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Category: Events

Published on Wednesday, 02 March 2016 12:09

[Talk, French] Démons de Loschmidt, Ondes et Holographie

Mathias FINK (ESPCI, Paris)

30 March 2016, 4.30pm

Télécom ParisTech

46 rue Barrault 75013 Paris, amphitheater "Émeraude"

En partant de la discussion célèbre qui opposa Boltzmann et Loschmidt sur la flèche du temps et sur certains paradoxes liés à la réversibilité des lois de la mécanique, nous introduirons différentes alternatives pour construire une « time machine » en physique classique. Nous nous intéresserons plus particulièrement aux cas des ondes (à large contenu spectral). Nous comparerons la façon de fabriquer une « time machine » pour différents types d’ondes : sonores, électromagnétiques, lumière et vagues à la surface de l’eau.

Nous montrerons qu’il existe en fait deux approches duales bien différentes pour obliger une onde à revivre sa vie passée. On peut d’une part manipuler une onde sur une surface bidimensionnelle échantillonnée par un réseau d’antennes réversibles. C’est l’approche des miroirs à retournement temporel dont les applications sont nombreuses. On peut aussi manipuler une onde très différemment en créant des « discontinuités temporelles » dans tout l’espace. C’est l’approche des « miroirs temporels » où les discontinuités spatiales des miroirs usuels sont remplacées par des discontinuités temporelles.

On verra que ces différentes approches permettent aussi de revisiter la méthode holographique et la limite de diffraction dans les méthodes d’imagerie et de contrôle d’ondes.

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Category: Spatial Lights Modulators (SLMs)

Published on Wednesday, 09 December 2015 16:23

[tutorial] How to control a lquid crystal SLM with Python

Most liquid crystal Spatial Light Modulators (SLMs) and some digital micromirror devices (DMDs) are controlled via an analog (VGA) or digital (HDMI/DVI) monitor standard communication protocol. In other words, you plug it to your computer and it is recognized as a monitor display. There is usually no useful tool or API provided with the device to dynamically control the SLM. I previously introduced a way to control an SLM using Matlab/Octave, now that I switched to Python, I present here a way to do this using Python.

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Category: Highlights

Published on Tuesday, 04 November 2014 09:41

[highligth] A microwave spatial modulator to improve in-home WiFi

[N. Kaina et al., Sci. Rep. (2014)]

Wavefront shaping is not limited to optical waves. Similar techniques can be used for any kind of wave for which one can control dynamically the phase over a large number of independent elements. In [N. Kaina et al., Sci. Rep. (2014)], the authors demonstrate the use of their Spatial Microwave Modulator (SMM) to control the propagation of radio frequency waves inside a room to improve the WiFi signal at any chosen position. The system is passive as there is no energy transfer from the modulator to the WiFi signal, it only controls the local phase of the waves reflected of the modulator. The device is thin and has the typical size of small poster, it can be conveniently placed on the wall of a typical room without any loss of space.

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Category: Multimode fibers

Published on Monday, 27 October 2014 00:08

[tutorial] Modes of step index multimode fibers

Scattering media were the first type of "complex media" for which wavefront shaping techniques were applied. Quickly, applications were developed for multimode fibers as well. One can consider multimode fiber as a complex media; because of its inherent modal dispersion (different modes travel at different speeds) and also because of the possible coupling between modes, the output field of the fiber does not resemble its input one. Wavefront shaping in multimode fibers has had a fast development because of its applications in biomedical endoscopic imaging and for telecommunications, where the exploitation of the spatial modes in multimode fibers offers a promising way to increase data rates compared to single mode fibers. 

I present here a quick tutorial on the calculation of the modes of a step index multimode fiber and how to find the so called linearly polarized modes, that are convenient for manipulation using shaping techniques.

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Category: Spatial Lights Modulators (SLMs)

Published on Friday, 24 October 2014 10:43

[tutorial] How to use a binary amplitude Deformable Miror Device (DMD) as a phase modulator: The "superpixel" method

I previously presented a technique based on the Lee hologram that allows to use a binary amplitude modulator (like a DLP chip you find in standard projectors) to perform a phase modulation (or amplitude and phase modulation). Recently, a new technique was introduced in [S.A. Goorden et al., Opt. Express (2014)] that allows an accurate complex modulation with less loss in term of spatial resolution. This post is more a highlight on this paper than a proper tutorial. In a nutshell, while the Lee hologram only take advantage of one dimension to encode the amplitude and phase in fringes, this technique exploits both dimensions of the pixel array using "superpixels".

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Category: Highlights

Published on Tuesday, 01 July 2014 11:45

[highlight] From diffusive to ballistic-like transport in absorbing media

• • • [S.F. Liew et al., Phys. Rev. B, 89, (2013)]

Intuitively, absorption of light is detrimental for imaging as it reduces the intensity of the image we see. On the other hand, scattering is also an known obstacle for imaging as it mixes light sending it in all the dirrections. In the present paper, S.F. Liew and his collaborators from Yale University (CT, USA) and the University of Twente (The Netherlands) show that, contrary to appearances, absorption can in fact help light to follow a direct path through disordered media.

Without absorption, spatial information of an object transmitted through an opaque material is totally mixed and difficult to recover. The reason is that the photons are multiply scattered, hence their propagation directions are randomized at every scattering event. In their recent numerical calculation study, the authors noticed that when absorption becomes strong, the transport of light occurs via much straighter paths.

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