LABORATOIRE DE PHYSIQUE THEORIQUE DE LA MATIERE CONDENSEE

 

 

Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle  523 du LPTMC - Tour 12-13 


 

Kirone Mallick (IPhT CEA Saclay)

Continuous-time Quantum Walks


Quantum analogs of classical random walks have been defined in quantum information theory as a useful concept to implement  algorithms. Due to  interference effects, statistical properties of quantum walks can drastically differ from their classical counterparts, leading to much faster computations.
In this talk, we  shall discuss  various  statistical properties of continuous-time quantum walks on a  lattice, such as: survival properties of quantum  particles in the presence of traps (i.e. a quantum generalization of the Donsker-Varadhan stretched exponential law), the growth of a quantum  population in the presence of a  source, quantum return probabilities and  Loschmidt echoes.

Introduction de propriétés topologiques dans un métamatériau

Fabrice Lemoult (Institut Langevin, ESPCI, Paris)


Les métamatériaux sont des milieux artificiels qui offrent des propriétés de propagation exotiques, comme par exemple des effets de réfraction négative. Ces effets sont la conséquence de l’interaction des ondes avec la cellule unité composant ces métamatériaux, celle-ci étant beaucoup plus petite que la longueur d'onde de travail. Dans le domaine des micro-ondes, un exemple de cellule unité est une simple tige métallique placée sur un plan de masse, et dans le cas de l’acoustique une simple canette de soda jouant le rôle de résonateur de Helmholtz. Dans un milieu composé de nombreuses cellules unités, la propagation de l'onde est décrite simplement avec le modèle du polariton, c'est-à-dire par une relation de dispersion résultant de l'hybridation de l'onde et de la résonance individuelle de chaque résonateur. Dans cet exposé, nous montrerons comment aller au-delà de cette description habituelle permet d’introduire des propriétés topologiques à la propagation des ondes.

Laura Messio (LPTMC)

Mini-cours sur GIT

Je propose de faire un mini-cours complètement informel sur git (niveau débutant) lundi 11 février à 10h, dans la salle de cours. GIT est un outil de suivi de version (comme svn, pour ceux qui connaissent), qui peut être très utile lors de l'écriture de codes ou lors d'un travail collaboratif quelconque (article écrit à plusieurs par exemple). Cela permet de voir facilement et de garder des traces des modifications apportées par les différents contributeurs. On peut utiliser des espace de stockage en ligne (bitbucket). On peut faire des "branches" (si on veut développer un bout de code sans casser le code principal par exemple), et plein d'autres choses.

Vous pouvez amener votre portable pour faire des tests.

Nicolas Pavloff (LPTMS Orsay)

Analogues acoustiques de l'horizon d'un trou noir


L'analogue acoustique d'un trou noir peut être réalisé par l'écoulement d'un liquide dans un tuyau: si le flot est super-sonique dans une région de l'espace, une onde sonore émise dans cette région ne pourra pas remonter le courant et ne sera donc pas entendue en amont. On parle de "trou muet".
En 1981, Unruh a suggéré que les trous muets doivent permettre d'observer l’analogue acoustique du rayonnement des trous noirs, qui est un effet quantique prévu par Hawking en 1975. Cette idée a récemment connu un regain d'intérêt dans le domaine de la condensation de Bose-Einstein des vapeurs ultra-froides. Une première raison en est la très grande précision du contrôle expérimental qu'on peut obtenir sur ces systèmes. Il y a également une motivation théorique que je discuterai en détail: l'étude des corrélations de densité permet d'identifier très clairement le rayonnement de Hawking. Je présenterai les résultats enthousiasmants d'une expérience récente.

Alexei Kornyshev (Imperial College London)

Electrochemical plasmonics: A path to electrotunable self-assembling optical metamaterials (scenarios navigated by theory)

This talk will overview a new direction of research based on voltage-controlled self-assembly of plasmonic nanoparticles at electrochemical interfaces, the optical properties of which can dramatically vary with assembly structure and density.

Progress in photonic metamaterials was made possible by advances in nanotechnology. Many such materials, however, can only perform a single function. Not surprisingly, a premier conference in the field opened with a provocative statement: “The time of meta-materials is over… It is the time of tuneable metamaterials” (N. Zheludev). Realization of such platforms would allow properties of such functional metamaterials to be tuned in real-time, with major implications for absorbers in solar cells, antennae, super-lenses, cloaking, sensors – amongst others.

Tunability can be reached by utilization of fine physical effects, but also changing the structure of materials in real time. Our team at Imperial (led by J.B. Edel – nanofluidics, optics, analytical science; A.R. Kucernak – experimental electrochemistry; and myself – theory development, theory-based navigation of experiments and analysis of the data) in collaboration with M. Urbakh at Tel Aviv University (theory) have responded to this challenge first with developing the ‘nanotechnology-free’ concept of chemically tuneable self-assembly of plasmonic nanoparticles, such as quasi-2D arrays NPs at a liquid|liquid (LLI) and solid|liquid (SLI) interfaces [1,2]. We have demonstrated that such arrays could be used for ultrasensitive SERS detection of trace analytes –e.g. proxies for pollutants, illegal substances, terror agents – that get into ‘hot spots’ between NP’s [3]. The array structure was controlled by tuning the composition of the solutions (electrolyte concentration or pH). We further performed a complex study of the structure and optical properties of such arrays within the same setup, via a combination of grazing incidence, small angle X-ray scattering and in situ optical reflectivity. From the X-ray and optical data, we could determine (from a combination of experimental [4] and original theoretical [5] results) the average distance between NPs, the long-range order, and reflectivity –all as a function of electrolyte concentration. Incorporating the obtained values of array’s ‘lattice constants’ into the theory of optical reflectance from such arrays [5], we could calculate the reflectance spectra for each electrolyte concentration and compare them with those measured in the same system [4]. Excellent match between the theory and experiments has demonstrated that the underlying physics works exactly as expected! These studies gave us confidence that we could chemically control these nanoplasmonic platforms, i.e. generate tuneable self-assembled metamaterials. However, this did not incorporate real-time reversible control.

It was clear, that electrochemistry will be the game changer here. At electrochemical interfaces, with tiny voltage variation, one can create localised electric fields that may dramatically change the structures of adsorbed charged NP arrays and their optical properties. We demonstrated this by creating the first electrically switchable mirror based on voltage controlled self-assembly of gold NPs at the interface of two immiscible electrolytic solutions [6]. We have shown that it is possible to transition between a mirror and window and back using just 0.5 V - voltage variation through its effect on the density of the NP arrays and their resulting optical response. Furthermore, a different kind of switch based on voltage-controlled adsorption-desorption of NPs on a metal substrate, the principle of which has been described theoretically in [7], was reported in [8]. A set of other scenarios were also considered (see e.g.[9]).

 

[1] J.B. Edel, A.A. Kornyshev, M. Urbakh, “Self-Assembly of Nanoparticle Arrays for Use as Mirrors, Sensors, and Antennas”, ACS Nano 7, 9526-9632 (2013).

[2] J. Edel, A.A. Kornyshev, A. Kucernak, M. Urbakh, “Fundamentals and applications of self-assembled plasmonic nanoparticles at interfaces”, Chemical Society Reviews 45, 1581-1596 (2016).

[3] M.P. Cecchini, V.A. Turek, J. Paget, A.A. Kornyshev, J.B. Edel, “Self-assembled nanoparticle arrays for multi-phase trace analyte detection”, Nature Materials 12, 165-171 (2013).

[4] L.Velleman, D. Sikdar, V.A. Turek, S.J. Roser, A.R., Kucernak, A.A. Kornyshev, J.B. Edel, “Tuneable 2D self-assembly of plasmonic nanoparticles at liquid | liquid interfaces”, Nanoscale , 8  19229 (2016).

[5] D. Sikdar, A.A. Kornyshev, “Theory of tailorable optical response of two dimensional arrays of plasmonic nanoparticles at dielectric interfaces”, Sci. Rep. 6, #33712 (2016).

[6] Y. Montelongo, D. Sikdar, Y. Ma, A.J.S. McIntosh, L. Velleman, A.R. Kucernak, J.B. Edel, A.A. Kornyshev, “Electrotuneable nanoplasmonic liquid mirror”, Nature Materials 16, 1127-1135 (2017). https://youtu.be/68J0yLvrvJE

[7] D. Sikdar, S. Bin Hasan, M. Urbakh, J. Edel, A.A.Kornyshev, “Unravelling the optical responses of nanoplasmonic mirror-on-mirror metamaterials”, Phys.Chem.Chem.Phys. 18, 20486-20498 (2016).

[8] Y. Ma, C. Zagar, D. Klemme, D. Sikdar, L. Velleman, Y. Montelongo, S.-H. Oh, A.R. Kucernak; J.B. Edel, A.A. Kornyshev,

"A tunable nanoplasmonic mirror at an electrochemical interface", ACS Photonics 5, 4604-4616 (2018).

[9] H. Weir, J.B. Edel, A.A. Kornyshev, D. Sikdar, “Towards electrotuneable Fabry-Perot Interferometer”, Sci.Rep. 8, 2045-2322 (2018); D. Sikdar, A.Bucher, C.Zagar, A.A. Kornyshev, “Electrochemical plasmonic metamaterials: towards, fast electrotuneable reflecting nanoshutters, Faraday Disc. 199, 585-602 (2017).