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Bose-Einstein condensation (BEC) is a thermodynamic phase transition of an interacting Bose gas. Its key signatures are remarkable quantum effects like superfluidity and a phonon-like Bogoliubov excitation spectrum, which have been verified for atomic BECs. In the solid state, BEC of exciton-polaritons has been reported. Polaritons are strongly coupled light-matter quasiparticles in semiconductor microcavities and composite bosons. However, they are subject to dephasing and decay and need external pumping to reach a steady state. Accordingly the polariton BEC is a nonequilibrium process of a degenerate polariton gas in self-equilibrium, but out of equilibrium with the baths it is coupled to and therefore deviates from the thermodynamic phase transition seen in atomic BECs. Here we show that key signatures of BEC can even be observed without fulfilling the self-equilibrium condition in a highly photonic quantum degenerate nonequilibrium system. photon statistics | quantum optics | semiconductor photon sources M icrocavity polaritons are composite bosons, which are partly photons and partly excitons as quantified by the Hopfield coefficients jC 2 j and jX 2 j giving the relative photonic and excitonic content (1), respectively, and are expected to condense at high temperatures because of their light mass (2). Moreover, the photonic and excitonic contents of polaritons can be precisely adjusted by changing the detuning Δ ¼ E c − E x between the bare cavity mode and the bare exciton mode. Unlike Bose-Einstein condensates (BECs) in atomic gases, solid-state systems are subject to strong dephasing and decay on timescales on the order of the particle lifetimes. As a consequence, external pumping is required to achieve a steady state. Despite this nonequilibrium character, degenerate polariton systems show several textbook features of BECs (3), including spontaneous build up of coherence (4) and polarization (5), quantized vortices (6, 7), spatial condensation (8), and superfluidity (9, 10). Usually this behavior is attributed to the system undergoing an equilibrium phase transition toward a condensed state: Although the polariton gas is not necessarily in equilibrium with the lattice, it is in selfequilibrium, if the relaxation kinetics of excited carriers is fast enough compared to the leakage of the photonic component out of the cavity, which is usually the case for positive detunings Δ ≥ 0. In this case jX 2 j is larger than 50%, an effective temperature can be defined and the polariton gas can be considered as a thermodynamic equilibrium state, which is out of equilibrium with the baths it is coupled to. This degenerate polariton gas is distinguishable from a simple photon laser (11). It is often pointed out that this intrinsic nonequilibrium situation and the two-dimensional order parameter of polariton BECs give rise to interesting phenomena like half-vortices (12) and a diffusive Goldstone mode (13, 14), which do not occur in equilibrium condensates. Accordingly, the next interesting questions are whether the sa...

Abstract:We present a detailed discussion of a recently demonstrated experimental technique capable of measuring the correlation function of a pulsed light source with picosecond time resolution. The measurement involves a streak camera in single photon counting mode, which is modified such that a signal at a fixed repetition rate, and well defined energy, can be monitored after each pulsed laser excitation. The technique provides further insight into the quantum optical properties of pulsed light emission from semiconductor nanostructures, and the dynamics inside a pulse, on the subnanosecond time scale. ©2010 Optical Society of America

The second-order correlation function g (2) (τ = 0), input-output curves and pulse duration of the emission from a microcavity exciton-polariton system subsequent to picosecond-pulsed excitation are measured for different temperatures. At low temperatures a two-threshold behaviour emerges, which has been attributed to the onset of polariton lasing and conventional lasing at the first and the second threshold, respectively. We observe that polariton lasing is stable up to temperatures comparable with the exciton binding energy. At higher temperatures a single threshold displays the direct transition from thermal emission to photon lasing.

The experimentally measured input-output characteristics of optically pumped semiconductor microcavities exhibits unexpected oscillations modifying the fundamentally linear slope in the excitation power regime below lasing. A systematic microscopic analysis reproduces these oscillations, identifying them as a genuine quantum-memory effect, i.e., a photon-density correlation accumulated during the excitation. With the use of projected quantum measurements, it is shown that the input-output oscillations can be controlled and enhanced by an order of magnitude when the quantum fluctuations of the pump are adjusted.