Breakdown patterns in Branly's coheror

We use thermal imaging of Joule heating to see for the first time electrical conducting paths created by the so-called Branly effect in a two-dimensional metallic granular medium (aluminium). Multiple breakdowns are shown to occur when the medium is submitted to high voltage increases (more than 500 V) with rise times close to one hundred of microseconds.Comment: 4 pages, 5 figures, related informations at http://chemphys.weizmann.ac.il/~damien/index.htm

Retrieving time-dependent Green's functions in optics with low-coherence interferometry

We report on the passive measurement of time-dependent Green's functions in the optical frequency domain with low-coherence interferometry. Inspired by previous studies in acoustics and seismology, we show how the correlations of a broadband and incoherent wave-field can directly yield the Green's functions between scatterers of a complex medium. Both the ballistic and multiple scattering components of the Green's function are retrieved. This approach opens important perspectives for optical imaging and characterization in complex scattering media.Comment: 5 pages, 4 figure

Angular and Linear Speed Cells in the Parahippocampal Circuits

An essential role of the hippocampal region is to integrate information to compute and update representations. How this transpires is highly debated. Many theories hinge on the integration of self-motion signals and the existence of continuous attractor networks (CAN). CAN models hypothesise that neurons coding for navigational correlates – such as position and direction – receive inputs from cells conjunctively coding for position, direction and self-motion. As yet, such conjunctive coding had not been found in the hippocampal region. Here, we report neurons coding for angular and linear velocity, distributed across the medial entorhinal cortex, the presubiculum and the parasubiculum. These self-motion neurons often conjunctively encoded position and/or direction, yet lacked a structured organisation, calling for the revision of current CAN models. These results offer insights as to how linear/angular speed – derivative in time of position/direction – may allow the updating of spatial representations, possibly uncovering a generalised algorithm to update any representation

Defocus test and defocus correction in full-field optical coherence tomography

We report experimental evidence and correction of defocus in full-field OCT of biological samples due to mismatch of the refractive index of biological tissues and water. Via a metric based on the image quality, we demonstrate that we are able to compensate this index-induced defocus and to recover a sharp image in depth.Comment: 7 pages, 3 figures, minor changes, 1 figure adde

Anomalous Thermal Diffusivity of Amorphous Semiconductor Superlattices

Since the development of amorphous semiconductor superlattice films, the great interesting in fundamental and technological is attracted by their special properties and applications. The optical and electronic properties of the films have been studied by conventional spectroscope, photoluminescence and photoconductance techniques etc. Some anomalous phenomena, such as, the quantum size effect and the permenent photoconductivity (PPC) effect, have been discovered [1–2]. Recently the investigations of the characteristics and parameters of amorphous semiconductor superlattice films by photoacoustic and photothermal techniques are being got attention [3,4]. In this paper, we describe the study of the thermal diffusivity of a series samples of amorphous semiconductor superlattices a-Si:H/a-SiNx:H. The thermal diffusivity of the samples is experimentally measured by using mirage detection technique [5,6]. Up to date, the mirage detection method has been used successfully to determine the thermal diffusivity of both transparent and opaque materials. Specially it is available to measure directly the thermal parameters of thin films which are usually difficalt to be, evaluated. In the theoretical calculation, one dimensional model with “ the thormal resistors in series “ and ” the weighted average ” is used. Comparing the experimental and theoretical results, we find that both are in good agreement for the most of the samples. However, some anomalous thermal diffusivity phenomena have been observed in some special samples

Heterodyne detection of multiply scattered monochromatic light with a multipixel detector

International audienceA new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a CCD as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer

Physics of brain dynamics: Fokker-Planck analysis reveals changes in EEG delta-theta interactions in anaesthesia

We use drift and diffusion coefficients to reveal interactions between different oscillatory processes underlying a complex signal and apply the method to EEG delta and theta frequencies in the brain. By analysis of data recorded from rats during anaesthesia, we consider the stability and basins of attraction of fixed points in the phase portrait of the deterministic part of the retrieved stochastic process. We show that different classes of dynamics are associated with deep and light anaesthesia, and we demonstrate that the predominant directionality of the interaction is such that theta drives delt

Controlling Light Through Optical Disordered Media : Transmission Matrix Approach

We experimentally measure the monochromatic transmission matrix (TM) of an optical multiple scattering medium using a spatial light modulator together with a phase-shifting interferometry measurement method. The TM contains all information needed to shape the scattered output field at will or to detect an image through the medium. We confront theory and experiment for these applications and we study the effect of noise on the reconstruction method. We also extracted from the TM informations about the statistical properties of the medium and the light transport whitin it. In particular, we are able to isolate the contributions of the Memory Effect (ME) and measure its attenuation length