Study of exciton-polaritons in 2D semiconductor via strong light-matter coupling

Abstract

Strong light-matter coupling in semiconductors coupled with optical microcavities results in the half-light and half-matter bosonic quasiparticles known as exciton-polaritons. Two-dimensional (2D) transition metal dichalcogenides (TMDs) have received great attention as an active material for exploring cavity quantum electrodynamics due to their intriguing excitonic properties, including a large exciton binding energy of 0.5 – 1 eV and the valley-dependent optical selection rules. However, an atomically thin dimensionality of TMD materials impedes the formation of exciton-polaritons due to the limited spatial overlap between 2D excitons and cavity photon fields. In conventional planar cavity devices, in particular, achieving a strong coupling between excitons and photons is challenging. It requires not only the fabrication of sophisticated cavity structures but also the precise placement of TMD materials at the antinodes of the photon fields. Additionally, the inhomogeneous broadening of TMD excitons, induced by their surrounding external disorders such as the adsorbates, strain, and substrate roughness, poses an obstacle to examining polariton properties based on TMD materials. In this thesis, we explored the light-matter coupling properties of WS2 integrated into cavities supporting the whispering gallery and guided photon modes, eliminating the need for sophisticated cavity fabrication or precise placement of monolayer WS2, and we have successfully demonstrated strong light-matter coupling based on 2D semiconductors. As the first work, we designed ZnO-WS2 hybrid structures as an optical system, which can be fabricated by simply transferring ZnO microrods onto monolayer WS2 and demonstrated the polarization-controlled amplification of excitonic emission in the monolayer WS2 coupled with ZnO microcavity. In the second work, we revealed that encapsulation of WS2 using hexagonal boron nitride (h-BN) passivates defects of WS2 crystals, significantly reducing inhomogeneous broadening of the excitons. Finally, we achieved the strong coupling in one-dimensional waveguide structures composed of h-BN encapsulated monolayer WS2 and demonstrated valley polarization-controlled chiral transport of waveguide valley polaritons. |본 논문은 2차원 반도체를 이용한 엑시톤-폴라리톤 상태 구현을 위해 수행된 단계적인 연구 결과를 제시한다. 2차원 반도체의 원자 층 수준의 얇은 두께는 2차원 엑시톤과 광 공진기에 구속된 정상파 간 공간적인 중첩을 극히 제한하며, 특히 일반적으로 사용되는 평면 공진기의 경우 2차원 반도체를 정상파의 진폭이 최대인 배의 위치에 정확히 배치해야만 강한 빛-물질 상호작용을 유도할 수 있다. 이러한 문제를 극복하기 위하여 ZnO-WS2 하이브리드 구조를 도입했고, ZnO에서 형성되는 높은 Q-factor를 갖는 whispering gallery modes와 WS2 엑시톤의 약한 빛-물질 상호작용 효과로 2차원 반도체 엑시톤의 편광 제어된 자연증폭방출을 증명하였다. 하지만, 본 구조의 경우 WS2층과 ZnO 공진기가 접하는 계면에서만 엑시톤과 광자의 상호작용이 가능하여, 강한 빛-물질 상호작용을 유도하는 것에 한계가 있음을 확인하였다. 이에 h-BN/WS2/h-BN 도파관 구조를 도입하였고, 선행 연구에서 증명된 h-BN encapsulation에 의한 WS2 층에서의 안정적인 엑시톤 형성, 그리고 WS2층을 따라 전파하는 도파관 구조의 광자 특성으로부터 엑시톤과 광자 간의 강한 상호작용을 유도하여 최종적으로 2차원 반도체 엑시톤-폴라리톤 상태를 관찰하였다.