EAGER: Terahertz near-field studies of quantum sensing

This project will investigate quantum phenomena using innovative techniques to probe low-frequency excitations in the terahertz spectral region. Recent advances at Brown have established new approaches for studying these low-energy excitations through light scattering from a nanoscale metallic tip positioned in close proximity to the sample. This technique enables nanometer-scale spatial resolution and, when combined with ultrashort optical pulses, also picosecond temporal resolution. The project will address two fundamental open questions with significant implications for understanding terahertz dynamics in condensed matter systems. The first research thrust will examine how crystalline defects, such as grain boundaries, influence the vibrational dynamics of molecular crystals in the terahertz regime. Because these vibrational modes are generally believed to exhibit long-range coherence, they are expected to be highly sensitive to such structural inhomogeneities. The second thrust will develop and apply new methods for probing charge transport across surfaces, enabling detailed studies of charge flow in the presence of engineered inhomogeneities. These measurements will provide new insights into charge transport across imperfect semiconductor surfaces. This research will advance fundamental understanding of quantum dynamics in materials and establish powerful new capabilities for investigating terahertz phenomena at the nanoscale. This research builds on recent NSF-supported advances at Brown in scattering-type nearfield optical and terahertz nanoscopy, particularly the development of non-local nanoscale measurement techniques. In this approach, an optical pump pulse excites the sample at a location that is laterally displaced from the probing region defined by the atomic force microscope (AFM) tip, which scatters the terahertz probe field. This non-local pump-probe configuration enables scattering-type scanning near-field optical microscopy (s-SNOM) to probe lateral charge transport in semiconductors, and to investigate the spatial coherence of phonon modes in the vicinity of localized crystalline defects. These capabilities represent a significant advance beyond conventional terahertz s-SNOM, which is generally sensitive only to out-of-plane charge transport (through dipoles aligned with the AFM tip and to vibrational dynamics directly beneath the tip. By extending s-SNOM to measure laterally displaced excitations and in-plane transport, this research establishes a new experimental framework for investigating quantum phenomena with both nanoscale spatial resolution and ultrafast temporal resolution. 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.