We investigate dynamics of resonantly excited excitons in single-layers of MoSe2 and WS2 down to 4.5K. To this end, we measure the delay dependence of the heterodyne four-wave mixing (F M) amplitude induced by three, short laser pulses. This signal depends not only on the population of optically active excitons, which affects the absorption of the probe, but also on the population of optically inactive states, by interaction-induced energy shift, influencing the refractive index experienced by the probe. As such, it offers insight into density dynamics of excitons which do not directly couple to photons. Reproducing the coherent signal detected in amplitude and phase, the FWM delay dependence is modeled by a coherent superposition of several exponential decay components, with characteristic time constants from 0.1 picosecond up to 1 nanosecond. With increasing excitation intensity and/or temperature, we observe strong interference effects in the FWM field amplitude, resulting in progressively more complex and nonintuitive signal dynamics. We attribute this behaviour to increasingly populated exciton dark states, which change the FWM field phase by the relative effect on absorption and refractive index. We observe that exciton recombination occurs on a significantly longer timescale in WS2 with respect to MoSe2, which is attributed to the dark character of exciton ground state in the former and the bright in the latter.
Light and matter can be unified under the strong coupling regime, creating superpositions of both, called dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems ,strong coupling phenomena have been realized in solid state optical microcavities. They form an essential ingredient in the exciting physics spanning from many-body quantum coherence phenomena, like Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics (cQED). A widely used approach within cQED is the
Jaynes-Cummings (JC) model that describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling (QSC), a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently published. In our latest report we applied advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity
system. Specifically, we measured and simulated its four-wave mixing (FWM) response, granting direct access to the first two rungs of the JC ladder. This paper summarizes the main results of Ref. 15 and adds
FWM experiments obtained on a micropillar cavity in which a doublet of quantum dot (QD) excitons interacts with the cavity mode in the limit of weak to strong coupling.
We review our recent advances in four-wave mixing spectroscopy of individual semiconductor quantum dots
using heterodyne spectral interferometry, a novel implementation of transient nonlinear spectroscopy allowing
the study of the transient nonlinear polarization emitted from individual electronic transitions in both amplitude
and phase. We present experiments on individual excitonic transitions localized in monolayer islands
of a GaAs/AlAs quantum well. The detection of amplitude and phase allows the implementation of a twodimensional
femtosecond spectroscopy, in which mutual coherent coupling of single quantum dot states can
be observed and quantified. By combining two-dimensional femtosecond spectroscopy with four-wave mixing
mapping in the real space we found coherent coupling between spatially separated excitons by up to ~ 0.8 μm.
Conference Committee Involvement (2)
Ultrafast Phenomena in Semiconductors and Nanostructure Materials XV
23 January 2011 | San Francisco, California, United States
Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIV
24 January 2010 | San Francisco, California, United States
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