[论文解读] Lightwave-driven electrons in a Floquet topological insulator

Tobias Weitz, Daniel Lesko+5 more|arXiv (Cornell University)|Jul 25, 2024

Topological Materials and Phenomena被引用 8

一句话总结

本文展示了在光照下的石墨烯中实现全光控制的光电流，形成 Floquet 拓扑绝缘体（FTI），并测量光波驱动的异常霍尔电流、圆二色性以及谷极化，理论与实验高度一致。

ABSTRACT

Topological insulators offer unique opportunities for novel electronics and quantum phenomena. However, intrinsic material limitations often restrict their applications and practical implementation. Over a decade ago it was predicted that a time-periodic perturbation can generate out-of-equilibrium states known as Floquet topological insulators (FTIs), hosting topologically protected transport and anomalous Hall physics, and opening routes to optically tunable bandstructures and devices compatible with petahertz electronics. Although such states have not yet been directly observed, indirect signatures such as the light-induced anomalous Hall effect were recently measured. Thus far, much remained experimentally unclear and fundamentally unknown about solid-state FTI and whether they can be employed for electronics. Here we demonstrate coherent control of photocurrents in light-dressed graphene. Circularly-polarized laser pulses dress the graphene band structure to obtain an FTI, and phase-locked second harmonic pulses drive electrons in the FTI. This approach allows us to measure resulting all-optical anomalous Hall photocurrents, FTI-valley-polarized currents, and photocurrent circular dichroism, all phenomena that put FTIs on equal footing with equilibrium topological insulators. We further present an intuitive description for the sub-optical-cycle light-matter interaction, revealing dynamical symmetry selection rules for photocurrents. All measurements are supported by strong agreement with ab-initio and analytic theory. Remarkably, the photocurrents show a strong sub-cycle phase-sensitivity that can be employed for ultrafast control in topotronics and spectroscopy. Our work connects Floquet and topological physics with attoscience and valleytronics, and goes beyond band structure engineering by initiating lightwave-driven dynamics in FTI states.

研究动机与目标

• 通过对石墨烯进行光 dressing，在固态系统中推动并探索 Floquet 拓扑绝缘体（FTIs）的创建与研究。
• 展示使用两色（ω 与 2ω）光源实现对光电流的全光控制，以探针 FTI 的性质。
• 测量由 Floquet 工程化能带产生的圆二色性、异常霍尔电流和谷极化。
• 结合对称性考虑，发展一个对亚周期光物质相互作用的直观描述，以解释光电流的产生。
• 用从头 TDDFT 的数值模拟和解析理论来验证实验结果，将电流归因于 FTI 状态。

提出的方法

• 用圆偏振的基频场 ω 对石墨烯进行包裹，产生带有非零能隙和 Berry 曲率的 Floquet-Bloch 能带。
• 使用同向传播的二次谐波 2ω 脉冲驱动超快电子动力学并生成光电流。
• 控制 ω 与 2ω 之间的相对相位 φ_{ω-2ω}，以探测亚周期动力学并测量相位相关的电流。
• 改变 2ω 脉冲的椭圆度，以研究圆二色性和拓扑信号。
• 将实验结果与从头 TDDFT 模拟及解析理论进行比较，以识别谷机制和拓扑驱动的贡献。

实验结果

研究问题

• RQ1 光照如何将石墨烯转换为 Floquet 拓扑绝缘体，光电流中会出现哪些拓扑特征？
• RQ2 通过相对相位和椭圆度，两色 ω 与 2ω 的场控如何表现圆二色性和异常霍尔电流？
• RQ3 在 Floquet 驱动下，谷自由度（K 和 K′）对光电流和谷极化的贡献有多大？
• RQ4 哪些动态对称性选择规则支配亚周期电子运动和 FTIs 中的光电流产生？
• RQ5 从头计算模拟是否支持实验观察，即电流来自 Floquet 物理而非边缘态？

主要发现

• 通过改变 2ω 的椭圆度引发的圆二色性显示出与 TDDFT 模拟一致的强电流调制。
• 当 ω-dressed FTI 在 2ω 垂直偏振驱动时，出现全光异常霍尔电流。
• 观察到高达约 25% 的谷极化电流，表明在 FTI 中存在谷选择性传输。
• 通过改变 φ_{ω-2ω} 实现对异常霍尔电流的相位控制翻转。
• TDDFT 与从头理论与实验一致，指向 Floquet 带起源而非边缘态贡献。
• 在特定配置下展示了高达 31 pA 的谷电流，凸显 FTIs 中谷电子学的潜力。