216 research outputs found
Splitting between Bright and Dark excitons in Transition Metal Dichalcogenide Monolayers
The optical properties of transition metal dichalcogenide monolayers such as
the two-dimensional semiconductors MoS and WSe are dominated by
excitons, Coulomb bound electron-hole pairs. The light emission yield depends
on whether the electron-hole transitions are optically allowed (bright) or
forbidden (dark). By solving the Bethe Salpeter Equation on top of wave
functions in density functional theory calculations, we determine the sign and
amplitude of the splitting between bright and dark exciton states. We evaluate
the influence of the spin-orbit coupling on the optical spectra and clearly
demonstrate the strong impact of the intra-valley Coulomb exchange term on the
dark-bright exciton fine structure splitting.Comment: 6 pages, 2 figure
Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers
We investigate valley exciton dynamics in MoSe2 monolayers in polarization-
and time-resolved photoluminescence (PL) spectroscopy at 4K. Following
circularly polarized laser excitation, we record a low circular polarization
degree of the PL of typically . This is about 10 times lower than the
polarization induced under comparable conditions in MoS2 and WSe2 monolayers.
The evolution of the exciton polarization as a function of excitation laser
energy and power is monitored in PL excitation (PLE) experiments. Fast PL
emission times are recorded for both the neutral exciton of ps and for
the charged exciton (trion) of 12 ps.Comment: 4 pages, 3 figure
Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides
We study the neutral exciton energy spectrum fine structure and its spin
dephasing in transition metal dichalcogenides such as MoS. The interaction
of the mechanical exciton with its macroscopic longitudinal electric field is
taken into account. The splitting between the longitudinal and transverse
excitons is calculated by means of the both electrodynamical approach and
perturbation theory. This long-range exciton
exchange interaction can induce valley polarization decay. The estimated
exciton spin dephasing time is in the picosecond range, in agreement with
available experimental data.Comment: 5 pages, 3 figure
Exciton dynamics in WSe2 bilayers
We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved
photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for
both the direct exciton with ~ 3 ps and the indirect optical
transition with ~ 25 ps. For temperatures between 4 to 150 K
remains constant. Following polarized laser excitation, we observe
for the direct exciton transition at the K point of the Brillouin zone
efficient optical orientation and alignment during the short emission time
. The evolution of the direct exciton polarization and intensity as a
function of excitation laser energy is monitored in PL excitation (PLE)
experiments.Comment: 4 pages, 3 figure
Exciton Valley Dynamics probed by Kerr Rotation in WSe2 Monolayers
We have experimentally studied the pump-probe Kerr rotation dynamics in
WSe monolayers. This yields a direct measurement of the exciton valley
depolarization time . At T=4K, we find ps, a fast
relaxation time resulting from the strong electron-hole Coulomb exchange
interaction in bright excitons. The exciton valley depolarization time
decreases significantly when the lattice temperature increases with
being as short as 1.5ps at 125K. The temperature dependence is well explained
by the developed theory taking into account the exchange interaction and a fast
exciton scattering time on short-range potentials.Comment: 5 pages, 3 figure
Carrier and polarization dynamics in monolayer MoS2
In monolayer MoS2 optical transitions across the direct bandgap are governed
by chiral selection rules, allowing optical valley initialization. In time
resolved photoluminescence (PL) experiments we find that both the polarization
and emission dynamics do not change from 4K to 300K within our time resolution.
We measure a high polarization and show that under pulsed excitation the
emission polarization significantly decreases with increasing laser power. We
find a fast exciton emission decay time on the order of 4ps. The absence of a
clear PL polarization decay within our time resolution suggests that the
initially injected polarization dominates the steady state PL polarization. The
observed decrease of the initial polarization with increasing pump photon
energy hints at a possible ultrafast intervalley relaxation beyond the
experimental ps time resolution. By compensating the temperature induced change
in bandgap energy with the excitation laser energy an emission polarization of
40% is recovered at 300K, close to the maximum emission polarization for this
sample at 4K.Comment: 7 pages, 7 figures including supplementary materia
Exciton states in monolayer MoSe2: impact on interband transitions
We combine linear and non-linear optical spectroscopy at 4K with ab initio
calculations to study the electronic bandstructure of MoSe2 monolayers. In
1-photon photoluminescence excitation (PLE) and reflectivity we measure a
separation between the A- and B-exciton emission of 220 meV. In 2-photon PLE we
detect for the A- and B-exciton the 2p state 180meV above the respective 1s
state. In second harmonic generation (SHG) spectroscopy we record an
enhancement by more than 2 orders of magnitude of the SHG signal at resonances
of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our
post-Density Functional Theory calculations show in the conduction band along
the direction a local minimum that is energetically and in k-space
close to the global minimum at the K-point. This has a potentially strong
impact on the polarization and energy of the excitonic states that govern the
interband transitions and marks an important difference to MoS2 and WSe2
monolayers.Comment: 8 pages, 3 figure
Spectrally narrow exciton luminescence from monolayer MoS2 exfoliated onto epitaxially grown hexagonal BN
The strong light-matter interaction in transition Metal dichalcogenides
(TMDs) monolayers (MLs) is governed by robust excitons. Important progress has
been made to control the dielectric environment surrounding the MLs, especially
through hexagonal boron nitride (hBN) encapsulation, which drastically reduces
the inhomogeneous contribution to the exciton linewidth. Most studies use
exfoliated hBN from high quality flakes grown under high pressure. In this
work, we show that hBN grown by molecular beam epitaxy (MBE) over a large
surface area substrate has a similarly positive impact on the optical emission
from TMD MLs. We deposit MoS and MoSe MLs on ultrathin hBN films (few
MLs thick) grown on Ni/MgO(111) by MBE. Then we cover them with exfoliated hBN
to finally obtain an encapsulated sample : exfoliated hBN/TMD ML/MBE hBN. We
observe an improved optical quality of our samples compared to TMD MLs
exfoliated directly on SiO substrates. Our results suggest that hBN grown
by MBE could be used as a flat and charge free substrate for fabricating
TMD-based heterostructures on a larger scale.Comment: 5 pages, 3 figure
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