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.

Indium (iii) selenide is currently one of the most actively studied materials in the two-dimensional family due to its remarkable ferroelectric and optical properties. This study focuses on the luminescent properties of few-layer In2Se3 flakes with thicknesses ranging from 7 to 100 monolayers. To explore the photoluminescence features and correlate them with changes in crystal symmetry and surface potential, we employed a combination of techniques, including temperature-dependent micro-photoluminescence, time-resolved photoluminescence, Raman spectroscopy, atomic force microscopy, and Kelvin probe force microscopy. X-ray diffraction and Raman spectroscopy confirmed that the samples studied possess the alpha-phase structure. The micro-photoluminescence spectrum consists of two bands, A and B, with band B almost completely disappearing at room temperature. Temperature-dependent photoluminescence and time-resolved measurements helped us to elucidate the nature of the observed bands. We find that peak A is associated with emission from interband transitions in In2Se3, while peak B is attributed to defect-related emission. Additionally, the photoluminescence decay times of In2Se3 flakes with varying thicknesses were determined. No significant changes were observed in the decay components as the thickness increased from 7 to 100 monolayers, suggesting that there are no qualitative changes in the band structure.

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.

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.

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.

Experiments on the growth of self-assembled InP/GaInP2 quantum dots in dielectric mask 0.1–1 μm apertures by MOVPE epitaxy have been carried out. A sequence of operations for the implementation of the lift-off lithography method is proposed and implemented. The possibility of obtaining apertures with 100 nm diameter and less is shown. Combination of thermally deposited SiO2 and wet etching is shown to produce minimal amount of nonradiative defects and results in a stable PL signal from single QDs in the aperture.

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.

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.

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.

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.

Thin GaTe films were grown by molecular beam epitaxy (MBE) on GaAs(001) substrates. X-ray powder diffraction confirmed the coexistence of the h- and m-GaTe phases in all grown layers. A quantitative correlation between the MBE growth conditions and the phase composition of the grown films is established, and the upper limit of the MBE growth temperature for thin GaTe/GaAs(001) films is experimentally determined. New data are presented confirming the defect-related origin of the broad emission line with an energy maximum of 1.45–1.46 eV, which dominates the low-temperature photoluminescence spectra of the grown GaTe/GaAs(001) layers.

We report non-contact local doping of a monolayer WSe2 transferred onto a piezoelectric substrate having surface potential wells (SPWs) induced by structural inhomogeneities. We used epitaxial GaN and InP/GaInP2 structures, in which there are SPWs ∼0.2 V deep and 0.1–2 μm in size. Using surface topography and potential scanning probe microscopy, as well as optical reflectance, photoluminescence, and Raman spectroscopy measurements, we observed strong enhancement of charged exciton emission and Raman intensity in the SPW regions of the monolayer WSe2, which indicate on piezoelectric doping at a level n ≥ 1012 cm−2 on a length scale ∼0.2–1 μm. Our results can be used to create electron/hole quantum puddles with anyon states in transition metal dichalcogenides, promising for the development of room temperature and magnetic-field-free fault-tolerant topological quantum computing.

This paper reports on molecular beam epitaxy of GaTe thin films grown on GaAs(001) and GaAs(1 1 1)B substrates at temperatures of TS = 450-550 degrees C under weak Te-rich conditions (Te/Ga approximate to 10-18) as well as studies of their structural and optical properties. The results obtained by transmission electron microscopy and X-ray diffraction techniques have established a correlation between the growth conditions and the GaTe polymorphic transition from hexagonal to monoclinic phase. The critical temperature of the polymorphic transition was found to be -540-550 degrees C. The monoclinic GaTe layers demonstrate an excellent excitonic emission that was confirmed by both micro-photoluminescence and time-resolved photoluminescence techniques. The strong anisotropy of the GaTe excitonic emission has been demonstrated.

The optical characteristics of vertical cylindrical micropillars with AlGaAs distributed Bragg reflectors and InAs/GaAs quantum dots, which are designed for the fabrication of single-photon sources, have been studied. The effect of parameters such as the inclination angle of sidewalls, partial oxidation of AlGaAs layers, and deviation of quantum dots from the central axis of a micropillar on the Purcell factor and the radiation extraction efficiency has been numerically simulated by the finite-difference time-domain method. The allowable ranges of the listed parameters have been determined for cylindrical vertical 920-nm micropillars. The comparison of the calculations performed with the refined refractive indices of the used materials at cryogenic temperatures with the measured characteristics of the fabricated micropillar structures has confirmed the adequacy of the used models.

This work presents the results of the growth, structural characterization and magneto-photoluminescence spectroscopy measurements of GaAs/AlGaAs/InP/GaInP 2 quantum well-quantum dot structures. We demonstrate that GaAs/AlGaAs QDs in these structures are formed above InP/GaInP 2 QDs. This allows us to measure the internal magnetic field in the InP/GaInP 2 QD by monitoring the Zeeman splitting of the excitonic transition in the GaAs/AlGaAs QD.

The characteristics of single photons emitted by columnar microstructures based on epitaxial microcavity heterostructures with distributed Bragg reflectors, which include self-organized InAs/GaAs quantum dots and have a comparatively low Q factor in the range of 2000–3000, have been studied. It has been shown that a state with a given spin configuration—exciton with a certain polarization or trion—can be initialized in a single quantum dot under the coherent resonant linearly polarized optical pumping by a π pulse. The measurement of two-photon interference by the Hong–Ou–Mandel scheme has demonstrated that the degree of indistinguishability of successively emitted single photons is 97 and 93% at a time delay of 2 and 250 ns, respectively. Prospects of application of such sources in optical quantum computing schemes have been discussed.