3 research outputs found
Exciton-phonon-scattering: A competition between bosonic and fermionic nature of bound electron-hole pairs
The question of macroscopic occupation and spontaneous emergence of coherence
for exciton ensembles has gained renewed attention due to the rise of van der
Waals heterostructures made of atomically thin semiconductors. The hosted
interlayer excitons exhibit nanosecond lifetimes, long enough to allow for
excitonic thermalization in time. Several experimental studies reported
signatures of macroscopic occupation effects at elevated exciton densities.
With respect to theory, excitons are composite particles formed by fermionic
constituents, and a general theoretical argument for a bosonic thermalization
of an exciton gas beyond the linear regime is still missing. Here, we derive an
equation for the phonon mediated thermalization at densities above the
classical limit, and identify which conditions favor the thermalization of
fermionic or bosonic character, respectively. In cases where acoustic,
quasielastic phonon scattering dominates the dynamics, our theory suggests that
transition metal dichalcogenide (TMDC) excitons might be bosonic enough to show
bosonic thermalization behaviour and decreasing dephasing for increasing
exciton densities. This can be interpreted as a signature of an emerging
coherence in the exciton ground state, and agrees well with the experimentally
observed features, such as a decreasing linewidth for increasing densities
Lasing of Moir\'e Trapped MoSe/WSe Interlayer Excitons Coupled to a Nanocavity
Moir\'e trapped interlayer excitons (IXs) in heterobilayer transition metal
dichalcogenides currently attract strong interest due to their potential for
non-classical light generation, coherent spin-photon interfaces and exploring
novel correlated phases of electrons. Here, we report lasing of moir\'e trapped
IXs by integrating a pristine hBN-encapsulated MoSe/WSe heterobilayer
in a high-Q () nanophotonic cavity. We control the detuning between the
IX line and the cavity mode with a magnetic field and measure the dipolar
coupling strength to the cavity mode to be , fully
consistent with the 82 predicted by theory. The emission from
the cavity mode shows clear threshold-like behaviour. We observe a superlinear
power dependence accompanied by a narrowing of the linewidth as the distinct
features of lasing. The onset and prominence of these threshold-like behaviours
are significant at resonance whilst weak off-resonance. Our results show that a
lasing transition can be induced in interacting moir\'e trapped IXs with
macroscopic coherence extending over the lengthscale of the cavity mode. Such
systems raise interesting perspectives for low-power switching and synaptic
nanophotonic devices using 2D materials
Optical dipole orientation of interlayer excitons in MoSe<sub>2</sub>-WSe<sub>2</sub> heterostacks
We report on the far-field photoluminescence intensity distribution of
interlayer excitons in MoSe-WSe heterostacks as measured by back
focal plane imaging in the temperature range between 1.7 K and 20 K. By
comparing the data with an analytical model describing the dipolar emission
pattern in a dielectric environment, we are able to obtain the relative
contributions of the in- and out-of-plane transition dipole moments associated
to the interlayer exciton photon emission. We determine the transition dipole
moments for all observed interlayer exciton transitions to be (99 1)%
in-plane for R- and H-type stacking, independent of the excitation power and
therefore the density of the exciton ensemble in the experimentally examined
range. Finally, we discuss the limitations of the presented measurement
technique to observe correlation effects in exciton ensembles