Tantalum STJ for Photon Counting Detectors

Superconducting Tunnel Junctions (STJ's) are currently being developed as photon detectors for a wide range of applications. Interest comes from their ability to cumulate photon counting with chromaticity (i.e. energy resolution) from the near infrared (2 $\mu$m) to the X-rays wavelengths and good quantum efficiency up to 80%. Resolving power can exceed 10 in the visible wavelength range. Our main goal is to use STJ's for astronomical observations at low light level in the near infrared. This paper put the emphasis on two main points: the improvement of the tantalum absorber epitaxy and the development of a new version of the fabrication process for making Ta/Al-AlOx-Al/Ta photon counting STJ's. The main features of this process are that pixels have aligned electrodes and vias patterned through a protecting SiO2 layer. These vias are then used to contact the top electrode layer. We use a double thin aluminum trapping layer on top of a 150 nm thick Ta absorber grown epitaxially. Photon counting experiments with Ta junction array are presented at \lambda = 0.78 $\mu$m. Digital filtering methods are used to compute the photon counting data in order to minimize the effects of noise

High-resolution spatial mapping of a superconducting NbN wire using single-electron detection

Superconducting NbN wires have recently received attention as detectors for visible and infrared photons. We present experiments in which we use a NbN wire for high-efficiency (40 %) detection of single electrons with keV energy. We use the beam of a scanning electron microscope as a focussed, stable, and calibrated electron source. Scanning the beam over the surface of the wire provides a map of the detection efficiency. This map shows features as small as 150 nm, revealing wire inhomogeneities. The intrinsic resolution of this mapping method, superior to optical methods, provides the basis of a characterization tool relevant for photon detectors.Comment: 2009 IEEE Toronto International Conference, Science and Technology for Humanity (TIC-STH

Champs électromagnétiques et vulnérabilité cérébrale

Parmi les innovations récentes, la téléphonie mobile a connu une progression fulgurante. Une conséquence en est que les expositions aux champs électromagnétiques (CEM) apparaissent de plus en plus tôt au cours de la vie (lors de l’adolescence, voire dès l’enfance), ce qui suscite des questionnements quant à leurs effets potentiels sur la santé humaine, en particulier sur le cerveau. Le projet "INFLAREF" visait à tester l’hypothèse d’une interaction entre les radiofréquences et le système nerveux, lorsque celui-ci se trouve en état de vulnérabilité parce qu’il est en phase de développement ou en lien avec l’exposition à des agents agresseurs

Effets des RF sur le système nerveux central : sommeil, EEG, vascularisation, cognition

National audienceLa plupart des études cliniques avec des champs RF ont étudié les effets d'expositions caractéristiques des téléphones portables, habituellement au niveau de la tête, sur un certain nombre de paramètres physiologiques comprenant le sommeil, l'activité électrique du cerveau, la cognition, la vascularisation cérébrale et plus généralement les systèmes cardiovasculaire et endocrinien. La majorité des études ont été effectuées chez des adultes en bonne santé. Il apparaît maintenant important d'étudier les effets des champs RF chez les enfants et les adolescents, étant donné la connaissance croissante d'une maturation continue du cerveau jusqu'à un stade avancé de l'adolescence, et quelques études récentes ont été effectuées avec des écoliers. Plusieurs études ont également porté sur des adultes qui déclarent être" électrosensibles "

Effects of 900 MHz radiofrequency on corticosterone, emotional memory and neuroinflammation in middle-aged rats

International audienceThe widespread use of mobile phones raises the question of the effects of electromagnetic fields (EMF, 900 MHz) on the brain. Previous studies reported increased levels of the glial fibrillary acidic protein (GFAP) in the rat's brain after a single exposure to 900 MHz global system for mobile (GSM) signal, suggesting a potential inflammatory process. While this result was obtained in adult rats, no data is currently available in older animals. Since the transition from middle-age to senescence is highly dependent on environment and lifestyle, we studied the reactivity of middle-aged brains to EMF exposure. We assessed the effects of a single 15 min GSM exposure (900 MHz ; specific absorption rate (SAR)=6 W/kg) on GFAP expression in young adults (6 week-old) and middle-aged rats (12 month-old). Brain interleukin (IL)-1ß and IL-6, plasmatic levels of corticosterone (CORT), and emotional memory were also assessed. Our data indicated that, in contrast to previously published work, acute GSM exposure did not induce astrocyte activation. Our results showed an IL-1ß increase in the olfactory bulb and enhanced contextual emotional memory in GSM-exposed middle-aged rats, and increased plasmatic levels of CORT in GSMexposed young adults. Altogether, our data showed an age dependency of reactivity to GSM exposure in neuro-immunity, stress and behavioral parameters. Reproducing these effects and studying their mechanisms may allow a better understanding of mobile phone EMF effects on neurobiological parameters

Effets des radiofréquences sur le système nerveux central chez l'homme : EEG, sommeil, cognition, vascularisation

International audienceMost of clinical studies on radiofrequency electromagnetic fields (RF) were directed at mobile phone-related exposures, usually at the level of the head, at their effect on some physiological functions including sleep, brain electrical activity (EEG), cognitive processes, brain vascularisation, and more generally on the cardiovascular and endocrine systems. They were frequently carried out on healthy adults. Effects on the amplitude of EEG alpha waves, mainly during sleep, look reproducible. It would however be important to define more precisely whether and how the absence of electromagnetic disturbance between RF exposure and the recording systems is checked. No consensus arises about cognitive effects. Some effects on cerebral vascularisation need complementary work.La plupart des études cliniques avec des champs radiofréquences (RF) ont porté sur les effets d'expositions caractéristiques des téléphones portables, habituellement au niveau de la tête, sur un certain nombre de fonctions physiologiques comprenant le sommeil, l'activité électrique du cerveau (EEG), la cognition, la vascularisation cérébrale et, plus généralement, les systèmes cardiovasculaire et endocrinien. La majorité des études a été effectuée chez des adultes en bonne santé. L'effet décrit sur l'amplitude des ondes alpha de l'EEG, notamment pendant le sommeil, semble reproductible. Il semble cependant important de préciser si et comment l'absence d'interférence entre l'exposition RF et l'enregistrement est vérifiée. Il ne se dégage pas de consensus sur des effets cognitifs. Quelques effets décrits sur la vascularisation cérébrale nécessitent des études complémentaires

Magnetic Screening of NbN Multilayers Samples

6 pagesInternational audienceIn 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > Hc^Nb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities [1]. We have deposited high quality "model" samples by magnetron sputtering on monocrystalline sapphire substrates. A 250 nm layer of niobium figures the bulk Nb. It was coated with a single and multi-stacks of NbN layers (25 or12 nm) separated by 15 nm MgO barriers, and characterized by X-Ray reflectivity and DC transport measurements. DC or AC measurement of HC1 is an important goal for multilayer evaluation during the sample evaluation phase. A clear increase of HC1 at low frequency is promising indication since HC1 is expected to increase with frequency (see e.g. [2] and references therein). We have measured the first penetration field (HP~HC1) on DC magnetization curves in a SQUID system. HP of NbN covered sample is increased compared to Nb alone. We have also developed a set-up that allows measuring a large range of field and temperature with a local probe method based on 3rd harmonic analysis. We have confirmed the screening behavior of a single 25 nm NbN layer placed on the top of a Nb Layer

Characterization of superconducting multilayers samples

Best RF bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H 200 mT close to the thermodynamic critical field Hc. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > HcNb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities [1]. Depositing good quality layers inside a whole cavity is rather difficult but we have sputtered high quality samples by applying the technique used for the preparation of superconducting electronics circuits and characterized these samples by X-ray reflectivity, dc resistivity (PPMS) and dc magnetization (SQUID). Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating clearly exhibit different behaviors. Because SQUID measurements are influenced by edge and shape effects we propose to develop a specific local magnetic measurement of HC1 based on ac third harmonic analysis in order to reveal the screening effect of multilayers

Intrinsic nonlinearity probed by intermodulation distortion microwave measurements on high quality MgB2 thin films

The two tone intermodulation distortion arising in MgB2 thin films synthesized by hybrid physical-chemical vapour deposition is studied in order to probe the influence of the two bands on the nonlinear response of this superconductor. The measurements are carried out by using a dielectrically loaded copper cavity operating at 7 GHz. Microwave data on samples having critical temperatures above 41 K, very low resistivity values, and residual resistivity ratio larger than 10, are shown. The dependence of the nonlinear surface losses and of the third order intermodulation products on the power feeding the cavity and on the temperature is analyzed. At low power, the signal arising from distortion versus temperature shows the intrinsic s-wave behavior expected for this compound. Data are compared with measurements performed on Nb and YBCO thin films using the same technique.Comment: Revised version. Submitted to APL on October 7th 200

Spectral dependency of superconducting single photon detectors

International audienceWe investigate the effect of varying both incoming optical wavelength and width of NbN nanowires on the superconducting single photondetectors (SSPD) detection efficiency. The SSPD are current biased close to critical value and temperature fixed at 4.2 K, far from transition. The experimental results are found to verify with a good accuracy predictions based on the "hot spot model," whose size scales with the absorbed photon energy. With larger optical power inducing multiphoton detection regime, the same scaling law remains valid, up to the three-photon regime. We demonstrate the validity of applying a limited number of measurements and using such a simple model to reasonably predict any SSPD behavior among a collection of nanowire device widths at different photon wavelengths. These results set the basis for designing efficient single photondetectors operating in the infrared (2-5 μm range)