Coherent Ultrafast Excitonic Oscillations in Monolayer WS$_2$

Abstract

Monolayer transition metal dichalcogenides are a suitable platform for studying excitonic coherence in the light-matter coupling regime. We present an ab initio time-dependent GW-Bethe-Salpeter equation (GW-BSE) investigation of coherent excitonic dynamics in monolayer WS$_2$. By solving the coherent coupling between the A, A$^{*}$, and B excitons under linearly polarized pump fields, we identify the microscopic origin of the resulting oscillatory dynamics and rationalize it using an effective theoretical model. Our results provide the interpretation of recently reported coherent excitonic phenomena in monolayer WS$_2$ (Nano Lett. 24, 8117 (2024)). Building on this first-principles time-resolved framework, we propose a tailored pump-probe scheme that enables the controlled generation and regeneration of coherent oscillations between excitonic states. These findings establish a predictive route for controlling excitonic coherence in two-dimensional materials, with direct relevance for ultrafast optoelectronic switches and solid-state quantum logic devices.

Figures (3)

• Figure 1: Pump-probe setup and excitonic response in monolayer WS$_{2}$ with pump at 2.24 eV and pump-probe time delay of 50 fs. (a) Side and top views of the monolayer WS$_{2}$ crystal structure, along with its Brillouin zone. (b) Absorption spectra at equilibrium (black line), out-of-equilibrium after the pump (orange line) and the resulting $\Delta$R/R signal (blue dashed line). The pump pulse (orange shaded area) is detuned equally from the A and B excitonic resonances. (c) Time evolution of the excited density (black), polarization (blue), and pump pulse (red). (d) Electronic band structure of WS$_{2}$ showing the A, A$^{*}$, and B resonances, and the corresponding bands that contribute to their formation.
• Figure 2: Ultrafast pump-probe simulations on monolayer WS$_{2}$. (a) Pump-probe map of $\Delta$R/R signal. Resonances A, A$^{*}$ and B are highlighted in blue, green and red, respectively. (b) Time-domain coherent oscillations and (c) their corresponding frequency components for the A, A$^{*}$ and B resonances.
• Figure 3: Pump-probe configuration for realizing a generator of coherent oscillations in monolayer WS$_{2}$. (a) $\Delta$R/R signal monitored at the A resonance, shown alongside the temporal profiles of the applied pump pulses. Shaded regions indicate the preparation regime before the signal stabilizes, while the colored lines highlight the time windows relevant for the generator operation. The inset shows the pump pulse energies to excite resonantly resonances A and B. (b) Time evolution of the excited population densities of the A (blue) and B (red) resonances. The coherence term $\Gamma_{AB}$ develops when pump A (pump B) acts on the system in presence of a coherent B state (A state) previously induced by the action of pump B (pump A). (c) Amplitudes of the $\Delta$R/R signal in the different generator configurations. The coherent oscillations exhibit an amplitude approximately one order of magnitude larger than in the non-oscillatory regime. The inset depicts the frequency components of the signals, illustrating that the dominant oscillation frequency matches the energy splitting between the A and B excitonic resonances.