Septembre 2017

Vendredi, 8 Septembre, 2017


11h - Site Sophia

Sensitizing rare earth ions in optical materials: a challenge with high potential impact

The spectroscopic properties of rare earth ions (RE3+), which are characterized by specific narrow photoluminescence emissions and long luminescence lifetimes, make them suitable for many optical applications. Their use is well established in phosphors for lighting and light-emitting devices, light amplifiers, lasers, spectral conversion layers (up- and down-conversion), and optical biomarkers. However, major limitations are related to their small absorption cross section and limited absorption spectral bandwidth, which reduce their effective implementation and use. Therefore, a significant field of research is devoted to the development of sensitizing strategies, from the synthesis of organic ligands for rare earth complexes, to the coupling with other rare earth ions (like in Tb-Yb or Yb-Er codoped systems) or with semiconductor or metal nanoaggregates that act as energy-transfer centers.

This seminar will be focused on the sensitizing process of rare earth ions, highlighting how the photoluminescence characterization techniques can deepen the understanding of the physics of the interaction. As an example, I will illustrate the advantages that can be provided by silicon or silver nanoaggregates for a broadband enhancement of the efficiency of Er3+ ions for optical amplification. Moreover, I will present the increasing of quantum cutting efficiency by proper optimization of Tb3+/Yb3+-codoping in down-converting glasses and glass-ceramics for spectral conversion in solar cells and the possibilities of further improvements by the introduction of Ag nanoaggregates in the system.

These and other examples will be discussed, with a focus on the high potential impact of this field of investigation, suggesting perspectives for future research developments.


Jeudi, 14 Septembre, 2017


11h - Site Sophia

Nonlinear Physics of Bose Einstein Condensates in optical Ring Resonators

A Bose Einstein condensate inside the light mode of an optical resonator is probably the most fundamental scenario of light interacting with a bunch of atoms. In fact, this scenario turned out to be a surprisingly rich test bed to investigate basic phenomena such as quantum self organization, super and sub radiance or Dicke type phase transitions. I will report about our latest experimental results with atoms in a ring resonator which can be related to the physics of "exceptional points" and to transitions between stable and unstable phases of open systems.

Vendredi, 15 Septembre, 2017


11h - Site Valrose

Fibre and Waveguide Lasers: from materials to devices (with an eye on applications)

This seminar reviews the work done in fibre and waveguide laser and amplifier and propose future research lines. Emphasize will be on the generation of new wavelength regimes, as Mid-Infrared laser action using different pumping schemes and its potential impact on healthcare. The talk will also cover generation of visible/UV continuum, based on non-linear tailored microstructured fibres where high-order mode coupling and propagation is exploited, and its applications. The seminar also reviews the work done on photodarkening investigation, measurements and mitigation in fiber lasers. We present the outcomes were we link the glass composition to active fiber properties and we propose a model for photodarkening in Yb-doped fibers. We demonstrate a mitigation method based on photobleanching and we show how an accurate spectroscopic measurement of Yb excited state absorption spectrum lead to its optimization. We finally describe a set-up to provide fiber manufacturers a reliable method of evaluation of photodarkening in active fiber.

Jeudi, 21 Septembre, 2017


10h30 — Jean DECAMP

Titre : Symmetries of One-dimentional Strongly Correlated Quantum Mixtures
Encadrement : Mathias Albert, Patrizia Vignolo
Résumé : Ultracold atom experiments allow to engineer and probe, with an incredible and always improving precision, a yet inaccessible variety of many-body quantum systems. One-dimensional models, which display several unique features associated to the reduced dimensionality, are the object of intense theoretical and experimental interest. Quantum gases in one dimension can be realized in actual experiments by trapping atoms in tight optical waveguides. Moreover, these experiments offer the possibility to tune the interactions between the atoms, and hence to access the strongly correlated regime for various kinds of quantum mixtures. In these systems, the exchange symmetry between identical particles is fixed by their bosonic or fermionic nature, but not between distinguishable particles. Thus, a natural question is this one : How to characterize, both theoreti- cally and experimentally, the global exchange symmetry of the many-body wavefunction in our system ?
In this talk, I will first try to define all these concepts, and to explain how this question is interesting. I will then briefly present the theoretical results obtained during my two first years of PhD, which bring some elements of a response.


11h — Guido SCHIFANI

Titre : Shape and dynamics of semi-conductor islands in hetero-epitaxy
Encadrement : Médéric Argentina, Thomas Frisch
Résumé : The shapes of nano-crystals have a great interest in science both from fundamental and applied perspectives. The formation, morphology and properties of a nano-crystal (islands) results from the competition between several subtle effects, such as for example temperature, mechanical stresses and surface energy anisotropy.
In this work, I describe the morphology and the dynamic of an elastically strained semi-conductor film in hetero-epitaxy using the framework of continuum elasticity. This island undergoes a Grinfeld instability and their morphology play an important role in the optical properties of quantum dots.
I investigate the formation and the coarsening dynamics of an array of islands in a strained epitaxial semiconductor film using a new analytical method. This model takes into account the presence of the wetting layer, the effect of elasticity and capillarity (surface energy) and it includes surface diffusion. I describe the dynamic of coarsening using both an isotropic and anisotropic form for the surface energy.
Using an analytical method, I describe the shape of an island by solving the equation of elasticity. Secondly using a new dynamical punctual model, I show that the coarsening of two islands can be described analytically. I find that the characteristic time for the coarsening of two islands increases linearly with the distance between the two islands [1]. Furthermore, I find that the coarsening time increases due to the anisotropy of surface energy [2]. These results pave the way for a deeper understanding of coarsening in strained semi-conductor film and for the formation of quantum dots in hetero-epitaxy.
I will also present my future work on the effect of anisotropy of the surface energy using three-dimensional simulation.

[1] Shape and coarsening dynamics of strained islands, G Schifani, T Frisch, M Argentina, J.-N. Aqua, Physical Review E 94 (4), 042808, 2016
[2] Dynamics of anisotropic strained islands, G Schifani, M Argentina, T Frisch, submitted to Physical Review E, 2017.



Titre : Synchronization of a network of lasers as coupled oscillators
Encadrement : Stéphane Barland
Résumé : There are many examples of oscillators in nature : they can range from biological systems like fireflies, heart cells and neurons, to electronic components or physical systems such as lasers. In this study we investigate a system of lasers as a network of coupled oscillators under different kinds of coupling (all to all, nearest neighbors or random) and we study the dynamics of the overall population in order to observe some synchronization pattern between of a part or the whole population.
Before we plunge into the study of the whole system, we analyze the behavior of a single laser with an injected signal. We already know that a laser with an injected signal behaves as an excitable medium, whose phase dynamics can be basically described by the Adler equation. In particular, this system reacts to an input perturbation by producing a spike in intensity, where the control of these spikes was only achieved recently. As a first approximation, the system behaves as a leaky integrate-and-fire type of neuron, so that it will integrate all the incoming pulses and generate a spike only when the integral surpasses a certain threshold. However, a more refined analysis shows that this picture is too simple, as it was soon noticed that whenever the pure phase dynamics reduction ceases to be valid, more complex dynamical phenomena can take place, leading to multipulse excitability and a switch of dynamics from an integrator type of neuron to a resonator one.
We then extend our system of study to that of a network of oscillators, namely a matrix of around 500 Vcsels laser. After a characterization of the threshold and wavelength distribution of the population of lasers, we couple them with different configurations, and we observe some first proof of a synchronization of the population.

Vendredi, 22 Septembre, 2017


11h - Site Sophia

Mathematical relationships between resonances resolve key modes and novel definitions of ’thermodynamic’ state variables in human brain

A mechanistic understanding of the mind remains one of the most substantive problems in science, but the probabilistic nature of neural signals seems to oc- clude a deep understanding of brain activity. Between behavioral states and individual subjects, neural electromagnetic signals vary in magnitude, complex- ity, and spectral composition [1,2]. D.O. Hebb [3] posited that reverberations produced by transiently communicating cell assemblies could underpin neural network communication. G.M. Edelman [4] hypothesized that re-entrant, reit- erated activity in recurrent neural architectures (loops) could select communi- cating neural assemblies. W. Singer and others have shown cognition-dependent phase coherence of disparate neural networks on conserved frequencies [5] . How- ever, no consensus has been reached even regarding the spectral organization of human brain activity and organizing principles remain somewhat obscured in skewed, high-dimensional signals. Neural network activity is comprised of rhythmic and arrhythmic components. The arrhythmic component reflects fluc- tuation of brain activity at all discernible frequencies [1], described by power spectra of the form 1/f. Such heavy-tailed, power law distributions are often found in self-organizing, complex systems [6]. Deviating above the power law, resonant modes have particular time constants, representing rhythmic brain ac- tivity with relatively more energy than expected by their frequency position in the heavy-tailed distribution, however, the organization of modes has remained highly controversial [1,7–14]. Largely, this controversy rests on ad hoc definition of bandwidths (δ ≈ 1 : 4 Hz, θ ≈ 4 : 8 Hz, α ≈ 8 : 16 Hz, β ≈ 16 : 32 Hz, and γ ≥ 32 Hz) in which modes are assumed, perhaps erroneously, to be general- ized. In this lecture I will first outline the state of the field and relevant prior findings, then present my analyses of human magnetoencephalography (MEG) data. These analyses have led to the identification of cognitive state dependent, self-similar structures of neural resonance revealing keystone positions for es- pecially powerful modes with distinct mathematical relationships. Next, I will show how a natural definition of entropy and energy in the modes is consistent with ‘thermodynamic’ state variables, opening the possibility that definitions of ‘work’ and ‘order’ may also be useful for understanding basic functions of the brain.

1. Buzsaki, G. Rhythms of the Brain. (Oxford University Press, 2006).
2. Destexhe, A. Rudolph-Lilith, M. Neuronal noise. 8, (Springer Science Busi- ness Media, 2012).
3. Hebb, D. O. The Organization of Behavior. (John Wiley Sons, 1949).
4. Edelman, G. M. Darwinism, N. Selection and reentrant signaling in higher brain function. Neuron 10, 115–125 (1993).
5. von der Malsburg, C., Phillips, W. A. Singer, W. Dynamic Coordination in the Brain : From Neurons to Mind. (MIT Press, 2010).
6. Bak, P. Paczuski, M. Complexity, contingency, and criticality. Proc. Natl. Acad. Sci. U. S. A. 92, 6689–6696 (1995).
7. Penttonen, M. Buzs ́aki, G. Natural logarithmic relationship between brain oscillators. Thalamus Relat. Syst. 2, 145–152 (2003).
8. Belluscio, M. A., Mizuseki, K., Schmidt, R., Kempter, R. Buzs ́aki, G. Cross- frequency phase-phase coupling between and oscillations in the hippocampus. J. Neurosci. 32, 423–435 (2012).
9. Carlqvist, H., Nikulin, V. V., Str ̈omberg, J. O. Brismar, T. Amplitude and phase relationship between alpha and beta oscillations in the human electroen- cephalogram. Med. Biol. Eng. Comput. 43, 599–607 (2005).
10. van Albada, S. J. Robinson, P. A. Relationships between Electroencephalo- graphic Spectral Peaks Across Frequency Bands. Front. Hum. Neurosci. 7, 56 (2013).
11. Ju ̈rgens, E., R ̈osler, F., Henninghausen, E. Heil, M. Stimulus-induced gamma oscillations : harmonics of alpha activity ? Neuroreport 6, 813–816 (1995).
12. Haegens, S., Cousijn, H., Wallis, G., Harrison, P. J. Nobre, A. C. Inter- and intra-individual variability in alpha peak frequency. Neuroimage 92, 46–55 (2014).
13. Atasoy, S., Donnelly, I. Pearson, J. Human brain networks function in connectome-specific harmonic waves. Nat. Commun. 7, 10340 (2016).
14. Pletzer, B., Kerschbaum, H. Klimesch, W. When frequencies never synchro- nize : the golden mean and the resting EEG. Brain Res. 1335, 91–102 (2010).

Lundi, 25 Septembre, 2017


15h - Site Valrose

Self-propelled motion of biological cells and polymeric fibers


Jeudi, 28 Septembre, 2017


10h30 — Khalid OUBAHA

Titre : TBA
Encadrement : Ulrich Kuhl, Olivier Legrand
Résumé : TBA

11h — Aurélien ELOY

Titre : TBA
Encadrement : Robin Kaiser, Mathilde Fouché
Résumé : TBA

11h30 — Florent MAZEAS

Titre : TBA
Encadrement : Laurent Labonté, Sébastien Tanzilli
Résumé : TBA


14h - Site Sophia

Mise en forme temporelle d’impulsions femtosecondes

During this defense, I will present an overview of my research activities over the past 10 years, in LOA (Laboratoire d’Optique Appliquée, France) and INPHYNI. The framework of my research is the temporal shaping of ultra-short (femtosecond) and intense pulses, by means of ultrafast non linear optics and spectro-temporal shaping.

Vendredi, 29 Septembre, 2017


11h - Site Valrose