이차원 전이 금속 화합물 이종접합체의 이차 조화파 간섭 연구

Abstract

광학 이차 조화파는 입사한 광자의 두 배의 에너지로 새로운 광자가 형성되는 비선형 광학 현상으로, 대칭성이 없는 물질에서만 생성될 수 있다. 따라서 예전부터 필연적으로 대칭성이 깨지는 표면을 연구하는데 많은 활용이 되었으며, 최근에는 이차원 전이 금속 화합물을 연구하는데도 활용되고 있다. 특히 이차원 전이 금속 화합물은 위상정합을 무시할 만한 얇은 두께를 가지고 있고 이종접합시에 다양한 물질 및 적층각도를 선택할 수 있어 이상적인 이차조화파 발생 물질로 각광받고 있다. 본 학위논문에서는 편광 분해 이차조화파 분광학 및 간섭계를 활용하여 이차원 전이금속 화합물 이종접합체인 MoS2/WS2의 SHG 생성 원리에 대해 보고하였다. MoS2/WS2의 편광 분해 이차 조화파 결과가 선행모델로는 설명할 수 없음을 확인하였으며 이는 두 물질간의 SHG 위상차를 고려해야만 설명이 된다는 것을 확인하였다. 두 가지 독립적인 방법인 편광 분해 이차 조화파 분광학 및 간섭계 분광학을 활용하여 실험값으로부터 위상차를 정량화하였으며 두 방법으로 얻은 위상차가 일치하여 해당 가설을 입증할 수 있었다. 학위논문의 두 번째 주제에서는 해당 현상을 다른 종류의 이종접합체 조합으로 확장하고 SHG의 세기 및 위상이 외부 요인에 의해 어떻게 영향을 받는지 탐구하였다. 그 중 MoSe2/WS2 이종접합체에서 적층되지 않은 영역과 적층된 영역의 이차 조화파 세기 및 위상이 차이가 나는 것을 확인하여, 이차 조화파가 층간 상호작용에 영향을 받는 것을 확인하였다. 본 연구는 전이 금속 화합물의 이차 조화파 생성에 관한 이해도를 증진시킬 뿐만 아니라 저차원 물질을 활용한 비선형 광학 소자를 개발하는데도 기여할 것으로 생각된다.

Second-harmonic generation (SHG) is nonlinear optical phenomenon where two-photons with same frequency interact with nonlinear materials to produce new photon with twice the energy of original photon. SHG is well known to be proportional to second-order susceptibility and this term becomes zero in the presence of inversion symmetry. As a result, SHG can be only active in the non-centrosymmetric materials. It has been widely utilized to investigate the chemistry at the surface where symmetry is necessarily broken and utilized in bio-imaging field due to their capability to detect non-centrosymmetric bio-molecules, such as collagen. Recently, SHG started to be used to study two-dimensional transition metal dichalcogenides (2D TMDs). Transition metal dichalcogenides refers to two-dimensional materials that consist of transition metal and chalcogen atoms which include molybdenum, tungsten, and sulfide, selenide, respectively. While graphene exhibits gapless bandgap, which restricts their optoelectronic application, direct bandgap of TMDs makes them to be attractive candidates for applications such as photodetector and atom-thin field effect transistor. Besides, their strong optical nonlinearity was recently demonstrated in SHG spectroscopy. Their atomic thickness defying phase-matching condition and excitonic resonance at near-IR region contributes to efficient SHG in TMDs monolayer. However, SHG study have been limited in intensity aspects while SHG can be delayed with respect to fundamental wave because of complex nature of second-order susceptibility. Although it was reported that polarization-resolved SHG spectroscopy enables determination of crystal orientation of TMDs and SHG can be modulated by electrical gating, phase-resolved SHG spectroscopy has not been carried out until now. In this dissertation, I present that SHG from TMDs heterostructure is governed by optical interference between two SHG fields with different phase delay with respect to fundamental wave. I verified this phenomenon by both polarization-resolved SHG spectroscopy and SHG spectral interferometry in that SHG phase difference from two independent methods coincided with each other. In chapter 1, I will briefly talk about general introduction of transition metal dichalcogenides and overview of SHG. And then, I will introduce experimental methods including light source, sample preparation and SHG spectroscopy. SHG spectroscopy is divided into two parts. One is intensity spectroscopy and the other is phase-resolved SHG spectroscopy. In SHG intensity spectroscopy, I will explain performance of polarization-resolved SHG spectroscopy set-up and how to perform analyzer rotation experiment circumventing optics-induced phase shift and reflectance difference between p- and s-polarization. In phase-resolved SHG spectroscopy, I will talk about principle of spectral interferometry and performance of spectral interferometry that I used. Spectral interferometry was operated in two types, one is interferometry with convex-lens pair and the other is interferometry with off-axis parabolic mirror. The latter was adopted to reduce time delay between two SHG fields from sample and reference. Last, I will explain how to calculate SHG phase difference from SHG interferograms.

In chapter 3, I will talk about SHG interference in atom-thin heterocrystals composed of MoS2 and WS2. In comparison with previous reports about SHG polar plot from MoS2 bilayers, SHG polar plot from MoS2/WS2 exhibited striking difference. In case of heterostructures, SHG node intensity did not converge to zero and this node intensity depended on stack angle. We suggested that this anomaly comes from SHG phase difference and confirmed this idea by polarization-resolved SHG spectroscopy and SHG spectral interferometry. SHG phase differences from two independent method were consistent with each other in the wide range of excitation wavelength. Even, DFT calculation was performed to calculate SHG phase difference and this result also described experimental results very well. In chapter 4, I will expand previous discussion into other combinations of TMDs. Like MoS2 and WS2, MoSe2 and WSe2 also exhibit strong SHG and same SHG polar plot because they belong to same symmetry group of sulfide-based TMDs. As a result, six types of heterostructures are possible out of four different TMDs. Depending on relative SHG intensity and characteristic SHG phase difference, SHG polarization in TMDs heterostructure becomes different. Among these combinations, we found that SHG polar plot from MoSe2/WS2 changed after vacuum annealing, which implies interlayer coupling effect in SHG behavior of heterostructure. We revealed that SHG intensity and phase of each monolayer was different in heterostack region using polarization-resolved spectroscopy and spectral interferometry, respectively. These works will contribute to not only deepening understanding of nonlinear optics in TMDs but also creating novel nonlinear application using low-dimensional materials.