The LCAR (Laboratoire Collisions-Agrégats-Réactivité) is a laboratory centered in fundamental physics organized around two main fields
The study of laser-matter interaction focuses on matter waves studies and strong-field physics
The study of molecular structures and dynamics develops the study and measurement of the properties of clusters, molecules of biological interest, and nano-objects in their environment
[hal-02904427] The sonorous beat of optical waves in a Michelson interferometer (28/07/2020) Moins
This paper describes time-dependent phase modulation experiments using an optical Michelson interferometer. Changes in the path length of the interferometer arms induce phase shifts in time and, thanks to a loudspeaker, make the interferometric signal audible. We produce time-fluctuating refractive indices and mechanical motions of one reflecting mirror. A sinusoidal motion is applied, thus creating a phase modulation of the wave in one of the interferometer arms. Sidebands of the laser frequency appear, which interfere with the other interferometer wave. Our hearing sense is quite developed, and we use it to analyze the non-linear character by the acoustic timbre. Finally, we transfer information by phase modulation. Our setup benefits the visually impaired community as temporal phase changes can be analyzed by the auditory sense, but this unconventional use of an optical interferometer is also attractive to a general audience.
[hal-02895475] Accuracy of neural networks for the simulation of chaotic dynamics: precision of training data vs precision of the algorithm (16/07/2020) Plus
We explore the influence of precision of the data and the algorithm for the simulation of chaotic dynamics by neural networks techniques. For this purpose, we simulate the Lorenz system with different precisions using three different neural network techniques adapted to time series, namely reservoir computing (using ESN), LSTM and TCN, for both short and long time predictions, and assess their efficiency and accuracy. Our results show that the precision of the algorithm is more important than the precision of the training data for the accuracy of the predictions. This result gives support to the idea that neural networks can perform time-series predictions in many practical applications for which data are necessarily of limited precision, in line with recent results. It also suggests that for a given set of data the reliability of the predictions can be significantly improved by using a network with higher precision than the one of the data.
[hal-02881940] Fragmentation dynamics of Ar4He1000 upon electron impact ionization: competition between ion ejection and trapping (09/07/2020) Plus
The fragmentation upon electron impact ionization of Ar4He1000 is investigated by means of mixed quantum-classical dynamics simulations. The Ar4+ dopant dynamics is described by a surface hopping method coupled with a diatomics-in-molecules model to properly take into account the multiple Ar4+ electronic surfaces and possible transitions between them. Helium atoms are treated individually using the zero-point averaged dynamics (ZPAD), a method based on the building of an effective He-He potential. Fast electronic relaxation is observed, from less than 2 ps to ∼ 30 ps depending on initial conditions. The main fragments observed are Ar2+Heq and Ar3+Heq (q ≤ 1000), with a strong contribution of the bare Ar2+ ion, and neither Ar+ nor Ar+Heq fragments are found. The smaller fragments (q ≤ 50) are found to mostly come from ion ejection whereas larger fragments (q > 500) originate from long-term ion trapping. Although the structure of the trapped Ar2+ ions is the same as in the gas phase, trapped Ar3+ and Ar4+ are rather slightly bound Ar2+ · · · Ar and Ar2+ · · · Ar · · · Ar structures (i.e., an Ar2+ core with one or two argon atoms roaming within the droplet). These loose structures can undergo geminate recombination and release Ar3+Heq or Ar4+Heq (q ≤ 50) in the gas phase and/or induce strong helium droplet evaporation. Finally, the translational energy of the fragment center of mass was found suitable to provide a clear signature of the broad variety of processes at play in our simulations.
[hal-02481944] Spontaneous emission and energy shifts of a Rydberg rubidium atom close to an optical nanofiber (11/06/2020) Plus
In this paper, we report on numerical calculations of the spontaneous emission rates and Lamb shifts of a $^{87}\text{Rb}$ atom in a Rydberg-excited state $\left(n\leq30\right)$ located close to a silica optical nanofiber. We investigate how these quantities depend on the fiber's radius, the distance of the atom to the fiber, the direction of the atomic angular momentum polarization as well as the different atomic quantum numbers. We also study the contribution of quadrupolar transitions, which may be substantial for highly polarizable Rydberg states. Our calculations are performed in the macroscopic quantum electrodynamics formalism, based on the dyadic Green's function method. This allows us to take dispersive and absorptive characteristics of silica into account; this is of major importance since Rydberg atoms emit along many different transitions whose frequencies cover a wide range of the electromagnetic spectrum. Our work is an important initial step towards building a Rydberg atom-nanofiber interface for quantum optics and quantum information purposes.