Collective dynamics of excitons and polaritons in semiconductor nanostructures

arXiv:0912.1937v1 [cond-mat.mes-hall] 10 Dec 2009 Collective dynamics of excitons and polaritons in semiconductor nanostructures A Amo‡, D Sanvitto and L Vi˜na Departamento de F´ısica de Materiales, Universidad Auton´oma de Madrid, 28049 Madrid, Spain E-mail: alberto.amo@spectro.jussieu.fr Abstract. Time resolved photoluminescence is a powerful technique to study the collective dynamics of excitons and polaritons in semiconductor nanostructures. We present a two excitation pulses technique to induce the ultrafast and controlled quenching of the exciton emission in a quantum well. The depth of the dip is given by the magnitude of the warming of the carriers induced by the arrival of a laser pulse when an exciton population is already present in the sample. We use this technique to study the relaxation mechanisms of polaritons in semiconductor microcavities, which are of great importance to enhance the conditions for their condensation under non-resonant excitation. We also explore the dynamics of polariton fluids resonantly created in the lower polariton branch in a triggered optical parametric oscillator configuration, showing evidence of polariton superfluidity, and opening up the way to the real-time study of quantum fluids. 1. Introduction Semiconductor nanostructures offer a privileged workbench for the study of many fundamental properties of the light-matter interaction and of the collective excitations in solids. Due to single atomic monolayer resolution achieved with epitaxial growth techniques, semiconductor devices can be designed into heterostructures in which the dimensionality of the excitations, the strength of the light-matter interaction and the particle character according to its statistics (bosonic or fermionic) can be finely controlled. Additionally, if the materials of choice in the structure present a direct gap, excitations in its basic form of electrons promoted from the valence to the conduction band can be easily created and detected by optical means. In quantum wells (QWs), optical excitation leads to the formation of two types of populations: free electrons and holes, and exciton complexes. The two types of populations coexist in quasi thermal equilibrium (Szczytko et al. 2004, Chatterjee et al. 2004, Bajoni et al. unpublished), with a temperature which decreases in time towards the lattice temperature when the excitation is pulsed (von der Linde & Lambrich 1979, Capizzi et al. 1984, Leo et al. 1988a, Yoon et al. 1996, Bajoni et al. unpublished). By varying the lattice temperature and density of the photoexcited carriers it is possible to control the ratio between the two populations, allowing for the observation of a transition from an exciton dominated phase (insulating ‡ Present address: Laboratoire Kastler Brossel, Universit´e Pierre et Marie Curie, Ecole Normale Sup´erieure et CNRS, UPMC Case 74, 4 place Jussieu, 75252 Paris Cedex 05, France Collective dynamics of excitons and polaritons in semiconductor nanostructures 2 due to the neutral character of these quasiparticles) to a free carrier phase (i. e. conducting) (Kaindl et al. 2003, Kappei et al. 2005, Amo et al. 2006, Stern et al. 2008). Additionally, the energy separation of the exciton and free carrier recombination enables the detailed study of phase-space filling effects associated to the fermionic character of the free electron-hole populations (Kappei et al. 2005). For instance, Pauli blockade is one of the typical effects in a fermionic degenerate system (Warburton et al. 1997, Kalevich et al. 2001, Ono et al. 2002), and has been shown to greatly alter the electron spin-flip dynamics in semiconductors and, consequently, the polarisation dynamics of the light emitted by the system (Potemski et al. 1999, Dzhioev et al. 2002, Nemec et al. 2005, Amo et al. 2007). Semiconductor nanostructures allow also for the study of the many body properties of boson ensembles. In particular, microcavities consitute an excellent playground. In these systems the fundamental excitations are polaritons: bosons formed from the linear combination of quantum well excitons embedded in a cavity, and the photon modes confined by Bragg mirrors. Due to their partially photonic nature, polaritons have a very small mass, (∼10−5 me, the free electron mass) and, consequently, a very high critical temperature for Bose-Einstein condensation (BEC) (Kasprzak et al. 2006, Christopoulos et al. 2007, Christmann et al. 2008). Additionally, the properties of the ensemble can be easily probed through the light escaping from the cavity, which arises from the annihilation of polaritons and contains all the energy, coherence and density information of the polariton ensemble inside the cavity. Recent experiments have shown the achievement of polariton condensates in CdTe and GaAs based microcavities at temperatures of the order of ∼10 K (Kasprzak et al. 2006, Balili et al. 2007, Wertz et al. 2009). Despite their out of equilibrium character (their lifetimes range up to ∼

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