Optical refrigeration with coupled quantum wells

Refrigeration of a solid-state system with light has potential applications for cooling small-scale electronics and photonics. We show theoretically that two coupled semiconductor quantum wells are efficient cooling media for optical refrigeration because they support long-lived indirect electron-hole pairs. Thermal excitation of these pairs to distinct higher-energy states with faster radiative recombination allows an efficient escape channel to remove thermal energy from the system. This allows reaching much higher cooling efficiencies than with single quantum wells. From band-diagram calculations along with an experimentally realistic level scheme we calculate the cooling efficiency and cooling yield of different devices with coupled quantum wells embedded in a suspended nanomembrane. The dimension and composition of the quantum wells allow optimizing either of these quantities, which cannot, however, be maximized simultaneously. Quantum-well structures with electrical control allow tunability of carrier lifetimes and energy levels so that the cooling efficiency can be optimized over time as the thermal population decreases due to the cooling.Comment: 10 pages, 5 figure

Nouvelles observations sur la glaciation des montagnes du nord-ouest du Portugal

[Resumé] Les observations réalisées récemment par les auteurs confirment en les nuançant~ les conclusions antérieures de G. Coudé-Gaussen et ouvrent de nouvelles perspectives d'étude. Le haut plateau isolé de la Serra do Larouco (1527 m) et les Cotos de Fonte Fria qui attei gnent 1456 m Vers l'extrémité orientale du Geré n'ont révélé aucune trace d'englacement en territoire portugais mais seulement d'imposants pavages superficiels accompagnés de quelques bourrelets ou de niches ce qui implique une importante nivation mais sans formation de glace. Une route en construction a montré l'existence dans la profonde vallée de Gavieira (Serra da Peneda) de deux gros bourre lets de moraine frontale entaillés jusqu'a 580 et 505 m par le Rio Grande. C'est peut-etre l'altitude la plus basse atteinte par la glace dans le Nord-Ouest de la Péninsule Iberique. Sur le versant nord-ouest de la montagne la °langue glaciaire isolée de Corga dos Cortelhos ne descendait qu'a environ 750 m. Cette dissymétrie démontre l 'importance de la suralimentation neigeuse par les vents d'Ouest qui balayaient les croupes sommitales (14l5 m) dominant un large plateau établi Vers 1200 m. Plus al 'Ouest~ la Serra de Arga (823 m) n'a pas été englacée~ en dépit d'un modelé tres favorable. Ces faits confirment~ en accentuant légerement les schémas antérieurs~ la rapide montée d'Ouest en Est de la limite régionale des neiges persistantes; située Vers 1100 m dans l'Ouest de la Serra da Peneda~ Vers 1200 m dans l 'Ouest du Geres, elle atteignait 1400 m a l'Est du massif et environ 1500 m au Larouco. Les indices d'une possible glaciation antérieurede plus large extension et la distinction de plusieurs phases au cours de la seule glaciation actuellement démontrée exigeront de nouvelles recherches sur le terrain~ a propos desquelles queZques réfZexions méthodologiques sont présentées.[Resumen] Nuevas observaciones sobre la glaciación de las montañas del Noroeste de Portugal. Las observaciones recientemente hechas por los autores confirman y matizan las conclusiones anteriores de G. Coudé-Gaussen e abrem nuevas persvectivas de estudio. El altiplano aislado de la Serra do Larouco (1527 m) y los Cotos de Fonte Fria que alcanzan 1456 m cerca de la extremidad oriental del Geres no muestran ningún rasgo de cobertura de hielo en territorio portugués, mas solamente imponentes lanchas acompañadas de algunos caballones y nichos lo que indica una importante nivación pero sin formación de hielo. La carretera en construción en el hondo valle de Gavieira (Serra da Peneda) enseña dos grandes arcos de morrena terminal cortados hasta 580 m y 505 m por el Rio Grande. Es posiblemente la altitud la más baja alcanzada por el hielo en todo el Noroeste de la Península Ibérica. En la vertiente NW de la montaña la lengua glaciaria aislada de Corga dos Cortelhos no baja a más de cerca 750 m. Esta disimetría muestra la importancia de la sobrealimentación en nieve por los vientos del Oeste desde las cumbres (1416 m)~ que dominan un ancho altiplano situado a cerca de 1200 m. Más al Oeste la Serra de Arga (823 m) pese a un relieve muy favorable no estuvo cubierta por el hie lo. Estos hechos confirman acentuando ligeramente los esquemas'anteriores la rápida subida de Oeste para Este del limite regional de las nieves perpetuas; situado cerca de 1100 m al Oeste de la Serra da Peneda cerca de 1200 m al Oeste del Geres alcanza 1400 m al Este del mismo macizo y cerca de 1500 m en Larouco. Los indicios de una posible glaciación anterior más extensa y distinción de varias fases durante la única glaciación hasta hora reconocida exigen nuevas investigaciones de campo a propósito de las quales se plantean algunas reflexiones metodológicas

Tectonic control of the Tejo river fluvial incision during the late Cenozoic, in Ródão – central Portugal (Atlantic Iberian border)

Staircases of strath terraces and strongly incised valleys are the most typical landscape features of Portuguese rivers. This paper examines the incision achieved during the late Cenozoic in an area crossed by the Tejo river between the border with Spain and the small town of Gavia˜o. In the more upstream reach of this area, the Tejo crosses the Ro´da˜o tectonic depression, where four levels of terraces are distinguished. During the late Cenozoic fluvial incision stage, the Ro´da˜o depression underwent less uplift than the adjacent areas along the river. This is reflected by the greater thicknesses and spatial extent of the terraces; terrace genesis was promoted by impoundment of alluvium behind a quartzitic ridge and the local presence of a soft substratum. Outside this tectonic depression, the Tejo has a narrow valley incised in the Hercynian basement, with some straight reaches that probably correspond to NE–SW and NNW–SSE faults, the terraces being nearly absent. Geomorphological evidence of tectonic displacements affecting the Ro´da˜o dissected terrace remnants is described. Geochronological dating of the two younger and lower terrace levels of this depression suggests a time-averaged incision rate for the Tejo in the Ro´da˜o area, of ca. 1.0 m/ka over the last 60 thousand years. A clear discrepancy exists between this rate and the 0.1 m/ka estimated for the longer period since the end of the Pliocene. Although episodes of valley incision may be conditioned by climate and base-level changes, they may also have been controlled by local factors such as movement of small fault-bounded blocks, lithology and structure. Regional crustal uplift is considered to be the main control of the episodes of valley incision identified for this large, long-lived river. A model is proposed in which successive regional uplift events—tectonic phases—essentially determined the long periods of rapid river downcutting that were punctuated by short periods of lateral erosion and later by some aggradation, producing strath terraces

Single-Photon Superradiance from a Quantum Dot.

We report on the observation of single-photon superradiance from an exciton in a semiconductor quantum dot. The confinement by the quantum dot is strong enough for it to mimic a two-level atom, yet sufficiently weak to ensure superradiance. The electrostatic interaction between the electron and the hole comprising the exciton gives rise to an anharmonic spectrum, which we exploit to prepare the superradiant quantum state deterministically with a laser pulse. We observe a fivefold enhancement of the oscillator strength compared to conventional quantum dots. The enhancement is limited by the base temperature of our cryostat and may lead to oscillator strengths above 1000 from a single quantum emitter at optical frequencies

Indistinguishable and efficient single photons from a quantum dot in a planar nanobeam waveguide

We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasiresonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4% and a photon indistinguishability of up to 94±1% by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes, notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another means of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The paper opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip

Mouse model of carbon tetrachloride induced liver fibrosis: Histopathological changes and expression of CD133 and epidermal growth factor

<p>Abstract</p> <p>Background</p> <p>In the setting of chronic liver injury in humans, epidermal growth factor (EGF) and EGF receptor (EGFR) are up-regulated and have been proposed to have vital roles in both liver regeneration and development of hepatocellular carcinoma (HCC). Chronic liver injury also leads to hepatic stellate cell (HSC) differentiation and a novel subpopulation of HSCs which express CD133 and exhibit properties of progenitor cells has been described in rats. The carbon tetrachloride (CCl₄)-induced mouse model has been historically relied upon to study liver injury and regeneration. We exposed mice to CCl₄to assess whether EGF and CD133+ HSCs are up-regulated in chronically injured liver.</p> <p>Methods</p> <p>CCl₄in olive oil was administered to strain A/J mice three times per week by oral gavage.</p> <p>Results</p> <p>Multiple well-differentiated HCCs were found in all livers after 15 weeks of CCl₄treatment. Notably, HCCs developed within the setting of fibrosis and not cirrhosis. CD133 was dramatically up-regulated after CCl₄treatment, and increased expression of desmin and glial fibrillary acidic protein, representative markers of HSCs, was also observed. EGF expression significantly decreased, contrary to observations in humans, whereas the expression of amphiregulin, another EGFR ligand, was significantly increased.</p> <p>Conclusions</p> <p>Species-specific differences exist with respect to the histopathological and molecular pathogenesis of chronic liver disease. CCl₄-induced chronic liver injury in A/J mice has important differences compared to human cirrhosis leading to HCC.</p