Radiatively Limited Dephasing and Exciton Dynamics in MoSe₂ Monolayers Revealed with Four-Wave Mixing Microscopy

Abstract

By implementing four-wave mixing (FWM) microspectroscopy, we measure coherence and population dynamics of the exciton transitions in monolayers of MoSe2. We reveal their dephasing times T2 and radiative lifetime T1 in a subpicosecond (ps) range, approaching T2 = 2T1 and thus indicating radiatively limited dephasing at a temperature of 6 K. We elucidate the dephasing mechanisms by varying the temperature and by probing various locations on the flake exhibiting a different local disorder. At the nanosecond range, we observe the residual FWM produced by the incoherent excitons, which initially disperse toward the dark states but then relax back to the optically active states within the light cone. By introducing polarization-resolved excitation, we infer intervalley exciton dynamics, revealing an initial polarization degree of around 30%, constant during the initial subpicosecond decay, followed by the depolarization on a picosecond time scale. The FWM hyperspectral imaging reveals the doped and undoped areas of the sample, allowing us to investigate the neutral exciton, the charged one, or both transitions at the same time. In the latter, we observe the exciton–trion beating in the coherence evolution indicating their coherent coupling.

Realization of an Electrically Tunable Narrow-Bandwidth Atomically Thin Mirror Using MonolayerMoSe2

“…The resulting frequency dependent decay rate given by the imaginary part of the exciton self-energy ΣðωÞ, combined with transfer matrix calculations reproduces well the measured T, R, as well as R þ T data (see Supplemental Material [18]). We remark that our findings, demonstrating that disorder induced scattering is the dominant nonradiative exciton line-broadening mechanism, is in agreement with four wave mixing experiments [23].…”

Resonantly excited exciton dynamics in two-dimensional MoSe2 monolayers

“…The phonon activated term, representing a homogeneous broadening, amounts to 33 meV at T = 300 K, 7.8 meV at T = 150 K, and 0.79 meV at T = 77 K. In Ref. [1] an additional linear term of 0.03 meV/K was found, and a homogeneous broadening of about 3.5 meV at 77 K, which is consistent with our finding.…”

“…The exciton radiative lifetime within the radiative cone τ r scales with the square of the exciton oscillator strength, and is about 10 ps in the classical 2D system -GaAs quantum wells 37 . The large exciton binding energy in SL-MoSe 2 leads to a faster lifetime, in the sub-picosecond regime 1,17,35,38 . This situation is similar to the fast radiative lifetimes in the 1 ps range observed in two-dimensional CdSe platelets 32 , which have an exciton binding energy of 100-300 meV.…”

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

“…Also, all timeresolved experiments using pulsed laser, such as pumpprobe spectroscopy, time-resolved PL, and four-wave mixing [12,31,36,90,91], will benefit from these samples with much higher threshold for optical damage as compared to uncapped samples. Note added in proof.-Narrow linewidth emission in hBN encapsulated samples is also reported in [92].…”

“…The first member of the transition metal dichalcogenides (TMDC) to be established as a direct gap semiconductor in monolayer (ML) form was MoS 2 [1,2], which has resulted in a global research effort exploring this promising 2D semiconductor family [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. First prototype device applications, such as transistors [17][18][19] and light emitters [20][21][22], have shown the promise of this atomically thin material for electronics and optoelectronics.…”