Photon entanglement is indispensable for optical quantum technologies. Measurement-based optical quantum computing and all-optical quantum networks rely on multiphoton cluster states consisting of indistinguishable entangled photons. A promising method for creating such cluster states on demand is spin-photon entanglement using the spin of a resident charge carrier in a quantum dot, precessing in a weak external magnetic field. In this work, we show theoretically and experimentally that spin-photon entanglement is strongly affected by the hidden anisotropy of quantum dots, which can arise from mechanical stress, shape anisotropy and even specific crystal structure. In the measurements of time-resolved photoluminescence and the cross-polarized second-order photon correlation function in a magnetic field, the anisotropy manifests itself in the spin dynamics and, as a consequence, in the spin-photon concurrence. The measured time-filtered spin-photon Bell state fidelity depends strongly on the excitation polarization and reaches an extremely high value of 94% at maximum. We specify the magnetic field and excitation polarization directions that maximize spin-photon entanglement and thereby enhance the fidelity of multiphoton entangled states.

Photon entanglement is indispensable for optical quantum technologies. Measurement-based optical quantum computing and all-optical quantum networks rely on multiphoton cluster states consisting of indistinguishable entangled photons. A promising method for creating such cluster states on demand is spin-photon entanglement using the spin of a resident charge carrier in a quantum dot, precessing in a weak external magnetic field. In this work, we show theoretically and experimentally that spin-photon entanglement is strongly affected by the hidden anisotropy of quantum dots, which can arise from mechanical stress, shape anisotropy and even specific crystal structure. In the measurements of time-resolved photoluminescence and cross-polarized second-order photon correlation function in a magnetic field, the anisotropy manifests itself in the spin dynamics and, as a consequence, in the spin-photon concurrence. The measured time-filtered spin-photon Bell state fidelity depends strongly on the excitation polarization and reaches an extremely high value of 94

A single nanotube synthesized from a transition metal dichalcogenide (TMDC) exhibits strong exciton resonances and, in addition, can support optical whispering gallery modes. This combination is promising for observing exciton-polaritons without an external cavity. However, traditional energy-momentum-resolved detection methods are unsuitable for this tiny object. Instead, we propose to use split optical modes in a twisted nanotube with the flattened cross-section, where a gradually decreasing gap between the opposite walls leads to a change in mode energy, similar to the effect of the barrier width on the eigenenergies in the double-well potential. Using micro-reflectance spectroscopy, we investigated the rich pattern of polariton branches in single MoS$_2$ tubes with both variable and constant gaps. Observed Rabi splitting in the 40 - 60 meV range is comparable to that for a MoS$_2$ monolayer in a microcavity. Our results, based on the polariton dispersion measurements and polariton dynamics analysis, present a single TMDC nanotube as a perfect polaritonic structure for nanophotonics.

The ability to emit narrow exciton lines, preferably with a clearly defined polarization, is one of the key conditions for the use of nanostructures based on III-VI monochalcogenides and other layered crystals in quantum technology to create non-classical light. Currently, the main method of their formation is exfoliation followed by strain and defect engineering. A factor limiting the use of epitaxy is the presence of different phases in the grown films. In this work, we show that control over their formation makes it possible to create structures with the desired properties. We propose Ga$_2$Se$_3$/GaSe nanostructures by van der Waals epitaxy with a high VI/III flux ratio as a source of narrow exciton lines. Actually, these nanostructures are a combination of allotropes: GaSe and Ga$_2$Se$_3$, consisting of the same atoms in different arrangements. The energy position of the narrow lines is determined by the quantum confinement in Ga$_2$Se$_3$ inclusions of different sizes in the GaSe matrix, similar to quantum dots, and their linear polarization is due to the ordering of Ga vacancies in a certain crystalline direction in Ga$_2$Se$_3$. Such nanostructures exhibit single-photon emission with second-order correlation function $g^{(2)}(0)\sim$0.1 at 10 K that makes them promising for quantum technologies.

InSe is a promising material for a next-generation of two-dimensional electronic and optical devices, characteristics of which are largely determined by the type of band structure, direct or indirect. In general, different methods can be sensitive to different peculiarities of the electronic structure leading to different results. In this work, we will focus on the luminescent properties of few-layer $\beta$-InSe with a thickness of 6 to 75 monolayers (ML). Low-temperature micro-photoluminescence ($mu$-PL) studies show a sharp increase in PL intensity in the range of thicknesses from 16 to 20 monolayers, where, in addition, there is a singularity in the dependence of the work function on the thickness. Time-resolved photoluminescence spectroscopy (TRPL) reveals three characteristic PL decay times that differ from each other by about an order of magnitude. We associate the processes underlying the two faster decays with the recombination of electrons and holes between the band extrema, either directly or through the interband relaxation of holes. Their contributions to the total PL intensity increase significantly in the same thickness range, 16-20 MLs. On the contrary, the slowest contribution, which we attribute mainly to the defect-assisted recombination, prevails at a smaller number of monolayers and then noticeably decreases. These results indicate the indirect-to-direct bandgap transition near 16-20 MLs, which determines the range of applicability of a few-layer $\beta$-InSe for efficient light emitters.

Currently, two optical processes are mainly used to realize single photon sources: deterministic transitions in a semiconductor quantum dot (QD) placed in a microcavity and spontaneous frequency down-conversion in materials with intrinsic nonlinearity. In this work, we consider another approach that combines the advantages of both, such as high power with on-demand generation from QDs and the possibility of frequency tuning from nonlinear sources. For this purpose, we use stimulated frequency down-conversion occurring directly in the QD inside a microcavity designed not to the exciton frequency in the QD but to the target single photon frequency, which is set by the difference between the exciton resonance and the stimulating laser energies. This down-conversion arises from the second-order nonlinear interaction of an exciton (bright heavy-hole or dark) and a light-hole exciton in the stimulating laser field. We present an analytical model for such a down-conversion process and evaluate its efficiency for a widely sought-after single photon source for the telecom C-band (1530-1565 nm). We show that the emission rate of down-converted single photons can approach MHz. At certain conditions, this process is comparable in efficiency to direct emission from an InAs/GaAs QD at 920 nm, which is outside the cavity mode.

The paper reports on heterostructures for mid-ultraviolet (UVC) emitters with multiple (up to 400 periods) and single two-dimensional (2D)-GaN/AlN quantum disks/quantum wells with a nominal thickness below the critical thickness of ~2 monolayers (MLs) characterizing the transition of the 2D growth mode to 3D.The structures were grown by plasma-assisted molecular beam epitaxy (PA MBE) using low growth temperatures (~690C) in a wide range of gallium and activated nitrogen flux ratios Ga/N2* = 0.6  2.2 on various AlN/csapphire templates fabricated either by PA MBE or MOCVD.This made it possible to vary the surface topography from a 3D type under nitrogen-rich conditions to various types of 2D topographies in the structures grown under metal(Ga)-rich conditions.The absence of a Stranski-Krastanov transition in the latter structures was confirmed by a streaky RHEED pattern throughout the growth of QWs and barrier layers.The growth runs were monitored also by multi-beam optical stress sensor, which revealed an unusual stress relaxation in the ML-thick GaN/AlN heterostructures.Structural properties of GaN/AlN heterostructures were studied using X-ray diffraction analysis, including measurement of X-ray reflectance curves, atomic force microscopy, and high-resolution transmission microscopy.The results of these studies, together with the measurements of photoluminescence spectra, both cw and time-resolved, made it possible to suggest the formation of twodimensional GaN quantum disks with a thickness of either 1 or 2 ML and different lateral sizes on the stepped surface of the AlN barrier layers, which can lead to effective carrier localization.Moreover, we demonstrate a unique functional property of these atomically thin QW to maintain stable excitons, resulting in a particularly high radiation yield at room temperature.As a result, the emission energy (wavelength) from GaN/AlN 400QW structures could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) and was connected with a simultaneous increased of charge carrier localization.Using electron-beam pumping with a plasma cathode ferroelectric electron gun ensuring a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W, as shown in Fig. 1.In addition, we discussed the optical properties of cylindrical nanorods with diameters from 50 to 5000 nm, fabricated by the topdown technology from ML-thick GaN/AlN single QWs using a combination of wet and reactive ion etching.Photoluminescence measurements in an ultra-small QW region enclosed in a nanorod revealed narrow lines of individual excitons localized on potential fluctuations attributed to GaN quantum nano-disks 2-3 MLs high, which appear in a QW with an nominal thickness of 1.5 ML.A model that takes into account dark and bright exciton states was used to explain the features in the PL spectra, including their behavior with increasing temperature.This research was funded by

At present, the available experimental data on the optical properties of layered III–VI monochalcogenide compounds are scattered and somewhat contradictory, although they are the parent materials for promising two-dimensional (2D) structures. This work is devoted to optical studies of bulk GaSe crystals, whose perfect structural properties are confirmed by Raman studies, observation of singlet-triplet splitting of 1.5 meV, and the polarized photoluminescence measurements from the sample edge. We analyze the band structure of GaSe, namely the sequence and energies of direct and indirect exciton transitions, using cw and time-resolved micro-photoluminescence measurements with variation of temperature. It turns out that the direct band gap in bulk GaSe is at 2.13 eV, close to calculated values. The indirect exciton transition is located ∼15 meV below the direct exciton (2.11 eV). Its intensity quickly quenches and characteristic decay time strongly shortens with increasing temperature, while the contribution of the direct exciton is relatively enhanced.

Single photon sources based on semiconductor quantum dots are one of the most prospective elements for optical quantum computing and cryptography. Such systems are often based on Bragg resonators, which provide several ways to control the emission of quantum dots. However, the fabrication of periodic structures with many thin layers is difficult. On the other hand, the coupling of single-photon sources with resonant nanoclusters made of high-index dielectric materials is known as a promising way for emission control. Our experiments and calculations show that the excitation of magnetic Mie-type resonance by linearly polarized light in a GaAs nanopillar oligomer with embedded InAs quantum dots leads to quantum emitters absorption efficiency enhancement. Moreover, the nanoresonator at the wavelength of magnetic dipole resonance also acts as a nanoantenna for a generated signal, allowing control over its radiation spatial profile. We experimentally demonstrated an order of magnitude emission enhancement and numerically reached forty times gain in comparison with unstructured film. These findings highlight the potential of quantum dots coupling with Mie-resonant oligomers collective modes for nanoscale single-photon sources development.

Topic: 33. Bleeding disorders (congenital and acquired) Background: Hemorrhagic conditions of various genesis can lead to hemocoagulation complications due to impaired reactions of vascular-platelet and plasma hemostasis. This is a serious problem of physiology and medicine, because in such cases massive bleeding can occur, which threatens the lives of patients. Currently, various highly active hemostatic agents are widely used for the relief of hemorrhages. In this regard, it is of interest to study chitosan-based biopolymers that exhibit a wide range of biological activities, including hemostatic, while having good biocompatibility and non-toxicity. However, the effect of chitosan and its composites on the blood clotting system remains unclear. Aims: To study the effect of a chitosan composite with aspartic acid on vascular-platelet hemostasis in animals with experimental hypocoagulation. Methods: The study was performed on male Wistar rats in accordance with the “International Guiding Principles for Biomedical Involving Animals”. 4 groups of animals were used: rats treated with composite of chitosan (100 mg/kg) with aspartic acid (2 mg/kg); rats treated rats treated with either chitosan or aspartic acid in doses equivalent to their content in the composite; rats treated with saline solution (as a control). All drugs were administered orally to rats daily once a day for 14 days. After the application of the studied drugs, the animals of each group orally received acetylsalicylic acid (aspirin) at a dose of 1.5 mg/kg to simulate experimental hypocoagulation, since aspirin is the most widely used antiplatelet drug with a clinically proven mechanism of action. Additionally, a group of intact rats was used. Blood samples from each animal were obtained 20 hours after completion of the course of treatment, and then ADP-induced platelet aggregation (according to Born) was evaluated in platelet-rich blood plasma of rats (final concentration of ADP 10-6 M). Results: During the experiment, it was shown that in animals injected with aspirin, a decrease in platelet aggregation was revealed by 30% (p < 0.05) in relation to intact rats, which indicated an increase in the antiplatelet potential of the blood and the development of a hypocoagulation state. Treatment of animal composite of chitosan with aspartic acid contributed to an increase in the hemostatic potential of the blood, which was expressed in an increase in platelet aggregation by 58% (p < 0.05) versus control. In animals treated with either chitosan or aspartic acid, platelet aggregation increased by 22% and 36% (p < 0.05 in both cases), respectively, versus the control group. Consequently, the composite showed a greater effect on primary hemostasis compared to its constituent parts – chitosan and aspartic acid, significantly enhancing platelet aggregation. Summary/Conclusion: Thus, the chitosan-based composite exhibits a noticeable procoagulant effect, restoring the hemostatic balance reduced as a result of aspirin use. Activation of the vascular-platelet link of hemostasis, due to increased platelet aggregation processes, may be a potential mechanism by which the composite of chitosan with aspartic acid normalizes hemostatic reactions under hypocoagulation conditions in the experiment. Based on this, the composite of chitosan with aspartic acid can be considered a promising hemostatic agent. Keywords: Bleeding disorder, Aspirin, Platelet aggregation, Procoagulant

The characteristics of a single-photon emitter based on a semiconductor quantum dot, such as their indistinguishability and brightness, depend on the stability of the recombination channel, which can switch spontaneously between exciton and trion. We show that dominant recombination through neutral exciton states can be achieved by careful control of the doping profile near an epitaxial InAs/GaAs quantum dot placed in a columnar microcavity with distributed Bragg reflectors. The Hong-Ou-Mandel experiments carried out in the fabricated device demonstrate the degree of indistinguishability of 91% of successively emitted single photons within 242 ns at an efficiency of 10% inside a single-mode optical fiber. The achieved brightness made it possible to implement spatio-temporal demultiplexing of photons in six independent spatial modes with an in-fiber generation frequency of more than 0.1 Hz.

GaN/AlN heterostructures with thicknesses of one monolayer (ML) are currently considered to be the most promising material for creating UVC light-emitting devices. A unique functional property of these atomically thin quantum wells (QWs) is their ability to maintain stable excitons, resulting in a particularly high radiation yield at room temperature. However, the intrinsic properties of these excitons are substantially masked by the inhomogeneous broadening caused, in particular, by fluctuations in the QWs' thicknesses. In this work, to reduce this effect, we fabricated cylindrical nanocolumns of 50 to 5000 nm in diameter using GaN/AlN single QW heterostructures grown via molecular beam epitaxy while using photolithography with a combination of wet and reactive ion etching. Photoluminescence measurements in an ultrasmall QW region enclosed in a nanocolumn revealed that narrow lines of individual excitons were localized on potential fluctuations attributed to 2-3-monolayer-high GaN clusters, which appear in QWs with an average thickness of 1 ML. The kinetics of luminescence with increasing temperature is determined via the change in the population of localized exciton states. At low temperatures, spin-forbidden dark excitons with lifetimes of ~40 ns predominate, while at temperatures elevated above 120 K, the overlying bright exciton states with much faster recombination dynamics determine the emission.

In two-dimensional (2D) transition metal dichalcogenides, the sequence and splitting energy between spin-allowed (bright) and spin-forbidden (dark) excitons controls the optical and transport properties. In this paper, we discuss the effect of strain at both compression and tension on the band structure and fine spectrum of exciton states in MoS 2 nanostructures. Using a combination of micro-Raman and time-resolved micro-photoluminescence, we found that the exciton spectrum in unstrained layers in complete agreement with the theoretical predictions. In the A-exciton series, the bright state is the lowest in the monolayer, while in the bilayer the exciton states are spin-degenerate due to the even number of layers. However, strain can lift the degeneracy and increase the splitting value in the monolayer by several times. On folds subjected to local tension, the splitting decreases down to the reversed sequence of dark and bright excitons. With both types of strain, the band structure tends to transform towards the indirect type.

Synthesized micro- and nanotubes composed of transition metal dichalcogenides (TMDCs) such as MoS_2 are promising for many applications in nanophotonics, because they combine the abilities to emit strong exciton luminescence and to act as whispering gallery microcavities even at room temperature. In addition to tubes in the form of hollow cylinders, there is an insufficiently-studied class of twisted tubes, the flattened cross section of which rotates along the tube axis. As shown by theoretical analysis, in such nanotubes the interaction of electromagnetic waves excited at opposite sides of the cross section can cause splitting of the whispering gallery modes. By studying micro-photoluminescence spectra measured along individual MoS_2 tubes, it has been established that the splitting value, which controls the energies of the split modes, depends exponentially on the aspect ratio of the cross section, which varies in "breathing" tubes, while the relative intensity of the modes in a pair is determined by the angle of rotation of the cross section. These results open up the possibility of creating multifunctional tubular TMDC nanodevices that provide resonant amplification of self-emitting light at adjustable frequencies.

Atherosclerotic vascular changes are observed in violation of the lipid profile, carbohydrate metabolism and increased blood clotting, which may be associated with the development of metabolic syndrome (MS) in the body. This pathology is accompanied by endothelial dysfunction, which is manifested by a decrease in fibrinolytic activity due to reduce in the activity of the tissue plasminogen activator. Natural regulatory peptides are used in the development of drugs that promote the inclusion of compensatory and adaptive reactions of the body in damage of homeostatic balance. On their basis, new synthetic peptides of the glyproline series were synthesized. For them, the ability to exert regulatory effects on the processes of coagulation and fibrinolysis in normal and in various pathological conditions was shown

This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N2*) on c-sapphire substrates. An increase in the Ga/N2* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W.

We report on single-photon emitters for the telecommunication O-band (1260–1360 nm), which comprise an InAs/(In)GaAs quantum dot with asymmetric barriers, placed inside a semiconductor tapered nanocolumn acting as a photonic nanoantenna. The implemented design of the barriers provides a shift in the quantum dot radiation wavelength towards the O-band, while the nanoantenna collects the radiation and ensures its effective output. With non-resonant optical pumping, the average count rate of emitted single photons exceeds 10 MHz with the second-order correlation function g(2)(0) = 0.18 at 8 K.

We review recently discovered phenomena observed in the MoS2 tubular structures, which were synthesized by a lasting chemical transport reaction nearly at chemical equilibrium. Such MoS2 nanotubes are distinguished by low density of structural defects, thin walls and a high aspect ratio, which intrinsically provide a confined, edge-free geometry. Quantum confinement with single electron conductance was recorded and a bright exciton photoluminescence with appearance of whispering gallery modes was observed. In addition, the field-effect transistors and field emitters based on single MoS2 nanotubes were demonstrated. These discoveries revitalize research of MoS2 curved structures grown from vapour phase, which were reported several decades ago.

We report the emergence of a significant degree of intrinsic circular polarization of exciton photoluminescence in a twisted MoSe2/WSe2 heterostructure upon nonresonant driving with a linearly polarized laser. The effect is not related to the polarization of the incident light. Moreover, it is present at zero magnetic field, and reacts perceptibly to a perpendicularly applied magnetic field that, unexpectedly, can strongly diminish this effect. The giant magnitude of the polarization, which cannot be explained by natural optical activity or circular dichroism of the twisted lattice, suggests a kinematic origin arising from an emergent pyromagnetic symmetry in our structure, which we exploit to gain insight into the microscopic optical processes of our device.

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright–dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.