Biomedical subjects
Publications and source records attributed to Claude Amra.
A single optical technique is presented that allows direct and selective probing or imaging in the thickness of multilayers. It is based on tunable interferences of polarized light. Adequate vertical resolution is provided with this method.
We present the characterizations performed at the Institut Fresnel for the Measurement Problem of the Optical Interference Coatings 2004 Topical Meeting. A single layer coated on a fused-silica substrate of unknown composition and parameters is analyzed in terms of optogeometrical parameters, uniformity, and scattering. We determine the refractive index and the average thickness of the coating, then provide the localized determination of the thickness with a 2 mm spatial resolution. Topography measurements include atomic force microscopy and angle-resolved scattering measurements. These results are completed thanks to a Taylor Hobson noncontact 3D surface profiler.
Angle-resolved light scattering has been used for decades to quantify the surface roughness of optical components. However, because this technique is affected by the contribution of both interfaces of the sample, it cannot be applied to transparent substrates. We show how to overcome this issue and apply these principles to the characterization of superpolished samples.
Spectral properties of quarter-wave stacks deposited on microspheres are investigated with Mie theory. These properties are converted into colorimetric spaces to deal with visual applications. An ion-beam-sputtering process is then implemented to produce elementary stacks on the spheres, and the resulting powders are characterized.
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.
The refractive indices of optical materials are usually determined from spectrophotometric and ellipsometric measurements of specular beams. When the roughness of the interfaces increases, the energy in the specularly reflected and transmitted beams decreases and scattering becomes predominant. For strong roughness (compared to the incident wavelength) a surface does not exhibit specular reflection or transmission, making difficult the determination of the refractive index. We describe two techniques, based on scattering measurements, that one can use to determine the refractive indices of opaque inhomogeneous media.
Accurate threshold curves of laser-induced damage (7-ns single shot at 1.064 microm) are measured in bulk and at the surfaces of optical components such as substrates, thin films, multilayers, and liquids. The shapes and the slopes of the curves are related to the spot size and to the densities of the nanodefects that are responsible for damage. First, these densities are reported for bulk substrates. In surfaces and films the recorded extrinsic and intrinsic threshold curves permit the discrimination of the effects of microdefects and nanodefects. In all cases the density of nanocenters is extracted by means of a phenomenological approach. Then we test liquids and mixtures of liquids with controlled defect densities. The results emphasize the agreement between measurement and prediction and demonstrate the validity of the presence of different kinds of nanocenter as the precursors of laser damage.
Working with wavelengths shorter than the deep ultraviolet involves the development of dedicated optics for free-electron lasers with devoted coating techniques and characterizations. High-performance deep-ultraviolet optics are specially developed to create low-loss, high-reflectivity dielectric mirrors with long lifetimes in harsh synchrotron radiation environments. In February 2001, lasing at 189.7 nm, the shortest wavelength obtained so far with free-electron-laser oscillators, was obtained at the European Free-electron-laser project at ELETTRA Synchrotron Light Laboratory, Trieste, Italy. In July 2001, 330-mW extracted power at 250 nm was measured with optimized transmission mirrors. Research and development of coatings correlated to lasing performance are reported.
High-reflectivity dense multilayer coatings were produced for the ultraviolet spectral region. Thin-film single layers and UV mirrors were deposited by ion plating and plasma ion-assisted deposition high-energetic technologies. Optical characterizations of HfO2 and SiO2 single layers are made. The optical constants obtained for these two materials are presented. HfO2 and SiO2 mirrors with a reflectance of approximately 99% near 250 nm are reported.
Piezoelectric transparent thin films are of great interest for use in tunable filters. We present experimental results on Ta2O5 single layers coated on fused-silica substrates with an electron-beam deposition process. Above 450 degrees C, coatings change from an amorphous to a polycrystallized structure. When this structure shows a preferred orientation matching the piezoelectric tensor of the Ta2O5 crystal and the external electric field, variation in the piezoelectric layer thickness is expected. We detail experimental results in terms of optical (spectrophotometric and scattering measurements) and nonoptical characterizations (x-ray diffraction and scanning electron microscopy). Then the resultant thickness variation under oscillating applied voltage is measured with an extrinsic Fabry-Perot interferometer setup.
We provide a detailed analysis of the various problems connected with the development of tunable thin-film filters for wavelength-division multiplexing applications. We examine the relation between the change in layer thickness and the central wavelength shift for various configurations and point out the significance of the structure of the reflectors, the spacer thickness, and the location of the active layers. We describe and compare practical arrangements using either temperature or an electric field as the driving parameter.
We produce overcoated microspheres and try to obtain artificial optical powders with specific spectral properties. Electron-beam deposition is used with a particular vibration system. Calibration and characterization results are presented that validate the techniques and procedures for single-layers and quarter-wave mirrors.
Angle-resolved ellipsometry of light scattering is an original technique developed at the Fresnel Institute to identify scattering processes in substrates and multilayers. We extend the investigation because numerous experimental results proved that the technique can be of major interest for analysis of microcomponents and their scattering origins. Surface and bulk effects can be separated in most situations, as well as the oblique growth of materials and the presence of first-order contaminants.
Index-determination methods based on reflectance and transmittance measurements are developed for both opaque and semitransparent metallic films. Results are given concerning chromium and nickel layers manufactured by electron-beam deposition. To take account of the evolution of the optical constants versus layer thickness, an inhomogeneous layer model is used, which permits us to obtain a good agreement between measurements and calculations. Results are applied to the design and manufacture of light absorbers for which accurate index knowledge is required. Measured absorption is higher than 0.999 on both broadband and monochromatic components.