Ultrafast terahertz measurements of quantum dynamics in matter

相关电子和量子材料正在寻找高效能量收集设备的应用，从光伏和热电到低功率电子。然而，材料科学只触及了其潜力的表面，对其特性的深刻理解仍然是一个重大挑战。电荷、晶格、轨道和自旋自由度之间复杂的相互作用是它们丰富性质的基础，这使得微观层面的理解在所有方面都很困难。这些相互作用的自然能量尺度通常在太赫兹（THz）区域，时间分辨太赫兹技术是通过观察随光学扰动动态展开的涌现相来解开这种复杂性的强大工具。我的研究计划的长期目标是开发和应用新的时间分辨太赫兹技术来探测物质的量子动力学，并了解与能量收集应用相关的紧急特性的起源。目前人们感兴趣的材料是具有高热电性能的金属单硫族化合物。我们采取了三种不同的方法来完成这项工作，可以在短期内完成：第一种是使用时间分辨超宽带太赫兹光谱来观察准粒子诞生时的量子动力学，观察电荷和晶格之间的相关性和相互作用。我们最近利用这项研究研究了VO2中绝缘体到金属的转变，并观察了杂化金属卤化物钙钛矿中的极化子相干量子拍。我们将探索具有电子晶体但声子玻璃性质的热电，准二维金属单硫族化合物，具有强电子-声子耦合元素和依赖于相位的拓扑结构。第二个扩展了我们最近的工作，即由高场单光周期太赫兹光脉冲驱动的金属纳米尖的超快电子脉冲。在此基础上，我们开发了一种具有电子束电荷的超快点投影电子显微镜，可在飞秒时间和纳米尺度上进行单次成像。该系统具有无像差，振动不敏感的电子显微镜接近原子分辨率的潜力，将非常适合表征二维量子材料的结构动力学和电荷再分配。第三种方法是将定制太赫兹光脉冲应用于旨在直接操纵有序参数的新型量子控制实验。我的小组最近展示了一种新技术，可以任意控制太赫兹光脉冲的时间电场分布。由于该方法与新的高场太赫兹产生技术兼容，我们现在可以首次在量子材料上进行多脉冲、复杂波形的太赫兹实验。这些将应用于探索通过工程光物质相互作用对材料进行量子控制的新概念。

Correlated electron and quantum materials are finding applications in high efficiency energy harvesting devices, from photovoltaics and thermoelectrics to low power electronics. Material science has only scratched the surface of their potential, however, and a deep understanding of their properties remains a major challenge. The complex interplay between the charge, lattice, orbital and spin degrees of freedom underlying their rich properties makes a microscopic level understanding difficult on all fronts. The natural energy scale of these interactions is often in the terahertz (THz) region, and time-resolved THz techniques are powerful tools to disentangle this complexity by watching emergent phases dynamically unfold following optical perturbation. The long-term goal of my research program is to develop and apply new time-resolved THz techniques to probe the quantum dynamics of matter and understand the origin of emergent properties relevant for energy harvesting applications. Current materials of interest are the metal monochalcogenides showing high thermoelectric performance. We take three distinct approaches to this effort that can be accomplished in the short term: The first is to use time-resolved ultrabroadband THz spectroscopy to observe the quantum kinetics of quasiparticles as they are born, watching as correlations and interactions develop between charge and lattice. We have recently used this study the insulator-to-metal transitions in VO2 and to observe polaron coherent quantum beats in the hybrid metal halide perovskites. We will explore the thermoelectric, quasi-2D metal monochalcogenides exhibiting electron crystal but a phonon glass properties, with elements of strong electron-phonon coupling and topology depending on the phase. The second extends our recent work on ultrafast electron pulses from a metal nanotip driven by high field, single optical cycle THz light pulses. Based on this, we are developing an ultrafast point-projection electron microscope with electron bunch charges compatible with single-shot imaging on femtosecond time and nm length scales. This system holds the potential for aberration-free, vibration-insensitive electron microscopy approaching atomic resolution and will be ideally suited to characterize structural dynamics and charge redistribution in 2D quantum materials. The third approach is in the application of tailored THz light pulses for novel quantum control experiments aimed at manipulating order parameters directly. My group recently demonstrated a new technique to arbitrarily control the temporal electric field profile of THz light pulses. As this method is compatible with new high field THz generation techniques, we can now do multi-pulse, complex waveform THz experiments on quantum materials for the first time. These will be applied to explore a new concept of quantum control over materials by engineered light-matter interactions.