Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as MoS$_2$. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and $\mathbf k \cdot \mathbf p$ perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.Comment: 5 pages, 3 figure

Carrier and polarization dynamics in monolayer MoS2

In monolayer MoS2 optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical valley initialization. In time resolved photoluminescence (PL) experiments we find that both the polarization and emission dynamics do not change from 4K to 300K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature induced change in bandgap energy with the excitation laser energy an emission polarization of 40% is recovered at 300K, close to the maximum emission polarization for this sample at 4K.Comment: 7 pages, 7 figures including supplementary materia

Exciton dynamics in WSe2 bilayers

We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with $\tau_{D}$ ~ 3 ps and the indirect optical transition with $\tau_{i}$ ~ 25 ps. For temperatures between 4 to 150 K $\tau_{i}$ remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignment during the short emission time $\tau_{D}$. The evolution of the direct exciton polarization and intensity as a function of excitation laser energy is monitored in PL excitation (PLE) experiments.Comment: 4 pages, 3 figure

Exciton states in monolayer MoSe2: impact on interband transitions

We combine linear and non-linear optical spectroscopy at 4K with ab initio calculations to study the electronic bandstructure of MoSe2 monolayers. In 1-photon photoluminescence excitation (PLE) and reflectivity we measure a separation between the A- and B-exciton emission of 220 meV. In 2-photon PLE we detect for the A- and B-exciton the 2p state 180meV above the respective 1s state. In second harmonic generation (SHG) spectroscopy we record an enhancement by more than 2 orders of magnitude of the SHG signal at resonances of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our post-Density Functional Theory calculations show in the conduction band along the $K-\Gamma$ direction a local minimum that is energetically and in k-space close to the global minimum at the K-point. This has a potentially strong impact on the polarization and energy of the excitonic states that govern the interband transitions and marks an important difference to MoS2 and WSe2 monolayers.Comment: 8 pages, 3 figure

Surgery using plasma energy for deep endometriosis: A quality of life assessment

OBJECTIVE: The principal objective of our study was to assess women\u27s quality of life (QoL) after surgery for Deep Endometriosis (DE), according to the surgical technique used. MATERIAL AND METHODS: Qualitative single-center survey in the department of obstetrics and gynecology, Angers University Hospital Center, France. All women who underwent surgery for DE from January 2011 to December 2015 were contacted by phone. The Endometriosis Health Profile-5 score was used to assess QoL before and after the surgery. Fifty-two women (response rate=86%) were included and classified into 3 groups according to the surgical technique used: simple shaving, shaving exclusively or in part by plasma vaporization (plasma), and resection. RESULTS: The 3 groups were comparable for surgical history, preoperative QoL score, and characteristics of endometriotic lesions (size and site). All DE symptoms and QoL scores improved significantly after the surgery, all techniques combined (P<0.01). QoL scores for women who had plasma shaving or complete resection were significantly higher than those for women with simple shaving (respectively, 375 [225-800] and 450 [-50 to 725] vs 275 [-100 to 600]; P=0.04). Self-image significantly improved only in the plasma group (P=0.03). The complete resection group had longer hospitals stays than the other groups (P=0.001), as well as a higher surgical revision rate (23% vs 0%; P=0.02). CONCLUSION: Plasma and complete resection improved QoL similarly for women with DE, both more than shaving alone. The advantage of plasma vaporization lies in the lesser morbidity and better self-image, both better than in women with resection

Robust optical emission polarization in MoS2 monolayers through selective valley excitation

We report polarization resolved photoluminescence from monolayer MoS2, a two-dimensional, non-centrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys and close to 90% polarized emission at 4K is observed with 40% polarization remaining at 300K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9T indicating robust, selective valley excitation.Comment: 5 pages, 3 figure

La reconnaissance d'un capital environnemental autochtone ? Les « Parcs Nationaux à la française »

International audienceThis article contributes to the discussion about environmental capital through the lens of "local" user position in protected areas. More precisely, it focuses on French national parks, emblematic of the conception of nature conservation in France. Indigenous uses and "local" environmental knowledge were denied in the first national parks in former colonies and metropolitan France. But they are currently reintroduced in the context of the parks reform (2006) and the international background of participatory imperative, indigenous recognition and ecologization of the thought (Mormont, 2009 ; 2013). Is this movement like a recognition of an environmental capital for "local" users ? This hypothesis may be discussed. Under which conditions are these knowledge and uses recognized by the ecological vision, which is usually dominant in French national parks ? Who does participate in and benefit from this recognition ? Focusing on the cases of the young national park of Creeks (2012) and of Reunion Island (2007), we outline the components of environmental capital granted to "local" users in French national parks, by analyzing the combined effects of indigenousness capital recognition and ecological imperative. To what extent does this process broaden the definition of the "publics" of protected areas users and who does remain excluded ?Cet article contribue au questionnement général sur le capital environnemental au prisme de la place des usagers « locaux » dans les espaces protégés en France. Il traite, plus précisément, des parcs nationaux français, archétypes de la protection de l'environnement « à la française ». Les usages autochtones et les savoirs environnementaux « locaux », niés dans les premiers parcs nationaux des anciennes colonies, comme en France métropolitaine, se renégocient aujourd'hui, à la faveur de leur réforme (loi de 2006) et d'un contexte national et international marqué à la fois par l'impératif participatif, la reconnaissance de l'autochtonie et l'écologisation de la pensée (Mormont, 2009 ; 2013). Lire ce processus comme la reconnaissance d'un capital environnemental des usagers « locaux » dans les parcs nationaux soulève plusieurs interrogations. À quelles conditions ces usages et savoirs sont-ils reconnus à l'aune de la vision « environnementaliste » de la nature, traditionnellement hégémonique dans les parcs nationaux français ? Qui participe à ce processus et bénéficie de cette reconnaissance ? À travers les exemples des jeunes parcs nationaux des Calanques (2012) et de la Réunion (2007), nous proposerons un éclairage sur la nature des capitaux environnementaux reconnus aux usagers « locaux » dans les parcs nationaux français, en considérant les effets combinés de la reconnaissance de l'autochtonie et de l'impératif écologique. Jusqu'à quel point le processus en cours élargit-il la définition des « publics » des usagers de la nature protégée et lesquels en demeurent exclus