We report on the origin of energy-shifts in organic polariton condensates. The localised nature of Frenkel excitons in molecular semiconductors precludes interparticle Coulomb exchange interactionsthe latter being the dominant mechanism for blueshifts in inorganic semiconductor microcavities that bear Wannier-Mott excitons. We examine the contribution of optically induced change of the intracavity non-linear refractive index, gain induced frequency-pulling and quenching of the Rabi splitting, as well as the role of polariton-exciton and polariton-polariton scattering in the energy-shift of the polariton mode at condensation threshold in strongly coupled molecular dye microcavities. We conclude that blueshifts in organic polariton condensates arise from the interplay of the saturation of molecular optical transitions and intermolecular energy migration. Our model predicts the commonly observed step-wise increase of both the emission energy and degree of linear polarisation at polariton condensation threshold.

Inorganic cesium lead halide perovskite nanowires, generating laser emission in the broad spectral range at room temperature and low threshold, have become powerful tools for the cutting-edge applications in the optoelectronics and nanophotonics. However, to achieve high-quality nanowires with the outstanding optical properties, it was necessary to employ long-lasting and costly methods of their synthesis, as well as postsynthetic separation and transfer procedures that are not convenient for large-scale production. Here we report a novel approach to fabricate high-quality CsPbBr3 nanolasers obtained by rapid precipitation from dimethyl sulfoxide solution sprayed onto hydrophobic substrates at ambient conditions. The synthesis technique allows producing the well-separated nanowires with a broad size distribution of 2–50 μm in 5–7 min, being the fastest method to the best of our knowledge. The formation of nanowires occurs via ligand-assisted reprecipitation triggered by intermolecular proton transfer from (CH3)2­CHOH to H2O in the presence of a minor amount of water. The XRD patterns confirm an orthorhombic crystal structure of the as-grown CsPbBr3 single nanowires. Scanning electron microscopy images reveal their regular shape and truncated pyramidal end facets, while high-resolution transmission electron microscopy ones demonstrate their single-crystal structure. The lifetime of excitonic emission of the nanowires is found to be 7 ns, when the samples are excited with energy below the lasing threshold, manifesting the low concentration of defect states. The measured nanolasers of different lengths exhibit pronounced stimulated emission above 13 μJ cm–2 excitation threshold with quality factor Q = 1017–6166. Their high performance is assumed to be related to their monocrystalline structure, low concentration of defect states, and improved end facet reflectivity.

GaAs-based microcavities. [2] However, the applications of III-V and II-VI inorganic semiconductors in polaritonics are relatively limited due to the challenging growth techniques required to create wide bandgap semiconductors together with the necessity of using cryogenic temperatures to create Wannier-Mott excitons. [3,4] Organic semiconductors however comprise the broadest class of strongly coupled materials developed to date and include molecular dyes, crystalline organic molecules, oligofluorenes, and conjugated polymers. [5][6][7][8][9][10][11] Owing to the high quantum yield, large dipole moment of optical transitions and high-binding energy of excitons, organic semiconductors have permitted the physics and applications of polaritonics to be explored at room temperature. [12,13] Polariton lasing is one of the most distinctive nonlinear phenomena related to the collective behavior of exciton polaritons. In contrast to conventional photon lasers, a polariton laser does not necessitate the electronic inversion of population, but is instead driven by a stimulated relaxation to a coherent state during the process of condensation to the ground polariton state. This process has allowed polariton lasers to exhibit significantly lower thresholds compared to photon lasers that have been fabricated using the same device configuration. [14,15] Recently, we demonstrated polariton lasing in the yellow part of the spectrum in an organic microcavity containing the molecular dye bromine-substituted boron-dipyrromethene (BODIPY-Br). [5] In the present paper, we explore the incorporation of another molecular dye of the BODIPY family, namely BODIPY-G1 fluorescent dye, into a microcavity and show that by incorporating a wedged cavity-layer configuration, we can achieve strong coupling over a broad range of exciton-photon detuning conditions. Such structures allow us to controllably access different cavity lengths and thus select the energy of the ground polariton state. We then use this approach to provide evidence of polariton lasing over a broad range of wavelengths (>30 nm) utilizing a single material system.

We have developed a simplified approach to fabricate high-reflectivity mirrors suitable for applications in a strongly-coupled organic-semiconductor microcavity. Such mirrors are based on a small number of quarter-wave dielectric pairs deposited on top of a thick silver film that combine high reflectivity and broad reflectivity bandwidth. Using this approach, we construct a microcavity containing the molecular dye BODIPY-Br in which the bottom cavity mirror is composed of a silver layer coated by a SiO2 and a Nb2O5 film, and show that this cavity undergoes polariton condensation at a similar threshold to that of a control cavity whose bottom mirror consists of ten quarter-wave dielectric pairs. We observe, however, that the roughness of the hybrid mirror—caused by limited adhesion between the silver and the dielectric pair—apparently prevents complete collapse of the population to the ground polariton state above the condensation threshold.

A simple experimental method of light emitting diode (LED) injection efficiency (IE) determination was suggested. IE and internal quantum efficiency (IQE) calculation is an actual and difficult problem in LED science. In this paper IE and IQE of blue LEDs were determined separately. The method is based on electroluminescence data fitting by the modified rate equation model. Efficiency droop caused by Auger recombination and poor injection were taken into account. Only one reasonable assumption was accepted during the calculations: IE tends to 1 at low current densities.

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ZnCdSe/ZnSSe MQW structures for an electron beam pumped VCSEL with resonant periodic gain were grown by MOVPE at 425-470 °C. Strong contamination of the structure by Ga from a GaAs substrate was found and its effect on the growth rate and photoluminescence characteristics was studied. A protective thin ZnSSe layer deposited at lower temperature (350 °C) or thin layers of ZnS and ZnS/ZnSSe SL grown at temperature 425-470 °C prevent Ga penetration and allowed improving the quality and periodicity of the structure. Based on the grown MQW structure, green VCSEL was fabricated. Lasing at 542 nm with 3 W output power was achieved at RT and 40 keV. The threshold was as low as 8 A/cm 2 .Introduction E-beam pumped vertical cavity surface emitting laser (VCSEL) is an efficient high power emission source perspective for display applications [1]. Output power up to 10 W at efficiency of 12 % in red range (645 nm) was achieved in a VCSEL with a resonant periodic gain using MOVPE grown GaInP/AlGaInP structures [2]. Lasing in green range was also achieved in VCSELs on MOVPE grown ZnCdSe/ZnSSe MQW structures [3,4]. But the penetration of Ga into the structure in a considerable distance from the GaAs/ZnSSe interface leads to the decrease of the nonequilibrium charge carrier transport from barrier layers into the QWs and to the increase of the laser threshold. Moreover the growth rate of the structure depends strongly on Ga content and this makes it difficult to obtain a strictly periodic structure. To avoid these problems, we removed the Ga-doped part of the MQW structure by chemical etching after removing the GaAs substrate [3]. However this method does not allow us to control the QW position inside the optical cavity. To achieve high efficiency of the laser it is very important to place QWs in the antinodes of the cavity mode being at gain maxima. Here we study other approaches to preventing Ga-doping of the long period ZnCdSe/ZnSSe MQW structures at the initial stage of epitaxy. By e-beam pumping, high energy electrons must pass through one of the cavity mirrors. This limits the mirror thickness which results in low reflectance and high cavity losses. Hence, to compensate all losses the number of QWs in the cavity is to be more than 20. To decrease the laser threshold, it is necessary to increase the thickness of the barrier layers separating the QWs up to the value comparable with the diffusion length of nonequilibrium carriers. This requirement together with positioning of QWs in the antinodes of the cavity mode defines the optimal thickness for ZnSSe barriers as about 0.2 µm.