Littérature scientifique sur le sujet « Dark-Bright exciton splitting »

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright–dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

Lead-halide perovskite nanocrystals (NCs) are attractive nano-building blocks for photovoltaics and optoelectronic devices as well as quantum light sources. Such developments require a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. In this review, we give an overview of recent magneto-optical spectroscopic studies revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of their inhomogeneities in morphology and crystal structure. We highlight the prominent role of the electron-hole exchange interaction in the order and splitting of the bright triplet and dark singlet exciton sublevels and discuss the effects of size, shape anisotropy and dielectric screening on the fine structure. The spectral and temporal manifestations of thermal mixing between bright and dark excitons allows extracting the specific nature and strength of the exciton–phonon coupling, which provides an explanation for their remarkably bright photoluminescence at low temperature although the ground exciton state is optically inactive. We also decipher the spectroscopic characteristics of other charge complexes whose recombination contributes to photoluminescence. With the rich knowledge gained from these experiments, we provide some perspectives on perovskite NCs as quantum light sources.

Two-dimensional (2D) metal halide perovskites are natural quantum wells which consist of low bandgap metal-halide slabs, surrounded by organic spacers barriers. The quantum and dielectric confinements provided by the organic part lead to the extreme exciton binding energy which results in a huge enhancement of exciton fine structure in this material system. This makes 2D perovskites a fascinating playground for fundamental excitonic physics studies. In this review, we summarize the current understanding and quantification of the exciton fine structure in 2D perovskites. We discuss what is the role of exciton fine structure in the optical response of 2D perovskites and how it challenges our understanding of this fundamental excitation. Finally, we highlight some controversy related to particularly large bright-dark exciton states splitting and high efficiency of light emission from these materials. This can result from the unique synergy of excitonic and mechanical properties of 2D perovskites crystals.

The tubular nature of single-walled carbon nanotube (SWCNT) crystals allows them to exhibit non-intuitive quantum phenomena when threaded by a magnetic flux, which breaks the time reversal symmetry and adds an Aharonov-Bohm phase to the circumferential boundary conditions on the electronic wave function. We demonstrate that such a symmetry-breaking magnetic field can dramatically "brighten" an optically-inactive, or dark, exciton state at low temperature. This phenomenon, magnetic brightening, can be understood as a consequence of interplay between the strong intervalley Coulomb mixing and field-induced lifting of valley degeneracy. Most recently, we made the direct observation of the dark excitonic state in individual SWCNTs using low-temperature micro-photoluminescence (PL) and and verified the importance of a parallel, tube-threading magentic field with ensemble spectroscopy. For micro-PL, a magnetic field up to 5 T, applied along the nanotube axis, brightened the dark state, leading to the emergence of a new emission peak. The peak rapidly grew in intensity with increasing field at the expense of the originally-dominant bright exciton peak and finally became dominant at fields > 3 T. The directly measured dark-bright splitting values were 1-4 meV for tube diameters 1.0-1.3 nm. For ensemble PL, we used fields up to 55 T in two collection geometries to demonstrate the importance of the tube-threading component. These experiments have provided one of the most critical tests for recently-proposed theories of 1-D excitons taking into account the strong 1-D Coulomb interactions and unique band structure on an equal footing.

Owing to their flexible chemical synthesis and the ability to shape nanostructures, lead halide perovskites have emerged as high potential materials for optoelectronic devices. Here, we investigate the excitonic band edge states and their energies levels in colloidal inorganic lead halide nanoplatelets, particularly the influence of dielectric effects, in a thin quasi-2D system. We use a model including band offset and dielectric confinements in the presence of Coulomb interaction. Short- and long-range contributions, modified by dielectric effects, are also derived, leading to a full modelization of the exciton fine structure, in cubic, tetragonal and orthorhombic phases. The fine splitting structure, including dark and bright excitonic states, is discussed and compared to recent experimental results, showing the importance of both confinement and dielectric contributions.

Strain-free GaAs quantum dots (QDs) are fabricated by filling droplet-etched nanoholes in AlGaAs. Using a template of nominally identical nanoholes, the QD size is precisely controlled by the thickness of the GaAs filling layer. Atomic force microscopy indicates that the QDs have a cone-shell shape. From single-dot photoluminescence measurements, values of the exciton emission energy (1.58...1.82 eV), the exciton–biexciton splitting (1.8...2.5 meV), the exciton radiative lifetime of bright (0.37...0.58 ns) and dark (3.2...6.7 ns) states, the quantum efficiency (0.89...0.92), and the oscillator strength (11.2...17.1) are determined as a function of the dot size. The experimental data are interpreted by comparison with an atomistic model.

In this work, we present a comprehensive theoretical and computational investigation of exciton fine structures of WSe2-monolayers, one of the best-known two-dimensional (2D) transition-metal dichalcogenides (TMDs), in various dielectric-layered environments by solving the first-principles-based Bethe–Salpeter equation. While the physical and electronic properties of atomically thin nanomaterials are normally sensitive to the variation of the surrounding environment, our studies reveal that the influence of the dielectric environment on the exciton fine structures of TMD-MLs is surprisingly limited. We point out that the non-locality of Coulomb screening plays a key role in suppressing the dielectric environment factor and drastically shrinking the fine structure splittings between bright exciton (BX) states and various dark-exciton (DX) states of TMD-MLs. The intriguing non-locality of screening in 2D materials can be manifested by the measurable non-linear correlation between the BX-DX splittings and exciton-binding energies by varying the surrounding dielectric environments. The revealed environment-insensitive exciton fine structures of TMD-ML suggest the robustness of prospective dark-exciton-based optoelectronics against the inevitable variation of the inhomogeneous dielectric environment.

The far- and near-field chirality properties are usually characterized by circular dichroism (CD) and optical chirality (OC), respectively. As a light–matter interaction for the hybrid states consisting of plasmons and excitons, the strong coupling interactions can affect the original chiral electromagnetic modes. However, there are few works on this influence process, which prevents an in-depth understanding of chirality. Here, we theoretically investigate both the far-field and near-field characteristics of the chiral plasmonic gold helicoid nanoparticle (GHNP) to explore the chirality mechanism further. We found that the electromagnetic field distribution of GHNP consists of one dark mode and two bright modes. The dark mode is observed more clearly in CD than in extinction spectra. Two bright modes can strongly couple with excitons respectively, which is confirmed by the anticrossing behavior and mode splitting exhibited in the extinction and CD spectra. We also analyzed the near-field OC distribution of the GHNP hybrid system and obtained the chiral responses as well as the spectral correspondence between OC and CD. Furthermore, although the strong coupling interaction changes the energy levels, resulting in mode splitting, the chiral hotspot distributions of both the upper polariton branch and lower polariton branch are consistent with the original bright mode in OC maps. Our findings provide guidance for the design of structures with strong chiral responses and enhance the comprehension of chiral strong coupling systems.

Based on the atomistic tight-binding theory (TB) and a configuration interaction (CI) description, the electron-hole exchange interaction in the morphological transformation of CdSe/ZnS core/shell nanodisk to CdSe/ZnS core/shell nanorod is described with the aim of understanding the impact of the structural shapes on the change of the electron-hole exchange interaction. Normally, the ground hole states confined in typical CdSe/ZnS core/shell nanocrystals are of heavy hole-like character. However, the atomistic tight-binding theory shows that a transition of the ground hole states from heavy hole-like to light hole-like contribution with the increasing aspect ratios of the CdSe/ZnS core/shell nanostructures is recognized. According to the change in the ground-state hole characters, the electron-hole exchange interaction is also significantly altered. To do so, optical band gaps, ground-state electron character, ground-state hole character, oscillation strengths, ground-state coulomb energies, ground-state exchange energies, and dark-bright (DB) excitonic splitting (stoke shift) are numerically demonstrated. These atomistic computations obviously show the sensitivity with the aspect ratios. Finally, the alteration in the hole character has a prominent effect on dark-bright (DB) excitonic splitting.

Ces dernières années, les matériaux pérovskites bidimensionnels (2D) ont attiré une attention considérable en raison de leurs propriétés électroniques et optiques uniques et excellentes, qui en font un semi-conducteur extrêmement prometteur pour les applications d'émission de lumière et d'affichage. En outre, la pérovskite non magnétique peut devenir un semi-conducteur magnétique en incorporant des impuretés magnétiques dans les réseaux de la pérovskite hôte pour introduire des propriétés magnétiques. La coexistence d'excellentes propriétés optoélectroniques et magnétiques fait de la pérovskite 2D semi-magnétique un matériau très prometteur pour les dispositifs semi-conducteurs opto-spintronique pour le traitement de l'information et les communications.Dans cette thèse, nous explorons les propriétés électroniques et optiques des pérovskites 2D via la spectroscopie magnéto-optique. Nous commençons par effectuer des mesures de magnéto-photoluminescence (PL) et de magnéto-transmission sur des nanoplaquettes à base de CsPbBr3 avec une épaisseur différente de la dalle de plomb-halogénure, allant de 2 à 4 couches de plan octaédrique de plomb-halogénure. En appliquant des champs magnétiques dans le plan jusqu'à 65 T, l'état excitonique sombre optiquement inactif est éclairci. Cette approche nous permet d'observer directement une amélioration de l'émission PL du côté basse énergie du spectre PL, ce qui indique que l'état excitonique sombre optiquement inactif est l'état le plus bas dans ces nanoplaquettes. De plus, en combinant nos résultats de magnéto-PL et de magnéto-transmission avec les prédictions théoriques de la division de la structure fine de l'exciton, nous déterminons avec précision la division d'énergie entre les excitons sombres et brillants. Nous démontrons qu'en effet, la division des excitons sombres et brillants augmente avec la diminution des couches du plan octaédrique du plomb-halogénure. Nous démontrons également que l'émission efficace de ces nanoplatelles est due à un effet de goulot d'étranglement phononique, qui réduit considérablement la relaxation des excitons photoexcités vers l'état sombre optiquement inactif.Enfin, nous étudions les propriétés électroniques de la perovskite 2D (PEA)2PbI4 dopée au Mn par spectroscopie de magnéto-transmission pour différentes fractions molaires de Mn. Nous constatons que le facteur g de Lande des excitons peut être contrôlé par la concentration de Mn incorporé. Avec l'augmentation de la concentration x de Mn de 0 à 2%, le facteur g augmente, ce que nous attribuons à l'interaction d'échange sp-d entre les excitons de bord de bande et les spins hébergés dans les ions Mn. Si la concentration en Mn est encore augmentée, jusqu'à 5%, le facteur g des excitons diminue. Cette contre-tendance anormale est attribuée aux interactions Mn-Mn, qui résultent en un couplage anti-ferromagnétique efficace
Abstract: In recent years, two-dimensional (2D) perovskite materials have attracted considerable attention duo to their unique and excellent electronic and optical properties, which make them an extremely promising semiconductor for light-emitting and display applications. Furthermore, the nonmagnetic perovskite can be semi magnetic semiconductor by incorporating magnetic impurities into lattices of the host perovskite to introduce magnetic properties. The coexistence of both excellent optoelectronic and magnetic properties, makes semi magnetic 2D perovskite to be a considerably promising material for opto-spintronic semiconductor devices for information processing and communications.In this thesis, we explore the electronic and optical properties of 2D perovskites via magneto-optical spectroscopy. We start from performing magneto-photoluminescence (PL) and magneto-transmission measurements on CsPbBr3-based nanoplatelets with a different thickness of the lead-halide slab, ranging from 2 to 4 layers of lead-halide octahedral plane. By applying in-plane magnetic fields up to 65 T, the optically inactive dark excitonic state is brightened. This approach allows us to directly observe an improvement of the PL emission on the low-energy side of the PL spectrum, which indicates that the optically inactive dark excitonic state is the lowest-lying state in these nanoplatelets. Additionally, combining our magneto-PL and magneto-transmission results with theoretical predictions of the exciton fine structure splitting, we accurately determine the energy splitting between the dark and bright excitons. We demonstrate that indeed the dark-bright exciton splitting increases with decreasing layers of lead-halide octahedral plane. We also demonstrate that the efficient emission from these nanoplateltes is due to a phonon bottleneck effect, which significantly reduces the relaxation of the photo excited excitons to the optically inactive dark state.Finally, we investigate the electronic properties of Mn-doped 2D (PEA)2PbI4 perovskite via magneto-transmission spectroscopy for various Mn molar fractions. We find that the exciton Lande g-factor can be controlled by the incorporated Mn concentration. With increasing Mn concentration x from 0 to 2%, the g-factor increases, which we attribute to the sp-d exchange interaction between band-edge excitons and spins hosted in Mn ions. If the Mn concentration is increased further, up to 5%, the exciton g-factor decreases. This anomalous counter-trend is attributed to the Mn-Mn interactions, which result in an effective anti-ferromagnetic coupling