Department of Physics, Philipps-University of Marburg, Marburg, Germany.
Department of Physics, University of Regensburg, Regensburg, Germany.
Nature. 2018 Oct;562(7727):396-400. doi: 10.1038/s41586-018-0544-x. Epub 2018 Sep 26.
Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids, charge transport in nanostructures, attosecond-streaking experiments and atomic-resolution ultrafast microscopy. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity. The Dirac-like, quasi-relativistic band structure of topological insulators may allow these constraints to be lifted and may thus open a new era of lightwave electronics. To understand the associated, complex motion of electrons, comprehensive experimental access to carrier-wave-driven currents is crucial. Here we report angle-resolved photoemission spectroscopy with subcycle time resolution that enables us to observe directly how the carrier wave of a terahertz light pulse accelerates Dirac fermions in the band structure of the topological surface state of BiTe. While terahertz streaking of photoemitted electrons traces the electromagnetic field at the surface, the acceleration of Dirac states leads to a strong redistribution of electrons in momentum space. The inertia-free surface currents are protected by spin-momentum locking and reach peak densities as large as two amps per centimetre, with ballistic mean free paths of several hundreds of nanometres, opening up a realistic parameter space for all-coherent lightwave-driven electronic devices. Furthermore, our subcycle-resolution analysis of the band structure may greatly improve our understanding of electron dynamics and strong-field interaction in solids.
利用光载波作为交流偏置可能使电子设备达到光学时钟速率。光波驱动电流被认为对于固体中的高次谐波产生、纳米结构中的电荷输运、阿秒条纹实验和原子分辨率超快显微镜是必不可少的。然而，在传统的半导体和电介质中，电子的有限有效质量和超快散射限制了它们的弹道运动和速度。拓扑绝缘体的类狄拉克、准相对论能带结构可能允许消除这些限制，并因此开辟一个新的光波电子时代。为了理解相关的电子复杂运动，对载波驱动电流进行全面的实验研究至关重要。在这里，我们报告了具有亚周期时间分辨率的角分辨光发射谱，使我们能够直接观察太赫兹光脉冲的载波如何在 BiTe 的拓扑表面态的能带结构中加速狄拉克费米子。虽然光发射电子的太赫兹条纹追踪表面的电磁场，但狄拉克态的加速导致动量空间中电子的强烈再分布。无惯性的表面电流受到自旋-动量锁定的保护，峰值密度高达每厘米 2 安培，弹道平均自由程可达数百纳米，为全相干光波驱动电子设备开辟了一个现实的参数空间。此外，我们对能带结构的亚周期分辨率分析可能会极大地提高我们对固体中电子动力学和强场相互作用的理解。
