Resonantly excited exciton dynamics in two-dimensional MoSe2 monolayers

Abstract

We report on the exciton and trion density dynamics in a single layer of MoSe2, resonantly excited and probed using three-pulse four-wave mixing (FWM), at temperatures from 300 K to 77 K . A multi-exponential third-order response function for amplitude and phase of the heterodyne-detected FWM signal including four decay processes is used to model the data. We provide a consistent interpretation within the intrinsic band structure, not requiring the inclusion of extrinsic effects. We find an exciton radiative lifetime in the sub-picosecond range consistent to what has been recently reported 1 . After the dominating radiative decay, the remaining exciton density, which has been scattered from the initially excited bright radiative state into dark states of different nature by exciton-phonon scattering or disorder scattering, shows a slower dynamics, covering 10ps to 10ns timescales. This includes direct bright transitions with larger in-plane momentum, as well as indirect dark transitions to indirect dark states. We find that exciton-exciton annihilation is not relevant in the observed dynamics, in variance from previous finding under non-resonant excitation. The trion density at 77 K reveals a decay of the order of 1 ps, similar to what is observed for the exciton. After few tens of picoseconds, the trion dynamics resembles the one of the exciton, indicating that trion ionization occurs on this timescale.

Exciton–phonon coupling strength in single-layer MoSe2 at room temperature

“…We now analyze the origin of the oscillations in the 2D maps with the goal to deduce the EXPC strength. Previous experiments reported that the trion signal in 1L-MoSe2, located ~0.03 eV below the neutral exciton peak 54 , gradually dies out both in PL and absorption when T increases from 15 to 295 K 25,54 . The electronic coupling between excitons and trions, observed at 20 K in Ref.…”

“…Optical four-wave-mixing experiments in semiconductors provide access to coherent dynamics of excitons [23][24][25]10 . In photon echo experiments the polarization state of incident photons (circular or linear) allows one to uncover different mechanisms behind the signal formation 26,27 .…”

Coherence and Density Dynamics of Excitons in a Single-Layer MoS2 Reaching the Homogeneous Limit

“…The different relaxation processes contribute to the FWM amplitude with different phases, imprinting unusual signatures in the measured density dynamics of the FWM amplitude due to constructive and destructive interferences. 38…”

“…After the resonant excitation, several relaxation mechanisms play an important role. 38 They are governed by, on the one hand, the high oscillator strength 43 and thus fast radiative recombination rate and, on the other hand, by the EX conversion toward different dark states, resulting from the peculiar valley structure and available scattering channels with phonons, charge carriers, and EXs. Due to this scattering and radiative recombination, the EX phase is lost on a picosecond time scale, as shown by the results discussed in the previous sections.…”

Long Exciton Dephasing Time and Coherent Phonon Coupling in CsPbBr2Cl Perovskite Nanocrystals

“…For a quantitative analysis, the population decay data is fitted with the complex multi-exponential response function, as explained in detail in ref. 35 :…”