Local field effects in ultrafast light–matter interaction measured by pump-probe spectroscopy of monolayer MoSe₂

Abstract

Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects.

Prolonging valley polarization lifetime through gate-controlled exciton-to-trion conversion in monolayer molybdenum ditelluride

“…a b 5,6,27,37,44,45 . b Comparison of intervalley scattering time at different temperatures 9,10,15,[46][47][48][49] . The colored circles represent different TMDCs.…”

“…Figure 5a, b shows the CW-PL polarization and intervalley scattering time of ML-MoTe 2 in comparison with the corresponding quantities of other ML-TMDCs. The detailed test conditions, DVP values 5,6,27,37,44,45 , and intervalley scattering times 9,10,15,[46][47][48][49] unity polarization under near-resonant excitation. Through gating, the DVP value of ML-MoTe 2 ranges from 0 to 38% for excitons and 33% for trions.…”

“…a Comparison of CW-PL DVP as a function of the excess energy at low temperatures (4-125 K)5,6,27,37,44,45 . b Comparison of intervalley scattering time at different temperatures9,10,15,[46][47][48][49] . The colored circles represent different TMDCs.…”

Short excitonic lifetimes of MoSe2 monolayers grown by molecular beam epitaxy on the hexagonal boron nitride

“…Short excitonic lifetimes and relaxation times observed in monolayers of semiconducting TMDs [16][17][18][19][20][21] are promising for applications in nonlinear optics [22,23] that may now become viable due to the development of scalable growth process of high optical quality material [15,24]. Possible applications include higher harmonic generation [25,26], THz wave generation [27,28], parametric amplification [29] or nanoscale light sources [29,30].…”

Controlled coherent-coupling and dynamics of exciton complexes in a MoSe2 monolayer

“…Firstly, it is supposed that free carriers affect the relative absorption (oscillator or effective dipole strength) and the possible couplings between neutral excitons (X) and (negatively) charged excitons (X − ), also called trions. Secondly, the exciton's dephasing and therefore the spectral line shape should be sensitive to the electron density n e [39], in an analogous way as it depends on the total exciton density, through the mechanisms called excitation induced dephasing [20,40,41], sometimes described by a local field effect [42,43]. Thirdly, with increasing n e the exciton complexes are screened more efficiently from intrinsic disorder, which in turn should reduce their inhomogeneous broadening [3,44], further impacting the excitons' radiative decay rates [45].…”