近日，德国马堡大学的S.Ito与雷根斯堡大学的R.Huber以及马普物质结构与动力学研究所的M. A.Sentef等人合作，研究了Floquet-Bloch带在亚周期时间尺度上的建立和失相。相关成果已于2023年4月12日在国际权威学术期刊《自然》上发表。

该研究使用时间分辨和角度分辨光电发射光谱技术，可视化Floquet-Bloch带的超快建立过程，并提供了这种关联。研究人员利用中红外场在拓扑绝缘体表面态上驱动，产生足够强的高次谐波，并用亚周期时间分辨率直接监测瞬态带结构。从强带内电流开始，研究人员观察到Floquet侧带在一个光学周期内出现，多个侧带同时进行带内加速，直到高能电子散射到体态并耗散破坏Floquet带。量子非平衡计算解释了Floquet态与带内和带间动力学的同时发生。该研究的联合实验和理论研究提供了Floquet物理的直接时间域视角，并探索了超快带结构工程的基本前沿。

据悉，强光场提供了调控固体材料新功能的机会。周期性电子驱动可形成Floquet-Bloch态，产生异乎寻常的量子相位。在亚周期时间尺度内，光波可同时驱动带内电流和带间跃迁，实现高次谐波生成，为超快电子学铺平道路。然而，带内和带间激发的相互作用及其与Floquet物理的关系一直是关键的未解问题，因为Floquet态的动力学方面一直是难以捉摸的。

附：英文原文

Title: Build-up and dephasing of Floquet–Bloch bands on subcycle timescales

Author: Ito, S., Schler, M., Meierhofer, M., Schlauderer, S., Freudenstein, J., Reimann, J., Afanasiev, D., Kokh, K. A., Tereshchenko, O. E., Gdde, J., Sentef, M. A., Hfer, U., Huber, R.

Issue&Volume: 2023-04-12

Abstract: Strong light fields have created opportunities to tailor novel functionalities of solids^1,2,3,4,5. Floquet–Bloch states can form under periodic driving of electrons and enable exotic quantum phases^{6,7,8,9,10,11,12,13,14,15}. On subcycle timescales, lightwaves can simultaneously drive intraband currents^{16,17,18,19,20,21,22,23,24,25,26,27,28,29} and interband transitions^18,19,30,31, which enable high-harmonic generation^{16,18,19,21,22,25,28,29,30} and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet–Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator^32,33 with mid-infrared fields—strong enough for high-harmonic generation—and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.

DOI: 10.1038/s41586-023-05850-x

Source: https://www.nature.com/articles/s41586-023-05850-x