Gaelle Georges

Ellipsometry of reflected and scattered fields for the analysis of substrate optical quality

Specular ellipsometry is a well-known and efficient technique to characterize surfaces and coatings. This technique has been extended to the measurement of scattered light. We present an experimental setup, using a polarization modulator, which permits us to characterize transition layers and roughness without a calibration procedure. Experimental results are presented concerning transition layers for damage threshold applications and for rough surfaces or bulks.

Light scattering from human corneal grafts: Bulk and surface contribution

The cornea is the only transparent tissue in the body. The transparency is the main characteristic of the corneal tissue, and depends not only on the transmission coefficient but also on the losses by scattering and absorption. The scattering properties of the cornea tissues become one of the most important parameters in the case of the corneal graft. These scattering properties are studied in this paper in the reflected half area, similar to the diagnosis configuration. We quantify the influence of the cornea thickness and of the epithelial layer on scattering level. The technique of ellipsometry on scattered field is also used to analyze the polarization properties in order to determine the origin of scattering (surface and/or bulk).

Study of varnish layers with optical coherence tomography in both visible and infrared domains

Optical Coherence Tomography (OCT) is an attractive technique to study works of art because it allows non-destructive and contactless analysis. In the case of musical instruments, the study of wood finishes could give interesting information as the thicknesses of the layers, the number of layers and the presence of fillers. A time-domain full-field OCT, achieving high resolution, is used in both visible and near infrared ranges to characterize semi-transparent layers containing scattering particles as charged varnish layers. We present OCT measurements on wood varnished with different coatings. We show that the detection of pigment particles is dependent of the spectral range and that both spectral domains allow to reach micrometer-scale spatial resolutions.

From angle-resolved ellipsometry of light scattering to imaging in random media

A procedure is described to allow selective cancellation of polarized scattering within optical substrates and multilayers. It is shown how bulk scattering (respectively surface) can be directly eliminated while the remaining roughness (respectively bulk) signal is still measurable. The same procedure can be applied to isolate a single interface or bulk within a stack or to detect slight departure from perfect correlation within multilayers. Experiments and a procedure for selective imaging in random media are described.

Optical far field measurements applied to microroughness determination of periodic microelectronic structures

With device size reduction, variability induced by local micro roughness is becoming less and less negligible in terms of statistical control of critical dimensions (CD). We applied a recent approach developed at Fresnel Institute for the determination of micro roughness on periodic structures through optical far field characterization using an angle resolved scatterometer. Structure periodicity affects the diffraction orders, while roughness signature is mainly found between diffraction orders. Theoretical simulation was performed using two in-house computer codes based on differential method and on first order approximation. We will review the theoretical approach and show roughness data derived from measurement on glass gratings as well as poly silicon gate type structures.

Scattering properties and transparency characterization of human corneal grafts

The cornea is the single human tissue being transparent. This unique property may be explained by the particular structure of the cornea, but the precise role of each of its constituents remains unsolved. On other matter, prior to corneal transplant, graft must be evaluated during a sorting procedure where a technician assesses of its transparency quality. Nevertheless, this criterion remains subjective and qualitative. This study proposes to combine 3D imagery using Full-Field Optical Coherence Tomography jointly with angular resolved scattering measurement to achieve a quantitative transparency characterization of the cornea. The OCT provides micrometric resolution structural information about the cornea, and we observe the evolution occurring when oedema develops within the tissue. Scattering properties are evaluated and compared parallely, as the transparency of the graft. A close link between the scattering intensity level of the cornea and its thickness is highlighted through this study. Furthermore, the three-dimensional imagery offers a view over the structural modifications leading to a change in transparency, and the combination with scattering properties measurement provides clues over the characteristic scale of scatterers to consider for a better understanding of corneal transparency evolution. Achieving an objective and quantified parameter for the transparency would be helpful for a more efficient corneal graft sorting, and may be able to detect the presence of localized wounds as the ones related to a previous refractive surgery. However, the study of graft nearly eligible for corneal transplant would be needed to confirm the results this study presents.

Micro Roughness Determination Of Periodic Microelectronics Structures Using Optical Far Field Measurements

The recent advance in lithography process leads to microelectronics compound size reduction. As a direct consequence, micro roughness should not be negligible compared to critical dimension. That is why a novel approach has been developed using an angle resolved scatterometer to quantify roughness value.In order to compare and validate results from scattering measurement, a multiscale analysis will be performed with AFM measurement. Based on theory and numerical models, validation of this technique is performed.

Angle-Resolved Ellipsometric Data for Selective Imaging in Scattering Media

Angle-resolved ellipsometric data are recorded on light scattering and provide a real time process for selective imaging in scattering media. Surface and bulk effects are separated and could be used for a selective screening inside the tissues.

Light scattering to isolate a single interface within a multilayer

Light scattering is a current tool for characterization of defects in optical interferential coatings. However, this tool is not fully efficient for multilayer component. Indeed, in this case, the scattered light from multilayers originates from several interface roughnesses that cannot be separated a priori. In this paper, a technique which can isolate a single interface embedded within a stack is presented. It is based on destructive interferences between the polarization modes of the angular scattering. These interferences can be tuned in a selective way that allows the extraction of light issued from a specific scattering interface.

Efficiency of polarimetric z probing in optical multilayers

Numerical calculation is performed to validate the principles of a single optical technique devoted to real time probing or imaging of submultilayers within interferential coatings.