Dark Ground Research Articles

Phonon‐Bottleneck Enhanced Exciton Emission in 2D Perovskites

AbstractLayered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin‐orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher‐energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low‐temperature magneto‐optical measurements with sophisticated many‐particle theory, the origin of the bright exciton emission in perovskites is elucidated by tracking the thermalization of dark and bright excitons under a magnetic field. The unexpectedly high emission is clearly attributed to a pronounced phonon‐bottleneck effect, considerably slowing down the relaxation toward the energetically lowest dark states. It is demonstrated that this bottleneck can be tuned by manipulating the bright‐dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications.

A new remarkable species of the genus Hyperlais Marion, 1959 (Lepidoptera: Crambidae) from Kyrgyzstan.

A new species, Hyperlais orodruinella sp. n., is described. Its type locality is "Kyrgyzstan, Fergansky Mts., Razan Say Valley, Sary-Bel stream, 41°25'16.74"N, 72°18'59.19"E, 892 m". This new species differs from all other species of the genus by its patternless wings, by its very dark ground colour and by its male genitalia features, especially aedoeagus cornuti (it has three groups of cornuti instead of two groups in other species).

Size-Dependent Lattice Symmetry Breaking Determines the Exciton Fine Structure of Perovskite Nanocrystals.

The order of bright and dark excitonic states in lead-halide perovskite nanocrystals is debated. It has been proposed that the Rashba effect, driven by lattice-induced symmetry breaking, causes a bright excitonic ground state. Direct measurements of excitonic spectra, however, show the signatures of a dark ground state, bringing the role of the Rashba effect into question. We use an atomistic theory to model the exciton fine structure of perovskite nanocrystals, accounting for realistic lattice distortions. We calculate optical gaps and excitonic features that compare favorably with experimental works. The exciton fine structure splittings show a nonmonotonic size dependence due to a structural transition between cubic and orthorhombic phases. Additionally, the excitonic ground state is found to be dark with spin triplet character, exhibiting a small Rashba coupling. We additionally explore the effects of nanocrystal shape on the fine structure, clarifying observations on polydisperse nanocrystals.

Variation for turfgrass performance in a set of Lolium perenne germplasm evaluated under limited irrigation

AbstractPerennial ryegrass (Lolium perenne L.) is a major turfgrass species in temperate regions around the world. Unfortunately, it lacks drought tolerance compared with other cool‐season turfgrass species and often requires substantial inputs of freshwater as supplemental irrigation to achieve high turfgrass quality. To address the need for more drought‐tolerant perennial ryegrass, we conducted an evaluation of perennial ryegrass germplasm from the U.S. National Plant Germplasm System under deficit irrigation conditions and two mowing heights at Kaysville and Logan, UT during 2017 and 2018. We characterized turfgrass dark green color, ground cover, density, and quality using digital photography to capture images that we then converted to trait values using the TurfAnalyzer software. The higher mowing height resulted in greater ground cover, density, and quality. The collection of 49 germplasm accessions and 11 cultivars exhibited substantial variation for turfgrass dark green color, ground cover, density, and quality. Cultivars included as checks generally had greater turfgrass color and density, but the germplasm accessions generally had greater turfgrass ground cover and quality. Selection among these germplasm accessions will provide elite germplasm with utility for improving these traits in applied perennial ryegrass turfgrass breeding programs.

Excitons and trions in WSSe monolayers

The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO2/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the ‘parent’ WS2 and WSe2 MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane and out-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin- and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbidden complex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potential future quantum applications of S-TMDs.

Dark ground microscopy for identification of Treponema pallidum

Demonstration of Treponema pallidum under the dark ground microscope is a rapid onsite method for diagnosis in early syphilis. The steps involve obtaining patient consent, specimen collection from an infective lesion of early syphilis, the adjustments of dark field microscope, and identification of treponemes by virtue of their characteristic morphology and motility.

Brightening of a dark monolayer semiconductor via strong light-matter coupling in a cavity

Engineering the properties of quantum materials via strong light-matter coupling is a compelling research direction with a multiplicity of modern applications. Those range from modifying charge transport in organic molecules, steering particle correlation and interactions, and even controlling chemical reactions. Here, we study the modification of the material properties via strong coupling and demonstrate an effective inversion of the excitonic band-ordering in a monolayer of WSe2 with spin-forbidden, optically dark ground state. In our experiments, we harness the strong light-matter coupling between cavity photon and the high energy, spin-allowed bright exciton, and thus creating two bright polaritonic modes in the optical bandgap with the lower polariton mode pushed below the WSe2 dark state. We demonstrate that in this regime the commonly observed luminescence quenching stemming from the fast relaxation to the dark ground state is prevented, which results in the brightening of this intrinsically dark material. We probe this effective brightening by temperature-dependent photoluminescence, and we find an excellent agreement with a theoretical model accounting for the inversion of the band ordering and phonon-assisted polariton relaxation.

PM 7/53 (2) Liriomyza spp.

PM 7/53 (2) <i>Liriomyza</i> spp.

Model Predictions of Occupational Exposures to Diacetyl and 2,3-Pentanedione Emitted From Roasted Whole Bean and Ground Coffee: Influence of Roast Level and Physical Form on Specific Emission Rates.

Roasted coffee emits hazardous volatile organic compounds including diacetyl and 2,3-pentanedione. Workers in non-flavored coffee roasting and packaging facilities might inhale diacetyl and 2,3-pentanedione from roasted coffee above occupational exposure limits depending on their work activities and proximity to the source of emissions. Objectives of this laboratory study were to: (1) investigate factors affecting specific emission rates (SERs) of diacetyl and 2,3-pentanedione from freshly roasted coffee, (2) explore the effect of time on SERs of coffee stored in sealed bags for 10-days, and (3) predict exposures to workers in hypothetical workplace scenarios. Two roast levels (light and dark) and three physical forms (whole bean, coarse ground, and fine ground) were investigated. Particle size for whole bean and ground coffee were analyzed using geometric mean of Feret diameter. Emitted chemicals were collected on thermal desorption tubes and quantified using mass spectrometry analysis. SERs developed here coupled with information from previous field surveys provided model input to estimate worker exposures during various activities using a probabilistic, near-field/far-field model. For freshly roasted coffee, mean SER of diacetyl and 2,3-pentantedione increased with decreasing particle size of the physical form (whole bean < coarse ground < fine ground) but was not consistent with roast levels. SERs from freshly roasted coffee increased with roast level for diacetyl but did not change for 2,3-pentanedione. Mean SERs were greatest for diacetyl at 3.60 mg kg−1 h−1 for dark, fine ground and for 2,3-pentanedione at 3.88 mg kg−1 h−1 for light, fine ground. For storage, SERs of whole bean remained constant while SERs of dark roast ground coffee decreased and light roast ground coffee increased. Modeling demonstrated that near-field exposures depend on proximity to the source, duration of exposure, and air velocities in the near-field further supporting previously reported chemical air measurements in coffee roasting and packaging facilities. Control of source emissions using local exhaust ventilation especially around grinding activities as well as modification of work practices could be used to reduce exposures in this workforce.

Exciton absorption, band structure, and optical emission in biased bilayer graphene

Biased bilayer graphene (BBG) is a variable band gap semiconductor, with a strongly field-dependent band gap of up to $300 \, \text{meV}$, making it of particular interest for graphene-based nano-electronic and -photonic devices. The optical properties of BBG are dominated by strongly bound excitons. We perform ab initio density-functional-theory+Bethe-Salpeter-equation modelling of excitons in BBG and calculate the exciton band structures and optical matrix elements for field strengths in the range $30-300 \, \text{mV}/{\unicode{x212B}}$. The exciton properties prove to have a strong field dependence, with both energy ordering and dipole alignment varying significantly between the low and high field regions. Namely, at low fields we find a mostly dark ground state exciton, as opposed to high fields, where the lowest exciton is bright. Also, excitons preferentially align with a dipole moment opposite the field, due to the field-induced charge transfer in the ground state of BBG. However, in stronger fields, this alignment becomes energetically less favorable. Additionally, the bright excitons show particle- and light-like bands similar to monolayer transition metal dichalchogenides. Finally, we model the radiative lifetimes and emission properties of BBG, which prove to be strongly dependent on temperature in addition to field strength.

Urban Water Extraction with UAV High-Resolution Remote Sensing Data Based on an Improved U-Net Model

Obtaining water body images quickly and reliably is important to guide human production activities and study urban change. This paper presents a fast and accurate method to identify water bodies in complex environments based on UAV high-resolution images. First, an improved U-Net (SU-Net) model is proposed in this paper. By increasing the number of connections in the middle layer of the neural network, more image features can be retained through S-shaped circular connections. Second, aiming at the interference of mixed ground objects and dark ground objects on water detection, the fusion of a deep learning network and visual features is investigated. We analyse the influence of a wavelet transform and grey level cooccurrence matrix (GLCM) on water extraction. Using a confusion matrix to evaluate accuracy, the following conclusions are drawn: (1) Compared with existing methods, the SU-Net method achieves a significant improvement in accuracy, and the overall accuracy (OA) is 96.25%. The kappa coefficient (KC) is 0.952. (2) SU-Net combined with the GLCM has a higher accuracy (OA is 97.4%) and robustness in distinguishing mixed and dark objects. Based on this method, a distinct water boundary in urban areas, which provides data for urban water vector mapping, can be obtained.

Brightening of Dark States in CsPbBr3 Quantum Dots Caused by Light-Induced Magnetism.

Lead halide perovskite quantum dots (QDs) have shown great potential for optoelectronic and quantum photonic applications. Although controversy remains about the electronic fine structures of bulk perovskites due to the strong spin-orbit coupling affecting the conduction bands, compelling evidence indicates that the ground states of perovskite QDs remain dark, limiting their applications in optoelectronic devices. Here, it is demonstrated that photoexcitation can induce large intrinsic magnetic fields in Mn-doped CsPbBr3 perovskite QDs. Equivalent to applying an external magnetic field, the light-induced field causes giant Zeeman splitting to the bright triplet states and brightens the dark singlet ground state, thus effectively rendering a partially bright ground state in the doped QDs. These findings here may contribute to the understanding of the electronic fine structures in perovskite QDs and demonstrate a potential approach for creating semiconductor nanostructures that can serve as bright light sources.

PUMA – An open‐source 3D‐printed direct vision microscope with augmented reality and spatial light modulator functions

3D-printed microscopes are a topical emerging field in the literature. However most microscopes presented to date are quite novel re-imaginings of the microscope's mechanical design and they are either solely dependent on, or primarily geared towards, camera-based observations rather than ergonomic direct vision screening through an ocular lens. The reliance on camera, computer and monitor for observation introduces a compromise between portability, cost and the quality of an instant wide field of view. In this report, I introduce the Portable Upgradeable Modular and Affordable (PUMA) microscope which is an open-source 3D-printed multimodality microscope that employs a traditional upright design for ease of human direct visual observations and slide screening. PUMA uses standard RMS or C-mount objectives, with a tube length 160 mm, 170 mm or infinity and wide field high eye point ocular lenses. PUMA can use simple mirror-based illumination or can be configured to a full Köhler system with Abbe condenser for high numerical aperture observations including oil immersion. PUMA also has advanced digital/optical imaging features such as a digital spatial light modulator and - unique to any 3D printed microscope to date - an augmented reality heads-up display for interactive calibrated measurements. Digital camera imaging can also be used with PUMA - in fact PUMA can take up to three separate digital cameras simultaneously. PUMA can also function as a direct vision multi-header microscope for teaching or discussion. The illumination system is also modular and includes transillumination, epi-illumination, fluorescence, polarisation, dark ground and also Schlieren-based phase contrast and other Fourier optics filtering modalities. All these advanced features are available through an on-board, battery operated, microprocessor so no mains supply, smartphone, network connection, PC or external monitor are required making PUMA a truly portable system suitable for remote field work.

Anaemia Associated with Trypanosomes Infections in Cattle of West Gojjam Zone, Northwest Ethiopia

Background African animal trypanosomosis is a major veterinary problem over a large area of the tsetse belt region of Africa. Anaemia is a cardinal sign of trypanosome infections. The mechanism of anaemia due to trypanosomosis is complex and multifactorial in origin. Packed cell volume (PCV) usually gives an indication of the anaemia and disease status of a trypanosome-infected animal. Methods A cross-sectional study was conducted from December 2017 to January 2018 in West Gojjam zone, Northwest Ethiopia, to determine the trypanosome infections rate and the possible correlation between parasitic infection and anaemia using the dark ground buffy coat technique, Giemsa-stained thin blood smear, and PCV reading on a haematocrit reader. Results The overall trypanosomosis prevalence was 7.81%, 95% CI = 7.45–8.17. Trypanosoma congolense (4.25%) and T. vivax (3.56%) were the trypanosomes species identified in the studied area. PCV for all sampled cattle was analysed to estimate the degree of anaemia. From the total examined animals (N = 730), 356 (48.77%) were anaemic and 374 (51.23%) were nonanaemic. The mean PCV of parasitemic cattle was significantly lower (21.09%, 95% CI = 20.13–22.05) than that of aparasitemic ones (25.96%, 95% CI = 25.68–26.24). There was a positive association between trypanosome infection and anaemia. Although both trypanosome species are significantly associated with a decreased herd mean PCV (<24), the mean PCV of cattle infected with T. congolense (19.45%) was lower than that of infected with T. vivax (23.04%). The herd mean PCV was not significantly associated to locations, age, and sex of the studied animals. Conclusions The study confirms that the prevalence of trypanosomes infections and herd mean PCV has a significant association. The mean herd PCV can be a useful cheap tool to screen for possible trypanosome infection. However, there were cattle positive for trypanosomes having mean PCV within the reference interval and negative animals with anaemia. Furthermore, PCV reading should be confirmed by other diagnostic techniques to accurately conclude that trypanosomosis is the only cause of anaemia.

Analysis of evil in Schelling’s Freiheitsschrift through Heidegger’s account of dissemblance and Αλήθεια

ABSTRACT In this paper, I offer an analysis of evil in Friedrich W. J. Schelling’s Philosophische Untersuchungen über das Wesen der menschlichen Freiheit (1809). Schelling develops an account of the sui-genesis of God out of the two principles. These principles are 1) the dark ground (dunkler Grund) that belongs to God and 2) the self-revelation of God, who actualizes the dark ground, which grounds God antecedently. These two principles also contain in themselves the possibility and the intelligibility of the human world. In order to elucidate the ontological account of the possibility of evil in Schelling, I turn to Martin Heidegger’s analyses of Schelling’s Freiheitsschrift and especially to Heidegger’s account of self-will (Eigenwille) and put these analyses in conversation with Heidegger’s own thinking about αλήθεια. I establish a conceptual affinity between Schelling’s presentation of the dark ground, which for him is the ground of selfhood, and Heidegger’s insights into the prioricity of concealment (Verborgenheit).

A new species of Dichagyris Lederer, 1857 from China (Lepidoptera, Noctuidae).

Dichagyris is a diverse genus with a Holarctic distribution. Only a few taxa of the subgenus Albocosta Fibiger Lafontaine, 1997 occur in the northwesternmost part of the Oriental region. The genus is most diverse in Turkey, Iran and the Central Asiatic high mountains. However, there is a distinct Sino-Tibetan group of species, possibly representing an undescribed subgenus. However, this idea is provisional and awaits revision of the genus. For present purposes, we assign these species to the subgenus Dichagyris. They share the following putative synapomorphies: black, dark brown or dark greyish forewing ground colour (with one exception), with obscure, reduced wing pattern and a broad-based, short, conical harpe in the male genitalia. Species of this group can be subdivided to two species-groups. In the astigmata-group which was mentioned for the first time by Hreblay et al. (1998) but without description or diagnosis from the minuta-group, the vesica is elongate and tubular (males), the appendix bursae and corpus bursae are long, sack-like, almost equal in size (females). In the minuta-group, the vesica is more ample than in the astigmata-group, and coiled, and the appendix bursae is globular and much shorter than the corpus bursae. The astigmata-group includes Dichagyris astigmata (Hampson, 1906), D. gansuensis Hreblay Ronkay, 1998, D. geochroides (Boursin, 1948) and D. vargazoli (Gyulai Ronkay, 2001). The minuta-group comprises D. minuta Hreblay Plante, 1998 and D. kormos Gyulai Ronkay, 2001. During a research expedition in Sichuan in 2019, the existence of a third species in the minuta-group was recognized, which is described here. In addition, the female and its genitalia of D. gansuensis, which was described by Hreblay Ronkay (1998) based on a single male, are illustrated here for the first time.

Probing negatively charged and neutral excitons in MoS2/hBN and hBN/MoS2/hBN van der Waals heterostructures

High-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer transition metal dichalcogenides enable observations of subtle optical and spin-valley properties whose identification was beyond the reach of structures exfoliated directly on standard SiO2/Si substrates. Here, we describe different van der Waals heterostructures based on uncapped single-layer MoS2 stacked onto hBN layers of different thicknesses and hBN-encapsulated monolayers. Depending on the doping level, they reveal the fine structure of excitonic complexes, i.e. neutral and charged excitons. In the emission spectra of a particular MoS2/hBN heterostructure without an hBN cap we resolve two trion peaks, T1 and T2, energetically split by about 10 meV, resembling the pair of singlet and triplet trion peaks (T S and T T ) in tungsten-based materials. The existence of these trion features suggests that monolayer MoS2 has a dark excitonic ground state, despite having a ‘bright’ single-particle arrangement of spin-polarized conduction bands. In addition, we show that the effective excitonic g-factor significantly depends on the electron concentration and reaches the lowest value of −2.47 for hBN-encapsulated structures, which reveals a nearly neutral doping regime. In the uncapped MoS2 structures, the excitonic g-factor varies from −1.15 to −1.39 depending on the thickness of the bottom hBN layer and decreases as a function of rising temperature.

Dark exciton preparation in a quantum dot by a longitudinal light field tuned to higher exciton states

Several important proposals to use semiconductor quantum dots in quantum information technology rely on the control of the dark exciton ground states, such as dark exciton based qubits with a $\mu$s life time. In this paper, we present an efficient way to occupy the dark exciton ground state by a single short laser pulse. The scheme is based on an optical excitation with a longitudinal field component featured by, e.g., radially polarized beams or certain Laguerre-Gauss or Bessel beams. Utilizing this component, we show within a configuration interaction approach that high-energy exciton states composed of light-hole excitons and higher dark heavy-hole excitons can be addressed. When the higher exciton relaxes, a dark exciton in its ground state is created.

RESEARCH NOTE PREVALENCE OF BOVINE TRYPANOSOME INFECTIONS IN DAMBOA LOCAL GOVERNMENT AREA BORNO STATE

African animal trypanosomiasis is a disease complex caused by pathogenic trypanosomes which are cyclically transmitted by tsetse-fly or mechanically by other biting flies to domestic animals. The disease has for long been a limiting factor to livestock production in tsetse-fly infested regions of Africa (Anosa, 1983; Trail et al., 1985; Dwinger et al., 1986). It is now assuming significance in the tsetse-free vegetational zones (Nawathe et al., 1988) probably due to transhumance and ecological upset in favour of increasing insect population and activity in the area. The study was undertaken to ascertain the prevalence of bovine trypansome infection in the area and hoped that it will provide information on the epidemiology of the disease. A total of 151 blood samples were collected from 11 sedentary herds of cattle in Damboa Local Government Area between February to April, 1992. About 3ml of blood was collected from each animal inbijou bottle containing anticoagulant (Ethylene diamine tetra-acetic acid). The sex, age and breed of each animal were noted during blood collection. The blood samples were transported to the Laboratory on ice and examined using wet-film, stained thin smears, haematocrit centrifuge technique (Woo, 1969) and dark ground buffy coat. The packed cell volume was also recorded for each animal using Hawksley's haematocrit reader.

The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals

Cesium lead halide perovskites exhibit outstanding optical and electronic properties for a wide range of applications in optoelectronics and for light-emitting devices. Yet, the physics of the band-edge exciton, whose recombination is at the origin of the photoluminescence, is not elucidated. Here, we unveil the exciton fine structure of individual cesium lead iodide perovskite nanocrystals and demonstrate that it is governed by the electron-hole exchange interaction and nanocrystal shape anisotropy. The lowest-energy exciton state is a long-lived dark singlet state, which promotes the creation of biexcitons at low temperatures and thus correlated photon pairs. These bright quantum emitters in the near-infrared have a photon statistics that can readily be tuned from bunching to antibunching, using magnetic or thermal coupling between dark and bright exciton sublevels.