Claude Amra

Transformation thermodynamics: cloaking and concentrating heat flux

We adapt tools of transformation optics, governed by a (elliptic) wave equation, to thermodynamics, governed by the (parabolic) heat equation. We apply this new concept to an invibility cloak in order to thermally protect a region (a dead core) and to a concentrator to focus heat flux in a small region. We finally propose a multilayered cloak consisting of 20 homogeneous concentric layers with a piecewise constant isotropic diffusivity working over a finite time interval (homogenization approach).

Comparison of surface and bulk scattering in optical multilayers

Electromagnetic theories provide a tool to detect the origin of scattering in optical multilayers. Illumination and observation conditions that cause surface and bulk scatterings to have different behaviors are pointed out. Angular, wavelength, and polarization dependences are investigated for the location of structural irregularities at interfaces or in the bulk of a multilayer. Specific experiments can be designed.

From light scattering to the microstructure of thin-film multilayers.

It is shown how light scattering provides a powerful tool for thin-film characterization. The introduction of a roughness isotropy degree permits the extraction of structural parameters of the stacks. Replication functions and residual roughnesses are given for TiO(2), SiO(2), and Ta(2)O(5) materials produced by ion-assisted deposition and ion plating. Additional confirmation is given by measurements of scattering versus wavelength. The sensitivity of design to material and substrate effects is studied. At low-loss levels, surface and bulk phenomena are discussed together. Microstructure is characterized in the frequency bandwidth given by experiment.

Quantitative Microroughness Analysis Down To The Nanometer-Scale

Cross characterizations of surface roughness of glassy materials have been performed using Atomic-Force Microscopy (AFM) and optical scattering techniques. The AFM measurements provide images of the surface height contours from the micrometer down to the nanometer scale. From a two-dimensional (2D) Fourier analysis of the images, the roughness power spectrum is measured for a range of spatial frequencies from 0.04 μm−1 up to 400 μm−1. An excellent agreement is obtained with parallel light scattering measurements of the surface roughness over the spatial frequencies ranging from 0.05 μm−1 to 1.54 μm−1, corresponding to the overlap bandwidth reached by the two techniques. From the power law dependence of the roughness spectrum vs. spatial frequency found on the whole range of AFM analysis, fractal properties of this self-affine surface are discussed.

Statistical study of single and multiple pulse laser-induced damage in glasses

Single and multiple pulse laser damage studies are performed in Suprasil silica and BK-7 borosilicate glasses. Experiments are made in the bulk of materials at 1.064microm with nanosecond pulses, using an accurate and reliable measurement system. By means of a statistical study on laser damage probabilities, we demonstrate that the same nano-precursors could be involved in the multiple shot and single shot damage process. A damage mechanism with two stages is then proposed to explain the results. Firstly, a pre-damage process, corresponding to material changes at a microscopic level, leads the precursor to a state that can induce a one-pulse damage. And secondly a final damage occurs, with a mechanism identical to the single shot case. For each material, a law is found to predict the precursor life-time. We can then deduce the long term life of optical elements in high-power laser systems submitted to multipulse irradiation.

Anisotropic conductivity rotates heat fluxes in transient regimes

We present a finite element analysis of a diffusion problem involving a coated cylinder enabling the rotation of heat fluxes. The coating consists of a heterogeneous anisotropic conductivity deduced from a geometric transformation in the time dependent heat equation. In contrast to thermal cloak and concentrator, specific heat and density are not affected by the transformation in the rotator. Therein, thermal flux diffuses from region of lower temperature to higher temperature, leading to an apparent negative conductivity analogous to what was observed in transformed thermostatics. When a conducting object lies inside the rotator, it appears as if rotated by certain angle to an external observer, what can be seen as a thermal illusion. A structured rotator is finally proposed inspired by earlier designs of thermostatic and microwave rotators.

Theory and application of antiscattering single layers: antiscattering antireflection coatings

We recall the analytical expression that gives, for a rough surface illuminated at normal incidence, the light scattered in the half-space containing the specular reflection direction. Two cases are studied: the bare substrate and the substrate coated with one transparent layer. It is shown, for this specular direction, that the light scattered from a single layer can be equal to zero (perfect antiscattering) in certain conditions relative to the roughnesses of the two layer interfaces. Data calculation proves that this antiscattering effect occurs in all directions of the half-space of the reflected light. The experimental results are in good agreement with this theoretical analysis for five different dielectric materials. This study brings out most information about the grain of the material, which is responsible for the residual roughness.

Description of a scattering apparatus: application to the problems of characterization of opaque surfaces

We show how the complexity of a micropolished optical surface can be investigated in detail by measurement of the distribution of scattered light. We deal with problems of roughness anisotropy and uniformity together with cleaning problems. Experimental results concern numerous black glasses from different polishing shops and allow a determination of the polish inhomogeneity in a same glass set. After that, we present a detailed study of the apparatus function of the scatterometer, and we determine the limits of validity of our optical characterization method.

Spontaneous emission into planar multi-dielectric microcavities: Theoretical and experimental analysis of rare earth ion radiations

Abstract Spontaneous emission control for rare earth ions implanted inside planar multidielectric microcavities is investigated theoretically and experimentally. Rigorous classical electrodynamics analyses are presented and compared in order to compute the electromagnetic power provided by sources located inside planar dielectric multilayer structures. Radiation patterns and lifetime measurements are performed in Ta 2 O 5 /SiO 2 microcavities implanted with erbium and praseodymium ions. Although we demonstrate a significant enhancement of the spontaneous emission in a direction normal to the layer (up to 30%), we show that a large amount of the emitted power is carried by the guided modes of the structures.

Thermal invisibility based on scattering cancellation and mantle cloaking

We theoretically and numerically analyze thermal invisibility based on the concept of scattering cancellation and mantle cloaking. We show that a small object can be made completely invisible to heat diffusion waves, by tailoring the heat conductivity of the spherical shell enclosing the object. This means that the thermal scattering from the object is suppressed, and the heat flow outside the object and the cloak made of these spherical shells behaves as if the object is not present. Thermal invisibility may open new vistas in hiding hot spots in infrared thermography, military furtivity, and electronics heating reduction.