Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T4: Polariton Condensates |
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Sponsoring Units: DCMP DAMOP Chair: Jacqueline Bloch, CNRS Room: 306/307 |
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T4.00001: Theory of polariton condensation Invited Speaker: Lately, some novel experiments with planar optical microcavities make use of the mixing of excitons with photons to create composite bosons called polaritons that have a very light mass, and are thus a good candidate for high-temperature Bose condensation. Good evidence for spontaneous coherence has now been obtained.\footnote{J. Kasprzak, et al.Nature, 443, 409-415 (2006).} There are special issues to resolve\footnote{J. Keeling, F. M. Marchetti, M. H. Szymanska, P. B. Littlewood, Semiconductor Science and Technology, 22, R1-26 (2007).} considering the effects of low dimensionality, disorder, strong interactions, and especially strong decoherence associated with decay of the condensate into environmental photons\footnote{M. H. Szymanska, J. Keeling, P. B. Littlewood, Physical Review B 75, 195331 (2007).} --- since the condensate is a special kind of laser. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T4.00002: Bose-Einstein Condensation of Microcavity Polaritons in Harmonic Traps Invited Speaker: Polaritons in microcavities are a two-dimensional, weakly interacting boson gas, and their spatial distribution, momentum distribution, coherence properties, and excitation spectrum can all be observed. In several recent experiments, these observations are consistent with the interpretation of a quasiequilibrium Bose condensate of polaritons. In our experiments, we use a special stress geometry to create a harmonic potential for the polaritons in the plane of their motion, which is analogous to the traps used in experiments on BEC of cold atoms. Among other effects, we observe coherent light emission from the polariton condensate. Two questions arise: 1) Since the system emits coherent light, how can we distinguish it from a standard laser? 2) How do we distinguish it from some type of nonlinear amplification of the excitation light? In our experiments with a trapping potential, we can easily distinguish between these different effects. We demonstrate two transitions which occur in the same place in the same structure, one which is standard lasing and one which is polariton condensation. The quasiequilibrium polariton condensate in microcavities thus emerges as a new type of coherent light emitter. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 4:18PM |
T4.00003: Observation of Bogoliubov excitations in exciton-polariton condensates Invited Speaker: Particle-particle interaction and peculiar excitation spectra are keys for understanding BEC and superfluidity physics. A quantum field-theoretical formulation for a weakly interacting Bose condensed system was developed by Bogoliubov in 1947, which predicted the phonon-like excitation spectrum in the low- momentum regime. Exciton-polaritons in a semiconductor microcavity, which are elementary excitations created by strong coupling between quantum-well excitons and microcavity photons, were proposed as a new BEC candidate in solid-state systems. Recent experiments with exciton-polaritons have demonstrated several interesting signatures from the viewpoint of polariton condensation, such as quantum degeneracy at non-equilibrium conditions, the polariton-bunching effect at the condensation threshold, long spatial coherence and quantum degeneracy at equilibrium conditions. The particle-particle interaction and the Bogoliubov excitation spectrum are at the heart of BEC and superfluidity physics, but have only been studied theoretically for exciton-polaritons. In this talk, we report the first observation of interaction effects on the exciton-polariton condensate and the excitation spectra, which are in quantitative agreement with the Bogoliubov theory. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T4.00004: Quantum fluid dynamics and superfluid behaviour of polaritons in microcavities Invited Speaker: Achievement of polariton condensation in semiconductor microcavities [1,2] has opened the way to the study of new interesting phenomena related to the behaviour of non-equilibrium Bose particles in the quantum limit. In this talk we will see the formation of a coherent quantum state of polaritons created at a given momentum and at a given time using a combination of a continuous wave pump and a pulsed probe. This state is observed to persist in the cavity for a time much longer than the cavity lifetime [3]. Using this technique we are able to investigate the behaviour of a quantum state of polaritons with an extension of $\sim $ 20 $\mu $m moving a hundreds of microns within the cavity. One of the most striking effects of a moving polariton condensate is the observation of superfluid behaviour when crossing obstacles even at speeds only 100 times smaller than the speed of light [4]. Other interesting phenomena, which will be shown, are diffusion-less motion, due to the linearization of the polariton dispersion, and the formation of Cherenkov-like patterns for polaritons moving at supersonic velocities. \\[4pt] [1] J. Kasprzak et al. \textit{Nature} \textbf{443}, 409 (2006). \\[0pt] [2] R. Balili et al. \textit{Science} \textbf{316}, 1007 (2007). \\[0pt] [3] D. Ballarini et al. http://arxiv.org/abs/0807.3224 (arXiv:0807.3224) (2008). \\[0pt] [4] A. Amo et al. \textit{Nature}, forthcoming publication (2009). [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:30PM |
T4.00005: Theory of polariton condensation and superfluidity Invited Speaker: Exciton-polaritons in planar microcavities have two allowed spin projections on the axis of the structure, which is why they can be considered as a two-component weakly interacting Bose gas. The order parameter for BEC or the superfluid phase transition in this gas is a 2D vector analogous to the Jones vector of classical light. The build-up of the order parameter results is the build up of vector polarisation of light emitted by the polariton condensate. Recently observed appearance of the spontaneous vector polarisation stochastically changing from one experiment to another [1] manifests the spontaneous symmetry breaking in a polariton system and confirms observation of the polariton BEC at room temperature in bulk GaN microcavities. The spin dependence of polariton-polariton interactions in quantum well microcavities favours formation of linearly polarised superfluids. The sound velocity in such superfluids is polarisation-dependent [2]. We show that the static potential disorder provokes generation of vortices having a semi-integer topological charge [3]. These half-vortices are lowest energy excitations in two-component superfluids. They can be observed by polarisation-resolved near-field spectroscopy. We show also that the polarisation bistability in optically driven polariton condensates allows for optical excitation of the ``spin rings'' spreading in the real space [4] and paves way to fabrication of ``polariton neurons'' assembled in all-optical integrated circuits [5]. \\[3pt] [1] J. J. Baumberg, A. V. Kavokin, et al Phys. Rev. Lett. 101, 136409 (2008). \\[0pt] [2] I. A. Shelykh, et al, Phys. Rev. Lett. \textbf{97}, 066402 (2006). \\[0pt] [3] T. C. Liew, Yuri G. Rubo, and A. V. Kavokin, Phys. Rev. Lett. \textbf{101}, 187401 (2008). \\[0pt] [4] I. A. Shelykh, T. C. Liew, and A. V. Kavokin, Phys. Rev. Lett. 100, 116401 (2008). \\[0pt] [5] T. C. Liew, A. V. Kavokin, and I. A. Shelykh, Phys. Rev. Lett. 101, 016402 (2008). [Preview Abstract] |
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