150 research outputs found
Ultrafast Acousto-Plasmonics in Gold Nanoparticles Superlattice
We report the investigation of the generation and detection of GHz coherent
acoustic phonons in plasmonic gold nanoparticles superlattices (NPS). The
experiments have been performed from an optical femtosecond pump-probe scheme
across the optical plasmon resonance of the superlattice. Our experiments allow
to estimate the collective elastic response (sound velocity) of the NPS as well
as an estimate of the nano-contact elastic stiffness. It appears that the
light-induced coherent acoustic phonon pulse has a typical in-depth spatial
extension of about 45 nm which is roughly 4 times the optical skin depth in
gold. The modeling of the transient optical reflectivity indicates that the
mechanism of phonon generation is achieved through ultrafast heating of the NPS
assisted by light excitation of the volume plasmon. These results demonstrate
how it is possible to map the photon-electron-phonon interaction in
subwavelength nanostructures
Anisotropy in the dielectric function of BiTe from first principles: From the UV-visible to the infrared range
The dielectric properties of BiTe, a layered compound crystallizing
in a rhombohedral structure, are investigated by means of first-principles
calculations at the random phase approximation level. A special attention is
devoted to the anisotropy in the dielectric function and to the local field
effects that strongly renormalize the optical properties in the UV-visible
range when the electric field is polarized along the stacking axis.
Furthermore, both the Born effective charges for each atom and the zone center
phonon frequencies and eigenvectors needed to describe the dielectric response
in the infrared range are computed. Our theoretical near-normal incidence
reflectivity spectras in both the UV-visible and infrared range are in fairly
good agreement with the experimental spectras, provided that the free carriers
Drude contribution arising from defects is included in the infrared response.
The anisotropic plasmon frequencies entering the Drude model are computed
within the rigid band approximation, suggesting that a measurement of the
reflectivity in the infrared range for both polarizations might allow to infer
not only the type of doping but also the level of doping.Comment: 29 pages, 10 figure
Quantum size effect on charges and phonons ultrafast dynamics in atomically controlled nanolayers of topological insulators Bi2Te3
This work was supported by the French Ministry of Education and Research, the CNRS, Region Pays de la Loire (CPER Femtosecond Spectroscopy equipment program) and the LIA-CNRS (Laboratoire International Associé) IM-LED. The partial financial support from National Science Center under project 2016/21/B/ST5/02531 is acknowledged. R. Rapacz was supported by FORSZT PhD fellowship.Heralded as one of the key elements for next generation spintronics devices, topological insulators (TIs) are now step by step envisioned as nanodevices like charge-to-spin current conversion or as Dirac fermions based nanometer Schottky diode for example. However, reduced to few nanometers, TIs layers exhibit a profound modification of the electronic structure and the consequence of this quantum size effect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. Here, thanks to a complete study of a set of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~6 nm, below which a spectacular reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2 Te3 In addition, we also evidence an A1g optical phonon mode softening together with the appearance of a thickness dependence of the photoinduced coherent acoustic phonons signals. This drastic evolution of the carriers and phonons dynamics might be due an important electron-phonon coupling evolution due to the quantum confinement. These properties have to be taken into account for future TIs-based spintronic devices.Centre National de la Recherche Scientifiqu
Coherent acoustic phonons generated by ultrashort terahertz pulses in nanofilms of metals and topological insulators
We report the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This
is demonstrated in metals and topological insulators by exciting an acoustic eigenmode in nanometric-sized
thin films. The efficiency of the coupling is quadratic in the terahertz electric field strength within the range
of investigation. Owing to a quantitative comparison between terahertz and near-infrared ultrashort pulse
excitations, we show that the process of acoustic phonon generation by terahertz radiation is mainly driven
by thermoelastic stress. While for the near-infrared light excitation the lattice temperature increase comes
from a rapid energy transfer from the hot carriers to the phonon bath during carrier intraband relaxation, the
thermoelastic stress induced by the terahertz electric field is linked to the scattering of the accelerated electrons
leading to an ultrafast Joule effect
Characterization of antibody response in asymptomatic and symptomatic SARS-CoV-2 infection
SARS-CoV-2 pandemic is causing high morbidity and mortality burden worldwide with unprecedented strain on health care systems. To investigate the time course of the antibody response in relation to the outcome we performed a study in hospitalized COVID-19 patients. As comparison we also investigated the time course of the antibody response in SARS-CoV-2 asymptomatic subjects. Study results show that patients produce a strong antibody response to SARS-CoV-2 with high correlation between different viral antigens (spike protein and nucleoprotein) and among antibody classes (IgA, IgG, and IgM and neutralizing antibodies). The antibody peak is reached by 3 weeks from hospital admission followed by a sharp decrease. No difference was observed in any parameter of the antibody classes, including neutralizing antibodies, between subjects who recovered or with fatal outcome. Only few asymptomatic subjects developed antibodies at detectable levels
Non-thermal transport of energy driven by photoexcited carriers in switchable solid states of GeTe
Phase change alloys have seen widespread use from rewritable optical discs to
the present day interest in their use in emerging neuromorphic computing
architectures. In spite of this enormous commercial interest, the physics of
carriers in these materials is still not fully understood. Here, we describe
the time and space dependence of the coupling between photoexcited carriers and
the lattice in both the amorphous and crystalline states of one phase change
material, GeTe. We study this using a time-resolved optical technique called
picosecond acoustic method to investigate the \textit{in situ} thermally
assisted amorphous to crystalline phase transformation in GeTe. Our work
reveals a clear evolution of the electron-phonon coupling during the phase
transformation as the spectra of photoexcited acoustic phonons in the amorphous
(-GeTe) and crystalline (-GeTe) phases are different. In particular
and surprisingly, our analysis of the photoinduced acoustic pulse duration in
crystalline GeTe suggests that a part of the energy deposited during the
photoexcitation process takes place over a distance that clearly exceeds that
defined by the pump light skin depth. In the opposite, the lattice
photoexcitation process remains localized within that skin depth in the
amorphous state. We then demonstrate that this is due to supersonic diffusion
of photoexcited electron-hole plasma in the crystalline state. Consequently
these findings prove the existence of a non-thermal transport of energy which
is much faster than lattice heat diffusion
New Materials and Technologies for Durability and Conservation of Building Heritage
The increase in concrete structures’ durability is a milestone to improve the sustainability of buildings and infrastructures. In order to ensure a prolonged service life, it is necessary to detect the deterioration of materials by means of monitoring systems aimed at evaluating not only the penetration of aggressive substances into concrete but also the corrosion of carbon-steel reinforcement. Therefore, proper data collection makes it possible to plan suitable restoration works which can be carried out with traditional or innovative techniques and materials. This work focuses on building heritage and it highlights the most recent findings for the conservation and restoration of reinforced concrete structures and masonry buildings
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