二硫化鎢/二硒化鎢雙層異質結構中層間激子之谷偏振動力學

Abstract

過渡金屬二硫屬化物 (TMDs) 因其直接能隙和強烈的激子效應而聞名，當它們組成范德瓦耳斯異質結構時，展現出獨特的光學與電子特性。由於層間的晶格不匹配或扭轉角度，形成的莫爾超晶格會產生周期性位能井，對激子的行為產生顯著影響。此外，在許多實驗中也觀察到層間激子具有相當豐富的量子現象。 在本研究中，我們報導了WS₂/WSe₂ 異質結構中的谷極化動力學。透過反射對比和功率依賴的時間解析光致發光測量，我們觀察到了層間激子的不同能量狀態以及谷極化的壽命。為了進一步探索相關電子態，我們開發了一種時間解析的泵浦-推進光致發光測量方法。我們展示了莫爾位點內和位點間激子的交互作用如何主導整體的自旋-谷弛豫行為。光驅動的不可壓縮激子態(填充因子等於1)被發現能在 15 ns 內放鬆至普通激子態，而莫爾位點內交互作用迅速降低了兩個激子同時佔據一個莫爾位點時的谷極化現象。 根據實驗結果，我們推導出了一組耦合速率方程，用於描述相關系統中激子態隨時間的演化。這使我們能進一步理解填充因子變化如何影響每個躍遷的速率，以及不同躍遷在同一激子態中的相對貢獻。 結果強調了瞬態光致發光測量在理解相關激子動力學中的重要性。本研究不僅拓展了對范德瓦耳斯異質結構中谷極化動力學的基礎認識，還為開發下一代量子與光電子設備奠定了基礎。

Transition metal dichalcogenides (TMDs), known for their direct band gaps and strong excitonic effects, exhibit unique optical and electronic properties when combined into van der Waals heterostructures. The resulting moiré superlattices, formed by lattice mismatches or twist angles between layers, create periodic potentials that significantly influence exciton behaviors. Moreover, even richer phenomena have been observed from the interlayer exciton.[1], [2], [3], [4], [5], [6] Here, we report the observation of valley polarization dynamics in WS₂/WSe₂ heterostructures. Through reflectance contrast and power-depend time-resolved photoluminescence (TRPL), distinct energy states and valley polarization lifetimes from interlayer excitons were observed. A time-resolved pump-push TRPL measurement was developed to further explore correlated electronic states. We show that the interplay between on-site and inter-site exchange interactions governs the overall spin-valley relaxation. The optically driven incompressible exciton states (filling factor=1) were found to relax to normal exciton states within 15ns, and the on-site exchange interactions arising from incompressible exciton states rapidly reduced the valley polarization of doublons. Based on the experimental results, we derived a coupled rate equation to describe the temporal evolution of exciton states within the correlated system. This allowed us to further understand how changes in the filling factor affect each transition and the relative contributions of different transitions within the same exciton state. The results highlight the importance of transient PL measurements in understanding correlated excitonic dynamics. This work not only advances fundamental knowledge of the valley polarized dynamics in the van der Waals heterostructure but also provides a foundation for developing next-generation quantum and optoelectronic devices.