Low-threshold Density Research Articles

Pneumococcal competence is a populational health sensor driving multilevel heterogeneity in response to antibiotics

Competence for natural transformation is a central driver of genetic diversity in bacteria. In the human pathogen Streptococcus pneumoniae, competence exhibits a populational character mediated by the stress-induced ComABCDE quorum-sensing (QS) system. Here, we explore how this cell-to-cell communication mechanism proceeds and the functional properties acquired by competent cells grown under lethal stress. We show that populational competence development depends on self-induced cells stochastically emerging in response to stresses, including antibiotics. Competence then propagates through the population from a low threshold density of self-induced cells, defining a biphasic Self-Induction and Propagation (SI&P) QS mechanism. We also reveal that a competent population displays either increased sensitivity or improved tolerance to lethal doses of antibiotics, dependent in the latter case on the competence-induced ComM division inhibitor. Remarkably, these surviving competent cells also display an altered transformation potential. Thus, the unveiled SI&P QS mechanism shapes pneumococcal competence as a health sensor of the clonal population, promoting a bet-hedging strategy that both responds to and drives cells towards heterogeneity.

Polariton Bose-Einstein condensate from a bound state in the continuum.

Bound states in the continuum (BICs)1-3 are peculiar topological states that, when realized in a planar photonic crystal lattice, are symmetry-protected from radiating in the far field despite lying within the light cone4. These BICs possess an invariant topological charge given by the winding number of the polarization vectors5, similar to vortices in quantum fluids such as superfluid helium and atomic Bose-Einstein condensates. In spite of several reports of optical BICs in patterned dielectric slabs with evidence of lasing, their potential as topologically protected states with theoretically infinite lifetime has not yet been fully exploited. Here we show non-equilibrium Bose-Einstein condensation of polaritons-hybrid light-matter excitations-occurring in a BIC thanks to its peculiar non-radiative nature, which favours polariton accumulation. The combination of the ultralong BIC lifetime and the tight confinement of the waveguide geometry enables the achievement of an extremely low threshold density for condensation, which is reachednot in the dispersion minimum but at a saddle point in reciprocal space. By bridging bosonic condensation and symmetry-protected radiation eigenmodes, we reveal ways of imparting topological properties onto macroscopic quantum states with unexplored dispersion features. Such an observation may open a route towards energy-efficient polariton condensation in cost-effective integrated devices, ultimately suited for the development of hybrid light-matter optical circuits.

Extended percolation analysis of the cosmic web

Aims. We develop an extended percolation method to allow the comparison of geometrical properties of the real cosmic web with the simulated dark matter (DM) web for an ensemble of over- and under-density systems. Methods. We scanned density fields of DM model and Sloan Digital Sky Survey (SDSS) observational samples and found connected over- and under-density regions in a large range of threshold densities. Lengths, filling factors, and numbers of largest clusters and voids as functions of the threshold density were used as percolation functions. Results. We find that percolation functions of DM models of varying box sizes are very similar to each other. This stability suggests that properties of the cosmic web, as found in the present paper, can be applied to the cosmic web as a whole. Percolation functions depend strongly on the smoothing length. At smoothing length 1 h−1 Mpc the percolation threshold density for clusters is log PC = 0.718 ± 0.014, and for voids such density is log PV = −0.816 ± 0.015; this is very different from percolation thresholds for random samples, which are log P0 = 0.00 ± 0.02. Conclusions. The extended percolation analysis is a versatile method to study various geometrical properties of the cosmic web in a wide range of parameters. Percolation functions of the SDSS sample are very different from percolation functions of DM model samples. The SDSS sample has only one large percolating void that fills almost the whole volume. The SDSS sample contains numerous small isolated clusters at low threshold densities, instead of one single percolating DM cluster. These differences are due to the tenuous DM web, which is present in model samples but absent in real observational samples.

Towards closing the gap of the global surgeon, anaesthesiologist, and obstetrician workforce: thresholds and projections towards 2030

BackgroundBillions of people are without access to surgical care, in part because of the inequitable distribution of the surgical workforce. Drawing on recently collected data for the number of surgeons, anaesthesiologists, and obstetricians worldwide, we sought to show their global maldistribution by identifying thresholds of surgical workforce densities, and by calculating the number of additional providers needed to reach those thresholds. MethodsFrom the WHO Global Surgical Workforce Database, national data for the number of specialist surgeons, anaesthesiologists, and obstetricians per 100 000 population (density) were compared with the number of maternal deaths per 100 000 live births (maternal mortality ratio; MMR) in WHO member countries. A regression line was fit between density of specialist surgeons, anaesthesiologists, and obstetricians and the logarithm of MMR, and we explored the correlation for an upper and a lower density threshold. Based on previous estimates of the global volume of surgical procedures, a global average productivity per specialist was derived. We then multiplied the average productivity with the derived upper and lower threshold densities, and compared these numbers to previously estimated global need of surgical procedures (4664 procedures per 100 000 population). Finally, the numbers of additional providers needed to reach the thresholds in countries with a density below the respective threshold were calculated. FindingsEach 10-unit increase in density of surgeons, anaesthesiologists, and obstetricians, corresponded to a 13·1% decrease in MMR (95% CI 11·3–14·8). We saw particularly steep improvements in MMR from 0 to roughly 20 per 100 000 population. Above roughly 40 per 100 000 population, higher density was associated with relatively smaller improvements in MMR. These arbitrary thresholds of 20 and 40 specialists per 100 000 corresponded with a volume of surgery of 2917 and 5834 procedures per 100 000 population, respectively, and were symmetrically distributed around the estimated global need of 4664 surgical procedures per 100 000 population. Our density thresholds are slightly higher than the current average in lower-middle income countries (16 per 100 000) and upper-middle-income countries (38 per 100 000), respectively. To reach the threshold of at least 20 per 100 000 in each country today, another 440 231 (IQR 438 900–443 245) providers would be needed. To reach 40 per 100 000, 1 110 610 (IQR 1 095 376–1 183 525) providers would be needed. InterpretationAssuming uniform productivity, a global surgical workforce between 20 and 40 per 100 000 would suffice to provide the world's missing surgical procedures. We concede that causality cannot be implied, but our results suggest that countries with a workforce density above certain thresholds have better health outcomes. Although the thresholds cannot be interpreted as a minimum standard, they are useful to characterise the global surgical workforce and its deficits. Such thresholds could also be used as markers for health system capacity. FundingNone.

Photonic crystal-based flat lens integrated on a Bragg mirror for high-Q external cavity low noise laser

We demonstrate a high reflectivity (> 99%), low-loss (< 0.1%) and aberrations-free (2% of λ rms phase fluctuations) concave Bragg mirror (20mm radius of curvature) integrating a photonic crystal with engineered spherical phase and amplitude transfer functions, based on a III-V semiconductors flat photonics technology. This mirror design is of high interest for highly coherent high power stable external cavity semiconductor lasers, exhibiting very low noise. We design the photonic crystal for operation in the pass band. The approach incorporates spatial, spectral (filter bandwidth= 5nm) and polarization filtering capabilities. Thanks to the mirror, a compact single mode TEM(00) 2mm-long air gap high finesse (cold cavity Q-factor 10(6) - 10(7)) stable laser cavity is demonstrated with a GaAs-based quantum-wells 1/2-VCSEL gain structure at 1μm. Excellent laser performances are obtained in single frequency operation: low threshold density of 2kW/cm(2) with high differential efficiency (21%). And high spatial, temporal and polarization coherence: TEM(00) beam close to diffraction limit, linear light polarization (> 60dB), Side Mode Suppression Ratio > 46dB, relative intensity noise at quantum limit (< -150dB) in 1MHz-84GHz radio frequency range, and a theoretical linewidth fundamental limit at 10 Hz (Q-factor ∼ 3.10(13)).

An electrically pumped polariton laser

Conventional semiconductor laser emission relies on stimulated emission of photons, which sets stringent requirements on the minimum amount of energy necessary for its operation. In comparison, exciton-polaritons in strongly coupled quantum well microcavities can undergo stimulated scattering that promises more energy-efficient generation of coherent light by 'polariton lasers'. Polariton laser operation has been demonstrated in optically pumped semiconductor microcavities at temperatures up to room temperature, and such lasers can outperform their weak-coupling counterparts in that they have a lower threshold density. Even though polariton diodes have been realized, electrically pumped polariton laser operation, which is essential for practical applications, has not been achieved until now. Here we present an electrically pumped polariton laser based on a microcavity containing multiple quantum wells. To prove polariton laser emission unambiguously, we apply a magnetic field and probe the hybrid light-matter nature of the polaritons. Our results represent an important step towards the practical implementation of polaritonic light sources and electrically injected condensates, and can be extended to room-temperature operation using wide-bandgap materials.

Polariton laser based on a ZnO photonic crystal slab

We propose a scheme of a ZnO room-temperature polariton laser based on a photonic crystal slab. We predict a very low threshold density for optical pumping: 5 × 1016 cm−3. We have studied the threshold dependence on the parameters of the system, such as detuning. The main advantage of photonic-crystal-based polariton laser is expected to be the simplicity of production.

Carrier and photon analyses of photonic microlasers by two-dimensional rate equations

In photonic crystal (PC) microlasers with point defect cavities, effective carriers are reduced by the leakage to outside of the defect, surface recombination, spatial carrier hole burning, and Auger recombination. To estimate these effects, we calculated carrier and photon behavior by solving two-dimensional rate equations in space and time domains. The result clearly shows these effects and their dependence on cavity structure, pump area, and so on. Compared with that for the microdisk laser, higher threshold values are estimated for PC microlasers. However, a comparably low threshold density and a high efficiency are expected for the quasi-periodic PC microlaser, because the hole burning by the whispering gallery mode of this cavity suppresses the carrier leakage.

Accessibility and properties of ELMy H-mode and ITB plasmas in TCV

This paper presents experimental results on the accessibility and the properties of plasmas with improved confinement in TCV. First, the H-mode threshold power is measured in Ohmic plasmas. Above an Ohmic threshold density, the threshold power increases with the density. A lower threshold density is found when additional electron cyclotron heating (ECH) is applied. At these low densities, the threshold power increases dramatically with decreasing density. Only a small fraction of the wide operational domain leading to the Ohmic H-mode is found to lead to a stationary regime with edge localized modes (ELMs). The ELMs have an irregular frequency, but in TCV they can be triggered by an external magnetic perturbation that induces a rapid vertical movement of the plasma. With this perturbation, the ELM frequency can be increased. The ELM triggering mechanism is provided by the vertical movement of the plasma away from the X-point of a single null configuration. This movement induces a positive current at the plasma edge, and we deduce that the ELMs are being controlled by this modification of the plasma edge current.Electron internal transport barriers (eITBs) are produced deep in the plasma during the stationary phase of TCV discharges. Different scenarios of ECH or electron cyclotron current drive (ECCD) at different radial locations have been used to obtain eITBs with and without inductively driven current. The eITBs are characterized by steep electron temperature gradients, high confinement improvement and a large fraction of bootstrap current. In plasmas with fully non-inductively driven current the size and the strength of the eITB are controlled by the location of the power deposition and by the co- or counter-direction of the central ECCD. Finally, a small inductive perturbation of an otherwise non-inductively driven plasma current profile progressively shrinks the eITB, confirming the link between current profiles and eITBs.

8-W high-efficiency continuous-wave semiconductor disk laser at 1000 nm

We demonstrate more than 8-W continuous-wave output power with good beam quality (M2&amp;lt;1.8) from an optically pumped semiconductor disk laser. The combination of low threshold density of 470 W/cm2 and high differential efficiency of 60% results in an optical-to-optical conversion efficiency of 46% for this high output level. Good epitaxial quality and low thermal resistance allow the scaling of output power with pump spot area.

Early detection of declining populations using floor and ceiling models

Summary Within Caughley’s (1994) declining population paradigm for conservation, we develop a realistic method of detecting population changes using floor and ceiling population models. A theoretical framework for detecting declining populations is presented and applied to four ungulate species from Kruger National Park, South Africa. Census data are fitted to a non‐linear autoregressive population model. Based on this model two auxiliary models, called the floor and ceiling models, are derived. These models predict the upper and lower threshold densities (or abundance values) for the non‐linear population model. Threshold floor and ceiling estimates can be used as a benchmark for monitoring population status. In particular, when abundance values (densities) tend to fall below the floor estimate, this may merit concern that a population is declining. In conjunction with appropriate response by management, the presence of such an early warning system may prevent a population from becoming low in numbers, possibly making it less susceptible to stochastic processes and a less likely candidate for local extinction.

Membrane association induces a conformational change in the Ebola virus matrix protein.

The matrix protein VP40 from Ebola virus is targeted to the plasma membrane, where it is thought to induce assembly and budding of virions through its association with the lipid bilayer. Ebola virus VP40 is expressed as a monomeric molecule in solution, consisting of two loosely associated domains. Here we show that a C-terminal truncation of seven residues destabilizes the monomeric closed conformation and induces spontaneous hexamerization in solution, as indicated by chemical cross-linking and electron microscopy. Three-dimensional reconstruction of electron microscopy images shows ring-like structures consisting of the N-terminal domain along with evidence for flexibly attached C-terminal domains. In vitro destabilization of the monomer by urea treatment results in similar hexameric molecules in solution. In addition, we demonstrate that membrane association of wild-type VP40 also induces the conformational switch from monomeric to hexameric molecules that may form the building blocks for initiation of virus assembly and budding. Such a conformational change induced by bilayer targeting may be a common feature of many viral matrix proteins and its potential inhibition may result in new anti-viral therapies.

Optical gain characteristics and excitonic nonlinearities in II–VI laser diodes

Gain characteristics and lasing of (Zn, Cd)Se/Zn(S, Se)/(Zn, Mg)(S, Se) separate confinement heterostructures are investigated by optical-gain spectroscopy and by high-excitation spectroscopy under quasi-stationary conditions. The optical gain is measured by means of the stripe-length method whereas excitonic bleaching under lasing conditions is analyzed through two-beam photoluminescence excitation (PLE) spectroscopy. At low lattice temperature, we find a rather low threshold density of 15–20 kW/cm 2 for both laser structures and the excitonic enhancement is still preserved at the onset of lasing. The red shift of the gain maximum with respect to the low-density PLE exciton peak increases with temperature. The results indicate a considerable influence of Coulomb correlations even at the high densities necessary for stimulated emission and are more adequately explained by a strongly correlated electron-hole plasma model that goes beyond the simple “excitonic lasing” concept.

Local-field effects in a dense collection of two-level atoms embedded in a dielectric medium: Intrinsic optical bistability enhancement and local cooperative effects

We investigate the effect of a linear host medium on the dynamics of a dense collection of resonant atoms. We find that near dipole-dipole interaction is enhanced by the presence of the host dielectric, resulting in a lower threshold density and a greater hysteresis area for intrinsic optical bistability. Cooperative decay terms appear that represent the interaction of near dipoles mediated by the absorptive component of the dielectric function of the host medium. These terms are part of the local response and correspond to local cooperative decay effects. \textcopyright{} 1996 The American Physical Society.

High quality GaAs quantum well lasers grown on InP substrates by organometallic chemical vapor deposition

High quality GaAs quantum well lasers grown on (100) and 3°-off (100)InP substrates by organometallic chemical vapor deposition were investigated for the first time. 50-μm-wide broad-area gain-guided lasers were fabricated using preferential proton implantation. Low threshold densities, 800 and 1080 A/cm2, were obtained at room temperature for lasers with 1.25-mm-long cavities grown on 3°-off (100) and (100) oriented InP substrates, respectively. High quantum efficiency of 36% and nearly single longitudinal mode emission were also achieved from these lasers.