Phase transitions to quantum condensed phases--such as Bose-Einstein condensation (BEC), superfluidity, and superconductivity--have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 10(9) times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase.

Cavity polaritons, the elementary optical excitations of semiconductor microcavities, may be understood as a superposition of excitons and cavity photons. Owing to their composite nature, these bosonic particles have a distinct optical response, at the same time very fast and highly nonlinear. Very efficient light amplification due to polariton-polariton parametric scattering has recently been reported in semiconductor microcavities at liquid-helium temperatures. Here we demonstrate polariton parametric amplification up to 120 K in GaAlAs-based microcavities and up to 220 K in CdTe-based microcavities. We show that the cut-off temperature for the amplification is ultimately determined by the binding energy of the exciton. A 5-micrometer-thick planar microcavity can amplify a weak light pulse more than 5,000 times. The effective gain coefficient of an equivalent homogeneous medium would be 107 cm-1. The subpicosecond duration and high efficiency of the amplification could be exploited for high-repetition all-optical microscopic switches and amplifiers. 105 polaritons occupy the same quantum state during the amplification, realizing a dynamical condensate of strongly interacting bosons which can be studied at high temperature.

We report on the realization of polariton quantum boxes in a semiconductor microcavity under strong coupling regime. The quantum boxes consist of mesas, etched on the top of the spacer of a microcavity, that confine the cavity photon. For mesas with sizes of the order of a few microns in width and nanometers in depth, we observe quantization of the polariton modes in several states, caused by the lateral confinement. We evidence the strong exciton-photon coupling regime through a typical anticrossing curve for each quantized level. Moreover, the growth technique permits one to obtain high-quality samples, and opens the way for the conception of new optoelectronic devices. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2172409͔Confining semiconductor structures allows the study of various fundamental effects, ranging from the Purcell effect to the full quantum confinement. Such confinement is also used for applications in many fields, from optoelectronics to quantum information. Previous works have focused on different aspects: on the one hand, on the matter part, with the confinement of the excitonic resonances in quantum wells, quantum wires, and quantum dots. On the other hand, environment for the electromagnetic field has been modified by optical confinement in different types of cavities. Additionally since the middle of the 90s, low dimensional devices have been realized in the strong coupling regime. 1 Confinement can enhance the interactions, modify the real and imaginary parts of the resonance's energy, or open access to new interaction processes. It is also often considered as a possible way to obtain a condensed phase of bosons in semiconductors, 2 but so far the fermionic nature of excitons has always become dominant upon increasing density. In this sense, polaritons are of great interest as, despite their excitonic content, they have a very small effective mass in comparison to the exciton ͑thanks to their photonic component͒, which theoretically increases their temperature of condensation ͑above 0.1 K͒. 3 The peculiar trap shape of the lower microcavity polariton dispersion curve has motivated several relaxation experiments towards the bottom of this "trap" 4,5 but no clear evidence for the formation of spontaneous coherence formation has been given yet.Zero-dimensional ͑0D͒ Polariton confinement can be achieved either through their excitonic or through their photonic component. Recently, evidence for 0D polaritons has been given with single quantum dots in micropillars, 6 photonic nanocavities, 7 or microdisks, 8 and for a large number of excitations in micropillar structures. 9-11 Here we consider a novel system under strong coupling regime, where 0D confinement is achieved through the photonic part of polaritons in high Q cavities. Our original structure contains polariton quantum boxes, constituted by mesas in the spacer layer of a semiconductor microcavity, allowing to keep the strong coupling regime. Each mesa, by acting on the two degrees of freedom of the photonic component of the two...

We demonstrate three-dimensional spatial confinement of exciton-polaritons in a semiconductor microcavity. Polaritons are confined within a micron-sized region of slightly larger cavity thickness, called mesa, through lateral trapping of their photon component. This results in a shallow potential well that allows the simultaneous existence of extended states above the barrier. Photoluminescence spectra were measured as a function of either the emission angle or the position on the sample. Striking signatures of confined states of lower and upper polaritons, together with the corresponding extended states at higher energy, were found. In particular, the confined states appear only within the mesa region, and are characterized by a discrete energy spectrum and a broad angular pattern. A theoretical model of polariton states, based on a realistic description of the confined photon modes, supports our observations.

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We report femtosecond pump and probe experiments in a semiconductor microcavity containing quantum wells. At high pump fields, the exciton-polariton Rabi doublet changes into a triplet structure. The triplet splitting increases as the square root of the input intensity. The transmitted probe intensity is modulated with the corresponding frequency versus pump-probe delay. The experimental results are discussed in terms of Mollow spectra of a saturated transition of a two-level system. [S0031-9007(98)06194-8] 71.36. + c, 73.20.Dx, 78.47. + p Most of the studies reported on the strong excitonphoton coupling in semiconductor microcavities (MCs) [1] concern the regime of linear response [2]. The investigation of the excitonic nonlinearities under an intense optical field is also of great interest, especially when tuned at the energy of the exciton (X) and Fabry-Perot mode. The resonant nonlinear response is important for both applied purposes and fundamental physics. Potential applications of MCs concern optoelectronic devices, as, e.g., ultrafast alloptical switching, which should operate under strong and resonant excitation conditions [3]. From a fundamental point of view, previous studies concern nonlinearities due to a high density of carriers nonresonantly injected into the MC [4], while the number of available experimental and theoretical investigations in the high and resonant excitation regime is quite limited [5].In semiconductors, the atomic two-level model has been widely used for modeling the coherent response of Xs [6], and even for X-MC systems [7]. In atomic physics, the two-level resonances have been extensively studied during the past decades. Mollow calculated both the free-space emission pattern (the so-called Mollow triplet) as well as the optical susceptibility (just called Mollow spectrum) of a strongly driven two-level system [8]. Fluorescence and absorption-gain measurements in atomic beams have verified these early theoretical predictions [9]. For the case of atoms inside a cavity that are strongly coupled to the electromagnetic field, to our knowledge the experimental results available concern only the normal-mode doublet and the Jaynes-Cummings dynamics observable at low fields [10]. With increasing field strength, theory predicts a transition from the Rabi doublet to an ac Stark triplet (or to even more complex spectra) [11].The ac Stark effect of quantum well (QW) Xs, as it has been observed in pump-probe experiments, represents the analog of the Mollow absorption spectrum for a nonresonant low-energy pumping [12]. In principle, it should be possible to observe the resonant ac Stark splitting of Xs in high-quality samples at low temperatures, using ps laser pulses in order to avoid the excitation of continuum states which would make unstable the X resonance. However, such experiments have not yet been reported.In this Letter, we show that a high-finesse semiconductor MC containing QWs is a much more suitable system. Our MC, featuring strong X-photon coupling, has been resonantly excite...

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In a pump-probe experiment, we have been able to control, with phase-locked probe pulses, the ultrafast nonlinear optical emission of a semiconductor microcavity, arising from polariton parametric amplification. This evidences the coherence of the polariton population near k 0, even for delays much longer than the pulse width. The control of a large population at k 0 is possible although the probe pulses are much weaker than the large polarization they control. With rising pump power the dynamics of the scattering get faster. Just above threshold the parametric scattering process shows unexpected long coherence times, whereas when pump power is risen the contrast decays due to a significant pump reservoir depletion. The weak pulses at normal incidence control the whole angular emission pattern of the microcavity. In the past few years, microcavities working in the strong coupling regime have attracted quite a lot of attention [1][2][3]. The excitonic transition of the embedded quantum well is strongly coupled to the cavity photon mode. In the radiative region near k 0 the resonant exciton and photon modes split and give rise to composite bosons, the so-called microcavity polaritons [4]. The dispersion of the lower polariton (LP) strongly deviates from the unperturbed exciton dispersion. The particular shape of the lower polariton dispersion allows for a parametric polariton scattering process conserving energy and in-plane momentum [2,5,6]. Two polaritons with an in-plane momentum k p scatter into a signal-idler pair with zero and 2k p momenta. The microcavity can thus be understood as an optical parametric oscillator.It is well-known that the parametric oscillation in a classical optical parametric oscillator (OPO) is a fully coherent process. The crystal is pumped in the transparency region and the coherence time of the process is given by the duration of the pump pulses. The pump intensity required to achieve parametric oscillation is very high because the involved electronic states are virtual [7,8]. The semiconductor microcavity system exhibits three major differences to the classical OPO. First, the signal, pump, and idler states are real and thus very efficiently coupled to external laser light. Second, the excitations are interacting via a real Coulomb interaction which results in high parametric scattering rates. These two features illustrate the high efficiency of the process [2,5,6]. The third difference is that for our system the coherence time should not be given by the external laser pulses but by the properties of the excitations and the scattering themselves. The coherent control technique allows one to sense these coherence properties and to manipulate the scattering within its coherence time [9][10][11].In this Letter we report the coherent control of the parametric polariton scattering. The dynamics of the parametric scattering are governed by the lifetime of the real polariton states and the applied pump power [12]. Especially just above threshold the dynamics of the polariton scattering are ...