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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.