178 research outputs found
Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides
The remarkably strong Coulomb interaction in atomically thin transition metal
dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics
including the formation of tightly bound excitons. Besides optically accessible
bright excitonic states, these materials also exhibit a variety of dark
excitons. Since they can even lie below the bright states, they have a strong
influence on the exciton dynamics, lifetimes, and photoluminescence. While very
recently, the presence of dark excitonic states has been experimentally
demonstrated, the origin of these states, their formation, and dynamics have
not been revealed yet. Here, we present a microscopic study shedding light on
time- and energy-resolved formation and thermalization of bright and dark
intra- and intervalley excitons as well as their impact on the
photoluminescence in different TMD materials. We demonstrate that intervalley
dark excitons, so far widely overlooked in current literature, play a crucial
role in tungsten-based TMDs giving rise to an enhanced photoluminescence and
reduced exciton lifetimes at elevated temperatures
Assignment of the NV0 575 nm zero-phonon line in diamond to a 2E-2A2 transition
The time-averaged emission spectrum of single nitrogen-vacancy defects in
diamond gives zero-phonon lines of both the negative charge state at 637 nm
(1.945 eV) and the neutral charge state at 575 nm (2.156 eV). This occurs
through photo-conversion between the two charge states. Due to strain in the
diamond the zero-phonon lines are split and it is found that the splitting and
polarization of the two zero-phonon lines are the same. From this observation
and consideration of the electronic structure of the nitrogen-vacancy center it
is concluded that the excited state of the neutral center has A2 orbital
symmetry. The assignment of the 575 nm transition to a 2E - 2A2 transition has
not been established previously.Comment: 5 pages, 5 figure
Phonon Sidebands in Transition Metal Dichalcogenides
Excitons dominate the optical properties of monolayer transition metal
dichalcogenides (TMDs). Besides optically accessible bright exciton states,
TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we
show that efficient exciton-phonon scattering couples bright and dark states
and gives rise to an asymmetric excitonic line shape. The observed asymmetry
can be traced back to phonon-induced sidebands that are accompanied by a
polaron redshift. We present a joint theory-experiment study investigating the
microscopic origin of these sidebands in different TMD materials taking into
account intra- and intervalley scattering channels opened by optical and
acoustic phonons. The gained insights contribute to a better understanding of
the optical fingerprint of these technologically promising nanomaterials
Enhanced Visibility of MoS2, MoSe2, WSe2 and Black Phosphorus: Making Optical Identification of 2D Semiconductors Easier
We explore the use of Si3N4/Si substrates as a substitute of the standard
SiO2/Si substrates employed nowadays to fabricate nanodevices based on 2D
materials. We systematically study the visibility of several 2D semiconducting
materials that are attracting a great deal of interest in nanoelectronics and
optoelectronics: MoS2, MoSe2, WSe2 and black phosphorus. We find that the use
of Si3N4/Si substrates provides an increase of the optical contrast up to a
50%-100% and also the maximum contrast shifts towards wavelength values optimal
for human eye detection, making optical identification of 2D semiconductors
easier.Comment: 4 figures + 3 supp.info. figure
Temperature dependence of the electron spin g factor in GaAs
The temperature dependence of the electron spin factor in GaAs is
investigated experimentally and theoretically. Experimentally, the factor
was measured using time-resolved Faraday rotation due to Larmor precession of
electron spins in the temperature range between 4.5 K and 190 K. The experiment
shows an almost linear increase of the value with the temperature. This
result is in good agreement with other measurements based on photoluminescence
quantum beats and time-resolved Kerr rotation up to room temperature. The
experimental data are described theoretically taking into account a diminishing
fundamental energy gap in GaAs due to lattice thermal dilatation and
nonparabolicity of the conduction band calculated using a five-level kp model.
At higher temperatures electrons populate higher Landau levels and the average
factor is obtained from a summation over many levels. A very good
description of the experimental data is obtained indicating that the observed
increase of the spin factor with the temperature is predominantly due to
band's nonparabolicity.Comment: 6 pages 4 figure
Ultrafast dynamics in monolayer TMDCs: the interplay of dark excitons, phonons and intervalley Coulomb exchange
Understanding the ultrafast coupling and relaxation mechanisms between
valleys in transition metal dichalcogenide semiconductors is of crucial
interest for future valleytronic devices. Recent ultrafast pump-probe
experiments showed an unintuitive significant bleaching at the excitonic
transition after optical excitation of the energetically lower excitonic
transition. Here, we present a possible microscopic explanation for this
surprising effect. It is based on the joint action of exchange coupling and
phonon-mediated thermalization into dark exciton states and does not involve a
population of the B exciton. Our work demonstrates how intra- and intervalley
coupling on a femtosecond timescale governs the optical valley response of 2D
semiconductors
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