Control of the Bright-Dark Exciton Splitting Using the Lamb Shift in a Two-Dimensional Semiconductor.

We investigate the exciton fine structure in atomically thin WSe_{2}-based van der Waals heterostructures where the density of optical modes at the location of the semiconductor monolayer can be tuned. The energy splitting Δ between the bright and dark exciton is measured by photoluminescence spectroscopy. We demonstrate that Δ can be tuned by a few meV as a result of a significant Lamb shift of the optically active exciton that arises from emission and absorption of virtual photons triggered by the vacuum fluctuations of the electromagnetic field. We also measure strong variations of the bright exciton radiative linewidth as a result of the Purcell effect. All these experimental results illustrate the strong sensitivity of the excitons to local vacuum field fluctuations. We find a very good agreement with a model that demonstrates the equivalence, for our system, of a classical electrodynamical transfer matrix formalism and quantum-electrodynamical approach. The bright-dark splitting control we demonstrate here in the weak light-matter coupling regime should apply to any semiconductor structures.

Optical Detection of Long Electron Spin Transport Lengths in a Monolayer Semiconductor.

Using a spatially resolved optical pump-probe experiment, we measure the lateral transport of spin-valley polarized electrons over very long distances (tens of micrometers) in a single WSe_{2} monolayer. By locally pumping the Fermi sea of 2D electrons to a high degree of spin-valley polarization (up to 75%) using circularly polarized light, the lateral diffusion of the electron polarization can be mapped out via the photoluminescence induced by a spatially separated and linearly polarized probe laser. Up to 25% spin-valley polarization is observed at pump-probe separations up to 20 µm. Characteristic spin-valley diffusion lengths of 18±3 µm are revealed at low temperatures. The dependence on temperature, pump helicity, pump intensity, and electron density highlight the key roles played by spin relaxation time and pumping efficiency on polarized electron transport in monolayer semiconductors possessing spin-valley locking.

Measurement of Conduction and Valence Bands g-Factors in a Transition Metal Dichalcogenide Monolayer.

The electron valley and spin degree of freedom in monolayer transition-metal dichalcogenides can be manipulated in optical and transport measurements performed in magnetic fields. The key parameter for determining the Zeeman splitting, namely, the separate contribution of the electron and hole g factor, is inaccessible in most measurements. Here we present an original method that gives access to the respective contribution of the conduction and valence band to the measured Zeeman splitting. It exploits the optical selection rules of exciton complexes, in particular the ones involving intervalley phonons, avoiding strong renormalization effects that compromise single particle g-factor determination in transport experiments. These studies yield a direct determination of single band g factors. We measure g_{c1}=0.86±0.1, g_{c2}=3.84±0.1 for the bottom (top) conduction bands and g_{v}=6.1±0.1 for the valence band of monolayer WSe_{2}. These measurements are helpful for quantitative interpretation of optical and transport measurements performed in magnetic fields. In addition, the measured g factors are valuable input parameters for optimizing band structure calculations of these 2D materials.

Control of the Exciton Radiative Lifetime in van der Waals Heterostructures.

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strengths. Encapsulation of monolayers such as MoSe_{2} in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allows us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

Unraveling the role of zinc complexes on indium phosphide nanocrystal chemistry.

The addition of zinc complexes to the syntheses of indium phosphide nanocrystals (InP NCs) has become commonplace, due to their ability to alter and significantly improve observed optical properties. In this paper, the role of zinc complexes on the synthesis and observed properties of InP is carefully examined. Produced InP and InP:Zn2+ NCs are thoroughly characterized from both structural (core and surface) and optical perspectives over a wide range of Zn2+ compositions (0%-43% atomic content). We find no differences in the physical (NC size and polydispersity) and structural properties (crystallographic phase) of InP and InP:Zn2+ NCs. Optically, significant changes are observed when zinc is added to InP syntheses, including blueshifted absorption edges and maxima, increased quantum yields, and the near elimination of surface state emission. These improved optical properties result from surface passivation by zinc carboxylate moieties. Changes to the optical properties begin at zinc concentrations as low as 5%, demonstrating the high sensitivity of InP optical properties to exogenous species.

In-Plane Propagation of Light in Transition Metal Dichalcogenide Monolayers: Optical Selection Rules.

The optical selection rules for interband transitions in WSe_{2}, WS_{2}, and MoSe_{2} transition metal dichalcogenide monolayers are investigated by polarization-resolved photoluminescence experiments with a signal collection from the sample edge. These measurements reveal a strong polarization dependence of the emission lines. We see clear signatures of the emitted light with the electric field oriented perpendicular to the monolayer plane, corresponding to an interband optical transition forbidden at normal incidence used in standard optical spectroscopy measurements. The experimental results are in agreement with the optical selection rules deduced from group theory analysis, highlighting the key role played by the different symmetries of the conduction and valence bands split by the spin-orbit interaction. These studies yield a direct determination of the bright-dark exciton splitting, for which we measure 40±1 meV and 55±2 meV in WSe_{2} and WS_{2} monolayer, respectively.

Giant enhancement of the optical second-harmonic emission of WSe(2) monolayers by laser excitation at exciton resonances.

We show that the light-matter interaction in monolayer WSe_{2} is strongly enhanced when the incoming electromagnetic wave is in resonance with the energy of the exciton states of strongly Coulomb bound electron-hole pairs below the electronic band gap. We perform second harmonic generation (SHG) spectroscopy as a function of laser energy and polarization at T=4 K. At the exciton resonance energies we record an enhancement by up to 3 orders of magnitude of the SHG efficiency, due to the unusual combination of electric dipole and magnetic dipole transitions. The energy and parity of the exciton states showing the strong resonance effects are identified in 1- and 2-photon photoluminescence excitation experiments, corroborated by first principles calculations. Targeting the identified exciton states in resonant 2-photon excitation allows us to maximize k-valley coherence and polarization.

Carrier and polarization dynamics in monolayer MoS2.

In monolayer MoS2, optical transitions across the direct band gap 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 4 to 300 K 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 4 ps. 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 band gap energy with the excitation laser energy, an emission polarization of 40% is recovered at 300 K, close to the maximum emission polarization for this sample at 4 K.

Surfactant-free CZTS nanoparticles as building blocks for low-cost solar cell absorbers.

A process route for the fabrication of solvent-redispersible, surfactant-free Cu2ZnSnS4 (CZTS) nanoparticles has been designed with the objective to have the benefit of a simple sulfide source which advantageously acts as (i) a complexing agent inhibiting crystallite growth, (ii) a surface additive providing redispersion in low ionic strength polar solvents and (iii) a transient ligand easily replaced by an carbon-free surface additive. This multifunctional use of the sulfide source has been achieved through a fine tuning of ((Cu²âº)(a)(Zn²âº)(b)(Sn4⁺)(c)(Tu)(d)(OH⁻)(e))(t⁺), Tu = thiourea) oligomers, leading after temperature polycondensation and S²â» exchange to highly concentrated (c > 100 g l⁻¹), stable, ethanolic CZTS dispersions. The good electronic properties and low-defect concentration of the sintered, crack-free CZTSe films resulting from these building blocks was shown by photoluminescence investigation, making these building blocks interesting for low-cost, high-performance CZTSe solar cells.

Full electrical control of the electron spin relaxation in GaAs quantum wells.

The electron spin dynamics in (111)-oriented GaAs/AlGaAs quantum wells is studied by time-resolved photoluminescence spectroscopy. By applying an external electric field of 50 kV/cm a two-order of magnitude increase of the spin relaxation time can be observed reaching values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one. The measurements under a transverse magnetic field demonstrate that the electron spin relaxation time for the three space directions can be tuned simultaneously with the applied electric field.

Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots.

In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum dots in longitudinal magnetic fields applied along the growth axis, we observe in addition to the expected bright states also nominally dark transitions for both charged and neutral excitons. We uncover a strongly nonmonotonic, sign-changing field dependence of the bright neutral exciton splitting resulting from the interplay between exchange and Zeeman effects. Our theory shows quantitatively that these surprising experimental results are due to magnetic-field-induced ±3/2 heavy-hole mixing, an inherent property of systems with C(3v) point-group symmetry.

Room-temperature defect-engineered spin filter based on a non-magnetic semiconductor.

Generating, manipulating and detecting electron spin polarization and coherence at room temperature is at the heart of future spintronics and spin-based quantum information technology. Spin filtering, which is a key issue for spintronic applications, has been demonstrated by using ferromagnetic metals, diluted magnetic semiconductors, quantum point contacts, quantum dots, carbon nanotubes, multiferroics and so on. This filtering effect was so far restricted to a limited efficiency and primarily at low temperatures or under a magnetic field. Here, we provide direct and unambiguous experimental proof that an electron-spin-polarized defect, such as a Ga(i) self-interstitial in dilute nitride GaNAs, can effectively deplete conduction electrons with an opposite spin orientation and can thus turn the non-magnetic semiconductor into an efficient spin filter operating at room temperature and zero magnetic field. This work shows the potential of such defect-engineered, switchable spin filters as an attractive alternative to generate, amplify and detect electron spin polarization at room temperature without a magnetic material or external magnetic fields.

Lost in translation: translational interference from a recurrent mutation in exon 1 of MECP2.

BACKGROUND: Rett syndrome (RTT) is an X linked neuro-developmental disorder affecting mostly girls. Mutations in the coding region of MECP2 are found in 80% of classic RTT patients. Until recently, the region encoding MECP2 was believed to comprise exons 2, 3, and 4 with the ATG start site located at the end of exon 2 (MeCP2_e2). METHODS: Recent reports of another mRNA transcript transcribed from exon 1 (MeCP2_e1) prompted us to screen exon 1 among RNA samples from 20 females with classic or atypical RTT. RESULTS: A previously reported 11 base pair deletion in exon 1 was detected in one subject with a milder phenotype. Although RNA expression for both protein isoforms was detected from the mutant allele, evaluation of MeCP2 protein in uncultured patient lymphocytes by immunocytochemistry revealed that MeCP2 protein production was restricted to only 74-76% of lymphocytes. X chromosome inactivation studies of genomic DNA revealed similar XCI ratios at the HUMARA locus (73:27 with HpaII and 74:26 with McrBC). We have demonstrated that translation but not transcription of the MeCP2_e2 isoform is ablated by the 11 nucleotide deletion, 103 nucleotides upstream of the e2 translation start site. CONCLUSIONS: These findings reveal that nucleotides within the deleted sequence in the 5'-UTR of the MeCP2_e2 transcript, while not required for transcription, are essential for translation.

[Pharmacological approach to desychronization of the sleep-wakefulness cycle in the military and sport environment]. / Approche pharmacologique de la désynchronisation du rythme veille-sommeil en milieu militaire et sportif.

Sports and military environments have many common features - intense physical activity, rigorous physical environment (heat, cold, high or low pressure, hypoxia, acceleration...), specific psychosocial atmosphere, team spirit. If combined with jet lag syndrome, these specific conditions can favor altered physical and mental performance. There is always the temptation to use drugs as a simple way to reduce the penalizing effects. The available compounds known to affect sleep and wakefulness include hypnotics, benzodiazepines and non benzodiazepines such as temazepam, zolopidem, and zopiclone, stimulants such as amphetamine and amphetamine-like agents, adrafinil, modafinil, caffeine and chronobiotics substances such as melatonin and, more recently, slow release caffeine. In the sports area, all of these substances except caffeine are on the list of forbidden products, although special authorizations linked to known disease conditions are allowed. In the military setting, the environment may be similar, but the context of use is very different. In the context of a rescue mission, the current practice in the French military organization is to place modafinil pills in the ejection seat of fight planes and in rescue boats. A second context is the use of anti-sleep agents under orders; the debate continues on this and the appropriate recommendations in this context. Self-medication is a third condition, in which case no rules have been defined.

The zeaxanthin biosynthesis enzyme beta-carotene hydroxylase is involved in myxoxanthophyll synthesis in Synechocystis sp. PCC 6803.

Beta-carotene hydroxylase is known to be involved in zeaxanthin synthesis. Disruption of the crtR gene encoding beta-carotene hydroxylase in Synechocystis sp. PCC 6803 resulted in the absence of both zeaxanthin synthesis and myxoxanthophyll accumulation in the mutant strain. A new carotenoid was detected in this strain. Its chemical structure was close to that of myxoxanthophyll, but the hydroxyl group on the beta-ring was lacking. This compound, deoxy-myxoxanthophyll, most likely is an intermediate in the myxoxanthophyll biosynthesis pathway. Therefore, beta-carotene hydroxylase is involved not only in zeaxanthin synthesis but also in myxoxanthophyll synthesis in Synechocystis. Furthermore, myxoxanthophyll in Synechocystis was found to have a higher molecular mass than what was determined in other species. This difference is likely to be due to a difference in the sugar moiety.

Construction and characterization of hybrid hepatitis B antigen particles carrying a poliovirus immunogen.

The hepatitis B surface antigen (HBsAg) has the unique property of assembling with cellular lipids into spherical or elongated particles of 22 nm diameter which are secreted by mammalian cells expressing HBsAg. We have studied the structural requirements for particle formation and secretion by creating in-phase insertions into different regions of the S gene of the hepatitis B virus, coding for HBsAg. Modified genes were integrated into an appropriate vector and expressed in mouse L cells. Various single and double inserts in the two major hydrophilic domains of HBsAg were compatible with particle synthesis and secretion. The level of secretion was influenced by the length of the insert, its primary structure, and the site of insertion into the HBsAg molecule. One of the inserted sequences was a synthetic DNA fragment encoding a continuous type 1 poliovirus neutralization epitope (the C3 epitope). Mammalian cells expressing the modified hepatitis B virus S gene secreted hybrid particles carrying the poliovirus antigen. The hybrid polio-HBsAg particles reacted with a monoclonal antibody specific for the C3 epitope and induced poliovirus neutralizing antibodies at low, but significant, titers in mice and at high titers in rabbits. However, the immune response to HBsAg was weaker to hybrid particles than to unmodified HBsAg particles. By cotransfection with two different plasmids carrying either modified or unmodified genes, we obtained phenotypically mixed particles containing both polio-HBsAg and HBsAg molecules. Inoculated into rabbits, the mixed particles induced high antibody titers against both poliovirus and HBsAg.

Long term regeneration of cis-platinum and X ray treated human tumor nodules maintained in continuous organotypic culture.

A simple method for maintaining tumor cells in continuous three-dimensional culture, derived from Wolff's organotypic technique, has been used to study the effects of cis-platinum and X rays on growth inhibition, regrowth and long term regeneration of cultures maintained in low traumatizing conditions (absence of enzymatic dissociation of the cells and the possibility of avoiding subculturing, if necessary). The tumor nodules were derived from cells of the A 549 lung carcinoma cell line. The nodules developed an alveolar structure. After 1 h treatment with 15 micrograms/ml cis-platinum the growth of the nodules was slightly inhibited during the first 10 days, and then resumed normal growth. After treatment with 100 micrograms/ml cis-platinum, growth inhibition lasted longer and regrowth was observed after about 30 days. Treatment with 300 micrograms/ml of cis-platinum induced cell necrosis and loss of alveolar structures. Forty days later, regeneration occurred; two months after the drug treatment, the reconstituted nodules could be routinely subcultured. A single 15 Gy X ray dose (inducing a 0.005% survival in A 549 monolayer cells, n = 8; D0 = 1.4 Gy; Dq = 1.48) caused an early growth inhibition of about 25%. The alveolar structures disappeared. Alveolar structures reappeared in 17% of the nodules 50 days after irradiation. A slight regrowth was observed 90 days after the irradiation. A 549 nodules supported an about 20-fold higher cis-platinum concentration than monolayer cells. An almost lethal X ray dose (15 Gy, inducing a survival of 0.005%) for monolayer cells induced a prolonged lag phase in nodules followed by a slight but regular regrowth. These results support the idea that cells maintained in three dimensional culture are more resistant to radiation and drug-induced injury than monolayer cells. The organotypic culture method may be a useful tool for determining the activity of antitumoral agents.

Electroencephalographic effects of modafinil, an alpha-1-adrenergic psychostimulant, on the sleep of rhesus monkeys.

An electroencephalographic (EEG) study on the rhesus monkey, primate model of human sleep, showed the significant wakening effect of a new psychostimulant, Modafinil. The first experiment, with single administration of three increasing doses of Modafinil (3, 6, and 12 mg/kg), was to determine the efficient threshold dose; the second experiment, with repeated administration of 22.5 mg/kg during 4 days, was to determine whether continuous wakefulness could be obtained without apparent behavioral disorders. Results of the first experiment showed a wakening effect above 6 mg/kg, but 12 mg/kg induced a sharp wakening effect with a significant decrease in all sleep stages. The second experiment induced important insomnia in all subjects for 4 days and 4 nights. No behavioral disorder was observed. Modafinil has a wakening effect at the dose of 6 mg/kg in rhesus monkeys and induces quasicontinuous wakefulness for 4 total days and nights with daily administration of 22.5 mg/kg, with no behavioral disorders. Modafinil should therefore find interesting applications in sleep disorder treatments.

Slow-release caffeine: a new response to the effects of a limited sleep deprivation.

STUDY OBJECTIVES: The aim of this study is to assess the interest of the intake of a new galenic form of caffeine called "slow-release" caffeine (SR caffeine) during a decrease of vigilance due to a limited sleep deprivation. DESIGN: The controlled method used compared three doses of SR caffeine (150, 300 and 600 mg) with a placebo. Tests were performed 2, 9 and 13 hours after each treatment. Wakefulness level was assessed subjectively through questionnaires and analog visual scales, and objectively with the Multiple Sleep Latency Test. Performance level was also assessed regularly with an attention test, a grammatical reasoning test, a spatial recognition test, a mathematical processing test, a visual tracking test, a memory search test, and a dual task. The motor activity was evaluated by wrist actimeter and safety of treatment was observed by regular clinical examination. SETTING: NA. PARTICIPANTS: Twenty-four young and healthy volunteers (12 men and 12 women) participated in a 32-hour sleep deprivation. INTERVENTIONS: NA. RESULTS: The results show a significant effect of slow-release caffeine vs. placebo, and on vigilance and performance when subjects became tired. The effects of SR caffeine lasted 13 hours after treatment. SR caffeine 300 and 600 mg are efficacious doses but the optimal dose (maximum effect without any side effects) for both men and women is after all 300 mg. Globally, there is no difference between placebo and caffeine during the recovery night period. CONCLUSIONS: SR caffeine (300 mg) seems to be an efficient and safety substance to maintain a good level of vigilance and performance during limited sleep deprivation.

Caffeine or melatonin effects on sleep and sleepiness after rapid eastward transmeridian travel.

We measured the effects of slow-release caffeine (SRC) and melatonin (Mlt) on sleep and daytime sleepiness after a seven-time zone eastbound flight. In a double-blind, randomized, placebo-controlled study, each of three groups of nine subjects was given either 300 mg SRC on recovery day 1 (D1) to D5 (0800) or 5 mg Mlt on preflight D-1 (1700), flight day D0 (1600), and from D1 to D3 (2300), or placebo (Pbo) at the same times. Nighttime sleep was evaluated by polysomnography and daytime sleepiness from measurements of sleep latencies and continuous wrist actigraphy. Compared with baseline, we found a significant rebound of slow-wave sleep on night 1 (N1) to N2 under Pbo and Mlt and a significant decrease in rapid eye movement sleep on N1 (Pbo) and N1-N3 (Mlt). Sleepiness was objectively increased under Pbo (D1-D6) and Mlt (D1-D3). SRC reduced sleepiness but also tended to affect sleep quality until the last drug day. In conclusion, both drugs have positive effects on some jet lag symptoms after an eastbound flight: SRC on daytime sleepiness, and Mlt on sleep.