phys. stat. sol. (c) 2, No. 2, 738–743 (2005) / DOI 10.1002/pssc.200460310
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Evidence for a spontaneous coherent phase transition of polari-tons in CdTe microcavitiesM. Richard, J. Kasprzak, R. Andre, R. Romestain, and L. S. Dang
CEA-CNRS-UJF group ”Nanophysique et semiconducteurs”, Laboratoire de Spectrométrie Physique(CNRS UMR5588), Université J. Fourier-Grenoble, BP87, 38402 St Martin d’Hères, France
We report on the non linear laser-like regime occuring in CdTe microcavities under strong non-resonantexcitation. We find that the sharp peak responsible for the nonlinearities is dispersionless. This feature isdiscussed in terms of coherent polariton-polariton scattering. A phase transition on the coherence of theemission is demonstrated experimentally : firstly, images of the luminescence in the Fourier plane exhibitsa contrasted speckle patterning that doesn’t exist under weak excitation. Secondly, a direct measurementof the transverse coherence is realized. It shows that the coherence is strongly enhanced at the transitiontoward the nonlinear regime.
1 Introduction Microcavity exciton-polariton [1] is a very suitable quantum state for the achievement ofhigh occupancy effect in solids [2]. Sharing both characteristics of the exciton and photon, including thebosonic character, it exhibits a distorted dispersion curve that acts as a trap for the population in the low-kzone. Moreover, this trap contains very few states (typically around 300 for a 50µm2 spot within the ex-ternal light cone) in comparison with the pure exciton. Thus, in spite of very short polariton lifetime (typi-cally one picosecond) the trapped polaritons can easily accumulate in order to reach the high occupancylevel which is necessary to trigger the processes of final state stimulation and coherent phase transition.Such effects have been clearly observed under resonant excitation. The most clear evidence of polaritoncollective effect in this configuration was presented by Saba et al. [3, 4]. They found out that the scatter-ing rate between polaritons could be drastically increased by the number of polaritons preexistent in the finalstate. This is a typical stimulation phenomena that relies on the high occupancy of the final states. In CdTebased microcavities excited by a non-resonant laser, it has been shown that a similar stimulation effectoccurs [5, 6] from a threshold on the excitation power. In this paper we present a study of this regime, andparticularly the changes that occurs in the emission coherence at the transition. A first part of this paperis dedicated to the observation of the luminescence in the Fourier plane where a very contrasted speckledpatterning appears above threshold. We discuss also the peculiar changes of the polariton dispersion abovethreshold. The second part presents a direct measurement of the transverse coherence of the emission. Wefind it to be strongly enhanced when entering the stimulation regime.
2 Experimental setup The sample consists of 16 CdTe QWs embedded in a 2λ microcavity whichexhibits a 26meV vacuum Rabi splitting. It is cooled down to 5K in a cold finger cryostat. The exciton-photon detuning is set to zero in this experiment. The experimental setup is sketched in fig.(1) : aTi:sapphire laser delivering 100fs long pulses is focused on the sample with an f = 9mm focal lengthmicroscope objective on a 3µm diameter spot. The laser energy is set far from the resonance : 80meVabove the lower polariton branch. The luminescence is collected through the same objective and refocusedby an f2 = 180mm achromatic lens after filtering out the laser reflections. A 1m focal length monochro-mator or a CCD camera can be placed either in the image plane or in the Fourier plane [7].
The observation of the Fourier plane provides some crucial informations on the emission that we reportin this paper. Recorded with a CCD camera, the position and brightness of a pixel in such an imagecorresponds to the direction (θ, φ) of the emitted light and to its intensity respectively (see fig. (2.a) and
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received 29 June 2004, revised 18 October 2004, accepted 12 January 2005Published online 15 February 2005
PACS 42.55.Sa,71.36.+c, 78.55.Et
∗ Corresponding author: e-mail: [email protected]
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