Claude Amra

Gradual loss of polarization in light scattered from rough surfaces: Electromagnetic prediction

Electromagnetic theory is used to calculate the gradual loss of polarization in light scattering from surface roughness. The receiver aperture is taken into account by means of a multiscale spatial averaging process. The polarization degrees are connected with the structural parameters of surfaces.

Enpolarization of light by scattering media

The polarization of a coherent depolarized incident light beam passing through a scattering medium is investigated at the speckle scale. The polarization of the scattered far field at each direction and the probability density function of the degree of polarization are calculated and show an excellent agreement with experimental data. It is demonstrated that complex media may confer high degree of local polarization (0.75 DOP average) to the incident unpolarized light.

Partial polarization of light induced by random defects at surfaces or bulks

Partial polarization may be the result of a scattering process from a fully polarized incident beam. It is shown how the loss of polarization is connected with the nature of scatterers (surface roughness, bulk heterogeneity) and on the receiver solid angle. These effects are theoretically predicted and confirmed via multiscale polarization measurements in the speckle pattern of rough surfaces. Full polarization can be recovered when reducing the receiver aperture.

Optimal design for 100% absorption and maximum field enhancement in thin-film multilayers at resonances under total reflection

Dielectric optical coatings are designed at resonances to reach total absorption, whatever the low value of the imaginary index. The corresponding field enhancement within the stack can be arbitrarily increased with the optimization procedure. Applications concern optical sensors and threshold lasers.

Polarization analysis of speckle field below its transverse correlation width : application to surface and bulk scattering

An experimental method for accurate polarimetric characterization of speckle field below its transverse correlation width is proposed. Using a polarimetric analyzer, the speckle field under investigation is probed by a set of polarimetric projections describing the full Poincaré sphere surface. Spatial polarimetric variations of the speckle field are thus observed with an accuracy of 1% for each Stokes parameter. Moreover, all the experimental data can be guaranteed by a validity criterion. Using white paper sheet and rough metal samples, the method exhibits strong potential to analyze and differentiate speckle fields generated by bulk and surface scattering.

Anti-Reflecting Absorbing Layers for Electrochemical and Biophotonic Applications

Anti-reflecting layers deposited on flat surfaces make molecular films visible in reflecting light microscopy. For centuries, single Anti-Reflecting layers have been implicitly associated with dielectric materials. We recently demonstrated that anti-reflecting layers could be achieved out of absorbing materials such as metals as well, but only when used as backside layers where illumination and detection are performed through a supporting window. Fortunately, this corresponds to the best geometry when envisaging biophotonic or electrochemical applications at the solid/liquid interface. Here we explain how single absorbing anti-reflecting layers can serve each of these applications, and both simultaneously.

Multiscale spatial depolarization of light: electromagnetism and statistical optics

Electromagnetism theory is used to calculate the effect of aperture receiver on the polarization of scattered light. The result is a spatial average (several speckle grains within the aperture) responsible for a depolarization process similar to that of temporal situation. The associated DOP is calculated versus surface parameters (roughness, slope...) and reveals new signatures for identification of samples. The DOP is also calculated in a multi-scale manner, that is, versus the receiver aperture. Examples are given to separate surface and bulk scattering.

Random media confer high polarization degree to unpolarized light

It is shown how scattering media may allow to confer high polarization degree (DOP ≈ 0.75) to an incident depolarized beam (DOP ≈ 0). The statistics of the polarization degree versus scattering directions or space location are investigated with a CCD camera at a speckle resolution. Numerical calculation and experimental data are compared and show high agreement for a sample highly inhomogeneous in the bulk. The probability density function of the DOP follows a 3u2 variation.