We present the design and experimental demonstration of a micropillar metamorphic cavity structure with an embedded InAs/InGaAs quantum dot, intended for single-photon generation in the telecommunication C-band. The microcavity, incorporating a single GaAs/AlGaAs distributed Bragg reflector, is designed to provide a photon extraction efficiency of 15

Heterostructures with InAs/InGaAs quantum dots and InxGa1–xAs/GaAs(001) metamorphic buffer layers are grown by molecular-beam epitaxy. The structures are designed to obtain single-photon emission in the telecommunication C-band wavelength range. The possibility of reducing the thickness of the InxGa1–xAs graded layer in order to form efficient microcavity structures with a cavity length as small as two wavelengths is examined. The structures with metamorphic buffer layers grown on top of an Al0.9Ga0.1As/GaAs distributed Bragg reflector are grown and characterized by cross-sectional transmission electron microscopy and photoluminescence spectroscopy.

We optimize the geometric parameters of the single-photon source operated in the red spectral range, which is based on a photonic nanoantenna with InAs/AlGaAs quantum dots. This made it possible to determine the range of the transverse dimensions of the cylindrical nanoantenna, within which the maximum efficiency of radiation extraction into the numerical aperture NA = 0.42 is achieved. An increase in the extraction efficiency as compared to a planar structure was demonstrated for the optimized nanostructure. The statistics of single photon correlations under optical pumping with participation of acoustic phonons is studied. The value of the zero-delay second-order correlation function g(2)(0) was 0.065.

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

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.

Heterostructures with InAs/InGaAs quantum dots grown by molecular beam epitaxy on the surface of InGaAs metamorphic buffer layers with a linearly graded composition profile on GaAs(001) substrates have been studied by X-ray diffraction, transmission electron microscopy, and, upon the growth of an additional quantum-dot layer on the surface of the structure, by atomic force microscopy. The tendency to the formation of quantum objects elongated along the [1–10] direction (so-called quantum dashes), caused by asymmetry in the surface migration of In along different crystallographic directions, is confirmed. It is established that the surface density of both quantum dots and quantum dashes is as high as (2‒4) × 1010 cm–2. At the same time, narrow lines associated with emission from individual quantum dots are observed in the spectra of low-temperature (T = 10 K) microphotoluminescence in a wide wavelength range (1.30–1.55 µm). The size and shape of quantum dots have been estimated from atomic-force microscopy and transmission electron microscopy data and good agreement with the previously reported parameters is demonstrated.

The effect of lattice relaxation instability (martensitic transition) on piezoelectric fields (EPE) in spontaneously ordered GaInP2/GaAs epitaxial layers was demonstrated using scanning Kelvin probe microscopy in combination with electron microscopy and optical spectroscopy measurements. The transition manifests itself in the dependence of the surface potential of the epi-layer on the mechanical (cleavage) and thermal (annealing) impacts. This is associated with a switching of the crystal lattice between relaxed and strained martensitic states, corresponding to a change in EPE in the epi-layer. The measured surface potential values (0.2–2.4 V) correspond to EPE within ±100 kV/cm and a strong decrease in |EPE| with increasing layer thickness, indicating the pinning of the Fermi level and piezoelectric doping. Our results open up the prospects for using spontaneously ordered semiconductor alloys to control electronic states in semiconductor nanostructures by controlling their piezoelectric fields.

Fluorescence spectra of nanodiamonds synthesized at high pressure from adamantane and other organic compounds show narrow (approximate to 1 nm) lines of unknown origin over the spectroscopic range from approximate to 500 to 800 nm. The study proposes and experimentally confirms the hypothesis that these lines are related to radiative recombination of donor-acceptor pairs (DAP). According to the experimental data, these pairs can be formed from donor-like substitutional nitrogen present in the diamond lattice and 2D acceptor layer resulting from the effect of transfer doping on the nanodiamond surface. A peculiar behavior of the narrow lines is identified within the temperature range of 100-10 K: their energy position slightly shifts downward, and the majority of the lines divide into two or more components as the temperature decreases. The lines are shown to be predominantly associated with single photon emitters, with an emission rate exceeding 1 million counts s-1 at room temperature. A new narrowband source of room-temperature fluorescence found in hydrogen-terminated nanodiamonds push horizons for quantum optical technologies related to the development of single photon emitters and temperature nanosensors. This study proposes and experimentally confirms the hypothesis that mysterious narrow lines, observed at room temperature in 500-800 nm range of fluorescence spectra of H-terminated nanodiamonds, originate from radiative recombination of donor-acceptor pairs. It is proved that the pairs are formed by donor-like substitutional nitrogen and 2D acceptor layer resulting from the effect of transfer doping on the nanodiamond surface. image

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

In fluorescence spectra of nanodiamonds (NDs) synthesized at high pressure from adamantane and other organic compounds, very narrow (~1 nm) lines of unknown origin are observed in a wide spectroscopic range from ~500 to 800 nm. Here, we propose and experimentally substantiate the hypothesis that these mysterious lines arise from radiative recombination of donor-acceptor pairs (DAPs). To confirm our hypothesis, we study the fluorescence spectra of undoped and nitrogen-doped NDs of different sizes, before and after thermal oxidation of their surface. The results obtained with a high degree of confidence allowed us to conclude that the DAPs are formed through the interaction of donor-like substitutional nitrogen present in the diamond lattice, and a 2D layer of acceptors resulting from the transfer doping effect on the surface of hydrogen-terminated NDs. A specific behavior of the DAP-induced lines was discovered in the temperature range of 100-10 K: their energy increases and most lines are split into 2 or more components with decreasing temperature. It is shown that the majority of the studied DAP emitters are sources of single photons, with an emission rate of up to >1 million counts/s at room temperature, which significantly surpasses that of nitrogen-vacancy and silicon-vacancy centers under the same detection conditions. Despite an observed temporal instability in the emission, the DAP emitters of H-terminated NDs represent a powerful room-temperature single-photon source for quantum optical technologies.

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.

We report a design and implementation of a resource-efficient spatial demultiplexer which produces four indistinguishable photons with efficiency of 39.7% per channel. Our scheme is based on a free-space storage/delay line which accumulates four photons and releases them by a controlled polarization rotation using a single Pockels cell.

In recent years, single-photon sources (SPSs) based on the emission of a single semiconductor quantum dot (QD) have been actively developed. While the purity and indistinguishability of single photons are already close to ideal values, the high brightness of SPSs remains a challenge. The widely used resonant excitation with cross-polarization filtering usually leads to at least a two-fold reduction in the single-photon counts rate, since single-photon emission is usually unpolarized, or its polarization state is close to that of the exciting laser. One of the solutions is the use of polarization-selective microcavities, which allows one to redirect most of the QD emission to a specific polarization determined by the optical mode of the microcavity. In the present work, elliptical micropillars with distributed Bragg reflectors are investigated theoretically and experimentally as a promising design of such polarization-selective microcavities. The impact of ellipticity, ellipse area and verticality of the side walls on the splitting of the optical fundamental mode is investigated. The study of the near-field pattern allows us to detect the presence of higher-order optical modes, which are classified theoretically. The possibility of obtaining strongly polarized single-photon QD radiation associated with the short-wavelength fundamental cavity mode is shown.

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.

A biexciton radiative cascade was recorded in the photoluminescence spectra of an InAs/GaAs QD embedded in a λ-cavity with a relatively high-quality factor of 4600, formed in a micropillar with GaAs/AlGaAs distributed Bragg reflectors. The spectrum and kinetics of the radiation, measured under conditions of two-photon excitation, demonstrated a significant effect of the microcavity on the emission process. A possible improvement based on this effect in the generation of entangled photon pairs is discussed.

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.

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.