Subcycle control of valley-selective excitations via the dynamical Franz-Keldysh effect in a WSe₂ monolayer

This study performed first-principles calculations based on the time-dependent density functional theory to control the valley degree of freedom relating to the dynamical Franz-Keldysh effect (DFKE) in a monolayer of transition metal dichalcogenide. By mimicking the attosecond transient absorption spectroscopy, we performed numerical pump-probe experiments to observe DFKE around the K or K ' valley in WSe2 monolayer with a linearly-polarized pump field and a circularly-polarized probe pulse. We found that the circularly-polarized probe pulse with a given helicity can selectively observe the transient conductivity modulated by DFKE in each valley. The transient conductivity and excitation probability around each valley oscillate with the pump field frequency Omega. The phases of the Omega oscillation for the K and K ' valleys are opposite to each other. Furthermore, the pump-driven DFKE alters the absorption rate of WSe2 monolayer and yields the valley-dependent Omega oscillation of the electron excitation induced by the pump plus probe field. With a simplified two-band model, we identified the Omega oscillation of the off-diagonal conductivity caused by the band asymmetry around the valleys as the physical mechanism responsible for the valley-selective DFKE.