Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton–bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing.

The experimental study of edge states in atomically-thin layered materials remains a challenge due to the difficult control of the geometry of the sample terminations, the stability of dangling bonds and the need to measure local properties. In the case of graphene, localised edge modes have been predicted in zig-zag and bearded edges, characterised by flat dispersions connecting the Dirac points. Polaritons in semiconductor microcavities have recently emerged as an extraordinary photonic platform to emulate 1D and 2D Hamiltonians, allowing the direct visualization of the wavefunctions in both real-and momentum-space as well as of the energy dispersion of eigenstates via photoluminescence experiments. Here we report on the observation of edge states in a honeycomb lattice of coupled micropillars. The lowest two bands of this structure arise from the coupling of the lowest energy modes of the micropillars, and emulate the π and π* bands of graphene. We show the momentum space dispersion of the edge states associated to the zig-zag and bearded edges, holding unidimensional quasi-flat bands. Additionally, we evaluate polarisation effects characteristic of polaritons on the properties of these states.

We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially- and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we measure a continuous red shift of the emission, as excitons propagate away from the excitation spot. This shift corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved micro-photoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons. We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 microns is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the exciton emission pattern on the scale up to 100 microns. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.Comment: 11 pages, 8 figure

Optical control of exciton fluxes is realized for indirect excitons in a crossed-ramp excitonic device. The device demonstrates experimental proof of principle for all-optical excitonic transistors with a high ratio between the excitonic signal at the optical drain and the excitonic signal due to the optical gate. The device also demonstrates experimental proof of principle for all-optical excitonic routers.

Indirect excitons in coupled quantum wells are long-living quasi-particles, explored in the studies of collective quantum states. We demonstrate, that despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons, but also one of free electrons by two orders of magnitude.

In this work, we investigate the temperature dependence of the photoluminescence decay and integrated photoluminescence of oleic acid capped PbS quantum dots with diameters ranging from 2.3 to 3.5 nm over a broad temperature range (6–290 K). All the investigated samples exhibit similar behavior, consisting of three different temperature regimes. The low-temperature regime (<180 K) is characterized by an increase in the average decay rate and a decrease in integrated photoluminescence. The intermediate regime (∼180–250 K) is described by an enhancement in the photoluminescence intensity and a decrease in the average decay rate. The high-temperature regime (>250 K) is governed by quenched photoluminescence intensity and acceleration in the average lifetimes. We propose a three-level system, composed of bright, dark, and surface states, which describes the observed photoluminescence dynamics of oleic acid capped PbS QDs.

We report on spatially-and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.An indirect exciton (IX) in a semiconductor quantum well (QW) structure is composed of an electron and a hole confined to spatially separated QW layers. IXs were realized in wide single quantum wells (WSQW) [1][2][3][4] and in coupled quantum wells (CQW) [5][6][7][8] [2-4, 6, 7]. Their long lifetimes allow IXs to travel over large distances before recombination, providing the opportunity to study exciton transport by optical imaging [9][10][11][12][13][14][15] and explore excitonic circuit devices based on exciton transport, see [16] and references therein.Materials with a high IX binding energy allow extending the operation of the excitonic devices to high temperatures [17][18][19]. Furthermore, such materials can allow the realization of high-temperature coherent states of IXs [19]. These properties make materials with robust IXs particularly interesting. However, so far, studies of IX transport mainly concerned GaAs-based CQW. In this paper, we probe transport of IXs in ZnO/MgZnO WSQW structures. IXs in these structures are much more robust than in GaAs structures: their binding energy ∼ 30 meV [4] is considerably higher than that in GaAs/AlGaAs and GaAs/AlAs CQW (∼ 4 and ∼ 10 meV, respectively [7,20]). The binding energy of IXs is smaller than that of excitons in bulk ZnO (∼ 60 meV), however it is large enough to make the IXs stable at room temperature. At the same time, the measurements reported in this work show that transport lengths of IXs in WSQW ZnO structures reach ∼ 4 µm. In comparison, for excitons in bulk ZnO and direct excitons in ZnO structures, transport lengths are within ∼ 0.2 µm [21,22].In this work, we study polar and semipolar ZnO/MgZnO QW structures. The samples were grown by molecular beam epitaxy as in Refs. [4,23] Fig. 1a. The charges on the interfaces between ZnO and MgZnO result in a built-in electric field in the structure, which is stronger for polar samples [4,23]. The built-in electric field pulls the electron and the hole toward opposite borders of the QW, resulting in the spatial separation required for an IX.

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