Oriented polaritons in strongly-coupled asymmetric double quantum well microcavities

Replacing independent single quantum wells inside a strongly-coupled semiconductor microcavity with double quantum wells produces a special type of polariton. Using asymmetric double quantum wells in devices processed into mesas allows the alignment of the electron levels to be voltage-tuned. At the resonant electronic tunnelling condition, we demonstrate that `oriented polaritons' are formed, which possess greatly enhanced dipole moments. Since the polariton-polariton scattering rate depends on this dipole moment, such devices could reach polariton lasing, condensation and optical nonlinearities at much lower threshold powers.Comment: 3 figure

Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities

We report on the direct observation of biexcitons in a III nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.This work was supported by the NCCR Quantum Photonics, research instrument of the Swiss National Science Foundation, through Grant No. 129715 and Grant No. 200020-113542, and by the EU-project Clermont4 (Grant No. FP7-235114)

USP 10 as a Deubiquitinase for β-Catenin

Der WNT-Signalweg ist ein hochkonservierter Signalweg, dessen zentraler intrazellulärer Regulationsschritt die Proteinstabilität des Proteins β-Catenin ist. Deregulierende Mutationen in diesem sind frühe Ereignisse bei der Entstehung von Darmtumoren. Ist der Abbau von β-Catenin gestört, so ist unabhängig von äußerer Kontrolle der Signalweg konstitutiv aktiviert und liefert ein Wachstumssignal. Untersuchungen haben aber gezeigt, dass beim Vorliegen solcher Mutationen immer noch eine – unzureichende – Ubiquitinylierung und ein Abbau von β-Catenin stattfindet. Ziel dieser Studie war Deubiquitinasen (DUBs) zu finden, die durch ihre Aktivität den Abbau von β-Catenin verhindern. Mithilfe eines siRNA Screens in der Vorarbeit konnten DUBs als Kandidaten für einen CRISPR Ansatz ausgewählt werden. APC Wildtyp HEK293T Zellen und Darmkrebszellen wurden mit lentiviralen CRISPR/Cas9 Vektoren infiziert, in welche sgRNAs gegen exonische Sequenzen von DUBs geklont waren. Einzelne Zellklone von USP10 CRISPR Zellen wurden weiter untersucht. In Western Blots und Immunofluoreszenz zeigte sich bei den USP10 CRISPR Zellen eine verminderte Expression von USP10 und damit einhergehend β-Catenin. Proteinstabilitätsversuche mit MG132 und Cycloheximid zeigten einen erhöhten Abbau von β-Catenin in HEK293T USP10 CRISPR Zellen, vor allem nach Stimulierung des WNT-Signalwegs durch LiCl. In Aktivierungsassays (Luciferase und TOP-GFP FACS) des WNT-Signalwegs zeigte sich in HEK293T Zellen nach Behandlung mit LiCl eine geringere Aktivierung in den USP10 CRISPR Zellen. In einem Wachstumsassay zeigten HT29 USP10 CRISPR ein geringeres Wachstum als Kontrollzellen. Während in einer histologischen Färbung von Mausgewebe eine erhöhte Expression von USP10 nachweisbar war, zeigten sich in einer TMA Färbung kein eindeutiger Unterschied zwischen gesundem Gewebe und Tumorgewebe. Die Studie identifiziert USP10 als eine mögliche DUB für β-Catenin und potenzielles Ziel für eine Beeinflussung des mutierten WNT-Signalwegs in Darmkrebszellen.The WNT- signaling pathway is a highly preserved pathway. Its central intracellular regulation measure is the protein stability of the protein β-Catenin. Mutations that cause a deregulation in said pathway occur early within the development of intestinal tumors. If the breakdown of β-Catenin is impaired, the WNT- signaling pathway will get activated constitutively, providing growth signals, regardless of outer controls. Investigations have shown, that if such mutations exist there will still be a (insufficient) ubiquitinylation and a breakdown of β-Catenin. It was the aim of this research to identify deubiquitinases (DUBs) whose activity prevent the breakdown of β-Catenin. It was possible to select DUBs as potential candidates for a CRISPR approach by using sIRNA Screens. APC Wildtype HEK293T cells and intestinal tumor cells where infected by lentiviral CRISPR/Cas9 vectors which contained sgRNAs clones that worked against exonic sequences of DUBs. Individual cell clones of USP10 CRISPR cells where further investigated. Within western blots and immunofluorescence USP10 CRISPR cells showed a reduced expression of USP10 and thus, β- Catenin. Protein stability trials using MG132 and cycloheximide showed an increased breakdown of β-Catenin in HEK293T USP10 CRISPR cells, especially in response to stimulation of the WNT- signaling pathway using LiCl. Within activation assays (Luciferase and TOP-GFP FACS) of the WNT- signaling pathway, a reduced activation in USP10 CRISPR cells after treatment with LiCl was shown within HEK293T cells. Within a growth assay HT29 USP10 CRISPR showed a reduced growth when compared to control cells. While there was evidence of an increased expression of USP10 within the histological coloration of mouse tissue, the TMA coloration did not show a significant difference between healthy tissue and tumor tissue. This study identified USP10 as a possible DUB for β- Catenin and as a possible target for the manipulation of the mutations that cause a deregulation within the WNT- signaling pathway in intestinal tumor cells

III-nitride based microcavities:towards polariton condensation at room temperature

This PhD thesis work describes the study of III-nitride based planar microcavities operating in the strong coupling regime at room temperature. The aim is to use the nonlinear emission properties of such samples in realistic devices. Furthermore these studies allow investigating macroscopic quantum phenomena at room temperature. Strongly coupled semiconductor microcavities are characterized by the non-perturbative coupling between the photon associated to the cavity mode and the semiconductor first excited state, namely the exciton. The quasiparticle resulting from this interaction is called cavity polariton and exhibits a peculiar in plane wave vector (k‖) dispersion. This results from the strong effective mass difference between the cavity photon and the exciton. Consequently, polaritons are subject to trapping in reciprocal space around k‖ = 0. Polaritons are thus characterized by a light effective mass in this trap. Theoretically, this point is favorable for the observation of macroscopic coherent polariton phases at low densities and high temperatures, in particular at room temperature. So far such phenomena have been mainly studied in III-arsenides and II-VI semiconductors. The main outcome is the recent demonstration of polariton Bose-Einstein condensation at cryogenic temperatures. This last point is the main limitation of III-arsenide semiconductors: the strong coupling regime is hardly maintained at room temperatures. This prevents the observation of such macroscopic coherent phases of polaritons in such conditions. In this context, the use of these materials for devices based on such physical phenomena appears strongly compromised. Indeed a potential use would be the realization of low threshold compact coherent light sources, and for such applications, an operation at cryogenic temperatures is not realistic. In this context, the use of III-nitride semiconductors is a very promising approach. Indeed, thanks to better intrinsic material parameters, the strong coupling regime is kept at room temperature. It has thus been observed in microcavities whose quality is much poorer than their III-arsenide or II-VI counterparts. Using high quality planar microcavities is however a prerequisite to observe these nonlinear effects. The aim of this work is to study such samples that can now be obtained thanks to the use of Bragg mirrors made of AlInN layers lattice matched to GaN. The strong coupling regime has been demonstrated at room temperature on several samples grown using this approach through angle resolved photoluminescence and reflectivity measurements. Furthermore, a strong nonlinear emission is observed at room temperature under nonresonant optical excitation for two different samples. This emission exhibits a strong spectral narrowing indicating the onset of an increased temporal coherence. These results are fully consistent with a room temperature polariton lasing picture. With the recent demonstration of the electrical injection of polaritons in III-arsenides, this latter result indicates that the realization of a coherent light source based on cavity polaritons and operating at room temperature is now realistic. In addition to this first major result, numerous properties of this nonlinear emission have been investigated. Thus, some preliminary evidences indicating that the transition is driven by thermodynamics have been obtained. Then some intriguing properties related to the macroscopic polarization buildup have been observed. One of the samples exhibiting this nonlinear emission is indeed characterized by the spontaneous buildup of a random polarization vector for each experimental realization. This result is interpreted in terms of spontaneous symmetry breaking at the transition, which is a fundamental property of phase transition theory. These latter results pave the way to the observation of room temperature Bose-Einstein condensation in the solid state

Grass sickness: a Belgian reality

peer reviewe

Palliating the efficiency loss due to shunting in perovskite/silicon tandem solar cells through modifying the resistive properties of the recombination junction

As the efficiency of commercial crystalline silicon solar cells approaches its maximum theoretical value, tandem architectures are becoming increasingly popular to continue the push to higher photovoltaic performances. Thin-film materials are particularly interesting partners for silicon wafers due to their potential cost effectiveness and ease of fabrication. However, in large scale thin-film coatings, particularly for perovskite materials, avoiding the formation of point shunts is a challenge. This study investigates the sensitivity of perovskite/silicon tandems to such shunts and whether or not optimising the lateral and transverse resistances of the recombination junction can reduce the negative effects of these defects. To do so, the inhomogeneous characteristic of shunts is reproduced by modelling tandem cells with an array of scaled equivalent circuit elements connected in parallel. It is shown that by optimising the resistive properties of the interconnection, there can be an important quenching effect on shunts present in the top cell, resulting in a significant increase in the overall cell efficiency at STC and under low light conditions. These findings give a clear pathway on how to bridge the efficiency gap between small laboratory cells, which can be selected shunt free, and industry scale devices, which are more prone to localised shunting