Inverting Singlet and Triplet Excited States using Strong Light-Matter Coupling

In organic microcavities, hybrid light-matter states can form with energies that differ from the bare molecular excitation energies by nearly 1 eV. A timely question, given recent advances in the development of thermally activated delayed fluorescence materials, is whether strong light-matter coupling can be used to invert the ordering of singlet and triplet states and, in addition, enhance reverse intersystem crossing (RISC) rates. Here, we demonstrate a complete inversion of the singlet lower polariton and triplet excited states. We also unambiguously measure the RISC rate in strongly-coupled organic microcavities and find that, regardless of the large energy level shifts, it is unchanged compared to films of the bare molecules. This observation is a consequence of slow RISC to the lower polariton due to the delocalized nature of the state across many molecules and an inability to compete with RISC to the dark exciton reservoir, which occurs at a rate comparable to that in bare molecules

Organic Photodiodes with an Extended Responsivity using Ultrastrong Light-Matter Coupling

In organic photodiodes (OPDs) light is absorbed by excitons, which dissociate to generate photocurrent. Here, we demonstrate a novel type of OPD in which light is absorbed by polaritons, hybrid light-matter states. We demonstrate polariton OPDs operating in the ultra-strong coupling regime at visible and infrared wavelengths. These devices can be engineered to show narrow responsivity with a very weak angle-dependence. More importantly, they can be tuned to operate in a spectral range outside that of the bare exciton absorption. Remarkably, we show that the responsivity of a polariton OPD can be pushed to near infrared wavelengths, where few organic absorbers are available, with external quantum efficiencies exceeding those of a control OPD

Triplet harvesting in the polaritonic regime: a variational polaron approach

We explore the electroluminescence efficiency for a quantum mechanical model of a large number of molecular emitters embedded in an optical microcavity. We characterize the circumstances under which a microcavity enhances harvesting of triplet excitons via reverse intersystem-crossing (R-ISC) into singlet populations that can emit light. For that end, we develop a time-local master equation in a variationally optimized frame which allows for the exploration of the population dynamics of chemically relevant species in different regimes of emitter coupling to the condensed phase vibrational bath and to the microcavity photonic mode. For a vibrational bath that equilibrates faster than R-ISC (in emitters with weak singlet-triplet mixing), our results reveal that significant improvements in efficiencies with respect to the cavity-free counterpart can be obtained for strong coupling of the singlet exciton to a photonic mode, as long as the singlet to triplet exciton transition is within the inverted Marcus regime; under these circumstances, we show the possibility to overcome the detrimental delocalization of the polariton states across a macroscopic number of molecules. On the other hand, for a vibrational bath that equilibrates slower than R-ISC (i.e., emitters with strong singlet-triplet mixing), we find that while enhancemnents in photoluminiscence can be obtained via vibrational relaxation into polaritons, this only occurs for small number of emitters coupled to the photon mode, with delocalization of the polaritons across many emitters eventually being detrimental to electroluminescence efficiency. These findings provide insight on the tunability of optoelectronic processes in molecular materials due to weak and strong light-matter coupling

Les stéréotypes de genre en mathématiques et en langues : recension critique en regard de la réussite scolaire

Cet article théorique expose l’étendue des stéréotypes de genre favorisant les garçons en mathématiques et les filles en langues, ainsi que leur influence sur la réussite et le cheminement scolaire. En mathématiques, l’analyse des écrits révèle qu’en dépit de stéréotypes explicites pro masculins moins saillants qu’auparavant, les élèves entretiennent toujours implicitement ces croyances. Les mesures explicites et implicites révèlent que les langues sont perçues comme un domaine plus féminin que masculin. Les quelques études sur l’impact des stéréotypes soulignent leur effet sur la motivation et le rendement scolaire. Enfin, les résultats recensés sont interprétés en regard du milieu scolaire québécois.This theoretical article exposes the extent of gender stereotypes favouring boys in mathematics and girls in language as well as their influence on students’ achievement and school trajectory. Comparison of different mathematics and language stereotype measures revealed that although students explicitly report low levels of stereotypes favouring boys in mathematics, they implicitly endorse these beliefs. Explicit and implicit measures also revealed that language is perceived as a female rather than male domain. The few studies conducted on the impact of gender stereotypes underline their effect on students’ motivation and school performance. The reviewed results are then discussed with regard to the Quebec school environment.Este artículo teórico presenta la magnitud de los estereotipos de género a favor de los niños en matemáticas y de las niñas en lenguas, así como su influencia sobre el éxito y la trayectoria escolar. En matemáticas, el análisis de los textos revela que a pesar de que los estereotipos explícitos a favor de los niños sean menos destacados que anteriormente, los alumnos siguen teniendo implícitamente estas creencias. Las medidas explícitas e implícitas muestran que las lenguas son percibidas como un dominio más femenino que masculino. Los pocos estudios que tratan del impacto de los estereotipos subrayan el efecto sobre la motivación y el rendimiento escolar. Por terminar, se interpretan los resultados recolectados respecto al ámbito escolar quebequense

Surface bound waves and optical interactions in excitonic thin films

Excitonic films such as J-aggregate-doped polymer films can exhibit sharp Lorentzian dispersions and thus have various optical features in the visible region. They can even show an optically metallic response and can be considered as alternative plasmonic materials. However, there were no systematic studies on optical interactions in such excitonic films. Here, we perform theoretical investigations on optical modes and interactions in planar excitonic thin films. We gradually vary the dye concentration and the film thickness and study optical coupling to surface bound waves (surface polariton modes and epsilon-near-zero modes). We also investigate thin-film interferences in high-loss and low-loss regimes. Finally, we discuss the freespace impedance matching that can result from the Lorentzian dispersion in excitonic films. Our work enables in-depth understandings on optical interactions in excitonic films and it can provide guidelines for various nanophotonic applications in the visible region

Quelles dynamiques de l'emploi en milieu rural : peut-on oser l'expression de « vitalité cachée » ?

La controverse joue un rôle essentiel dans les résultats et le progrès des sciences sociales. L'Emploi rural, une vitalité cachée, cette recherche collective et pluraliste sur l'avenir des emplois dans l'espace rural français publiée fin 1995, n'a pas épuisé les divergences d'interprétation sur le sujet et continue, une fois achevée, à opposer même les auteurs entre eux. L'objectif de cette table ronde, qui s'est tenue en décembre 1996, fut d'approfondir ces désaccords par leur discussion. Dans ce débat, chacun cherche, dans une langue vive et polémique, à préciser sa pensée en même temps que ce qui fait contradiction. On aboutit pourtant à un consensus : pour avancer plus loin dans l'analyse de phénomènes sociaux encore incertains, il faut théoriser les désaccords et prolonger la recherche à plusieurs voies.Dynamics of the employment in rural areas : may we speak of a vitality ? This paper relates the debate organized after the publication of L’Emploi rural, une vitalité cachée ?, in order to cross the contradictory points of view of the authors in the face of an outward sociologist reader. Though it was quite hot, the discussion has deepened the analysis and opened up to new approaches and questions

Van der Waals-Casimir-Polder interaction of an atom with a composite surface

We study the dispersion interaction of the van der Waals and Casimir-Polder (vdW-CP) type between a neutral atom and the surface of a metal by allowing for nonlocal electrodynamics, i.e. electron diffusion. We consider two models: (i) bulk diffusion, and (ii) diffusion in a surface charge layer. In both cases the transition to a semiconductor is continuous as a function of the conductivity, unlike the case of a local model. The relevant parameter is the electric screening length and depends on the carrier diffusion constant. We find that for distances comparable to the screening length, vdW-CP data can distinguish between bulk and surface diffusion, hence it can be a sensitive probe for surface states.Comment: v2: expanded references, statements on current status in the field. 10 pages, 6 figure

Manipulating molecules with strong coupling: harvesting triplet excitons in organic exciton microcavities

Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic semiconductors are of current interest for their room-temperature polariton formation. However, within organic optoelectronic devices, it is often the 'dark' spin-1 triplet excitons that dominate operation. These triplets have been largely ignored in treatments of polariton physics. Here we demonstrate polariton population from the triplet manifold via triplet-triplet annihilation, leading to polariton emission that is longer-lived (>microseconds) even than exciton emission in bare films. This enhancement arises from spin-2 triplet-pair states, formed by singlet fission or triplet-triplet annihilation, feeding the polariton. This is possible due to state mixing, which -in the strong coupling regime- leads to sharing of photonic character with states that are formally non-emissive. Such 'photonic sharing' offers the enticing possibility of harvesting or manipulating even states that are formally dark

Préface

Que voici un petit livre tonique, qui nous apprend au moins deux bonnes nouvelles. La première est que pour l’instant, par delà ce que l’on appelle l’américanisation, forme majeure de l’uniformisation mondiale, des différences importantes existent toujours en ce qui concerne les goûts et les couleurs ! ! ! La deuxième, à vrai dire complément de la première, est que l’unification européenne ne signifie et ne signifiera pas la fin des diversités à l’intérieur même de notre espace commun, au moi..

Nouveaux ouvriers, nouvelles formes de lutte

Eizner Nicole. Nouveaux ouvriers, nouvelles formes de lutte. In: Raison présente, n°32, Octobre – Novembre – Décembre 1974. Le rationalisme face a deux illusions. pp. 27-34