M. T. Portella-Oberli

Determination 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 setup, allowing us 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 denote the dominant contribution of excitons to the luminescence signal, and the lack of thermodynamical equilibrium at low densities.

Polaritonic Feshbach resonance

Feshbach resonances provide a powerful tool for engineering interactions in ultracold atomic gases. The strong exciton–photon coupling in semiconductor microcavities facilitates the demonstration of a polaritonic Feshbach resonance with promising implications for manipulating polariton quantum fluids. A Feshbach resonance occurs when the energy of two interacting free particles comes into resonance with a molecular bound state. When approaching this resonance, marked changes in the interaction strength between the particles can arise. Feshbach resonances provide a powerful tool for controlling the interactions in ultracold atomic gases, which can be switched from repulsive to attractive1,2,3,4, and have allowed a range of many-body quantum physics effects to be explored5,6. Here we demonstrate a Feshbach resonance based on the polariton spinor interactions in a semiconductor microcavity. By tuning the energy of two polaritons with anti-parallel spins across the biexciton bound state energy, we show an enhancement of attractive interactions and a prompt change to repulsive interactions. A mean-field two-channel model quantitatively reproduces the experimental results. This observation paves the way for a new tool for tuning polariton interactions and to move forward into quantum correlated polariton physics.

Ultrafast tristable spin memory of a coherent polariton gas

Non-linear interactions in coherent gases are not only at the origin of bright and dark solitons and superfluids; they also give rise to phenomena such as multistability, which hold great promise for the development of advanced photonic and spintronic devices. In particular, spinor multistability in strongly coupled semiconductor microcavities shows that the spin of hundreds of exciton-polaritons can be coherently controlled, opening the route to spin-optronic devices such as ultrafast spin memories, gates or even neuronal communication schemes. Here we demonstrate that switching between the stable spin states of a driven polariton gas can be controlled by ultrafast optical pulses. Although such a long-lived spin memory necessarily relies on strong and anisotropic spinor interactions within the coherent polariton gas, we also highlight the crucial role of non-linear losses and formation of a non-radiative particle reservoir for ultrafast spin switching.

From Single Particle to Superfluid Excitations in a Dissipative Polariton Gas

Using an 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 soundlike 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, soundlike shapes for the excitations dispersion, intensity, and linewidth ratio between the two branches in agreement with the theory. The influence of the nonequilibrium character of the polariton gas is shown by a careful analysis of its dispersion.

Coherent optical control of the wave function of zero dimensional exciton polaritons

R. Cerna1∗, D. Sarchi, T. K. Paräıso, G. Nardin, Y. Léger, M. Richard, B. Pietka, O. El Daif, F. Morier-Genoud, V. Savona, M. T. Portella-Oberli, B. Deveaud-Plédran Institut de Photonique et d’Électronique Quantiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne; ∗roland.cerna@epfl.ch Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne EPFL, CH-1015 Lausanne, Switzerland Institut Néel-CNRS, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France Institut des Nanotechnologies de Lyon (INL), UMR CNRS 5270, Ecole Centrale de Lyon, 36 Avenue Guy de Collongue, 69134 Ecully Cedex, France (Dated: April 28, 2009)

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.

Heterodyne spectroscopy of polariton spinor interactions

We report on spinor polariton interactions in GaAs based microcavities. This investigation is carried out by means of heterodyne polarized pump-probe spectroscopy. We show the dependence of the energy renormalization of the lower and upper polariton resonances with cavity detuning for different polariton densities. We use the exciton-photon based Gross-Pitaevskii equation to model the experiment for both lower and upper polariton modes. The theoretical results reproduce qualitatively the experimental observations revealing the magnitude and sign of the parallel and antiparallel spin interaction strength. We evidence the strong influence of the biexciton resonance on the antiparallel spin polariton energy shift and provide the exciton-biexciton coupling constant. We derive our results in the lower polariton basis using the Gross-Pitaevskii equation, from which we express analytically the spinor polariton interactions and identify the clear role of the biexciton resonance.

Temporally resolved second-order photon correlations of exciton-polariton Bose-Einstein condensate formation

Second-order time correlation measurements with a temporal resolution better than 3 ps were performed on a CdTe microcavity where spontaneous Bose-Einstein condensation is observed. After the laser pulse, the nonresonantly excited thermal polariton population relaxes into a coherent polariton condensate. Photon statistics of the light emitted by the microcavity evidences a clear phase transition from the thermal state to a coherent state, which occurs within 3.2 ps after the onset of stimulated scattering. Following this very fast transition, we show that the emission possesses a very high coherence that persists for more than 100 ps after the build-up of the condensate.

Stochastic Resonance in Collective Exciton-Polariton Excitations inside a GaAs Microcavity

We report the first observation of stochastic resonance in confined exciton polaritons. We evidence this phenomena by tracking the polaritons behavior through two stochastic resonance quantifiers namely the spectral magnification factor and the signal-to-noise ratio. The evolution of the stochastic resonance in the function of the modulation amplitude of the periodic excitation signal is studied. Our experimental observations are well reproduced by numerical simulations performed in the framework of the Gross-Pitaevskii equation under stochastic perturbation.

Two-dimensional 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-interactions, a lower-upper cross interaction. This appears as separated peaks in the on-diagonal and off-diagonal parts 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.