Garg Manish, Martin-Jimenez Alberto, Luo Yang, Kern Klaus
Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany.
Institut de Physique, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
ACS Nano. 2021 Nov 23;15(11):18071-18084. doi: 10.1021/acsnano.1c06716. Epub 2021 Nov 1.
Unification of the techniques of ultrafast science and scanning tunneling microscopy (STM) has the potential of tracking electronic motion in molecules simultaneously in real space and real time. Laser pulses can couple to an STM junction either in the weak-field or in the strong-field interaction regime. The strong-field regime entails significant modification (dressing) of the tunneling barrier of the STM junction, whereas the weak-field or the photon-driven regime entails perturbative interaction. Here, we describe how photons carried in an ultrashort pulse interact with an STM junction, defining the basic fundamental framework of ultrafast photon-induced tunneling microscopy. Selective dipole coupling of electronic states by photons is shown to be controllable by adjusting the DC bias at the STM junction. An ultrafast tunneling microscopy involving photons is established. Consolidation of the technique calls for innovative approaches to detect photon-induced tunneling currents at the STM junction. We introduce and characterize here three techniques involving dispersion, polarization, and frequency modulation of the laser pulses to lock-in detect the laser-induced tunneling current. We show that photon-induced tunneling currents can simultaneously achieve angstrom scale spatial resolution and sub-femtosecond temporal resolution. Ultrafast photon-induced tunneling microscopy will be able to directly probe electron dynamics in complex molecular systems, without the need of reconstruction techniques.
将超快科学技术与扫描隧道显微镜（STM）相结合，有可能在实空间和实时条件下同时追踪分子中的电子运动。激光脉冲可以在弱场或强场相互作用 regime 下与STM结耦合。强场 regime 会导致STM结的隧穿势垒发生显著改变（修整），而弱场或光子驱动 regime 则会产生微扰相互作用。在此，我们描述了超短脉冲中携带的光子如何与STM结相互作用，定义了超快光子诱导隧穿显微镜的基本框架。通过调整STM结处的直流偏置，表明光子对电子态的选择性偶极耦合是可控的。建立了一种涉及光子的超快隧穿显微镜。该技术的整合需要创新方法来检测STM结处的光子诱导隧穿电流。我们在此介绍并描述了三种涉及激光脉冲色散、偏振和频率调制的技术，以锁定检测激光诱导的隧穿电流。我们表明，光子诱导的隧穿电流可以同时实现埃级空间分辨率和亚飞秒时间分辨率。超快光子诱导隧穿显微镜将能够直接探测复杂分子系统中的电子动力学，而无需重建技术。
