LMI Seminar: Ultrafast Photocurrents in a topological insulator

Abstract:

Topological materials have captured the imagination of scientists with unique electronic dispersions and surface states. While their potential seems huge – from advanced photodetectors to devices carrying dissipationless spin-polarized currents – so far it has not come to fruition. Much effort has been dedicated to harness topological states for technology, and one promising direction is to optically excite and control currents involving the topological surface states. 

In this talk I will discuss how we use time- and angle-resolved photoemission spectroscopy (trARPES) to study coupling of light to topological electronic states, and in particular gain better understanding of photocurrent generation in topological insulators.  We resolve – in energy, momentum and time - photocurrents in the excited electronic states of a topological insulator. By analyzing the rise times of the population following the optical excitation, we gain a complete view of the occupied and unoccupied electronic states, and how they are coupled by the light. Furthermore, at certain conditions, the pump-circular dichroism reveals quantum interference pathways in the two-photon photoemission process. We resolve the bands involved in the interfering photoemission paths and demonstrate control of the interference via changes in photon energy and polarization. Our work provides a microscopic understanding of the photocurrent generation process and paves the way to coherent control of currents in topological states.