[论文解读] Observation and active control of a collective polariton mode and polaritonic band gap in few-layer WS2 strongly coupled with plasmonic lattices

Wenjing Liu, Yuhui Wang+8 more|arXiv (Cornell University)|Aug 20, 2018

Strong Light-Matter Interactions被引用 4

一句话总结

本研究展示了在少层二硫化钨（WS2）与等离子体纳米颗粒超晶格强耦合体系中，观察到集体极化激元模式及完整的极化激元能带隙，并实现了其主动电调制。该集体模式源于由激子-激子相互作用增强的协同激子-等离子体耦合，其模式分裂随激子振子强度以超线性方式增加，从而可通过场效应晶体管集成实现主动调控。

ABSTRACT

Two-dimensional semiconductors host excitons with very large oscillator strengths and binding energies due to significantly reduced carrier screening. Two-dimensional semiconductors integrated with optical cavities are emerging as a promising platform for studying strong light-matter interactions as a route to explore a variety of exotic many-body effects. Here, in few-layered WS2 coupled with plasmonic nanoparticle lattices, we observe the formation of a collective polaritonic mode near the exciton energy and the formation of a complete polariton band gap with energy scale comparable to the exciton-plasmon coupling strength. A coupled oscillator model reveals that the collective mode arises from the cooperative coupling of the excitons to the plasmonic lattice diffraction orders via exciton-exciton interactions. The emergence of the collective mode is accompanied by a superlinear increase of the polariton mode splitting as a function of the square root of the exciton oscillator strength. The presence of these many body effects, which are enhanced in systems which lack bulk polarization, not only allows the formation of a collective mode with periodically varying field profiles, but also further enhances the exciton-plasmon coupling. By integrating the hybrid WS2-plasmonic lattice device with a field-effect transistor, we demonstrate active tuning of the collective mode and the polariton band gap. These systems provide new opportunities for obtaining a deeper and systematic understanding of many body cooperative phenomena in periodic photonic systems and for designing more complex and actively controllable polaritonic devices including switchable polariton lasers, waveguides, and optical logical elements.

研究动机与目标

• 研究二维半导体与等离子体超晶格耦合体系中的强光-物质相互作用。
• 探索低维体系中多体效应的出现，如集体极化激元模式与极化激元能带隙。
• 通过场效应晶体管结构演示对这些集体模式与能带隙的主动电调制。
• 理解在缺乏体极化的体系中，激子-激子相互作用在增强激子-等离子体耦合中的作用。

提出的方法

• 将少层WS2与周期性等离子体纳米颗粒超晶格集成，以实现强激子-等离子体耦合。
• 采用耦合振子模型，解释集体极化激元模式源于对衍射级次的协同耦合。
• 利用角度分辨光致发光光谱测量，绘制极化激元色散关系并识别能带隙。
• 制备具有背栅场效应晶体管结构的WS2-等离子体超晶格异质结构器件，实现电调制。
• 测量模式分裂与激子振子强度平方根的关系，以确认多体效应增强。
• 利用场效应门控调节载流子密度，电控调节极化激元模式能量与能带隙。

实验结果

研究问题

• RQ1 在少层WS2与等离子体超晶格耦合体系中，集体极化激元模式如何产生？其微观起源是什么？
• RQ2 激子-激子相互作用在增强激子-等离子体耦合强度并促成集体行为中起什么作用？
• RQ3 能否通过外部电场门控主动调节极化激元能带隙的能量与宽度？
• RQ4 模式分裂如何随激子振子强度变化？这反映了何种多体效应？
• RQ5 该体系的光学响应在多大程度上可被调控，以适用于有源极化激元器件？

主要发现

• 在接近激子能量处观察到集体极化激元模式，其源于激子通过激子-激子相互作用对等离子体超晶格衍射级次的协同耦合。
• 形成完整的极化激元能带隙，其能量尺度与激子-等离子体耦合强度相当，表明处于强耦合 regime。
• 极化激元模式分裂随激子振子强度平方根的超线性增加，证实了多体效应增强。
• 由于等离子体超晶格的周期性与相干耦合，集体模式表现出周期性变化的场分布。
• 与场效应晶体管集成，实现了对集体模式能量与极化激元能带隙的主动电调制。
• 该体系为可主动调控的极化激元器件（如可切换激光器、波导与光学逻辑元件）提供了实现路径。