Direct and indirect excitation of Nd 3+ ions sensitized by Si nanocrystals embedded in a SiO 2 thin film

International audienceThe luminescence properties of Nd-doped SiO x layers containing silicon nanocrystals (Si-ncs) were investigated by steady state, time-dependent and power-dependent photoluminescence spectrometry and photoluminescence excitation experiments. Both direct and indirect excitation processes of Nd 3þ ions have been evidenced. The energy transfer mechanism between Si-ncs and Nd 3þ ions is favored by the overlap between the emission spectrum of confined excitons in Si-ncs and the Nd 3þ absorption from the ground state to 4 F 5=2 electronic level. The more intense Nd-related emission was obtained in samples containing 0.5 at. % of Nd and characterized by an indirect excitation cross section equal to 8 Â 10 À15 cm 2

Indirect excitation of Er3+ ions in silicon nitride films prepared by reactive evaporation

International audienceEr-doped silicon nitride films were obtained by reactive evaporation of silicon under a flow of nitrogen ions and were annealed at temperatures up to 1300°C. Samples were studied by infrared absorption and Raman spectrometries and by transmission electron microscopy. The 1.54 m Er-related photoluminescence ͑PL͒ was studied in relation with the structure with pump excitation at 488 and 325 nm. Steady-state PL, PL excitation spectroscopy, and time-resolved PL were performed. The results demonstrate that Er 3+ ions are indirectly excited both via silicon nanocrystals and via localized states in the silicon nitride matrix. Er-doped silicon-based materials have attracted much attention in the scientific community because of their potential use for optoelectronics. 1 Indeed, Er 3+ ions can emit sharp luminescence at 1.54 m, which is the commonly used wavelength for optical communications. The Er sensitization has been widely studied in Si rich SiO 2 layers. In silica containing silicon nanocrystals ͑Si-nc͒, the Er-related photolu-minescence is strongly improved due to a strong energy transfer from Si-nc to Er 3+ ions. 2-4 The Er 3+ ions can then be indirectly excited by Si-nc which have an absorption cross section several orders of magnitude higher than that of direct Er excitation. While SiN x is a particularly interesting host matrix for electrically pumped light-emitting devices, the Er excitation mechanism in silicon nitride films is still not clear. Similarly to the SiO x based samples, the sensitization of Er 3+ ions by Si nanoparticules has been reported in SiN x samples prepared by plasma enhanced chemical vapour deposition ͑PECVD͒ 5 or by magnetron sputtering. 6 However, some works have also demonstrated that indirect excitation of Er 3+ ions could occur via electronic states localized in the SiN x band tail states. 7,8 In this letter, we study the Er-related PL at 1.54 m in Er-doped silicon nitride thin films prepared by an ion-beam-assisted evaporation technique. The evolutions of the structure and of the PL properties with the annealing treatments are studied. It is demonstrated that the Er excitation is indirect and that Si-nc is able to improve the PL intensity. It is also shown that another indirect excitation path presumably exists in the amorphous SiN x matrix. Silicon was evaporated from an electron beam gun with a deposition rate equal to 0.1 nm/s. The 200 nm thick films were deposited on silicon substrates maintained at 100°C. The nitrogen ions were provided by an electron cyclotron resonance microwave plasma source. The nitrogen flow was regulated by maintaining the total pressure in the evaporation chamber at 2 ϫ 10 −5 Torr. The Er doping was performed from an effusion cell. Rutherford backscattering spectrom-etry was used to analyze the chemical content of the film. The Si, N, O, and Er atomic concentrations are equal to 47%, 48%, 5%, and 0.3%, respectively. The oxygen content is due to the low density of the layer and to exposure to the air. This concentration corresponds to a 12 at. % Si excess compared to the Si 3 N 4 equilibrium stoichiometry. The Fourier transform infrared ͑FTIR͒ experiments were carried out with a spectrometer with a resolution of 2 cm −1. Raman measurements were carried out with a mutichannel spectrometer equipped with a 1800 grooves mm −1 grating. The samples were excited by the 514 nm line from an argon laser. Transmission electron microscopy was performed with a 200 keV microscope. For the steady-state PL experiments, the samples were excited by a 30 mW He-Cd laser using the 325 nm line or by a 60 mW laser diode emitting at 488 nm. For the PL excitation ͑PLE͒ experiments, the samples were excited by an optical parametric oscillator laser. The PL signal was measured by a photomultiplier tube cooled at 190 K. For the time-resolved PL experiments, the samples were pumped by the 355 nm line of a frequency-tripled YAG:Nd laser. The laser pulse frequency, energy, and duration were typically equal to 10 Hz, 50 J, and 20 ns, respectively. The time response of the detection system was better than 1 s. Figure 1͑a͒ shows the FTIR spectra of the films for as-deposited sample and samples annealed at 1000 and 1100°C. The spectrum shows a very intense band at around 850 cm −1 , characteristic of the asymmetric stretching vibration of the SiN bonds. 9 The spectra are not significantly modified for annealing temperatures lower than 1000°C since only a 6 cm −1 shift occurs to higher wavenumbers. For higher annealing temperature, the peaks shift again a few cm −1 and a shoulder appears at high wavenumbers, demonstrating a modification of the SiN bonds, which could be correlated to the precipitation of Si-nc. 1

Radiation damage in the LHCb vertex locator

The LHCb Vertex Locator (VELO) is a silicon strip detector designed to reconstruct charged particle trajectories and vertices produced at the LHCb interaction region. During the first two years of data collection, the 84 VELO sensors have been exposed to a range of fluences up to a maximum value of approximately 45 × 1012 1 MeV neutron equivalent (1 MeV neq). At the operational sensor temperature of approximately −7 °C, the average rate of sensor current increase is 18 μA per fb−1, in excellent agreement with predictions. The silicon effective bandgap has been determined using current versus temperature scan data after irradiation, with an average value of Eg = 1.16±0.03±0.04 eV obtained. The first observation of n+-on-n sensor type inversion at the LHC has been made, occurring at a fluence of around 15 × 1012 of 1 MeV neq. The only n+-on-p sensors in use at the LHC have also been studied. With an initial fluence of approximately 3 × 1012 1 MeV neq, a decrease in the Effective Depletion Voltage (EDV) of around 25 V is observed. Following this initial decrease, the EDV increases at a comparable rate to the type inverted n+-on-n type sensors, with rates of (1.43±0.16) × 10−12 V/ 1 MeV neq and (1.35±0.25) × 10−12 V/ 1 MeV neq measured for n+-on-p and n+-on-n type sensors, respectively. A reduction in the charge collection efficiency due to an unexpected effect involving the second metal layer readout lines is observed

Performance of the LHCb vertex locator

The Vertex Locator (VELO) is a silicon microstrip detector that surrounds the proton-proton interaction region in the LHCb experiment. The performance of the detector during the first years of its physics operation is reviewed. The system is operated in vacuum, uses a bi-phase CO2 cooling system, and the sensors are moved to 7 mm from the LHC beam for physics data taking. The performance and stability of these characteristic features of the detector are described, and details of the material budget are given. The calibration of the timing and the data processing algorithms that are implemented in FPGAs are described. The system performance is fully characterised. The sensors have a signal to noise ratio of approximately 20 and a best hit resolution of 4 μm is achieved at the optimal track angle. The typical detector occupancy for minimum bias events in standard operating conditions in 2011 is around 0.5%, and the detector has less than 1% of faulty strips. The proximity of the detector to the beam means that the inner regions of the n+-on-n sensors have undergone space-charge sign inversion due to radiation damage. The VELO performance parameters that drive the experiment's physics sensitivity are also given. The track finding efficiency of the VELO is typically above 98% and the modules have been aligned to a precision of 1 μm for translations in the plane transverse to the beam. A primary vertex resolution of 13 μm in the transverse plane and 71 μm along the beam axis is achieved for vertices with 25 tracks. An impact parameter resolution of less than 35 μm is achieved for particles with transverse momentum greater than 1 GeV/c

Precision luminosity measurements at LHCb

Measuring cross-sections at the LHC requires the luminosity to be determined accurately at each centre-of-mass energy √s. In this paper results are reported from the luminosity calibrations carried out at the LHC interaction point 8 with the LHCb detector for √s = 2.76, 7 and 8 TeV (proton-proton collisions) and for √sNN = 5 TeV (proton-lead collisions). Both the "van der Meer scan" and "beam-gas imaging" luminosity calibration methods were employed. It is observed that the beam density profile cannot always be described by a function that is factorizable in the two transverse coordinates. The introduction of a two-dimensional description of the beams improves significantly the consistency of the results. For proton-proton interactions at √s = 8 TeV a relative precision of the luminosity calibration of 1.47% is obtained using van der Meer scans and 1.43% using beam-gas imaging, resulting in a combined precision of 1.12%. Applying the calibration to the full data set determines the luminosity with a precision of 1.16%. This represents the most precise luminosity measurement achieved so far at a bunched-beam hadron collider

Pleistocene Tectono-magmato-volcanic events recorded east of Mayotte - insights for the ongoing seismo-volcanic crisis

The offshore eastern Mayotte area has been extensively studied since the outbreak of the seismo-volcanic crisis in May 2018. Several oceanographic surveys have been carried out by REVOSIMA – https://doi.org/10.18715/MAYOTTE.REVOSIMA (MAYOBS cruises -https://doi.org/10.18142/291) for monitoring purpose, or for academic research (SISMAORE - https://doi.org/10.17600/18001331 and SCRATCH cruises - https://doi.org/10.17600/18002274). They offer a complete coverage of the eastern slope and abyssal plain with multibeam bathymetry and backscatter imagery, sub-bottom profiler (SBP), and several sediment cores. This set of data offers the opportunity to describe the morphology of the area and details the first tens of meters of the sedimentary succession on the abyssal plain.We discover several features including faults, domes and massive chaotic deposits that developed or occurred during Quaternary in the vicinity of the new Fani Maore Volcano. Domes are interpreted as forced folds related to the intrusion of a large sill at depth. The main fault crossing the largest forced fold is a succession of en-echelon segments with vertical throws of up to 8 meters and a preferential N130° strike compatible with the present day regional dextral context. Analysis of the SBP profiles reveals that faulting and doming associated to sill intrusion occurred simultaneously, and together with the deposition of two massive (several km3) and chaotic lobes at the foot of the Mayotte slope. Sediment cores collected over or close to the massive deposits indicate a mixed bioclastic-volcanoclastic content with a large amount of pumices, whose aspect and chemical composition are identical to volcanic edifices located on the upper slope (Horseshoe Volcano) or onshore (Petite-Terre). Backscatter imagery also reveals streaks that cover the lobes and trace back to the same upper slope area. Analysis of bioclasts from a core catcher stopped in the upper part of the deposit shows a large variety of foraminifers, shells fragments, with a little proportion originating from the upper-slope and shelf. We propose that these massive lobe deposits might have occurred as the pumice-dominated material ejected at HV or PT flows downslope, thus reworking and incorporating a substantial amount of hemipelagic deposits along the slope and over the abyssal plain. These observations suggests that a tectono-magmato-volcanic event occurred during recent geological time (Late Pleistocene according to early estimations). At least one comparable set of similar and synchronous features appears on our dataset thus implying the occurrence of a similar event, earlier in the Pleistocene. The correlations between these events and the activity at HV and PT volcanic centers are critical, as it would provide a recurring scenario to compare with present day seismo-volcanic crisis at Mayotte.This work is funded by REVOSIMA, ANR funded COYOTES project, and internal BRGM Project PDEV MAYOTTE.COmores & maYotte : vOlcanisme, TEctonique et Sismicit

Search for CP violation in D+→ϕπ+ and D+s→K0Sπ+ decays

A search for CP violation in D + → ϕπ + decays is performed using data collected in 2011 by the LHCb experiment corresponding to an integrated luminosity of 1.0 fb−1 at a centre of mass energy of 7 TeV. The CP -violating asymmetry is measured to be (−0.04 ± 0.14 ± 0.14)% for candidates with K − K + mass within 20 MeV/c 2 of the ϕ meson mass. A search for a CP -violating asymmetry that varies across the ϕ mass region of the D + → K − K + π + Dalitz plot is also performed, and no evidence for CP violation is found. In addition, the CP asymmetry in the D+s→K0Sπ+ decay is measured to be (0.61 ± 0.83 ± 0.14)%

Differential branching fraction and angular analysis of the decay B0→K∗0μ+μ−

The angular distribution and differential branching fraction of the decay B 0→ K ∗0 μ + μ − are studied using a data sample, collected by the LHCb experiment in pp collisions at s√=7 TeV, corresponding to an integrated luminosity of 1.0 fb−1. Several angular observables are measured in bins of the dimuon invariant mass squared, q 2. A first measurement of the zero-crossing point of the forward-backward asymmetry of the dimuon system is also presented. The zero-crossing point is measured to be q20=4.9±0.9GeV2/c4 , where the uncertainty is the sum of statistical and systematic uncertainties. The results are consistent with the Standard Model predictions

Model-independent search for CP violation in D0→K−K+π−π+ and D0→π−π+π+π− decays

A search for CP violation in the phase-space structures of D0 and View the MathML source decays to the final states K−K+π−π+ and π−π+π+π− is presented. The search is carried out with a data set corresponding to an integrated luminosity of 1.0 fb−1 collected in 2011 by the LHCb experiment in pp collisions at a centre-of-mass energy of 7 TeV. For the K−K+π−π+ final state, the four-body phase space is divided into 32 bins, each bin with approximately 1800 decays. The p-value under the hypothesis of no CP violation is 9.1%, and in no bin is a CP asymmetry greater than 6.5% observed. The phase space of the π−π+π+π− final state is partitioned into 128 bins, each bin with approximately 2500 decays. The p-value under the hypothesis of no CP violation is 41%, and in no bin is a CP asymmetry greater than 5.5% observed. All results are consistent with the hypothesis of no CP violation at the current sensitivity