Global optimization of an encapsulated Si/SiO[Formula: see text] L3 cavity with a 43 million quality factor.

We optimize a silica-encapsulated silicon L3 photonic crystal cavity for ultra-high quality factor by means of a global optimization strategy, where the closest holes surrounding the cavity are varied to minimize out-of-plane losses. We find an optimal value of [Formula: see text], which is predicted to be in the 2 million regime in presence of structural imperfections compatible with state-of-the-art silicon fabrication tolerances.

Observation of the Unconventional Photon Blockade.

We observe the unconventional photon blockade effect in quantum dot cavity QED, which, in contrast to the conventional photon blockade, operates in the weak coupling regime. A single quantum dot transition is simultaneously coupled to two orthogonally polarized optical cavity modes, and by careful tuning of the input and output state of polarization, the unconventional photon blockade effect is observed. We find a minimum second-order correlation g^{(2)}(0)≈0.37, which corresponds to g^{(2)}(0)≈0.005 when corrected for detector jitter, and observe the expected polarization dependency and photon bunching and antibunching; close by in parameter space, which indicates the abrupt change from phase to amplitude squeezing.

Dissociation dynamics of singly charged vortices into half-quantum vortex pairs.

The quest for identification and understanding of fractional vorticity is a major subject of research in the quantum fluids domain, ranging from superconductors, superfluid Helium-3 to cold atoms. In a two-dimensional Bose degenerate gas with a spin degree of freedom, the fundamental topological excitations are fractional vortical entities, called half-quantum vortices. Convincing evidence for the existence of half-quantum vortices was recently provided in spinor polariton condensates. The half-quantum vortices can be regarded as the fundamental structural components of singly charged vortices but, so far, no experimental evidence of this relation has been provided. Here we report on the direct and time-resolved observation of the dynamical process of the dissociation of a singly charged vortex into its primary components, a pair of half-quantum vortices. The physical origin of the observed phenomenology is found in a spatially inhomogeneous static potential that couples the two spin components of the condensate.

Polariton condensation in a one-dimensional disordered potential.

We study the coherence and density modulation of a nonequilibrium exciton-polariton condensate in a one-dimensional valley with disorder. By means of interferometric measurements we evidence a modulation of the first-order coherence function and we relate it to a disorder-induced modulation of the condensate density, that increases as the pump power is increased. The nonmonotonic spatial coherence function is found to be the result of the strong nonequilibrium character of the one-dimensional system, in the presence of disorder.

Phonon-mediated coupling of InGaAs/GaAs quantum-dot excitons to photonic crystal cavities.

We demonstrate that the emission characteristics of site-controlled InGaAs/GaAs single quantum dots embedded in photonic crystal slab cavities correspond to single confined excitons coupled to cavity modes, unlike previous reports of similar systems based on self-assembled quantum dots. By using polarization-resolved photoluminescence spectroscopy at different temperatures and a theoretical model, we show that the exciton-cavity interaction range is limited to the phonon sidebands. Photon-correlation and pump-power dependence experiments under nonresonant excitation conditions further establish that the cavity is fed only by a single exciton.

Optically erasing disorder in semiconductor microcavities with dynamic nuclear polarization.

The mean squared value of the photonic disorder is found to be reduced by a factor of 100 in a typical GaAs based microcavity when exposed to a circularly polarized continuous wave optical pump without any special spatial patterning. Resonant excitation of the cavity mode excites a spatially nonuniform distribution of spin-polarized electrons, which depends on the photonic disorder profile. Electrons transfer spin to nuclei via the hyperfine contact interaction, inducing a long-living Overhauser magnetic field able to modify the potential of exciton polaritons.

Probing the dynamics of spontaneous quantum vortices in polariton superfluids.

The experimental investigation of spontaneously created vortices is of utmost importance for the understanding of quantum phase transitions towards a superfluid phase, especially for two-dimensional systems that are expected to be governed by the Berezinski-Kosterlitz-Thouless physics. By means of time-resolved near-field interferometry we track the path of such vortices, created at random locations in an exciton-polariton condensate under pulsed nonresonant excitation, to their final pinning positions imposed by the stationary disorder. We formulate a theoretical model that successfully reproduces the experimental observations.

Single photons from coupled quantum modes.

Single photon emitters often rely on a strong nonlinearity to make the behavior of a quantum mode susceptible to a change in the number of quanta between one and two. In most systems, the strength of nonlinearity is weak, such that changes at the single quantum level have little effect. Here, we consider coupled quantum modes and find that they can be strongly sensitive at the single quantum level, even if nonlinear interactions are modest. As examples, we consider solid-state implementations based on the tunneling of polaritons between quantum boxes or their parametric modes in a microcavity. We find that these systems can act as promising single photon emitters.

Bose-Einstein condensation of exciton polaritons.

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.

Localization-dependent photoluminescence spectrum of biexcitons in semiconductor quantum wires.

We study exciton and biexciton spectra in disordered semiconductor quantum wires by means of nanophotoluminescence spectroscopy. We demonstrate a close link between the exciton localization length along the wire and the occurrence of a biexciton spectral line. The biexciton signature appears only if the corresponding exciton state extends over more than a few tens of nanometers. We also measure a nonmonotonous variation of the biexciton binding energy with decreasing exciton localization length. This behavior is quantitatively well reproduced by the solution of the single-band Schrödinger equation of the four-particle problem in a one-dimensional confining potential.

Enhanced resonant backscattering of excitons in disordered quantum wells.

A clear signature of enhanced backscattering of excitons is observed in the directional resonant Rayleigh scattering of light from localized two-dimensional excitons in disordered quantum wells. Its spectral dependence and time dynamics are measured and theoretically predicted in a quantitative way. The intensity enhancement has a large momentum span extending beyond the external light emission cone. This is a consequence of the small localization length of the exciton as a massive particle probed close to the band bottom. The localization length can be controlled by the photon kinetic energy. This constitutes a qualitative difference to backscattering phenomena in other branches of physics.

Quantum mechanical repulsion of exciton levels in a disordered quantum well.

Spatially resolved photoluminescence spectra of a single quantum well are recorded by near-field spectroscopy. A set of over four hundred spectra displaying sharp emission lines from localized excitons is subject to a statistical analysis of the two-energy autocorrelation function. An accurate comparison with a quantum theory of the exciton center-of-mass motion in a two-dimensional spatially correlated disordered potential reveals clear signatures of quantum mechanical energy level repulsion, giving the spatial and energetic correlations of excitons in disordered quantum systems.

Strongly driven exciton resonances in quantum wells: light-induced dressing versus coulomb scattering

The nonequilibrium dynamics of a two-dimensional electron-hole gas is studied in the regime of strong and resonant pumping of the exciton resonance. The Coulomb collision rates are consistently determined by taking into account the light-induced coherence of the two-band system that leads to a dressing of the carrier spectral functions. The light dressing dramatically reduces the Coulomb scattering efficiency. Results are presented for Rabi oscillations in the time domain and dynamical Stark splitting in the pump-probe absorption spectra.

Optical signatures of energy-level statistics in a disordered quantum system

Time-resolved measurements of the resonant Rayleigh scattering from quantum well excitons are shown to provide information on the energy-level statistics of the localized exciton states. The signal transients are reproduced by a microscopic quantum model of the exciton two-dimensional motion in presence of spatially correlated disorder. This model allows quantitative determination of the average energy separation between the localized states. Here this quantity turns out to be only a few times smaller than the average disorder amplitude, proving that spatial correlation and quantum mechanics are equally important in the description of the exciton localization process.

Concepts in cardiology - a historical perspective.

Our current knowledge of the anatomy and physiology of the circulatory system has been reached by deduction and reasoning over several centuries. In this article, we briefly outline the history of these theories.