By implementing four-wave mixing
(FWM) microspectroscopy, we measure coherence and population dynamics
of the exciton transitions in monolayers of MoSe2. We reveal
their dephasing times T2 and radiative
lifetime T1 in a subpicosecond (ps) range,
approaching T2 = 2T1 and thus indicating radiatively limited dephasing at a temperature
of 6 K. We elucidate the dephasing mechanisms by varying the temperature
and by probing various locations on the flake exhibiting a different
local disorder. At the nanosecond range, we observe the residual FWM
produced by the incoherent excitons, which initially disperse toward
the dark states but then relax back to the optically active states
within the light cone. By introducing polarization-resolved excitation,
we infer intervalley exciton dynamics, revealing an initial polarization
degree of around 30%, constant during the initial subpicosecond decay,
followed by the depolarization on a picosecond time scale. The FWM
hyperspectral imaging reveals the doped and undoped areas of the sample,
allowing us to investigate the neutral exciton, the charged one, or
both transitions at the same time. In the latter, we observe the exciton–trion
beating in the coherence evolution indicating their coherent coupling.
Optimized
light–matter coupling in semiconductor nanostructures
is a key to understand their optical properties and can be enabled
by advanced fabrication techniques. Using in situ electron beam lithography
combined with a low-temperature cathodoluminescence imaging, we deterministically
fabricate microlenses above selected InAs quantum dots (QDs), achieving
their efficient coupling to the external light field. This enables
performing four-wave mixing microspectroscopy of single QD excitons,
revealing the exciton population and coherence dynamics. We infer
the temperature dependence of the dephasing in order to address the
impact of phonons on the decoherence of confined excitons. The loss
of the coherence over the first picoseconds is associated with the
emission of a phonon wave packet, also governing the phonon background
in photoluminescence (PL) spectra. Using theory based on the independent
boson model, we consistently explain the initial coherence decay,
the zero-phonon line fraction, and the line shape of the phonon-assisted
PL using realistic quantum dot geometries.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.