61 research outputs found
Exciton-phonon relaxation bottleneck and radiative decay of thermal exciton reservoir in two-dimensional materials
We study exciton radiative decay in a two-dimensional material, taking into
account large thermal population in the non-radiative states, from which
excitons are scattered into the radiative states by acoustic phonons. We find
an analytical solution of the kinetic equation for the non-equilibrium
distribution function of excitons in the radiative states. Our estimates for
bright excitons in transition metal dichalcogenides indicate a strong depletion
of radiative state population due to insufficient exciton-phonon scattering
rate at low temperatures.Comment: 8 pages, 4 figure
Fine structure of -excitons in multilayers of transition metal dichalcogenides
Reflectance and magneto-reflectance experiments together with theoretical
modelling based on the approach have been employed to study
the evolution of direct bandgap excitons in MoS layers with a thickness
ranging from mono- to trilayer. The extra excitonic resonances observed in
MoS multilayers emerge as a result of the hybridization of Bloch states of
each sub-layer due to the interlayer coupling. The properties of such excitons
in bi- and trilayers are classified by the symmetry of corresponding crystals.
The inter- and intralayer character of the reported excitonic resonances is
fingerprinted with the magneto-optical measurements: the excitonic -factors
of opposite sign and of different amplitude are revealed for these two types of
resonances. The parameters describing the strength of the spin-orbit
interaction are estimated for bi- and trilayer MoS.Comment: 14 pages, 10 figure
Neutral and charged dark excitons in monolayer WS
Low temperature and polarization resolved magneto-photoluminescence
experiments are used to investigate the properties of dark excitons and dark
trions in a monolayer of WS encapsulated in hexagonal BN (hBN). We find
that this system is an -type doped semiconductor and that dark trions
dominate the emission spectrum. In line with previous studies on WSe, we
identify the Coulomb exchange interaction coupled neutral dark and grey
excitons through their polarization properties, while an analogous effect is
not observed for dark trions. Applying the magnetic field in both perpendicular
and parallel configurations with respect to the monolayer plane, we determine
the g-factor of dark trions to be -8.6. Their decay rate is close to 0.5
ns, more than 2 orders of magnitude longer than that of bright excitons.Comment: 6 pages, 6 figures, supplemental materia
Probing and manipulating valley coherence of dark excitons in monolayer WSe
Monolayers of semiconducting transition metal dichalcogenides are
two-dimensional direct-gap systems which host tightly-bound excitons with an
internal degree of freedom corresponding to the valley of the constituting
carriers. Strong spin-orbit interaction and the resulting ordering of the
spin-split subbands in the valence and conduction bands makes the lowest-lying
excitons in WX (X~being S or Se) spin-forbidden and optically dark. With
polarization-resolved photoluminescence experiments performed on a WSe
monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic
exchange interaction in combination with the applied in-plane and/or
out-of-plane magnetic fields enables one to probe and manipulate the valley
degree of freedom of the dark excitons.Comment: Manuscript: 6 pages, 3 figures; SM: 6 pages, 5 figure
Semiconductor Bloch equation analysis of optical Stark and Bloch-Siegert shifts in monolayers WSe and MoS
We report on the theoretical and experimental investigation of
valley-selective optical Stark and Bloch-Siegert shifts of exciton resonances
in monolayers WSe and MoS induced by strong circularly polarized
nonresonant optical fields. We predict and observe transient shifts of both 1sA
and 1sB exciton transitions in the linear interaction regime. The theoretical
description is based on semiconductor Bloch equations. The solutions of the
equations are obtained with a modified perturbation technique, which takes into
account many-body Coulomb interaction effects. These solutions allow to explain
the polarization dependence of the shifts and calculate their values
analytically. We found experimentally the limits of the applicability of the
theoretical description by observing the transient exciton spectra change due
to many-body effects at high field amplitudes of the driving wave.Comment: 20 pages, 9 figures, this manuscript is related to the "Giant
valley-selective Stark and Bloch-Siegert shifts of exciton resonances in
WSe and MoS monolayers" manuscrip
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