Excitonic Theory of the Ultrafast Optical Response of 2D-Quantum-Confined Semiconductors at Elevated Densities

TL;DR

This paper develops a microscopic excitonic theory for the ultrafast optical response of 2D quantum-confined semiconductors across densities up to elevated values below the Mott transition. Using Wannier–Mott excitons and a dynamics-controlled truncation up to fourth order, it jointly treats Coulomb interactions, light–matter coupling, and exciton–phonon effects, bridging coherent and incoherent regimes. The framework introduces and evolves the central excitonic quantities $P_{\,\mu}^{\xi}$, $N_{\mu,\mathbf{Q}}^{\xi,\xi'}$, $B_{\pm,\zeta}^{\xi,\xi'}$, and $Z_{\pm,\zeta,\rho,\mathbf{Q}}^{\xi_1,\xi_2,\xi_3,\xi_4,\xi_5,\xi_6}$, incorporating realistic material parameters and comparing two representative systems: GaAs quantum wells and MoSe$_2$ monolayers. Key findings show that, under circular excitation, incoherent Rabi oscillations are strongly suppressed in MoSe$_2$ due to exciton–exciton correlations and Coulomb renormalizations, while GaAs behaves closer to conventional SBEs; linear excitation further quenches oscillations by enhancing inter-valley and multi-exciton coupling. The results demonstrate the necessity of an excitonic description below the Mott density and provide insights for interpreting ultrafast experiments in 2D semiconductors.

Abstract

An excitonic approach to the ultrafast optical response of confined semiconductors at elevated densities below the Mott transition is presented. The theory is valid from the coherent regime, where coherent excitonic transitions and biexcitons dominate, to the incoherent regime, where excitonic occupations dominate. Numerical simulations of the $1s$ exciton dynamics during intense circularly polarized pump pulses in two different Coulomb-interaction regimes are performed for two-dimensional semiconductors: Moderate Coulomb interaction is compared with dominating Coulomb interaction with respect to the light-matter interaction strength. The different many-body contributions are disentangled and it is found, that excitonic Rabi oscillations in the Coulomb-dominated regime are considerably less strong. By also comparing circular and linear excitation in a MoSe$_2$ monolayer, it is found, that linear excitation creates a regime, where excitonic Rabi oscillations are almost completely suppressed.

Figures (10)

• Figure 1: Scheme of excitonic transitions in Eq. \ref{['eq:P']} at the $K$ and $K^{\prime}$ valley (left) and excitonic occupations from Eq. \ref{['eq:N']} in intravalley configuration at the $K$ and $K^{\prime}$ valleys (right) in a monolayer TMDC.
• Figure 2: Scheme of two-exciton transitions from Eq. \ref{['eq:B']} in the intravalley configuration ($K,K$/$K^{\prime},K^{\prime}$) and in the intervalley configuration $K,K^{\prime}/K^{\prime},K$ (left) and exciton-two-exciton transitions from Eq. \ref{['eq:Z']} in the intravalley configuration (right).
• Figure 3: Scheme of exciton-two-exciton transitions from Eq. \ref{['eq:Z']} in the two possible intervalley configurations.
• Figure 4: Scheme of the interaction processes via optical ($\Omega\overset{\wedge}{=}\Omega^{cv}$) and Coulomb ($V$) interaction up to second order DCT for the correlated expectation values in our theory. Additionally, we depict the exciton-phonon interaction in second-order Born-Markov approximation ($\Gamma$)
• Figure 5: Scheme of the interaction processes via optical ($\Omega\overset{\wedge}{=}\Omega^{cv}$) and Coulomb ($V$) interaction on third order DCT for the correlated expectation values considered in our theory.