Below a critical temperature, a sufficiently high density of bosons undergoes Bose-Einstein condensation (BEC). Under this condition, the particles collapse into a macroscopic condensate with a common phase, showing collective quantum behaviour like superfluidity, quantised vortices, interferences, etc. Up to recently, BEC was only observed for diluted atomic gases at μK temperatures. Following the recent observations of non-equilibrium BEC in semiconductor microcavities at temperatures of ~10 K, using momentum-1 and real-space 2 trapping, the quest is now towards the observation of the superfluid motion of a polariton BEC. For the same reasons that polaritons benefit from unusually favourable features for condensation, such as very high critical temperatures, it is expected that their superfluid properties would likewise manifest with altogether different magnitudes, such as very high critical velocities. Since they have shown many deviations in their Bose-condensed phase from the cold atoms paradigm, it is not clear a priori to which extent their superfluid properties would coincide or depart from those observed with atoms, among which quantised vortices 6 , frictionless motion 7 , linear dispersion for the elementary excitations 8 , or more recently Čerenkov emission of a condensate flowing at supersonic velocities 9 , are among the clearest signatures of quantum fluid propagation.Microcavity polaritons are two-dimensional bosons of mixed electronic and photonic nature, formed by the strong coupling of excitons-confined in semiconductor quantum wells-with photons trapped in a micron scale resonant cavity. First observed in 1992 10 , these particles have been profusely studied in the last fifteen years due to their unique features. Thanks to their photon fraction, polaritons can easily be excited by an external laser source and detected by light emission in the direction perpendicular to the cavity plane. However, as opposed to photons, they experience strong interparticle interactions owing to their partially electronic fraction. Due to the deep polariton dispersion, the effective mass of these particles is 10 4 -10 5 smaller than the free electron mass, resulting in a very low density of states. This allows for a high state 3 occupancy even at relatively low excitation intensities. However, polaritons live only a few 10 -12 s in a cavity before escaping and therefore thermal equilibrium is never achieved. In this respect, a macroscopically degenerate state of polaritons departs strongly from an atomic Bose-condensed phase. The experimental observations of spectral and momentum narrowing, spatial coherence and long range order-which have been used as evidence for polariton Bose-Einstein condensation-are also present in a pure photonic laser 11 . The recent observation of long range spatial coherence 12 , vortices 4 and the loss of coherence with increasing density in the condensed phase 13,14 , are in accordance with macroscopic phenomena proper of interacting, coherent bosons 15 . But a direct manifestation ...

In this Letter we demonstrate Mie resonances mediated transport of light in randomly arranged, monodisperse dielectric spheres packed at high filling fractions. By means of both static and dynamic optical experiments we show resonant behavior in the key transport parameters and, in particular, we find that the energy transport velocity, which is lower than the group velocity, also displays a resonant behavior.

In semiconductor microcavities, electron-polariton scattering has been proposed as an efficient process that can drive polaritons from the bottleneck region to the ground state, achieving Bose amplification of the optical emission. We present clear experimental observation of this process in a structure that allows control of the electron density and we report substantial enhancement of photoluminescence. We show that this enhancement is more effective at higher temperatures due to the different way that electron scattering processes either broaden or relax polaritons.

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We report the coexistence of low threshold lasing and strong coupling in a high-quality semiconductor microcavity under near-resonant optical pumping. A sharp laser mode splits from the lower-polariton branch and approaches the bare cavity mode frequency as the pump power increases. The lasing is produced by low density localized exciton states, which are weakly coupled to the cavity mode. The appearance of this lasing mode distinguishes quantum-well excitons into those which are strongly or weakly coupled with the cavity mode. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1643191͔A photon mode in a Fabry-Perot resonator and an excitonic resonance in a semiconductor quantum well ͑QW͒ resemble two harmonic oscillators which can be either weakly or strongly coupled together in semiconductor microcavities containing embedded quantum wells. In the weak-coupling ͑WC͒ regime, resonant tuning of the cavity mode and the exciton transition results in formation of two degenerate states ͑a crossing behavior͒, while for strong-coupling ͑SC͒ on resonance, an energy splitting between new eigenstates of the system occurs ͑anticrossing situation͒.1 WC exists in vertical cavity surface emitting lasers ͑VCSELs͒ and leads to enhancement of the spontaneous emission of excitons in resonance with the cavity mode. The SC regime has also been intensively studied in the past decade as it offers a unique opportunity to observe optical effects associated with exciton polaritons, i.e., quasiparticles composed of half a photon and half an exciton, coherently propagating in the microcavity plane. The transition between weak-/strongcoupling in microcavities has been carefully studied.2-5 This transition is governed by a condition on the exciton oscillator strength and decay rate, parameters that one can alter by changing the temperature, exciton density, or applying external fields. 6 Here we report clear evidence of the simultaneous presence of the perturbative ͑weak͒ and nonperturbative ͑strong͒ regimes when optically exciting semiconductor microcavities. We observe an unambiguous anticrossing behavior of the coupled cavity and exciton modes, characteristic of the strong-coupling regime, while between these eigenstates of the system a new strong line appears whose intensity is exponentially dependent on the pumping power. An intermediate regime is identified where this lasing mode, which fulfills the characteristics of the weak-coupling regime, coexists with the free polariton modes. This extra mode results from the inversion of a population of localized exciton states. 7Population inversion at pump densities lower than the transition threshold density is possible in microcavities possessing a very low concentration of localized states. Thus a VC-SEL and strongly coupled exciton polaritons are simultaneously present in the same cavity.The semiconductor structure used in this study is a conventional cavity with optical linewidth ␥ c ϭ0.41 meV, and a single GaAs QW with exciton linewidth ␥ x ϭ0.44 meV, fully discussed in Ref. 8 ͑studi...