4 research outputs found
Hybrid Exciton Signatures in ARPES Spectra of van der Waals Materials
Van der Waals heterostructures show fascinating physics
including
trapped moire exciton states, anomalous moire exciton transport, generalized
Wigner crystals, etc. Bilayers of transition metal dichalcogenides
(TMDs) are characterized by long-lived, spatially separated interlayer
excitons. Provided strong interlayer tunneling, hybrid exciton states
consisting of interlayer and intralayer excitons can be formed. Here,
electrons and holes are in a superposition of both layers. Although
crucial for optics, dynamics, and transport, hybrid excitons are usually
optically inactive and have therefore not yet been directly observed
yet. Based on microscopic and material-specific theory, we show that
time- and angle-resolved photoemission spectroscopy (tr-ARPES) is
a direct technique to visualize these hybrid excitons. Concretely,
we predict a characteristic double-peak ARPES signal arising from
the hybridized hole in the MoS2 homobilayer. The relative
intensity is proportional to the quantum mixture of the two hybrid
valence bands at the Γ point. Due to the strong hybridization,
a peak separation of more than 0.5 eV can be resolved in ARPES experiments.
Our study provides a concrete recipe for how to directly visualize
hybrid excitons and how to distinguish them from the usually observed
regular excitonic signatures
Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene
We present a joint theory-experiment
study on ultrafast photoluminescence from photoexcited graphene. On
the basis of a microscopic theory, we reveal two distinct mechanisms
behind the occurring photoluminescence: besides the well-known incoherent
contribution driven by nonequilibrium carrier occupations, we found
a coherent part that spectrally shifts with the excitation energy.
In our experiments, we demonstrate for the first time the predicted
appearance and spectral shift of the coherent photoluminescence
Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene
We present a joint theory-experiment
study on ultrafast photoluminescence from photoexcited graphene. On
the basis of a microscopic theory, we reveal two distinct mechanisms
behind the occurring photoluminescence: besides the well-known incoherent
contribution driven by nonequilibrium carrier occupations, we found
a coherent part that spectrally shifts with the excitation energy.
In our experiments, we demonstrate for the first time the predicted
appearance and spectral shift of the coherent photoluminescence
Strain Control of Exciton–Phonon Coupling in Atomically Thin Semiconductors
Semiconducting
transition metal dichalcogenide (TMDC) monolayers
have exceptional physical properties. They show bright photoluminescence
due to their unique band structure and absorb more than 10% of the
light at their excitonic resonances despite their atomic thickness.
At room temperature, the width of the exciton transitions is governed
by the exciton–phonon interaction leading to strongly asymmetric
line shapes. TMDC monolayers are also extremely flexible, sustaining
mechanical strain of about 10% without breaking. The excitonic properties
strongly depend on strain. For example, exciton energies of TMDC monolayers
significantly redshift under uniaxial tensile strain. Here, we demonstrate
that the width and the asymmetric line shape of excitonic resonances
in TMDC monolayers can be controlled with applied strain. We measure
photoluminescence and absorption spectra of the A exciton in monolayer
MoSe<sub>2</sub>, WSe<sub>2</sub>, WS<sub>2</sub>, and MoS<sub>2</sub> under uniaxial tensile strain. We find that the A exciton substantially
narrows and becomes more symmetric for the selenium-based monolayer
materials, while no change is observed for atomically thin WS<sub>2</sub>. For MoS<sub>2</sub> monolayers, the line width increases.
These effects are due to a modified exciton–phonon coupling
at increasing strain levels because of changes in the electronic band
structure of the respective monolayer materials. This interpretation
based on steady-state experiments is corroborated by time-resolved
photoluminescence measurements. Our results demonstrate that moderate
strain values on the order of only 1% are already sufficient to globally
tune the exciton–phonon interaction in TMDC monolayers and
hold the promise for controlling the coupling on the nanoscale