Exciton

Résumé

vignette|Représentation schématique d'un exciton de Frenkel, dans un cristal (points noirs). Un exciton est, en physique, une quasi-particule que l'on peut voir comme une paire électron-trou liée par des forces de Coulomb. Une analogie souvent utilisée consiste à comparer l'électron et le trou respectivement à l'électron et au proton d'un atome d'hydrogène. Ce phénomène se produit dans les semi-conducteurs et les isolants. En 2008, le premier dispositif électronique basé sur des excitons a été démontré, fonctionnant à des températures cryogéniques. En 2018, l'EPFL met au point un transistor basé sur les excitons qui peut fonctionner à temperature ambiante. Cette technique ouvre la voie à une panoplie de nouvelles possibilités en excitonique, branche de la physique appliquée qui, après celle de la photonique et de la spintronique, s’avère des plus prometteuses pour l'électronique du futur. Le terme est également utilisé en chimie pour désigner des états électroniquement excités des s

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https://fr.wikipedia.org/wiki/Exciton

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Interaction of Confined Polaritons in Microcavity Structures

Morteza Navadeh Toupchi

A polariton is a quasiparticle formed from the coupling of a confined photon in a cavity to electronic excitation, like exciton in a semiconductor. This dissertation reports on series of experiments in confined polariton interaction by design, fabrication, and characterization of semiconductor microcavity structures operating in strong or weak coupling regime. In the first part of the thesis, we mainly concentrate on the optical study of the 2D microcavity sample, including spin-dependent lower-upper polariton cross interactions by pump-probe spectroscopy technique, supported by theoretical analyses and numerical simulations based on Gross-Pitaevskii equations. In particular, we present a scattering resonance behavior via an exciton molecule (biexciton) when polaritons from both the upper and lower branches with anti-parallel spins are involved through a polaritonic cross Feshbach resonance. This demonstration will permit the control of the polariton interbranch scattering. The second part of the thesis is dedicated to the design and fabrication of the potentials where the photonic part of polaritons is confined laterally by adjusting the thickness of the cavity layer locally in so-called mesa structures. By engineering a periodic lattice of mesas on a two-dimensional microcavity, it is possible to couple confined polariton modes of nearby mesas to establish an optical lattice analogous to the crystalline semiconductorsâ electronic band structures. We especially demonstrate the localization of light with a lasing mode at the edge of the Brillouin zone in a two-dimensional triangular lattice. We produce a self-trapping of light by optically inducing a local breaking of the strong-coupling regime of excitons to photons. In the weak coupling regime, we control the confined modes by the shape of the generated defect. We also reveal a controllable localization degree and experimental signature of the Anderson localization in microcavity polaritons by inducing positional disorder in the triangular lattice. The last part is devoted to the fabrication of sub-micron size mesas to enhance polariton interaction by confining them tightly and discuss the quantum correlation of polaritons by a Hanbury Brown and Twiss (HBT) setting toward polariton blockade.

EPFL2021

Chargement

Only recently organic light-emitting diode (OLED) technology has successfully managed the transition from research labs into the consumer market, taking a 60% share of the global mobile display market in 2018. The latest discovery of thermally activated delayed fluorescence attracted a lot of attention in research and industry due to the potential to fabricate fluorescence-based OLEDs with high efficiencies comparable to the currently used phosphorescence-based OLEDs, but with the advantage of possibly cheaper and more sustainable emitter materials (no Ir-, Pt-complexes). For achieving high efficiencies in OLEDs, a substantial number of layers and interfaces of the multilayer stack have to be optimized. A particularly important role is assigned to the emission layer within which light is generated by charge recombination and subsequent energy transfer and radiative decay of excitons. The understanding of charge recombination and exciton dynamics and the determination of the position of light generation are essential for the fabrication of modern OLEDs and are the goal of this thesis. Therefore two different OLED types, phosphorescence-based OLEDs and state-of-the-art TADF exciplex host OLEDs incorporating a fluorescent emitter, are studied by electro-optical characterization and device modelling. In a first step the emission zones are determined and analyzed by angle-dependent steady-state measurements at different biases and optical simulations. In both OLED types split emission zones are obtained with densities of emissive excitons that decay way from both emission layer interfaces toward the center. For the phosphorescence-based OLEDs an additional bias-dependence of the split emission zone is observed, meaning that at low bias the main emission is located at the cathode side and shifts to the anode side for increasing bias. In a second step, with transient EL decay measurements and electro-optical simulations the split emission zones are correlated to an EL peak appearing after OLED turn-off. To study the influence of the emission zone and the exciton dynamics on the OLED efficiency an electro-optical device model is established to reproduce the experimentally obtained measurement data. As the model includes charge carrier dynamics, light outcoupling and time- and position-dependent exciton processes, such as the formation, diffusion, transfer, decay and quenching, the physical mechanisms in the OLEDs are elucidated. For the phosphorescence-based OLED a surprising current efficiency increase of up to 60% for increasing bias as well as a subsequent decrease is explained with the shift of the emission zone and its influence on exciton quenching and light outcoupling. Similarly, for the TADF exciplex host OLEDs a model parameter study illustrates promising EQE enhancement routes, which could lead to EQEs as high as 42%. This thesis emphasizes the need of accurate knowledge of the emission zone and its bias-dependence due to its potentially strong influence on the OLED efficiency and its importance for the optimization of the OLED layer stack. In addition, this thesis shows that full electro-optical device modelling (including electrons, excitons and photons) combined with advanced electro-optical characterization techniques is crucial for elucidating the physical mechanisms in state-of-the-art OLEDs as well as for the prediction of promising routes for future efficiency enhancements.

EPFL2019

Chargement

Two-dimensional (2D) materials are atomically thin crystals with exceptional mechanical, electrical and optical properties. Their unique characteristics originating from quantum confinement in the vertical dimension have attracted a strong interest for scientific research as well as technological applications. Moreover, the weak van der Waals coupling between layers facilitates the stacking of different materials to form heterostructures with new functionalities. This thesis focuses on semiconductor transition metal dichalcogenides (TMDCs), with a direct bandgap in the monolayer limit. The reduced dielectric screening leads to the formation of tightly bound electron-hole pairs (excitons) that are stable at room temperature and dominate the optical spectrum. The first part of the thesis explores the use of TMDC monolayers for developing transparent and tunable exciton light emitting diodes (LEDs) and photodetectors (PDs). The high modulation frequencies and integration capabilities with photonic structures make them promising candidates as optical/electrical interconnects. The properties of TMDC monolayers are determined by those of carriers close to the conduction and valence band extrema, located at the K and -K valleys in reciprocal space. The broken inversion symmetry and strong spin-orbit coupling links the valley and spin degrees of freedom, and enables optical excitation and detection of spin-valley accumulation with circularly polarized light. In the second part of the thesis, we explore the use of electric and magnetic fields to control the spin-valley dynamics of single carriers and excitons in TMDC-based optoelectronic devices. The spin-valley accumulation is probed by photoluminescence, electroluminescene and magneto-optical Kerr rotation.The third part of the thesis focuses on the integration of TMDCs in planar optical cavities. Their large oscillator strength induces a strong exciton-photon coupling, which results in the formation of polaritons. The observed polaritons propagate over long-distances and inherit the spin-valley properties of constituent excitons. By tuning the semiconductor carrier density, we achieve electrical control of exciton-photon coupling and polariton transport. Finally, we demonstrate a cavity integrated polariton light emitting diode where the location, directionality and polarization of emitted light can be electrically controlled at room temperature. Altogether, this thesis investigates the properties of charge carriers and excitons in 2D materials to realize practical optoelectronic devices where the spin-valley degree of freedom can be encoded in the polarization of light.

EPFL2022

Chargement

EPFL2022

EPFL2019

Interaction of Confined Polaritons in Microcavity Structures

Morteza Navadeh Toupchi

EPFL2021

Exciton

L'onde de polarisation est

Interaction of Confined Polaritons in Microcavity Structures

Interaction of Confined Polaritons in Microcavity Structures