Exciton Reservoir Research Articles

Dynamics of Long-Range Ordering in an Exciton-Polariton Condensate

We report on time-resolved measurements of the first order spatial coherence in an exciton-polariton Bose-Einstein condensate. Long-range spatial coherence is found to set in right at the onset of stimulated scattering, on a picosecond time scale. The coherence reaches its maximum value after the population and decays slowly, staying up to a few hundred picoseconds. This behavior can be qualitatively reproduced, using a stochastic classical field model describing interaction between the polariton condensate and the exciton reservoir within a disordered potential.

Exciton-phonon scattering and photoexcitation dynamics inJ-aggregate microcavities

We have developed a model accounting for the photoexcitation dynamics and the photoluminescence of strongly coupled $J$-aggregate microcavities. Our model is based on a description of the $J$-aggregate film as a disordered Frenkel exciton system, in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. In a strongly coupled microcavity exciton polaritons are formed, mixing super-radiant excitons and cavity photons. The calculation of the microcavity steady-state photoluminescence, following a cw nonresonant pumping, is carried out. The experimental photoluminescence intensity ratio between upper and lower polariton branches is accurately reproduced. In particular, both thermal activation of the photoluminescence intensity ratio and its Rabi splitting dependence are a consequence of the bottleneck in the relaxation, occurring at the bottom of the excitonic reservoir. The effects due to radiative channels of decay of excitons and to the presence of a particular set of discrete optical molecular vibrations active in relaxation processes are investigated.

Fluctuation-induced transitions of a bistable driven polariton system in the presence of damping

Fluctuation-induced transitions from the lower energy state of a bistable nonlinear driven microcavity oscillator are analyzed beyond the Fluctuation–Dissipation theorem. The sources of noise are: both fluctuations of the external pumping and inevitable interaction with the exciton reservoir in the cavity. We show that finite polariton lifetime strongly modifies the phase portrait and influences the temporal parameters of the transition within the bistable regime. To investigate the transient dynamics of the driven polariton system, three different approaches are realized: numerical experiment (1), i.e. direct solution of the quasiclassical dynamic equation for polariton amplitude driven by an external pump, (2) solution of two dimensional Fokker–Planck-equation and (3) effective one dimensional Fokker–Planck-equation, obtained within a low-damping approximation. We show that the escape times obtained within the numerical experiment and the 2D-Fokker–Planck-equation coincide. In contrast, the one dimensional Fokker–Planck-approximation fails for large damping parameters due to strong deviation of the phase trajectories from those obtained within the low-damping approximation. The range of the ratio between damping and detuning for which the 1d approximation is valid, is shown to be smaller than 0.04. Finally, we determine the impact of the fluctuations on experiments illustrated by the narrowing of the hysteresis cycle.

Polariton relaxation after resonant pumping at the upper polariton branch under doubly‐resonant Raman scattering conditions

AbstractWe have investigated the influence of polariton–phonon interaction on the relaxation of microcavity polaritons towards the bottom of the lower polariton branch (LPB). We have varied either the energy difference between the two polariton branches at K ∼ 0 (negative detuning) or the Rabi splitting (zero detuning), adjusting them to coincide with the energy of a LO‐phonon. We have observed a sub‐linear/super‐linear increase of the LPB emission with pump power for negative/zero detuning, respectively. These different power dependencies can be explained by the combined effect of the different polariton–phonon scattering efficiency, which is minimum/maximum for large negative/zero detunings, a competition between several relaxation mechanisms and the different coupling to the excitonic reservoir of large‐K LPB states. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum degeneracy of polaritons in a GaAs based Microcavity

AbstractPolariton quantum degeneracy is obtained in a GaAs Microcavity by pulsed resonant excitation of the excitonic reservoir. Time resolved emission is monitored in this stimulated regime along the polariton branch.Analyzing the polariton population distribution for different time delays, we conclude that polariton thermalization is not achieved and that the dynamics is governed by the scattering kinetics from the reservoir into the polariton trap. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamics of microcavity polaritons in the Markovian limit

A master equation for the reduced density matrix of the microcavity polaritons coupled with the reservoir of high energy excitons is derived. It is allowed both the polaritons and the excitons to be self-interacting systems. Long time asymptotic properties of the polariton population is studied in the whole range of the reservoir temperatures and the corresponding decoherence effects are reported.

Parametric scattering in semiconductor microcavities probed by four-wave mixing

We present a study of the polariton coherent dynamics in II–VI semiconductor microcavities in the parametric scattering regime. We perform angle resolved four-wave mixing, which allows us to study the polariton dephasing within the full reciprocal space. When polaritons are excited above the threshold in the phase matching conditions for parametric scattering, we observe a very efficient scattering towards the final states (signal and idler), inducing pump depletion. On the contrary, below threshold or when the phase matching conditions are not fulfilled, we observe an angle-dependent collision broadening that occurs when pump polaritons scatter towards the excitonic reservoir. We discuss the interplay between parametric scattering and collision broadening within the framework of a phenomenological numeric calculation. Last, we propose a new design for the realization of a vertical-cavity, surface emitting parametric oscillator/amplifier that provides triple resonance for the parametric frequencies together with cavity phase-matching for all waves at normal incidence.

Dynamics of the in-plane charge separation front in a two-dimensional electron-hole gas

We show both experimentally and theoretically that the recently observed optically induced in-plane charge separation in quantum well (QW) structures and the exciton ring emission pattern at this charge separation boundary have an extremely long lifetime. The oppositely charged carriers remain separated and provide a reservoir of excitons at their boundary with a persistent emission which lasts hundreds of microseconds (orders of magnitude longer than their recombination time) after the external excitation is removed. This long lifetime is due to an interplay between the slow in-plane carrier diffusion and slow carrier tunneling perpendicular to the QW plane.

Branch-entangled polariton pairs in planar microcavities and photonic wires

A scheme is proposed for the generation of branch-entangled pairs of microcavity polaritons through spontaneous inter-branch parametric scattering. Branch-entanglement is achievable when there are two twin processes, where the role of signal and idler can be exchanged between two different polariton branches. Branch-entanglement of polariton pairs can lead to the emission of frequency-entangled photon pairs out of the microcavity. In planar microcavities, the necessary phase-matching conditions are fulfilled for pumping of the upper polariton branch at an arbitrary in-plane wave-vector. The important role of nonlinear losses due to pair scattering into high-momentum exciton states is evaluated. The results show that the lack of protection of the pump polaritons in the upper branch is critical. In photonic wires, branch-entanglement of one-dimensional polaritons is achievable when the pump excites a lower polariton sub-branch at normal incidence, providing protection from the exciton reservoir.

Polariton parametric amplifier: pump dynamics in the coherent regime

We present a comprehensive study of the polariton coherent dynamics in II–VI semiconductor microcavities. An angle resolved four-wave mixing experiment has been developed, which allows us to study the polariton dephasing within the full reciprocal space. We observe two main trends: (i) pump polaritons that are excited above the threshold and in the phase matching condition for parametric amplification threshold are observed to scatter much more efficiently towards the final states (signal and idler), inducing pump depletion, and (ii) below this threshold or when the phase matching condition is not fulfilled, we observe an angle-dependent collision broadening, that occurs when pump polaritons can scatter towards the excitonic reservoir. Both dynamics are then analysed using a numerical model that reveals a complex interplay and competition between collision broadening and parametric amplification. In particular, it is shown how collision broadening must be taken into account in order to explain the gain saturation observed in the parametric amplification process. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Collision broadening in II-VI semiconductor microcavities

We present a comprehensive study of the collision broadening in II-VI semiconductor microcavities. The polariton nonlinear coherent dynamics was investigated using angle-resolved four-wave mixing, which allows us to study the polariton dephasing on a large scale within reciprocal space. The low-energy polariton dephasing time is found to present a threshold dependence with excitation angle as well as excitation density, associated with the possibility, for polaritons with large enough in-plane wave vectors to scatter towards the high density of states excitonic reservoir. The validity of the polariton-polariton scattering calculation based on the effective exciton-exciton interaction hamiltonian is also discussed.

II–VI semiconductor microcavity angle‐resolved coherent dynamics

We used angle-resolved time-integrated four-wave mixing to study the pump dynamics in a II–VI semiconductor microcavity polariton parametric amplifier. The low-energy polariton dephasing time decreases at large angles when the excitonic reservoir becomes accessible to polariton–polariton scattering. Additionally, in the parametric amplification regime, with phase matching for the polariton–polariton scattering at the magic angle, we observe a strong reduction of the FWM decay time. Numerical analysis of the polariton coherent dynamics in this regime allows to interpret the observed features in terms of time dependent pump depletion.

Polariton parametric amplifier pump dynamics in the coherent regime.

We studied the pump coherent dynamics in a II-VI microcavity parametric amplifier, using angle-resolved four-wave mixing. The polariton parametric amplification is found to result in a strong quenching and saturation of the pump coherence lifetime above the threshold. For the polariton scattering processes that remain below the amplification threshold, we find an angle-dependent collision broadening associated with the efficiency of the polariton scattering towards the excitonic reservoir.

Angle-dependent polariton collisional broadening in semiconductor microcavities

The polariton nonlinear dynamics in a II–VI semiconductor microcavity has been studied using angle-resolved four-wave mixing. The low-energy polariton dephasing time is found to decrease at large angles, when polariton can escape towards the excitonic reservoir, in good agreement with a calculation of the angle-dependent collisional broadening. Moreover, we find that the parametric amplification of k=0 polaritons is associated to a strong quenching of the coherence at the magic angle.

Suppression of exciton super-radiance due to spatially separated scatterers

We have found that the radiative lifetime of an exciton in a GaAs quantum well (QW) increases due to high-density excitons injected in an exciton reservoir spatially separated from the QW. The photoinjected excitons in the reservoir interact with the two-dimensional exciton in the QW via the Coulomb interaction without the phase-space filling effects and the exchange interaction. The radiative lifetime increases in proportion to the exciton density in the reservoir initially, while it saturates for the higher density. It is interpreted as suppression of the exciton super-radiance due to weak localization of the exciton center-of-mass motion caused by the excitons in the reservoir. A phenomenological two-level model reproduces the experimental observation, where an exciton state is represented as the scattering-induced superposition of small wave-vector (radiative) and large wave-vector (nonradiative) states.

Uncoupled excitons in semiconductor microcavities detected in resonant Raman scattering

We present an outgoing resonant Raman-scattering study of a GaAs/AlGaAs based microcavity embedded in a p-i-n junction. The p-i-n junction allows the vertical electric field to be varied, permitting control of exciton-photon detuning and quenching of photoluminescence which otherwise obscures the inelastic light scattering signals. Peaks corresponding to the upper and lower polariton branches are observed in the resonant Raman cross sections, along with a third peak at the energy of uncoupled excitons. This third peak, attributed to disorder activated Raman scattering, provides clear evidence for the existence of uncoupled exciton reservoir states in microcavities in the strong-coupling regime.

Polariton dynamics and Bose-Einstein condensation in semiconductor microcavities

We present a theoretical model that allows us to describe the polariton dynamics in a semiconductor microcavity at large densities, for the case of non-resonant excitation. Exciton-polariton scattering from a thermalized exciton reservoir is identified as the main mechanism for relaxation into the lower polariton states. A maximum in the polariton distribution that shifts towards lower energies with increasing pump-power or temperature is shown, in agreement with recent experiments. Above a critical pump-power, macroscopic occupancies (5 \times 10^4) can be achieved in the lowest energy polariton state. Our model predicts the possibility of Bose-Einstein Condensation of polaritons, driven by exciton-polariton interaction, at densities well below the saturation density for CdTe microcavities.

Pump Angle and Laser Energy Dependence of Stimulated Scattering in Microcavities

We show that stimulated polariton scattering in a semiconductor microcavity is observable over a wide range of laser pump angles. The ratio of idler to signal beams is found to vary strongly with angle, consistent with the expected variation of the photon fraction of the idler states, and the increasing probability of scattering of idler polaritons to the exciton reservoir with increasing wave-vector. The observed variation of threshold with angle is in good agreement with a semi-classical treatment of the parametric scattering process.

Evidence of Polariton Stimulation in Semiconductor Microcavities

Stimulated photoluminescence of the lower polariton in CdTe microcavities is studied at low temperature, using nonresonant excitation. The stimulation threshold is found to be independent of the number of quantum wells in the microcavities, which rules out the stimulation model based on population inversion of either localized excitons or electron–hole plasma. On the other hand, the bosonic final state stimulation process, or boser effect, is clearly evidenced by tuning the lower polariton to one LO phonon below the exciton reservoir. For detunings closer to zero, the stimulation is dominated by another physical mechanism, that could be the exciton–exciton scattering into the upper and lower polariton states.

Evidence of Polariton Stimulation in Semiconductor Microcavities

Stimulated photoluminescence of the lower polariton in CdTe microcavities is studied at low temperature, using nonresonant excitation. The stimulation threshold is found to be independent of the number of quantum wells in the microcavities, which rules out the stimulation model based on population inversion of either localized excitons or electron-hole plasma. On the other hand, the bosonic final state stimulation process, or boser effect, is clearly evidenced by tuning the lower polariton to one LO phonon below the exciton reservoir. For detunings closer to zero, the stimulation is dominated by another physical mechanism, that could be the exciton-exciton scattering into the upper and lower polariton states.