527 research outputs found
Temperature dependence of exciton recombination in semiconducting single-wall carbon nanotubes
We study the excitonic recombination dynamics in an ensemble of (9,4)
semiconducting single-wall carbon nanotubes by high sensitivity time-resolved
photo-luminescence experiments. Measurements from cryogenic to room temperature
allow us to identify two main contributions to the recombination dynamics. The
initial fast decay is temperature independent and is attributed to the presence
of small residual bundles that create external non-radiative relaxation
channels. The slow component shows a strong temperature dependence and is
dominated by non-radiative processes down to 40 K. We propose a quantitative
phenomenological modeling of the variations of the integrated photoluminescence
intensity over the whole temperature range. We show that the luminescence
properties of carbon nanotubes at room temperature are not affected by the
dark/bright excitonic state coupling
Ultra-coherent single photon source
We present a novel type of single photon source in solid state, based on the
coherent laser light scattering by a single InAs quantum dot. We demonstrate
that the coherence of the emitted single photons is tailored by the resonant
excitation with a spectral linewidth below the radiative limit. Our
ultra-coherent source opens the way for integrated quantum devices dedicated to
the generation of single photons with high degrees of indistinguishability
Convergence of a misanthrope process to the entropy solution of 1D problems
International audienceWe prove the convergence, in some strong sense, of a Markov process called "a misanthrope process" to the entropy weak solution of a one-dimensional scalar nonlinear hyperbolic equation. Such a process may be used for the simulation of traffic flows. The convergence proof relies on the uniqueness of entropy Young measure solutions to the nonlinear hyperbolic equation, which holds for both the bounded and the unbounded cases. In the unbounded case, we also prove an error estimate. Finally, numerical results show how this convergence result may be understood in practical cases
Time-resolved buildup of a photorefractive grating induced in Bi12SiO20 by picosecond light pulses
International audiencePhotorefractive gratings are induced with picosecond light pulses in a BSO crystal. Both experiment and calculations show a buildup of the effect governed by a diffusion of the excited charge carriers that occurs after illumination
Exploiting one-dimensional exciton-phonon coupling for tunable and efficient single-photon generation with a carbon nanotube
Condensed-matter emitters offer enriched cavity quantum electrodynamical
effects due to the coupling to external degrees of freedom. In the case of
carbon nanotubes a very peculiar coupling between localized excitons and the
one-dimensional acoustic phonon modes can be achieved, which gives rise to
pronounced phonon wings in the luminescence spectrum. By coupling an individual
nanotube to a tunable optical micro-cavity, we show that this peculiar
exciton-phonon coupling is a valuable resource to enlarge the tuning range of
the single-photon source while keeping an excellent exciton-photon coupling
efficiency and spectral purity. Using the unique flexibility of our scanning
fiber cavity, we are able to measure the efficiency spectrum of the very same
nanotube in the Purcell regime for several mode volumes. Whereas this
efficiency spectrum looks very much like the free-space luminescence spectrum
when the Purcell factor is small (large mode volume), we show that the
deformation of this spectrum at lower mode volumes can be traced back to the
strength of the exciton-photon coupling. It shows an enhanced efficiency on the
red wing that arises from the asymmetry of the incoherent energy exchange
processes between the exciton and the cavity. This allows us to obtain a tuning
range up to several hundred times the spectral width of the source
Importance and Sensitivity Analysis in Dynamic Reliability
International audienceIn dynamic reliability, the evolution of a system is governed by a piecewise deterministic Markov process, which is characterized by different input data. Assuming such data to depend on some parameter is an element of , our aim is to compute the first-order derivative with respect to each is an element of of some functionals of the process, which may help to rank input data according to their relative importance, in view of sensitivity analysis. The functionals of interest are expected values of some function of the process, cumulated on some finite time interval , and their asymptotic values per unit time. Typical quantities of interest hence are cumulated (production) availability, or mean number of failures on some finite time interval and similar asymptotic quantities. The computation of the first-order derivative with respect to is an element of is made through a probabilistic counterpart of the adjoint state method, from the numerical analysis field. Examples are provided, showing the good efficiency of this method, especially in case of a large
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