Ultrafast terahertz measurements of quantum dynamics in matter

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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.