Condensation and Lasing of Microcavity Polaritons: Comparison between two Models

Condensation of microcavity polaritons and the substantial influence of pair-breaking disorder and decoherence leading to a laser regime has been recently considered using two different models: a model for direct two band excitons in a disordered quantum well coupled to light and a model where the cavity mode couples instead to a medium of localised excitons, represented by two-level oscillators in the presence of dephasing processes. Even if complementary from the point of view of assumptions, the models share most of the main conclusions and show similar phase diagrams. The issue whether excitons are propagating or localised seems secondary for the polariton condensation and the way in which pair-breaking disorder and decoherence processes influence the condensation and drive the microcavity into a lasing regime is, within the approximations used in each model, generic. The reasons for the similarities between the two physical situations are analysed and explained.Comment: Proceeding of the First International Conference on Spontaneous Coherence in Excitonic Systems (ICSCE'04); 7 pages, 2 eps figure

The crossover between lasing and polariton condensation in optical microcavities

We study a model of a photon mode dipole-coupled to a medium of two-level oscillators in a microcavity in the presence of dephasing processes introduced by coupling to external baths. Decoherence processes can be classified as pair-breaking or non-pair-breaking in analogy with magnetic or non-magnetic impurities in superconductors. In the absence of dephasing, the ground state of the model is a polariton condensate with a gap in the excitation spectrum. Increase of the pair-breaking parameter $\gamma$ reduces the gap, which becomes zero at a critical value $\gamma_{C1}$; for large $\gamma$, the conventional laser regime is obtained in a way that demonstrates its close analogy to a gapless superconductor. In contrast, weak non-pair-breaking processes have no qualitative effect on the condensate or the existence of a gap, although they lead to inhomogeneous broadening of the excitations

The bird bones in the cave of Amalda

Análisis y estudio de los restos óseos de aves de los niveles paleolíticos y postpaleolíticos de la cueva de Amalda, y comparación de éstos con restos hallados en otras zonas de Cantabria, Pirineos, Languedoc y Norte de Cataluña.Amalda kobazuloko Paleolito eta Paleolito ondorengo mailetan aurkitutako hegaztien hezur-aztarnen azterketa eta ikerketa eta Kantabria, Pirinioak, Languedoc eta Katalunia iparraldean aurkitutako batzuekin konparazioa

Conical diffraction and the dispersion surface of hyperbolic metamaterials

Hyperbolic metamaterials are materials in which at least one principal dielectric constant is negative. We describe the refractive index surface, and the resulting refraction effects, for a biaxial hyperbolic metamaterial, with principal dielectric constants $\epsilon_1<0$, $0<\epsilon_2\neq\epsilon_3$. In this general case the two sheets of the index surface intersect forming conical singularities. We derive the ray description of conical refraction in these materials, and show that it is topologically and quantitatively distinct from conical refraction in a conventional biaxial material. We also develop a wave optics description, which allows us to obtain the diffraction patterns formed from arbitrary beams incident close to the optic axis. The resulting patterns lack circular symmetry, and hence are qualitatively different from those obtained in conventional, positive index materials.Comment: 10 pages, 7 figure

Mode-locking and mode-competition in a non-equilibrium solid-state condensate

A trapped polariton condensate with continuous pumping and decay is analyzed using a generalized Gross-Pitaevskii model. Whereas an equilibrium condensate is characterized by a macroscopic occupation of a ground state, here the steady-states take more general forms. Some are characterized by a large population in an excited state, and others by large populations in several states. In the latter case, the highly-populated states synchronize to a common frequency above a critical density. Estimates for the critical density of this synchronization transition are consistent with experiments.Comment: 5 pages, 2 figure

Economics of tandem mass spectrometry screening of neonatal inherited disorders

Objectives: The aim of this study was to evaluate the cost-effectiveness of neonatal screening for phenylketonuria (PKU) and medium-chain acyl-coA dehydrogenase (MCAD) deficiency using tandem mass spectrometry (tandem MS). Methods: A systematic review of clinical efficacy evidence and cost-effectiveness modeling of screening in newborn infants within a UK National Health Service perspective was performed. Marginal costs, life-years gained, and cost-effectiveness acceptability curves are presented. Results: Substituting the use of tandem MS for existing technologies for the screening of PKU increases costs with no increase in health outcomes. However, the addition of screening for MCAD deficiency as part of a neonatal screening program for PKU using tandem MS, with an operational range of 50,000 to 60,000 specimens per system per year, would result in a mean incremental cost of −£17,298 (−£129,174, £66,434) for each cohort of 100,000 neonates screened. This cost saving is associated with a mean incremental gain of 57.3 (28.0, 91.4) life-years. Conclusions: Cost-effectiveness analysis using economic modeling indicates that substituting the use of tandem MS for existing technologies for the screening of PKU alone is not economically justified. However, the addition of screening for MCAD deficiency as part of a neonatal screening program for PKU using tandem MS would be economically attractive