Rabi oscillations of a monolayer quantum emitter driven through its excited state

TL;DR

This work demonstrates coherent optical control of a solid-state quantum emitter in a WSe$_2$ monolayer by driving a Coulomb-coupled exciton ground state $|1\rangle$ through an excited state $|2\rangle$ using resonant picosecond pulses. A three-level exciton model with states $\ig\{|0\rangle,|1\rangle,|2\rangle\big\}$, detuning $\delta = E_L - E_2$, energy spacing $\Delta$, and Coulomb coupling $V$ describes the dynamics, with the pump described by $H_{\text{pump}}$ and the total dynamics governed by the Lindblad equation $\frac{\partial}{\partial t}\hat{\rho}(t)= -\frac{i}{\hbar}[H, \hat{\rho}(t)]+\mathcal{L}\hat{\rho}(t)$. Experiments reveal Rabi-like oscillations in the ground-state population as a function of pulse area, whose amplitude and frequency depend on the laser–exciton detuning $\delta$, and the results are reproduced by solving the model with a Gaussian pump envelope $f(t)$ and pure dephasing $\gamma$. The findings highlight detuning-dependent coherent population transfer in 2D TMD emitters and point toward integrating such emitters with optical cavities for enhanced coherence and on-demand single-photon generation.

Abstract

The interaction of a quantum two-level system with a resonant driving field results in the emergence of Rabi oscillations, which are the hallmark of a controlled manipulation of a quantum state on the Bloch sphere. This all-optical coherent control of solid-state two-level systems is crucial for quantum applications. In this work we study Rabi oscillations emerging in a WSe2 monolayer-based quantum dot. The emitter is driven coherently using picosecond laser pulses to a higher-energy state, while photoluminescence is probed from the ground state. The theoretical treatment based on a three-level exciton model reveals the population transfer between the exciton ground and excited states coupled by Coulomb interaction. Our calculations demonstrate that the resulting exciton ground state population can be controlled by varying driving pulse area and detuning which is evidenced by the experimental data. Our results pave the way towards the coherent control of quantum emitters in atomically thin semiconductors, a crucial ingredient for monolayer-based high-performance, on-demand single photon sources.

Figures (3)

• Figure 1: (a) Artistic render of the WSe$_2$ ML placed on a Si$_3$N$_4$/SiO$_2$/Au/GaAs slab. The laser beam is focused onto the QD area; the emitted single photons are depicted as white spheres. (b) PL spectrum of an emitter under study. (c) and (d) Second-order correlation of the emission under pulsed excitation. The solid red lines in (d) are fitted double exponential curves, while the black line is sum of these curves. (e) Scheme of the energy level structure of a QD. The emitter is driven via its excited state $\left| 2 \right>$, and PL emission from the exciton ground state $\left| 1 \right>$ is collected. The states $\left| 1 \right>$ and $\left| 2 \right>$ are separated by an energy difference $\Delta$ and coupled via Coulomb interaction with matrix element $V$.
• Figure 2: (a) PLE spectra of four different QD emitters. The ground-state energies $E_{1}$ are shown beside the respective spectra. The star symbol points to the resonance associated with the excited state of the QD. (b) Statistics of the energy spacing between PLE resonances, attributed to QD higher-energy states, and exciton ground state $E_\text{E}-E_\text{1}$ as a function of effective confinement potential $E_\text{FX}-E_\text{1}$.
• Figure 3: Normalized area of PL emission (gray squares) vs square root of laser power measured before microscope objective in cases of different detunings $\delta$ between laser and the excited state of the QD: (a) -1.15 $\pm$ 0.22 meV, (b) 0.24 $\pm$ 0.27 meV, (c) 1.61 $\pm$ 0.24 meV. Solid lines are based on an adapted saturation model. (d-f) display the outcome of a full quantum treatment of the coupled level model for the same detunings. (g) Population of the ground exciton state (color-coded) calculated at specific pulse areas ($x$-axis) and detunings $\delta$ ($y$-axis). The white dashed lines denote the detunings corresponding to the curves shown in (d-f).