Nonlocal Terahertz Nanospectroscopy and Nanoimaging

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In many emerging material systems which will be important for future device applications, the propagation of charges at the material surface can be the key determining factor in device performance. For example, devices based on gallium nitride, which includes most blue LEDs and blue diode lasers, are often limited in their performance by crystalline defects at their surfaces or interfaces, which perturb the transport of electrons between adjacent layers of the materials. In another example, the boundaries between crystalline micro-grains in a polycrystalline film of light-harvesting materials used in solar cells can strongly influence the speed at which charges created by absorbed sunlight are collected, ultimately setting a limit on the device efficiency. This project seeks to develop a suite of new experimental techniques to study such issues, with both high spatial and temporal resolution. These techniques rely on the very strong interaction between the charges moving in the material and electromagnetic radiation in the terahertz spectral range. In some cases, these moving charges can radiate an ultrashort burst of terahertz radiation, which contains important signatures of the charge carrier dynamics. In other cases, a short terahertz pulse reflected from the material surface can be used to characterize the properties of the mobile charges, with temporal resolution on the scale of a picosecond. Our work will extend these ideas to the nanoscale realm, allowing us to study the charge transport using terahertz techniques with spatial resolution of only a few tens of nanometers.The aim of this research program is to demonstrate revolutionary new measurement techniques in terahertz nanoscopy, and use them to glean new information about dynamical processes in materials. In particular, we will establish the idea of non-local THz nanoscopy, by developing a suite of methods such as non-local optical-pump THz-probe nanoscopy and non-local THz emission nanoscopy. We will then use these new experimental techniques in studies of several material systems of current technological relevance. We will initiate collaborations to leverage the expertise of colleagues in sample preparation and calculations of THz material properties. The proposed research will significantly advance the field of terahertz nanoscience by bringing the power of nonlinear optics to the nanoscale with sub-picosecond temporal resolution and nanoscale spatial resolution. Unlike traditional nanoscopy techniques, our new methods will reveal information about lateral charge transport, rather than vertical transport. We will study the transport of polaritons across an individual step edge in a multi-layer graphene film, and probe the effects of an individual grain boundary on charge transport in a polycrystalline perovskite thin film. We will also couple these new ideas to cutting-edge results in terahertz vibrational spectroscopy, by studying the effects of local nanoscale defects on vibrational modes with mesoscopic coherence lengths, and by observing the influence of nanoscale excitations on simple chemical reactions mediated by terahertz vibrations. These measurements will open up new possibilities for the study of nanoscale phenomena in materials, revealing important information that cannot be obtained using other methods.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.