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.

Optical bound states in the continuum (BICs) provide a way to engineer very narrow resonances in photonic crystals. The extended interaction time in these systems is particularly promising for the enhancement of nonlinear optical processes and the development of the next generation of active optical devices. However, the achievable interaction strength is limited by the purely photonic character of optical BICs. Here, we mix the optical BIC in a photonic crystal slab with excitons in the atomically thin semiconductor MoSe 2 to form nonlinear exciton-polaritons with a Rabi splitting of 27 meV, exhibiting large interaction-induced spectral blueshifts. The asymptotic BIC-like suppression of polariton radiation into the far field toward the BIC wavevector, in combination with effective reduction of the excitonic disorder through motional narrowing, results in small polariton linewidths below 3 meV. Together with a strongly wavevector-dependent Q -factor, this provides for the enhancement and control of polariton–polariton interactions and the resulting nonlinear optical effects, paving the way toward tuneable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics.

We address dynamics of a low polariton (LP) system excited resonantly in a wide range of wave vectors by converging 2.5-ps-long Gaussian pulses. The spatial coherence in an LP system excited by incoherent light is found to form very slowly in the absence of an exciton reservoir, the coherence length at the delay time of 250 ps and T=2 K being less than 2.5 μm. The LP fluid excited by coherent linearly polarized pulses does not lose the inherited high spatial coherence and polarization and demonstrates dynamic compression to a condensate state at the LP branch bottom. In the LP fluid excited by elliptically polarized pulses the components with opposite circular polarizations are compressed almost independently of each other.

The transition from the delocalized to the localized state has been investigated in a quasi-onedimensional exciton-polariton system excited nonresonantly in GaAs-based microcavity wire with disordered potential. The photoexcited polariton condensate has been found to spread along the wire with а velocity exceeding 1 μm/ps. The propagation along the wire is provided by high energy polaritons. The LP localization length decreases with decreasing blue shift of LPs in the excited spot. The polariton condensate returns to the Bose glass state when the blue shift of the LP resonance at the excitation spot decreases below the critical level that depends on the potential disorder.

The dynamics of a pure low polariton (LP) system created by resonant broadband excitation in a wide range of wave vectors was investigated in a high-Q GaAs-based microcavity. The LP system is shown to inherit the high spatial coherence from the laser pulse and does not lose it during decay. As a result, its dynamics is well controlled by the spatial and momentum distributions of photons in the exciting pulse and described by the Gross-Pitaevskii equations. In particular, the purely dynamic formation of the highly populated coherent LP state was implemented at the LP band bottom in the cavity excited in a large spot by converging ps-long Gaussian laser pulses when the active region of the cavity is in front of its waist. The formed state is found to persist for several picoseconds until the LP-LP repulsion leads to the creation of high-energy LPs dissipating from the ground state with high velocities.

The possibility of the dynamic compression of a polariton system in a planar microcavity after the end of a resonant pump pulse with the formation of the ground state of a condensate on the bottom of the polariton band has been studied. The studies of dynamics of a resonantly excited polariton gas in the mean field approximation have shown that such condensate state can be formed purely dynamically at excitation by coherent convergent Gaussian light pulses with a large aperture if the active region of the cavity is ahead of the waist of the Gaussian beam. The spatial distribution of polaritons in the formed high-density condensate has sharp edges and large jumps of the violet shift and quasimomentum on these edges prevent its monotonic expansion despite the repulsive interaction between polaritons. For this reason, the further evolution of the condensate is primarily due to the discharge of particles from its boundary and is accompanied by a decrease rather than an increase in the size of the high-density region at the initial stage. Thus, the self-sustained regime of the dynamic compression of the polariton condensate can be maintained for a relatively long time.

We demonstrate a method for controling polarization of exciton-polariton condensate emission in a planar semiconductor microcavity based on modification of the electromagnetic mode structure in the microcavity. The modification is realized by fabrication of a chiral photonic crystal slab with partial etching of the upper Bragg mirror lowering the microcavity symmetry to C4. A degree of circular polarization as high as 70% has been demonstrated experimentally in the structure with optimized parameters without the use of a static magnetic field or birefringent wave plates.

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

Transitional processes accompanying switches between steady states in multistable cavity polariton systems are studied experimentally in a low-symmetry high-$Q$ microcavity under resonant optical excitation at normal incidence. We show that the establishment of a high-energy polariton state is influenced by an optical parametric oscillation. Therefore, the emission spectrum reveals the energy-separated signal-idler pairs in both spin components. Accordingly, the time dependencies of the polariton emission exhibit oscillations whose periods correspond to the spectral positions of the scattered states. Thus, the sharp transitions between the steady states of a no-equilibrium polariton condensate enable one to visualize the near-condensate eigenmodes and explore their spectral and spin properties depending on the condensate amplitude and average spin.

We report an experimental verification of the recent prediction that sharp transitions between steady states in multistable cavity-polariton systems are mediated by intermode parametric scattering that triggers the accumulation of energy and, hence, lowers the threshold at the cost of extending the transition latency period [S. S. Gavrilov, Phys. Rev. B 90, 205303 (2014)]. The time-resolved measurements are performed using a high-Q GaAs microcavity pumped slightly above the lower polariton level at normal incidence.

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.