Grant: Discovery Grants Program - Individual

Ultrafast nanoscale quantum dynamics of materials The proposed research program uses leading-edge experimental tools to explore ultrafast quantum processes in materials and devices down to the atomic scale. One of the main objectives is to directly image and control ultrafast quantum processes and excitations on surfaces with atomic resolution. There are many fundamental interactions and dynamics of charge carriers and excitations in materials that occur over picosecond time scales (0.1 - 1000 ps) and nanometer length scales (1 - 1000 nm) all the way down to single molecules and atoms (< 0.3 nm). For example, electrons in semiconductors and device structures such as solar cells can diffuse distances of 100 nm over 4 ps time scales. However, direct imaging of how carrier diffusion in materials is affected by the local atomic environment has been challenging. In superconductors, the recombination time of normal-state electrons back into superconducting Cooper pairs ranges from 10-400 ps, and superconducting nanowire single-photon photodetectors that produce nanometer-sized normal-state regions upon absorption of a photon have exhibited ultrafast response times around 50 ps. However, no one has ever directly imaged picosecond nonequilibrium dynamics in superconductors on the nanoscale. In addition, one of the main goals in physics is to directly image the ultrafast dynamics of quantum mechanical wave functions in nanostructures and molecules. Clearly, the ability to directly probe materials and devices on sub-ps time scales and sub-nm length scales will have a significant impact on our understanding of why they behave the way they do and will enable us to develop new device technologies. In the proposed research program, we will use time-resolved terahertz spectroscopy (TRTS) and terahertz scanning tunneling microscopy (THz-STM) to probe and directly image ultrafast nanoscale quantum processes in materials. Femtosecond laser sources are used to generate optical excitation pulses as well as single-cycle, ps-duration terahertz (THz) pulses. TRTS will be used to explore nanoscale ultrafast conductivity dynamics and carrier transport in films of semiconductor nanowires and nanocrystals, as well as in quantum materials and nanostructures. THz-STM, which was developed in our lab, couples THz pulses to the sharp metal tip of an STM and will be used to directly image ultrafast quantum processes in materials down to the atomic scale. A recently awarded CFI Innovation Fund, Ultrafast Nanoscale Quantum Dynamics (UltraNanoQD), will allow for the development of (i) ambient THz-STM for rapid sample turnaround and (ii) a unique multi-probe THz-STM system with four independently-controlled STM tips for unprecedented imaging and control of ultrafast nanoscale dynamics in quantum materials and devices. This will revolutionize how we do THz-STM, providing a new window into fundamental ultrafast processes in materials and devices on the atomic scale.