IAMS Lecture Announcement

中研院原分所演講公告

Title: Excitonic effects in optical-field-driven systems from first-principles
Speaker: Dr. Yang-hao Chan 詹楊皓博士 (Department of Physics, University of California at Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, USA)

Time: 10:00 AM, February 14 (Friday), 2020
Place: Dr. Poe Lecture Hall, IAMS (本所浦大邦講堂　臺大校園內)

Contact: Dr. Charles Lai 賴品光博士

Abstract: Atomically thin quasi two-dimensional (2D) insulating materials exhibit novel exciton physicsdue to ineffective screening, quantum confinement, and topological effects. Such exciton physicshas recently been studied in details experimentally and theoretically. Going beyond near-equilibrium set-up, one expects that excitonic effects also dominate the responses of out-of-equilibrium systems and can lead to interesting phenomena in optically-driven 2D materials. Usinga newly developed real-time, non-equilibrium Green function method within the adiabatic GWapproximation, we show that, for non-centrosymmetric 2D semiconductors, excitonic effects giverise to a strong DC current, the so-called shift current, upon even sub-bandgap frequency CW lightillumination through a second-order nonlinear optical process. The frequency-dependent shiftcurrent coefficients can be enhanced by orders of magnitude by the strong e-h interactions,producing a bulk photovoltaic effect (i.e., without having to have a p-n junction) of promise forapplications with appropriate materials. Furthermore, we show that, in optical-field-driven angle-resolved photoemission spectroscopy (ARPES) experiments, the energy and wavefunction ofexcitons may be measured directly under achievable laboratory conditions. With optical pumpfrequencies close to the resonance frequency for exciton excitations, distinct excitonic featuresmanifest themselves dramatically as modulated replicas of the involved valence band states.