53 research outputs found
Light dressed-excitons in an incoherent-electron sea: Evidence for Mollow-triplet and Autler-Townes doublet
We demonstrate that the interaction between excitons and a sea of incoherent
electrons does not preclude excitons dressing by light. We investigate the role
of exciton-electron scattering in the light dressing by measuring the dynamical
absorption spectrum of a modulation-doped CdTe quantum well, which shows a
clear evidence for significant electron scattering of the excitonic states. We
show the occurrence of dressed and correlated excitons by detecting quantum
coherent interferences through excitonic Autler-Townes doublet and ac Stark
splitting, which evolves to Mollow triplet with gain. We also evidence the
partial inhibition of the electron-exciton scattering by exciton-light
coupling
Interacting many-body systems in quantum wells: Evidence for exciton-trion-electron correlations
We report on the nonlinear optical dynamical properties of excitonic
complexes in CdTe modulation-doped quantum wells, due to many-body interactions
among excitons, trions and electrons. These were studied by time and spectrally
resolved pump-probe experiments. The results reveal that the nonlinearities
induced by trions differ from those induced by excitons, and in addition they
are mutually correlated. We propose that the main source of these subtle
differences comes from the Pauli exclusion-principle through phase-space
filling and short-range fermion exchange.Comment: 5 pages, 4 figures. accepted for publications in Phys. Rev.
2D Fourier Transform Spectroscopy of exciton-polaritons and their interactions
We investigate polariton-polariton interactions in a semiconductor
microcavity through two-dimensional Fourier transform (2DFT) spectroscopy. We
observe, in addition to the lower-lower and the upper-upper polariton
self-interaction, a lower-upper cross-interaction. This appears as separated
peaks in the on-diagonal and off-diagonal part of 2DFT spectra. Moreover, we
elucidate the role of the polariton dispersion through a fine structure in the
2DFT spectrum. Simulations, based on lower-upper polariton basis
Gross-Pitaevskii equations including both self and cross-interactions, result
in a 2DFT spectra in qualitative agreement with experiments
Cross Feshbach resonance
Feshbach resonance occurs when a pair of free particles is resonantly coupled
to a molecular bound state. In the field of ultracold quantum gases, atomic
Feshbach resonances became a usual tool for tailoring atomic interactions
opening up many new applications in this field. In a semiconductor microcavity,
the Feshbach resonance appears when two lower polaritons are coupled to the
molecular biexciton state. Here, we demonstrate the existence of a cross
Feshbach resonance for which a pair of polaritons, lower together with upper,
effectively couples to the biexciton state. This demonstration is a crucial
step towards the efficient generation of entangled photon pairs in a
semiconductor microcavity. The existence of a Cross Feshbach resonance
establishes the condition to convert a pair of upper and lower polaritons via
the biexciton state into two lower polaritons, paving the way for the
generation of momentum and polarization entangled photons.Comment: 7 pages, 3 figure
From single particle to superfuid excitations in a dissipative polariton gas
Using angle-resolved heterodyne four-wave-mixing technique, we probe the low
momentum excitation spectrum of a coherent polariton gas. The experimental
results are well captured by the Bogoliubov transformation which describes the
transition from single particle excitations of a normal fluid to
sound-wave-like excitations of a superfluid. In a dense coherent polariton gas,
we find all the characteristics of a Bogoliubov transformation, i.e. the
positive and negative energy branch with respect to the polariton gas energy at
rest, sound-wave-like shapes for the excitations dispersion, intensity and
linewidth ratio between the two branches in agreement with the theory. The
influence of the non-equilibrium character of the polariton gas is shown by a
careful analysis of its dispersion.Comment: 4 pages, 3 figure
Direct measure of the exciton formation in quantum wells from time resolved interband luminescence
We present the results of a detailed time resolved luminescence study carried
out on a very high quality InGaAs quantum well sample where the contributions
at the energy of the exciton and at the band edge can be clearly separated. We
perform this experiment with a spectral resolution and a sensitivity of the
set-up allowing to keep the observation of these two separate contributions
over a broad range of times and densities. This allows us to directly evidence
the exciton formation time, which depends on the density as expected from
theory. We also evidence the dominant contribution of a minority of excitons to
the luminescence signal, and the absence of thermodynamical equilibrium at low
densities
Coherent and incoherent aspects of polariton dynamics in semiconductor microcavities
The interaction between coherent polaritons and incoherent excitons plays an important role in polariton physics. Using resonant pump-probe spectroscopy with selective excitation of single polariton branches, we investigate the different dephasing mechanisms responsible for generating a long-lived exciton reservoir. As expected, pumping the upper polariton results in a strong dephasing process that leads to the generation of a long lived reservoir. Unexpectedly, we observe an efficient reservoir creation while exciting only the lower polariton branch when the detuning is increased towards positive detuning. We propose a simple theoretical model, the polaritonic Bloch equations, to describe the dynamics of the system
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