Seminar

2021/09/30(Thu)     11:00 -12:30    五樓第一會議室5th Floor(1st meeting room)

Title

Excitonic effects in optical-field-driven quasi 2D materials from time-dependent GW approach

Speaker

詹楊皓助研究員 (中央研究院原子與分子科學研究所)

Dr. Yang-Hao Chan (Institute of Atomic and Molecular Sciences, Academia Sinica)

Abstract

Atomically thin quasi two-dimensional (2D) insulating materials exhibit novel exciton physics due to ineffective screening, quantum confinement, and topological effects. Such exciton physics has 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. Using a newly developed real-time, non-equilibrium Green function method within the adiabatic GW approximation, we show that, for non-centrosymmetric 2D semiconductors, excitonic effects give rise to a strong DC current, the so-called shift current, upon even sub-bandgap frequency CW light illumination through a second-order nonlinear optical process. The frequency-dependent shift current 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 for applications with appropriate materials. Furthermore, we show that, in optical-field-driven angle-resolved photoemission spectroscopy (ARPES) experiments, the energy and wavefunction of excitons may be measured directly under achievable laboratory conditions. With optical pump frequencies close to the resonance frequency for exciton excitations, distinct excitonic features manifest themselves dramatically as modulated replicas of the involved valence band states. We also find that, at higher pump intensity, the quasiparticle band energies are renormalized due to the driving optical fields.

Language

演講語言 (Language): in English