Abstract
By employing ultrafast transient reflection measurements based on two-color pump-probe spectroscopy, the population and valley polarization dynamics of trions in monolayer MoSe2 were investigated at relatively high excitation densities under near-resonant excitation. Both the nonlinear dynamic photobleaching of the trion resonance and the redshift of the exciton resonance were found to be responsible for the excitation-energy- and density-dependent transient reflection change as a result of many-body interactions. Furthermore, from the polarization-resolved measurements, it was revealed that the initial fast population and polarization decay process upon strong photoexcitation observed for trions was determined by trion formation, transient phase-space filling and the short valley lifetime of excitons. The results provide a basic understanding of the nonlinear dynamics of population and valley depolarization of trions, as well as exciton-trion correlation in atomically thin MoSe2 and other transition metal dichalcogenide materials.
Highlights
Monolayers of transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, are direct-band-gap semiconductors with two inequivalent valleys (±K points) at the corners of the hexagonal Brillouin zone
The population transfer from excitons to trions occurs when the photoexcited carrier density is in the linear-optical response regime or when the laser excitation energy lies above the free-carrier gap[31,32,33,34], but excitons and trions generated from free photocarriers can decay independently at the quasiresonant excitation when the photo-injected carrier density is as high as 1012–1013/cm[2 35,36]
Polarization-resolved differential reflectivity measurements further suggested that the initial fast population and polarization decay process observed for trions after photoexcitation was connected to trion formation, phase-space filling and the sub-picosecond intervalley scattering time of excitons resulting from strong exciton-trion correlations
Summary
Monolayers of transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, are direct-band-gap semiconductors with two inequivalent valleys (±K points) at the corners of the hexagonal Brillouin zone. The population transfer from excitons to trions occurs when the photoexcited carrier density is in the linear-optical response regime or when the laser excitation energy lies above the free-carrier gap[31,32,33,34], but excitons and trions generated from free photocarriers can decay independently at the quasiresonant excitation when the photo-injected carrier density is as high as 1012–1013/cm[2 35,36] These studies have indicated that exciton and trion dynamics upon optical excitation at a relatively high carrier density are much more complex, in addition to exhibiting intrinsic radiative and nonradiative recombination. Polarization-resolved differential reflectivity measurements further suggested that the initial fast population and polarization decay process observed for trions after photoexcitation was connected to trion formation, phase-space filling and the sub-picosecond intervalley scattering time of excitons resulting from strong exciton-trion correlations
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