）和（iii）强耦合的BL（后退火）并通过光致发光，拉曼光谱和反射率差光谱法进行监测来改变偶联强度和原子力显微镜。与解耦的BL不同，由于耦合引起的能带修改，耦合的BL中的SHG无法用简单的平方律来解释。还通过控制MoS2xSe2（x-1）中的Se浓度，研究了杂质BLs中激子共振偏移对SHG的影响。尽管这些VH通过垂直叠加组成ML的不同共振可以显着拓宽有效SHG的频谱范围，由于一个ML的谐振SHG场和另一个ML的失谐SHG场之间存在π/ 2相位差，因此基本上无法实现SHG的相干增强。然而，在退火后，重叠的共振方式表现出出乎意料的高SHG增强。当VH接近理想的3R型异质BL时，这可能是由于形成强共振而引起的。我们的方法可用于为高效宽带SHG应用充分利用TMDC VH。
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