Lasing from a Large-Area 2D Material Enabled by a Dual-Resonance Metasurface

Semiconducting transition metal dichalcogenides (TMDs) have gained significant attention as a gain medium for nanolasers, owing to their unique ability to be easily placed and stacked on virtually any substrate. However, the atomically thin nature of the active material in existing TMD lasers and the limited size due to mechanical exfoliation presents a challenge, as their limited output power makes it difficult to distinguish between true laser operation and other "laser-like" phenomena. Here, we present room temperature lasing from a large-area tungsten disulfide (WS2) monolayer, grown by a wafer-scale chemical vapor deposition (CVD) technique. The monolayer is placed on a dual-resonance dielectric metasurface with a rectangular lattice designed to enhance both absorption and emission, resulting in an ultralow threshold operation (threshold well below 1 W/cm2). We provide a thorough study of the laser performance, paying special attention to directionality, output power, and spatial coherence. Notably, our lasers demonstrated a coherence length of over 30 μm, which is several times greater than what has been reported for 2D material lasers so far. Our realization of a single-mode laser from a CVD-grown monolayer presents exciting opportunities for integration and the development of real-world applications.

Phase noise matching in resonant metasurfaces for intrinsic sensing stability

Interferometry offers a precise means of interrogating resonances in dielectric and plasmonic metasurfaces, surpassing spectrometer-imposed resolution limits. However, interferometry implementations often face complexity or instability issues due to heightened sensitivity. Here, we address the necessity for noise compensation and tolerance by harnessing the inherent capabilities of photonic resonances. Our proposed solution, termed "resonant phase noise matching," employs optical referencing to align the phases of equally sensitive, orthogonal components of the same mode. This effectively mitigates drift and noise, facilitating the detection of subtle phase changes induced by a target analyte through spatially selective surface functionalization. Validation of this strategy using Fano resonances in a 2D photonic crystal slab showcases noteworthy phase stability (σ < 10 - 4 π ). With demonstrated label-free detection of low-molecular-weight proteins at clinically relevant concentrations, resonant phase noise matching presents itself as a potentially valuable strategy for advancing scalable, high-performance sensing technology beyond traditional laboratory settings.

Perturbation approach to improve the angular tolerance of high-Q resonances in metasurfaces

The interest in high quality factor (high-Q) resonances in metasurfaces has been rekindled with the rise of the bound states in the continuum (BIC) paradigm, which describes resonances with apparently limitlessly high quality-factors (Q-factors). The application of BICs in realistic systems requires the consideration of the angular tolerance of resonances, however, which is an issue that has not yet been addressed. Here, we develop an ab-initio model, based on temporal coupled mode theory, to describe the angular tolerance of distributed resonances in metasurfaces that support both BICs and guided mode resonances (GMRs). We then discuss the idea of a metasurface with a perturbed unit cell, similar to a supercell, as an alternative approach for achieving high-Q resonances and we use the model to compare the two. We find that, while sharing the high-Q advantage of BIC resonances, perturbed structures feature higher angular tolerance due to band planarization. This observation suggests that such structures offer a route toward high-Q resonances that are more suitable for applications.

RGB Achromatic Metalens Doublet for Digital Imaging

Chromatic aberration is a major challenge faced by metalenses. Current methods to achieve broadband achromatic operation in metalenses usually suffer from limited size, numerical aperture, and working bandwidth due to the finite group delay of meta-atoms, thus restricting the range of practical applications. Multiwavelength achromatic metalenses can overcome those limitations, making it possible to realize larger numerical aperture (NA) and sizes simultaneously. However, they usually require three layers, which increases their fabrication complexity, and have only been demonstrated in small sizes, with low numerical aperture and modest efficiencies. Here, we demonstrate a 1 mm diameter red-green-blue achromatic metalens doublet with a designed NA of 0.8 and successfully apply the metalens in a digital imaging system. This work shows the potential of the doublet metasurfaces, extending their applications to digital imaging systems such as digital projectors, virtual reality glasses, high resolution microscopies, etc.

Beyond Q: The Importance of the Resonance Amplitude for Photonic Sensors

Resonant photonic sensors are enjoying much attention based on the worldwide drive toward personalized healthcare diagnostics and the need to better monitor the environment. Recent developments exploiting novel concepts such as metasurfaces, bound states in the continuum, and topological sensing have added to the interest in this topic. The drive toward increasingly higher quality (Q)-factors, combined with the requirement for low costs, makes it critical to understand the impact of realistic limitations such as losses on photonic sensors. Traditionally, it is assumed that the reduction in the Q-factor sufficiently accounts for the presence of loss. Here, we highlight that this assumption is overly simplistic, and we show that losses have a stronger impact on the resonance amplitude than on the Q-factor. We note that the effect of the resonance amplitude has been largely ignored in the literature, and there is no physical model clearly describing the relationship between the limit of detection (LOD), Q-factor, and resonance amplitude. We have, therefore, developed a novel, ab initio analytical model, where we derive the complete figure of merit for resonant photonic sensors and determine their LOD. In addition to highlighting the importance of the optical losses and the resonance amplitude, we show that, counter-intuitively, optimization of the LOD is not achieved by maximization of the Q-factor but by counterbalancing the Q-factor and amplitude. We validate the model experimentally, put it into context, and show that it is essential for applying novel sensing concepts in realistic scenarios.

Broadband c-Si metasurfaces with polarization control at visible wavelengths: applications to 3D stereoscopic holography

Visual arts and entertainment related industries are continuously looking at promising innovative technologies to improve users' experience with state-of-the-art visualization platforms. This requires further developments on pixel resolution and device miniaturization which can be achieved, for instance, with high contrast materials, such as crystalline silicon (c-Si). Here, a new broadband stereoscopic hologram metasurface is introduced, where independent phase control is achieved for two orthogonal polarizations in the visible spectrum. The holograms are fabricated with a birefringent metasurface consisting of elliptical c-Si nanoposts on Sapphire substrate. Two holograms are combined on the same metasurface (one for each polarization) where each is encoded with four phase levels. The theoretical bandwidth is 110 nm with a signal to noise ratio (SNR) >15 dB. The stereoscopic view is obtained with a pair of cross-polarized filters in front of the observers' eyes. The measured transmission and diffraction efficiencies are about 70% and 15%, respectively, at 532 nm (the design wavelength). The metasurfaces are also investigated at 444.9 nm and 635 nm to experimentally assess their bandwidth performance. The stereoscopic effect is surprisingly good at 444.9 nm (and less so at 635 nm) with transmission and diffraction efficiencies around 70% and 18%, respectively.

Highly efficient holograms based on c-Si metasurfaces in the visible range

This paper reports on the first hologram in transmission mode based on a c-Si metasurface in the visible range. The hologram shows high fidelity and high efficiency, with measured transmission and diffraction efficiencies of ~65% and ~40%, respectively. Although originally designed to achieve full phase control in the range [0-2π] at 532 nm, these holograms have also performed well at 444.9 nm and 635 nm. The high tolerance to both fabrication and wavelength variations demonstrate that holograms based on c-Si metasurfaces are quite attractive for diffractive optics applications, and particularly for full-color holograms.

Paths to light trapping in thin film GaAs solar cells

It is now well established that light trapping is an essential element of thin film solar cell design. Numerous light trapping geometries have already been applied to thin film cells, especially to silicon-based devices. Less attention has been paid to light trapping in GaAs thin film cells, mainly because light trapping is considered less attractive due to the material's direct bandgap and the fact that GaAs suffers from strong surface recombination, which particularly affects etched nanostructures. Here, we study light trapping structures that are implemented in a high-bandgap material on the back of the GaAs active layer, thereby not perturbing the integrity of the GaAs active layer. We study photonic crystal and quasi-random nanostructures both by simulation and by experiment and find that the photonic crystal structures are superior because they exhibit fewer but stronger resonances that are better matched to the narrow wavelength range where GaAs benefits from light trapping. In fact, we show that a 1500 nm thick cell with photonic crystals achieves the same short circuit current as an unpatterned 4000 nm thick cell. These findings are significant because they afford a sizeable reduction in active layer thickness, and therefore a reduction in expensive epitaxial growth time and cost, yet without compromising performance.

Insights into directional scattering: from coupled dipoles to asymmetric dimer nanoantennas

Strong and directionally specific forward scattering from optical nanoantennas is of utmost importance for various applications in the broader context of photovoltaics and integrated light sources. Here, we outline a simple yet powerful design principle to perceive a nanoantenna that provides directional scattering into a higher index substrate based on the interference of multiple electric dipoles. A structural implementation of the electric dipole distribution is possible using plasmonic nanoparticles with a fairly simple geometry, i.e. two coupled rectangular nanoparticles, forming a dimer, on top of a substrate. The key to achieve directionality is to choose a sufficiently large size for the nanoparticles. This promotes the excitation of vertical electric dipole moments due to the bi-anisotropy of the nanoantenna. In turn, asymmetric scattering is obtained by ensuring the appropriate phase relation between the vertical electric dipole moments. The scattering strength and angular spread for an optimized nanoantenna can be shown to be broadband and robust against changes in the incidence angle. The scattering directionality is maintained even for an array configuration of the dimer. It only requires the preferred scattering direction of the isolated nanoantenna not to be prohibited by interference.

Changing epidemiology of group B streptococcal infections among adults in Iceland: 1975-2014

We studied the bacterial characteristics and incidence of invasive infections caused by group B streptococci (GBS) in adults in Iceland in 1975-2014. A total of 145 isolates were characterized by serotyping, antimicrobial susceptibility, multilocus sequence typing and surface protein gene profiling. Disease incidence increased during the studied period (p <0.001), reaching 2.17 cases/100 000 person-years in 2013-14. Overall, serotype Ia was the most frequently found (23%), but serotypes Ib, II, III and V showed similar prevalence (14%-17%). Although there were notable changes in the proportion of most serotypes during the study period, only the decline of serotype III was statistically supported (p = 0.003) and was reflected in a decrease of clonal complexes CC17 and CC19 that included most serotype III isolates (p <0.04). On the other hand, the increase in frequency of CC1 was caused by two lineages expressing distinct serotypes: ST1/V/alp3 and ST196/IV/eps. Underlying the relative stability of serotype Ia were major changes in the lineages expressing this serotype, with an increase in the relative importance of CC23, including both ST23/Ia/eps and ST24/Ia/bca lineages, and a decrease in CC7. Nine cases of invasive GBS disease were caused by ST7, of possible zoonotic origin. All isolates were susceptible to penicillin. Rates of erythromycin and clindamycin resistance were 8.3% and 9.7%, respectively. An over-representation of resistance solely to clindamycin was associated with the unusual lsaC gene and serotype III ST19/rib lineage (p <0.001).

Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells

The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC₇₁BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10–50 nm wide and 200–400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.

The morphology of organic solar cells is crucial to their performance but is difficult to measure. Using a variety of probes, Hedley et al. map the morphology of polymer-fullerene solar cells and find that elongated fibre-like polymer- and fullerene-rich domains are desirable for high performance.

Experimental high numerical aperture focusing with high contrast gratings

We demonstrate high aperture (up to NA~0.64) three-dimensional focusing in free space based on wavefront-engineered diffraction gratings. The grating lens' optical response is tailored by spatially varying the grating ridge and groove width in two dimensions to achieve focal lengths of order 100 μm that are crucial for micro-optical applications. The phase profile of the lens includes multiple 2π phase jumps and was obtained by applying an algorithm for finding the optimal path for both phase and amplitude. Experimental measurements reveal a lateral spot size of 5 μm that is close to the size of a corresponding Airy disk.

Dual gratings for enhanced light trapping in thin-film solar cells by a layer-transfer technique

Thin film solar cells benefit significantly from the enhanced light trapping offered by photonic nanostructures. The thin film is typically patterned on one side only due to technological constraints. The ability to independently pattern both sides of the thin film increases the degrees of freedom available to the designer, as different functions can be combined, such as the reduction of surface reflection and the excitation of quasiguided modes for enhanced light absorption. Here, we demonstrate a technique based on simple layer transfer that allows us to independently pattern both sides of the thin film leading to enhanced light trapping. We used a 400 nm thin film of amorphous hydrogenated silicon and two simple 2D gratings for this proof-of-principle demonstration. Since the technique imposes no restrictions on the design parameters, any type of structure can be made.

Theoretical analysis of supercontinuum generation in a highly birefringent D-shaped microstructured optical fiber

This paper carries out a rigorous analysis of supercontinuum generation in an improved highly asymmetric microstructured fiber (MF) design. This geometry, defined simply as D-MF, has the advantage of being produced with a regular stacking and drawing technology. We have obtained birefringence values on the order of 4.87x10(-3) at the adopted pump wavelength and a significantly smaller effective area when compared to a whole MF, which makes this fiber quite attractive for SCG. Therefore, this D-MF design is a promising alternative for SCG since it provides new degrees of freedom to control field confinement, birefringence, and dispersion characteristics of MFs.

Analysis of group B streptococcal isolates from infants and pregnant women in Portugal revealing two lineages with enhanced invasiveness

The populations of group B streptococcus (GBS) associated with vaginal carriage in pregnant women and invasive neonatal infections in Portugal were compared. GBS isolates were characterized by serotyping, pulsed-field gel electrophoresis (PFGE) profiling, and multilocus sequence typing (MLST). Serotypes III and V accounted for 44% of all colonization isolates (n = 269), whereas serotypes III and Ia amounted to 69% of all invasive isolates (n = 64). Whereas serotype Ia was associated with early-onset disease (EOD), serotype III was associated with late-onset disease (LOD). Characterization by PFGE and MLST identified very diverse populations in carriage and invasive disease. Serotype Ia was represented mainly by a single PFGE cluster defined by sequence type 23 (ST23) and the infrequent ST24. In contrast, serotype III was found in a large number of PFGE clusters and STs, but a single PFGE cluster defined by ST17 was found to be associated with invasive disease. Although serotype III was associated only with LOD, ST17 showed an enhanced capacity to cause both EOD and LOD. Our data reinforce the evidence for enhanced invasiveness of ST17 and identify a lineage expressing serotype Ia capsule and represented by ST23 and ST24 as having enhanced potential to cause EOD.

Streptococcus agalactiae serotype Ib as an agent of meningitis in two adult nonpregnant women

Two temporally and geographically clustered cases of meningitis caused by Streptococcus agalactiae expressing the infrequent Ib serotype are reported. Characterization by pulsed-field gel electrophoresis and multilocus sequence typing revealed that the isolates were identical and represented the widely distributed ST10/ST8 lineage associated with serotype Ib.

Reevaluating the serotype II capsular locus of Streptococcus agalactiae

We report a novel sequence of the serotype II capsular locus of group B streptococcus that resolves inconsistencies among the results of various groups and the sequence in GenBank. This locus was found in diverse lineages and presents genes consistent with the complete synthesis of the type II polysaccharide.

Reproductive studies on ipecac (Cephaelis ipecacuanha (Brot.) A. Rich; Rubiaceae): meiotic behavior and pollen viability

Reproductive studies were carried out on Brazilian accessions of ipecac, Cephaelis ipecacuanha. Meiotic behavior was studied using the squashing technique. Irregular chromosome segregation in meiosis I and II, many sets of chromosomes in telophase II, micronuclei, incorrect cytoplasm division, incomplete cytokinesis and anomalous post-meiotic products, mainly polyads, were observed. The mean meiotic index was lower than 72%. Pollen viability was analyzed using Alexander solution, and the percentages ranged between brevistylous and longistylous floral morphs (85.3 to 93.1%), and among different localities (82.5 to 92.6%) analyzed. The size of pollen ranged between viable and sterile, and empty and shrunken sterile. In its natural habitat, this species is known to propagate by vegetative multiplication, but sexual reproduction seems to be as important as the vegetative propagation to this species.

[Bed baths--an initiation ritual for the nurse]

It's a experience, at the Fundamental Nursing Laboratory, for the "Bath in the Bed" practice with 22 graduating from the UERJ nursing course, the object was to record the teaching strategy procedure, identify the students representations and feelings with her own bath and characterize, after the sensibility dynamic developed in "LIVING EXPERIENCES", her feelings. The explorer-qualitative research was implemented in 1997. Among the results there are the proper categories of the knowledge construction on the "Bath in the Bed": Incentive for body dirt cleaner; Incentive to pleasure--the relax body; Incentive to comfort--free body and Incentive to sensations--dynamic body. Conclusion, the students carry from their reality common sense knowledge for "dirt" and "clean" when representing their own body. When experiment to "touch" humors and perceives the client bad odors, even of symbolic way, their reactions are different and re-extend a cultural and scientific knowledge.

[Mansonic neuroschistosomiasis: clinical, laboratory and therapeutic aspects]

The authors report 16 patients bearing the meningomyeloradicular form of Mansonic neuroschistosomiasis diagnosed according to clinical, laboratorial and image criteria. Patients have been observed at the Neurology Ambulatory of the Federal University of Bahia, within the period of April/91 to December/93. They have been treated with praziquantel, associated to corticosteroids. The aim has been to evaluate the drug's efficiency and safety in decreasing the neurological signs and symptoms.