Resonant Excitonic Optical Stark Effect In Gase

The time-resolved nonlinear transmission of bulk ε-GaSe has been studied in the femtosecond regime when resonantly exciting the material in the vicinity of the exciton at room temperature. Two regimes are evidenced. At early time delay, a blue shift of the exciton with no linewidth broadening can be related to optical Stark effect, while at longer time delay the usual exciton screening and band-gap renormalization due to real electronic transitions is observed. At resonance, a dependence of the Stark shift with the amplitude of the exciting field is obtained, as predicted by a simple "dressed- atom" model.55222307230

Photonic band-gap effects on photoluminescence of silicon nanocrystals embedded in artificial opals

Si nanocrystals were formed in synthetic opals by Si-ion implantation and their optical properties studied using microphotoluminescence and reflection techniques. The properties of areas with high crystalline quality are compared with those of disordered regions of samples. The photoluminescencespectrum from Si nanocrystals embedded in silica spheres is narrowed by the inhibition of emission at wavelengths corresponding to the opalphotonic pseudoband gap (∼690 nm). Measurements of photoluminescencespectra from individual implanted silica spheres is also demonstrated and the number of emitting Si nanocrystals in single brightly emitting spheres is estimated to be of the order of one thousand.This work was supported by GACR (202/03/0789), NATO (PST.CLG.978100), and by the Royal Swedish Academy of Sciences. One of the authors ~J.V.! appreciates financial support from the French government (program Echange)

Effects of electromagnetic waves on the electrical properties of contacts between grains

A DC electrical current is injected through a chain of metallic beads. The electrical resistances of each bead-bead contacts are measured. At low current, the distribution of these resistances is large and log-normal. At high enough current, the resistance distribution becomes sharp and Gaussian due to the creation of microweldings between some beads. The action of nearby electromagnetic waves (sparks) on the electrical conductivity of the chain is also studied. The spark effect is to lower the resistance values of the more resistive contacts, the best conductive ones remaining unaffected by the spark production. The spark is able to induce through the chain a current enough to create microweldings between some beads. This explains why the electrical resistance of a granular medium is so sensitive to the electromagnetic waves produced in its vicinity.Comment: 4 pages, 5 figure

Towards nanoprinting with metals on graphene

Graphene and carbon nanotubes are envisaged as suitable materials for the fabrication of the new generation of nanoelectronics. The controlled patterning of such nanostructures with metal nanoparticles is conditioned by the transfer between a recipient and the surface to pattern. Electromigration under the impact of an applied voltage stands at the base of printing discrete digits at the nanoscale. Here we report the use of carbon nanotubes as nanoreservoirs for iron nanoparticles transfer on few-layer graphene. An initial Joule-induced annealing is required to ensure the control of the mass transfer with the nanotube acting as a `pen' for the writing process. By applying a voltage, the tube filled with metal nanoparticles can deposit metal on the surface of the graphene sheet at precise locations. The reverse transfer of nanoparticles from the graphene surface to the nanotube when changing the voltage polarity opens the way for error corrections

Effect of Composition on Electrical and Optical Properties of Thin Films of Amorphous GaxSe100−x Nanorods

We report the electrical and optical studies of thin films of a-GaxSe100−x nanorods (x = 3, 6, 9 and 12). Thin films of a-GaxSe100−x nanorods have been synthesized thermal evaporation technique. DC electrical conductivity of deposited thin films of a-GaxSe100−x nanorods is measured as a function of temperature range from 298 to 383 K. An exponential increase in the dc conductivity is observed with the increase in temperature, suggesting thereby a semiconducting behavior. The estimated value of activation energy decreases on incorporation of dopant (Ga) content in the Se system. The calculated value of pre-exponential factor (σ0) is of the order of 101 Ω−1 cm−1, which suggests that the conduction takes place in the band tails of localized states. It is suggested that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges. On the basis of the optical absorption measurements, an indirect optical band gap is observed in this system, and the value of optical band gap decreases on increasing Ga concentration

Biological/Biomedical Accelerator Mass Spectrometry Targets. 2. Physical, Morphological, and Structural Characteristics

The number of biological/biomedical applications that require AMS to achieve their goals is increasing, and so is the need for a better understanding of the physical, morphological, and structural traits of high quality of AMS targets. The metrics of quality included color, hardness/texture, and appearance (photo and SEM), along with FT-IR, Raman, and powder X-ray diffraction spectra that correlate positively with reliable and intense ion currents and accuracy, precision, and sensitivity of fraction modern (Fm). Our previous method produced AMS targets of gray-colored iron−carbon materials (ICM) 20% of the time and of graphite-coated iron (GCI) 80% of the time. The ICM was hard, its FT-IR spectra lacked the sp2 bond, its Raman spectra had no detectable G′ band at 2700 cm−1, and it had more iron carbide (Fe3C) crystal than nanocrystalline graphite or graphitizable carbon (g-C). ICM produced low and variable ion current whereas the opposite was true for the graphitic GCI. Our optimized method produced AMS targets of graphite-coated iron powder (GCIP) 100% of the time. The GCIP shared some of the same properties as GCI in that both were black in color, both produced robust ion current consistently, their FT-IR spectra had the sp2 bond, their Raman spectra had matching D, G, G′, D+G, and D′′ bands, and their XRD spectra showed matching crystal size. GCIP was a powder that was easy to tamp into AMS target holders that also facilitated high throughput. We concluded that AMS targets of GCIP were a mix of graphitizable carbon and Fe3C crystal, because none of their spectra, FT-IR, Raman, or XRD, matched exactly those of the graphite standard. Nevertheless, AMS targets of GCIP consistently produced the strong, reliable, and reproducible ion currents for high-throughput AMS analysis (270 targets per skilled analyst/day) along with accurate and precise Fm values

FEMTOSECOND REFLECTIVITY MEASUREMENT OF HIGHLY PHOTOEXCITED SILICON

Nous décrivons dans une première partie les progrès récents qui ont permis de descendre la durée des impulsions laser dans le domaine femtoseconde. Nous montrons ensuite l'application de cette technique à la très forte photoexcitation du silicium et présentons des mesures de réflectivité résolues dans le temps, à diverses longueurs d'ondes que nous interprétons à travers des modèles simples.Recent advances have taken place in optical pulse generation techniques pushing optical pulse widths down into the femtosecond domain. Time resolved reflectivity measurements at various wavelength are presented and interpreted in the form of very simple models

Sur un préamplificateur laser femtoseconde de conception optique simple

We have designed a very simple, compact, all reflective optical set up for a multipass laser amplifier. This scheme allows an amplification factor of up to 10 000 for 100 fs light pulses, while minimizing group velocity dispersion.Nous avons réalisé un dispositif optique simple et compact permettant d'amplifier des impulsions lumineuses d'une durée de 100 fs en minimisant l'importance de la dispersion de la vitesse de groupe. Le montage permet le passage multiple des impulsions dans une même région de gain réalisée dans un jet et un gain total de l'ordre de 10 000 pour six passages

Quantum evaporation: An optical transposition

This work shows experimental evidence for a new quantum evaporation effect theoretically discovered by Bardou and Boosdt. In this effect the tunnelling probability of a particle through a barrier strongly increases when a small momentum is transferred to the particle during its reflection on the barrier. In our experiment we studied the induced tunnelling of a short optical pulse on a Newton-type optical barrier

Diffusion inélastique de la lumière à la résonance dans GaAs1-xP x

Resonant Raman scattering has been studied in Ga(AsP) by varying the energy level through the sample temperature, while leaving the incident photon energy constant. The scattered intensity variation due to the normal mode of vibrations in the first order spectrum and in the first replica shows that résonance takes place when the energy of the scattered photons is in coincidence with the energy of the direct band gap.La diffusion résonnante de la lumière a été étudiée dans Ga(AsP), en maintenant fixe la longueur d'onde du laser excitateur et en déplaçant les niveaux d'énergie de l'alliage par la variation de la température appliquée. Les courbes de résonance pour les modes normaux du spectre du premier ordre et de la première réplique, montrent que la résonance des répliques a lieu lorsque l'énergie du photon diffusé est en coïncidence avec la largeur de la bande d'énergie interdite