Energy relaxation of resonantly excited polaritons in semiconductor microcavities D.N. Krizhanovskii a , A.I. Tartakovskii a, * , A.V. Chernenko a , V.D. Kulakovskii a , M. Emam-Ismail b , M.S. Skolnick b , J.S. Roberts c a Institute of Solid State Physics, RAS, 142432 Chernogolovka, Russian Federation b Department of Physics and Astronomy, University of Shef®eld, Shef®eld S3 7RH, UK c Department of Electronic and Electrical Engineering, University of Shef®eld, Shef®eld S1 3JD, UK Received 12 January 2001; accepted 27 March 2001 by L.V. Keldysh Abstract Polariton emission has been investigated in a GaAs based microcavity under various excitation conditions which excite predominately hot free excitons and carriers He±Ne laser excitation), localized excitons Ti±sapphire laser excitation), or mixed system. Strong differences have been found both in the intensity, the energy distribution, and the temperature depen- dence of polariton emission for the different excitation conditions. The relaxation into the low polariton branch due to consecutive scattering via polariton states in the bottleneck region and via the states of localized excitons has been found ineffective. Instead, we found that the highly effective ®lling of low polariton states occurs via relaxation of mobile excitons delocalized due to either thermal activation or interaction with hot carriers. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 78.66; 78.60 Keywords: A. Quantum wells; A. Semiconductors; D. Recombination and trapping; E. Luminescence Excitons in quantum wells QW) embedded in a semi- conductor microcavity MC) are of signi®cant interest due to the possibility to control in such structures the properties of both excitons X) and photons C), and the coupling between them [1]. The coupled exciton±photon system is described in terms of cavity polaritons in analogy to bulk exciton polaritons [2,3]. However, in contrast to the bulk polaritons which are radiatively stable, two-dimensional cavity polaritons have an intrinsic radiative lifetime in a range of few ps for polariton modes with a large photon fraction. One of the major issues in the study of optical properties of polaritons is the description of their photoluminescence PL), which is the result of the interplay between radiative decay and energy relaxation through the interaction with phonons, excitons, hot carriers and defects. Calculations show that the rate of energy relaxation of polaritons via phonon scattering decreases strongly for photonlike polari- tons due to the reduced exciton content and due to the increased slope of their dispersion [4]. The combination of the enhanced radiative and reduced energy relaxation rates in passing from the excitonlike to photonlike modes results in a depletion of polaritons in the photonlike region referred to as a bottleneck effect. This effect has been studied with the use of both cw [5] and pulsed [6,7] photoexcitation technique. A depletion of about one order of magnitude has been found for photonlike polaritons in MCs with nega- tive detuning D  E C 2 E X < 25 meV [5] E C and E X are the energies of uncoupled photon and exciton modes, respec- tively). This depletion is much smaller than the value of ,10 24 obtained from calculations taking into account only phonon assisted LP scattering [4] which clearly indicates that other relaxation mechanisms must be also considered. In this paper, we present the investigation of energy relaxation mechanisms in MCs from the studies of PL under various excitation regimes providing different initial excited systems see the scheme in Fig. 1a). First, we have used a He±Ne laser providing the photoexcitation well above the QW energy gap E g and, hence, generating hot free carriers and free excitons FE) with large momenta k. Second, we have used an excitation with a Ti±sapphire laser Solid State Communications 118 2001) 583±587 0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-109801)00159-4 PERGAMON www.elsevier.com/locate/ssc * Corresponding author.