The effect of a vertical electric field on photoluminescence of a MoSe2 monolayer encapsulated with hexagonal boron nitride is investigated. In the spectra, there is a quadratic shift of the photoluminescence lines of excitons and trions from the applied potential difference, as well as a change in their intensity. It is found that the magnitude of the Stark shift significantly exceeds the theoretically predicted one. It is found that the energy distance between the trion and exciton lines in the spectra varies with the magnitude of the external field, which is due to the dependence of the density of free charge carriers in the monolayer on the field. This effect made it possible to determine the density of free charge carriers in the monolayer, which varies with the field and lies in the range from 0.3–3.4⋅1012 cm–2.

Excitons in MoSe2 and WSe2 monolayers encapsulated with hexagonal boron nitride have been studied using optical reflectance spectroscopy. The ground and excited states of A- and B-excitons have been studied at temperatures from liquid helium to room temperature. The lines of excitons A: 1s , B: 1s and their excited states А: 2s , А: 3s , and В: 2s are clearly observed in the reflectance spectrum. The observed line shapes of the reflection spectrum of transition metal dichalcogenide monolayers depend on the thickness of the hexagonal boron nitride layers used in the structure and are in good agreement with the numerical simulation using the transfer matrix method. For the first time, the values of the reduced masses of B-excitons have been obtained from experimental data and the performed calculations of the exciton binding energy.

The effect of a vertical electric field on the photoluminescence of a MoSe2 monolayer encapsulated with hexagonal boron nitride was investigated. The spectra showed a quadratic (in applied potential difference) shift of the photoluminescence lines of excitons and trions, as well as changes in their intensity. It was found that the magnitude of the Stark shift significantly exceeds the theoretically predicted one. It was determined that the energy distance between the trion and exciton lines in the spectra varies with the external field, which is due to the dependence of the density of free charge carriers in the monolayer on the field. This effect made it possible to determine the density of free charge carriers in the monolayer, which varies with the field and lies in the range (0.3–3.15) × 1012 cm–2.

The dependence of the width of exciton and trion photoluminescence lines in MoSe2 monolayers on the thickness of hexagonal boron nitride encapsulating layers has been investigated. The possibility of variation of the exciton photoluminescence linewidth due to the interaction of excitons with the modes of resonator cavities made up of a silicon substrate and a boron nitride top layer has been checked. This interaction may significantly change the photoluminescence linewidth owing to the Parcell effect. Measurements taken of samples with different thicknesses of the bottom and top boron nitride layers have not revealed any influence of the Parcell effect on the linewidth. It has turned out however that the linewidth narrows by several times with increasing boron nitride bottom layer thickness from 10 to 100 nm and reaches 2 meV at a thickness of 100 nm. Supposedly, such narrowing of the photoluminescence line is associated with a decrease in the density of submicron bubbles, which takes place at longitudinal stress relaxation in the thicker layer of boron nitride.

Photoluminescence spectra of excitons and trions in MoSe2 monolayers encapsulated with hexagonal boron nitride under nonresonant laser excitation were studied. When the size of the laser excitation spot decreases from 8 to 3 nm, individual peaks with a line width of ~2 meV emerge in the photoluminescence spectra, which were unresolved with a larger spot. Studies of the sample surface using a scanning electron microscope revealed the existence of a large number of features at the interfaces of structures with characteristic sizes ranging from submicron to micron and more. It was expected that the lines appearing in the spectrum at small excitation spot sizes were associated with similar submicron inhomogeneities. A study of a specially made heterostructure covered by a metal mask with holes of 1.6 microns in diameter confirmed this assumption.

A study is performed of the photoluminescence spectra of excitons and trions in MoSe2 monolayers encapsulated with hexagonal boron nitride upon nonresonant laser excitation. When the size of the laser spot of excitation is reduced from 8 to 3 nm, individual peaks with line widths of ~2 meV start to resolve in the photoluminescence spectra. Such peaks are unresolved when there is a larger spot. Studies of the surface of the sample using a scanning electron microscope reveal a great many features at the interfaces of structures that have characteristic sizes ranging from submicrometer to micrometer and larger. It is expected the lines that appear in the spectrum with small spots of excitation are associated with similar submicrometer inhomogeneities. Studies of a specially fabricated heterostructure that has a metal mask with holes 1.6 μm in diameter sputtered onto its surface confirm this assumption.

Doping II–VI semiconductors and low-dimensional structures based on them by manganese leads to the effective quenching of electroluminescence and photoluminescence under the condition that the electron excitation energy of the crystal exceeds the energy of the intracenter transition of the Mn2+ ion EMn ≈ 2.1 eV. Quenching implies effective energy transfer from a photoexcited crystal to Mn2+ ions. Three mechanisms of such nonradiative energy transfer are possible, notably, the dipole–dipole mechanism, the exchange mechanism, and a mechanism related to the latter, which is associated with sp–d mixing. Although it is thought that the dipole–dipole mechanism is not particularly efficient because of the forbidden intracenter transition for Mn2+, while the dominant mechanism is the spin-dependent exchange mechanism, not all experimental facts accumulated to date confirm this conclusion. Two experimental approaches that make it possible to reveal the dominant energy-transfer mechanism to Mn2+ ions and evaluate the partial contributions of various mechanisms are considered in the article. One of these approaches is associated with optically detected magnetic resonance at single semimagnetic quantum dots, and the second one is associated with plasmon enhancement of the energy transfer to Mn2+ ions by means of the dipole–dipole interaction.

Doping of group II-VI semiconductors and low-dimensional structures based on them with manganese leads to effective quenching of electro-and photoluminescence provided that the electron excitation energy of the crystal exceeds the energy of the intracenter transition of Mn2+ EMn и 2.1 eV. Quenching involves efficient energy transfer from the photoexcited crystal to Mn2 + ions. There are three possible mechanisms of this non-radiative energy transfer: dipole-dipole, exchange and related mechanism associated with sp-d mixing. Although the dipole-dipole mechanism is thought to be ineffective due to the forbidden intracenter transition in Mn2+, and the spin-dependent exchange mechanism is dominant, not all experimental facts accumulated to date supports this conclusion. The article discusses two experimental approaches to identify the dominant mechanism of energy transfer to Mn2 + ions and to evaluate the partial contributions of the mechanisms. One of these approaches is related to optically detectable magnetic resonance at single diluted magnetic semiconductor quantum dots (QD), the second - to plasmon amplification of energy transfer to Mn2 + ions by means of dipole-dipole interaction.

The photoluminescence of a nonequilibrium polariton condensate in cylindrical and rectangular micropillars etched on the surface of a high- Q GaAs microcavity is investigated in magnetic fields of up to 12 T. The measurements are carried out under different levels of nonresonant optical pumping with nanosecond laser pulses for a wide range of cavity detuning. As far as nonresonant excitation produces a high density of excitons in a reservoir, it should be expected that the exciton–polariton interaction, which depends on the pump level, has a considerable effect on the Zeeman splitting and polarization of the condensate. However, measurements of the Zeeman splitting and polarization in high magnetic fields demonstrate that only minor changes take place up to the highest available pump levels. This means that, in the case under study, the effect of exciton–polariton interaction on the polariton system is insignificant. At the same time, the data obtained provide an estimate for the exciton density in the reservoir. In contrast to cylindrical micropillars, the photoluminescence of the condensate in rectangular micropillars consists of two perpendicularly linearly polarized lines which retain a high degree of linear polarization even in a field as high as 12 T. The Zeeman splitting in this case is nearly independent of the pump power. The degrees of both circular and linear polarization change with pump power, but these changes are noticeably smaller than the ones predicted theoretically. This indicates that the system of polaritons in micropillars deviates considerably from thermodynamic equilibrium.

We analyze the photoluminescence of a nonequilibrium polariton condensate in cylindrical micropillars etched on the surface of a high-Q GaAs microcavity in a wide range of detunings in a magnetic field up to 12 T for various levels of nonresonant laser pumping by nanosecond pulses. With such a method of excitation, a considerable effect of the interaction of the reservoir of photoexcited excitons with the condensate on the Zeeman splitting of the polariton condensate levels can be expected, which can lead to a decrease in its value and even to sign reversal. However, the measurements of photoluminescence in a wide range of optical excitation densities show that Zeeman splitting weakly depends on the optical pumping (its variation does not exceed 15% of the splitting in a field of 12 T). The estimation of the exciton density in the reservoir based on these data gives a value lower than 108 cm–2. In addition, a noticeable decrease (by a factor of about 1.8) in the polariton condensation threshold in a magnetic field is detected.

Фотолюминесценция неравновесного конденсата поляритонов в микростолбиках цилиндрической и прямоугольной форм, вытравленных на поверхности высокодобротного микрорезонатора на основе GaAs, исследована в магнитном поле до 12 Тл. Измерения выполнены при различных уровнях нерезонансной импульсной оптической накачки в широком диапазоне значений расстройки резонатора. Нерезонансное возбуждение создает значительную плотность экситонов в резервуаре, что позволяет ожидать существенного влияния экситон-поляритонного взаимодействия, которое зависит от плотности накачки, на величину зеемановского расщепления и поляризацию. Измерения показали лишь незначительное изменение зеемановского расщепления и поляризации в сильных магнитных полях при максимально достижимом уровне накачки; следовательно, влияние экситон-поляритонного взаимодействия на поляритонную систему в нашем случае несущественно. Вместе с тем полученные данные позволили оценить плотность экситонов в резервуаре. В отличие от цилиндрических микростолбиков, фотолюминесценция конденсата из прямоугольных микростолбиков состоит из двух взаимно перпендикулярно линейно поляризованных линий, которые сохраняют высокую степень циркулярной поляризации даже в поле 12 Тл. Зеемановское расщепление в этом случае фактически не зависит от накачки, тогда как степени линейной и циркулярной поляризаций изменяются с накачкой, хотя эти изменения заметно меньше предсказанных теорией. Это указывает на значительное отклонение системы поляритонов в микростолбиках от термодинамически равновесной. DOI: 10.21883/FTP.2017.12.45172.35

В планарных GaAs-микрорезонаторах в магнитном поле до 5 Тл, перпендикулярном плоскости роста структур, в условиях резонансной импульсной накачки в точку, близкую к точке перегиба нижней дисперсионной кривой, наблюдается зеемановское расщепление спиновых подуровней конденсата поляритонов. Оно сопровождается значительным изменением степени циркулярной поляризации и коррелятора 2-го порядка g2(0). Оказалось, что коррелятор отличается для расщепленных в магнитном поле спиновых подуровней поляритонного конденсата. В частности, измерения коррелятора свидетельствуют о различии в порогах конденсации для спиновых подуровней. Изначально отличающиеся в отсутствие поля значения коррелятора растут, достигая максимального значения, а затем уменьшаются и сравниваются между собой для разных поляризаций в поле 5 Тл.

In planar GaAs microcavities in a magnetic field up to 5 T perpendicular to the structure growth plane, under conditions of resonant pulsed pumping to a point close to the inflection point of the lower dispersion curve, Zeeman splitting of the spin sublevels of the polariton condensate is observed. This is accompanied by a significant change in the degree of circular polarization and the second-order correlator g2(0). It is found that the correlator is different for the spin sublevels of the polariton condensate, split in a magnetic field. In particular, correlator measurements indicate different condensation thresholds for the spin sublevels. The correlator values initially differing in terms of the absence of a field increase, reach a maximum, and then decrease and become equal for different polarizations in a field of 5 T.

The mechanisms of the nonradiative recombination of excitons in neutral and charged quantum dots based on II–VI semimagnetic semiconductors are investigated. It is shown that, along with the dipole–dipole and direct-exchange mechanisms, there is one more mechanism referred to as the indirect-exchange mechanism and related to sp–d mixing. The selection rules for nonradiative recombination by exchange mechanisms are subsequently derived. The dependence of the efficiency of all recombination mechanisms on the quantum-dot size is studied. The experimentally observed growth in the intracenter photoluminescence intensity with decreasing size of dots and nanocrystals is accounted for. Methods for experimental determination of the contributions of different mechanisms to nonradiative recombination are discussed.

Despite their name polariton lasers do not rely on stimulated emission of cavity photons. The less stringent threshold conditions are the cause that bosonic polariton lasers can outperform standard lasers in terms of their threshold currents. The part-light and part-matter quasiparticles called polaritons, can undergo a condensation process into a common energy state. The radiated light from such a system shares many similarities with the light emitted from a conventional photon laser, even though the decay of the polaritons out of the finite lifetime cavity is a spontaneous process. We discuss properties of polariton condensates in GaAs based microcavities. The system’s response to an external magnetic field is used as a reliable tool to distinguish between polariton laser and conventional photon laser. In particular, we will discuss the realization of an electrically pumped polariton laser, which manifests a major step towards the exploitation of polaritonic devices in the real world.

Photoluminescence of ZnMnSe/ZnSSe multiple quantum wells under a bandgap continuous wave and fs-pulsed excitations is measured in magnetic fields up to 10 T in Faraday geometry at temperatures within the range of 1.6-20 K. The measurements reveal two dominant lines in the spectra and LO-phonon replicas of the lower-energy line. The photoluminescence and time-resolved studies show dramatically different behaviour of the lines. Analysis of their properties reveals that they correspond to recombination of indirect localized excitons and indirect acceptor-bound excitons (A0X). Crossing of exciton and A0X lines because of the difference in magnitudes of their Zeeman shifts is observed. Analysis of LO-phonon replicas of photoluminescence lines provides additional evidence for strong carrier localization bound to A0X. A model of phonon-assisted recombination of indirect acceptor-bound excitons is proposed. The fitting of photoluminescence lines with this model gives the Huang-Rhys factor S≃0.25 for A0X and the hole localization size ah≃30 Å. Contrary to expectations the exciton magnetic polaron effect is hardly observed in these structures.

Polariton lasers do not rely on stimulated emission of cavity photons, which sets stringent conditions on the threshold current in a conventional laser. Indeed, it has been demonstrated in optically pumped systems, that bosonic polariton lasers can outperform standard lasers in terms of their threshold power. The polaritons, which are part light and part matter quasiparticles, can undergo a condensation process into a common energy state. The radiated light from such a system shares many similarities with the light emitted from a conventional photon laser, even though the decay of the polaritons is a spontaneous process. We discuss properties of polariton lasers and condensates in GaAs based microcavities. Special emphasis is given to the system’s response to an applied magnetic field. We introduce the magnetic field interactions as a reliable tool to distinguish a polariton laser from a conventional photon laser device. In particular, we will discuss the first successful realization of an electrically pumped polariton laser, which marks a promising step towards the exploitation of polaritonic devices in the real world. We believe that our work can be extended to devices operated at room temperature by transferring the technology to large bandgap semiconductors, or even to GaAs samples with a modified layer design.

We observe a strong variation of the Zeeman splitting of exciton polaritons in microcavities when switching between the linear regime, the polariton lasing, and photon lasing regimes. In the polariton lasing regime the sign of Zeeman splitting changes compared to the linear regime, while in the photon lasing regime the splitting vanishes. We additionally observe an increase of the diamagnetic shift in the polariton lasing regime. These effects are explained in terms of the nonequilibrium "spin Meissner effect."