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Dissipationless Flow and Sharp Threshold of a Polariton Condensate with Long Lifetime

Bryan Nelsen, Gangqiang Liu, Mark Steger, and David W. Snoke*

Ryan Balili†

Ken West and Loren Pfeiffer

  • Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA

  • Department of Physics, MSU-Iligan Institute of Technology, Iligan 9200, Philippines

  • Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA

  • *snoke@pitt.edu
  • †Present address: Cavendish Laboratory, Cambridge University, Cambridge CB3 0HE, UK.
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Phys. Rev. X 3, 041015 – Published 21 November, 2013

DOI: https://doi.org/10.1103/PhysRevX.3.041015

Abstract

We report new results of Bose-Einstein condensation of polaritons in specially designed microcavities with a very high quality factor, on the order of 106, giving polariton lifetimes of the order of 100 ps. When the polaritons are created with an incoherent pump, a dissipationless, coherent flow of the polaritons occurs over hundreds of microns, which increases as density increases. At high density, this flow is suddenly stopped, and the gas becomes trapped in a local potential minimum, with strong coherence.

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When they are free, photons (particles of light) are massless and indifferent to each other’s existence. When they are put into a cavity where they interact with another type of quantum particle, however, they acquire a mass and begin to interact with each other. One class of such “dressed” photons is called exciton polaritons—photons interacting with excitons. (Excitons are bound electron-hole pairs in a semiconductor.) The polaritons are bosons. Like other interacting bosons (for example, bosonic atoms), they can self-organize into a coherent state in which almost all the polaritons have the same energy and the same wave properties. This phenomenon of self-organization, known as Bose-Einstein condensation (BEC), has been well established for polaritons in microcavities. In this experimental paper, we report a number of dramatic new effects demonstrated by polaritons in microcavities, including a very sharp transition to a highly coherent state.

One of the fundamental difficulties in observing new quantum effects associated with polaritons is that they live fewer than 10 ps in the typical microcavity setup. To enable our observations, we have succeeded in extending their lifetime 10 times, to the order of 100 ps, using extremely high-quality mirrors in the cavity. By manipulating the density of our collection of such long-living polaritons, we have observed not only BEC but also the emergence of a coherent beam of polaritons that travels hundreds of microns without losing their energy or coherence inside the structure—semiconductor layers sandwiched between the two parallel mirrors of the cavity. At higher polariton densities, the polaritons abruptly become trapped in a tiny spot, and all of them flow together to the place of lowest energy, very much like superfluid helium.

Our new observations will provide valuable input to the theoretical development of polariton physics, and the new structures we have developed should enable observations of qualitatively new physics on the millimeter (macroscopic) rather than the micron scale.

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Phys. Rev. X 3, 041015– Published 21 November, 2013
  • Received 8 May 2013
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This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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