Nanoscale Nonlinear Terahertz Spectroscopy

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As semiconducting devices become smaller and smaller, the question of how electrons move on shorter and shorter time scales becomes increasingly important. One way to investigate this question is to measure the radiation that is emitted by electrons which have been rapidly accelerated, as would typically be the case in a small device with a fast response. In many situations, it is possible to induce this rapid charge acceleration using a very short pulse of light (i.e., of 100 femtosecond duration); in this case, the emitted radiation typically falls in the terahertz range of the spectrum. Measuring this emitted terahertz radiation, a technique known as "laser terahertz emission spectroscopy" (or LTEM), has proven to be a very powerful method for studying many different kinds of materials, elucidating the earliest dynamical processes which influence the motion of charges in solids. However, it has an important limitation: since there is a limit to how tightly one can focus visible light, the spatial resolution of LTEM is limited to a few microns or larger (that is, the size of the optical spot on the sample). As a result, it has not been possible to leverage this spectroscopic tool in the study of single nanostructures, since they are far too small to be individually addressed by the incident light pulse. In this research program, we will develop a new way to apply LTEM which overcomes this limited spatial resolution. Our technique, which is based on scattering the terahertz radiation from a very small metal tip held near the sample's surface, will improve the spatial resolution of LTEM by three orders of magnitude. This will open up an entirely new realm of nanoscale phenomena for study using LTEM techniques. For example, we will study emerging photovoltaic materials, to see if the crystalline grain boundaries have a significant influence on charge transport which could limit their overall efficiency. We will study nanosized electronic materials such as gallium nitride, where surface defects and interfacial disorder are thought to strongly influence their performance. These measurements will reveal the fundamental physics of charge transport in many different materials, with both nanoscale spatial resolution and sub-picosecond temporal resolution.Technical description: The proposed research attacks the challenge of coupling millimeter and terahertz waves to nanostructures in a novel way, by using a scattering-type near-field microscope to implement tip-mediated nonlinear optics. Our preliminary work established for the first time the feasibility of tip-mediated terahertz generation (a second-order nonlinear optical process). Building on this early work, we will establish nanoscale laser terahertz emission microscopy as a valuable and versatile tool for spectroscopy of nanomaterials and nanostructures. We will develop several new methods for studying material systems based on this idea, including locally applied electric field bias modulation, and tapping amplitude modulation for studying buried interfaces. We will also broaden the scope of these techniques by combining them with a time-delayed pump pulse, for time-resolved studies of the evolution of the terahertz nonlinearities. This will all be done with a tip-size-limited spatial resolution of ~10 nanometers. Our work will bring the full power of both linear and nonlinear terahertz science to the realm of nanomaterials. We will use these new techniques to study several different material systems, including organometallic halide perovskites and GaN nanostructures and heterostructures. As well as answering open questions about the role of nanoscale morphology in charge transport in these important materials, these measurements will also serve as demonstrations of the power and versatility of this newly developed spectroscopic approach.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.