The ultrafast onset of exciton formation in 2D semiconductors

Abstract

The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.

Efficient phonon cascades in WSe2 monolayers

“…At low T (78 K), phonon absorption processes are suppressed because of the insufficient lattice thermal energy 53 . Optical excitation results in free e-h pair formation 56 , 57 or virtual formation of an exciton with small in-plane wavevector ( k ≲ ω i / c with ω i the excitation laser frequency) 1 . With the subsequent phonon emission, the e-h pair reaches a real final state (blue parabola in Fig.…”

Dissecting Interlayer Hole and Electron Transfer in Transition Metal Dichalcogenide Heterostructures via Two-Dimensional Electronic Spectroscopy

“… 49 , 51 , 52 At later times, all four peaks exhibit horizontal elongation along the excitation axis, consistent with previous observations, using ultrafast TA, of a delayed excitonic PB following above-resonance excitation. 53 …”

“…This result is supported by recent experiments showing that electron–hole pairs photoexcited above the optical gap of TMDs relax on the bottom (top) of the CB (VB) at the K point on time scale faster than 20 fs. 53 …”

Intertube excitonic coupling in nanotube van der Waals heterostructures

“…The dynamics following ultrafast excitation at 3.0 eV were also measured (Figure 2f) and fitted: the band-edge excitonic response formed extremely rapidly, within 100 fs, in line with recent results on monolayer MoS 2 , and attributed to rapid exciton cooling mediated by the strong exciton-phonon interaction. [26] Under IR excitation the A exciton's center energy undergoes a dynamic blueshift, evident in Figure 3a, which rises and decays with the IR laser pulse (shaded area, Gaussian pulse with σ = 35 fs). The peak blueshift increased with fluence.…”

“…The current state-of-the-art in this area includes studies of superradiant effects in coupled 2D TMD layers [29] and the rapid ultrafast formation dynamics of excitons in MoS 2 monolayers after nonresonant visible excitation. [26] Here we provide two simplified theoretical models that allow intuitive insights into intertube excitonic effects. First, a pair of coupled classical oscillators can represent the system: a first oscillator represents the excitonic polarization of an S 11 exciton in a CNT, the second oscillator denotes an A exciton in a MoS 2 NT, while a third spring models the Coulomb dipole-dipole coupling between excitons.…”