Polarizing and non-polarizing mirrors in far UV for the Hydrogen Lyman-α radiation (λ = 121.6 nm)

We demonstrate the feasibility of efficient polarizing and non-polarizing mirrors at λ = 121.6 nm designed within the framework of the Lyman α Lyot Coronograph Imager project. We have designed, realized and characterized such mirrors. Optical constants of the materials involved in the thin film coatings are determined experimentally. Reflectivity measurements in the VUV wavelength domain have been performed at PTB (Synchrotron at BESSY II, Berlin). A s-polarized reflectivity Rs as high as 68% is experimentally obtained with a coating made of a Fabry-Pérot resonator. To our knowledge this value is the highest ever reported for a polarizing mirror at this wavelength

Hétéro-épitaxie de Srx_{x} Ba1−x_{1-x} Nb2_{2} O6_{6} (SBN:xx) pour applications électro-optiques

Notre objectif est de préparer des couches minces de niobate de strontium baryum Sr$_{\rm x}$Ba$_{1 - {\rm x}}$Nb$_{2}$O$_{6}$ (SBN:x) qui possèdent des propriétés électro-optiques héritées de celles du matériau massif (r$_{33}$ voisin de 13 00pm/V) et permettent à terme d'intégrer traitement optique du signal (par effet Pockels) et guidage planaire. Nous avons exploré la possibilité d'obtenir de telles couches par pulvérisation R.F. magnétron

Miroirs pour l'UV lointain (VUV) entre 80 et 130 nm

Dans le domaine spectral entre 80 et 130 nm situé dans le VUV (“Vacuum Ultra-Violet”), les matériaux ne sont ni transparents, ni vraiment réfléchissants. Il est difficile de réaliser des systèmes optiques et, en conséquence, de déterminer les constantes optiques des matériaux. Les écarts entre la réflectivité calculée à partir des indices référencés dans les tables et la réflectivité expérimentale nous ont conduits à penser que certains de ces indices n'étaient pas exacts. Nous avons développé une méthode expérimentale de détermination des constantes optiques à partir des mesures de réflectivité au voisinage de l'incidence normale, en fonction de la longueur d'onde. A partir des valeurs ainsi déterminées, des composants tels que miroirs ou polariseurs peuvent être conçus et réalisés

Frequency and wavelength dependencies of the electro-optic coefficients in SBN:60 single crystal

International audienceWe report the dependence of the high and low frequency values of the linear electro-optic (EO) coefficient rc = r33 −(no/ne)3r13 of Sr0.6Ba0.4Nb2O6 (SBN:60) single crystal on the laser wavelength. The EO response has been carried out from dc to 150 MHz for four wavelengths ranging from 465 to 1321 nm. The clamped rS and unclamped rT coefficients have been determined and their dispersions as a function of wavelength have been established and discussed

Guides d'ondes planaires modulables dans SrxBa1-xNb2O6 (SBN:x) : deux approches

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Experimental determination of optical constants of MgF2 and AlF3 thin films in the vacuum ultra violet wavelength region 60 124 nm , and its application to optical designs

International audienceThe investigation of some solar radiations of interest for astrophysicists requires optics in the 80–130 nm vacuum ultra-violet spectral range (VUV). In this domain, where both transmittance and reflectance of most materials are very low, the measurement of optical constants is specifically difficult, and optical data are consequently often either inexistent or uncertain. Reliable modelling of optical components for VUV, like polarizing multi-layered mirrors, necessitates prior measurement of complex indices of the thin films involved in the coating. Fluorides like MgF2 or AlF3 are among the rare materials capable to contribute to multi-layer mirrors in the VUV. We have determined optical constants of thin films of these two materials by using a two media reflectance method at normal incidence and a graphical determination particularly suited to this VUV region, which we presented in a previous paper. Optical constants are determined in the range 60–124 nm with 2 nm step, and are compared to existing data. On the basis of these measured indices, polarizing mirrors for k1 = 121.6 nm or k2 = 103.2 nm have been modelled and fabricated. Their reflectance measured versus incidence angle by using monochromatized synchrotron radiation at the above wavelengths is found in agreement with the calculated predictions

Electrooptic and converse-piezoelectric properties of epitaxial GaN/Si structures for optoelectronic applications

International audienceIn order to take advantage in all-optic optoelectronic devices, we have investigated the optical and piezoelectric properties of GaN films deposited on (111) silicon. Films are epitaxially grown by MOCVD, thanks to a buffer made of (Al, Ga) N intermediate layers [1]. Structural properties of GaN are analyzed using TEM and the influence of threading dislocations density is discussed. Optical properties are investigated using a prism coupling [2]. Electrooptic measurements are performed using an original technique [3]. A semi-transparent gold electrode is deposited on top of GaN layer and an alternating voltage is applied between top and bottom electrodes. The electro-optic, converse piezoelectric, and electro-absorptive coefficients are simultaneously determined from the measurement of the electric field induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack versus incident angle. The method also enables to determine the GaN layer polarity. The results obtained for a Ga-face [0001] GaN layer when using a modulation frequency of 230 Hz are for the electro-optic coefficients r13 = +1 pm/V, r33 = +1.60 pm/V at 633 nm, and for the transverse piezoelectric coefficient d33 = +4.59 pm/V. The value measured for the electro-absorptive variation is Δko/ΔE = +0.77 pm/V. The electro-optic coefficients for GaN /Si and the electro-absorptive coefficient are measured for the first time. The converse piezoelectric value agrees with values previously reported

Electro-optic and converse-piezoelectric properties of epitaxial GaN grown on silicon by metal-organic chemical vapor deposition

We report the measurement of the (r13, r33) Pockels electro-optic coefficients in a GaN thin film grown on a Si(111) substrate. The converse piezoelectric (d33) and electro-absorptive coefficients are simultaneously determined. Single crystalline GaN epitaxial layers were grown with a AlGaN buffer layer by metal organic chemical vapor deposition, and their structural and optical properties were systematically investigated. The electro-optic, converse piezoelectric, and electro-absorptive coefficients of the GaN layer are determined using an original method. A semi-transparent gold electrode is deposited on the top of the GaN layer, and an alternating voltage is applied between top and bottom electrodes. The coefficients are simultaneously and analytically determined from the measurement of the electric-field-induced variation ΔR(θ) in the reflectivity of the Au/GaN/buffer/Si stack, versus incident angle and light polarization. The method also enables to determine the GaN layer polarity. The results obtained for a Ga-face [0001] GaN layer when using a modulation frequency of 230 Hz are for the electro-optic coefficients r13 = +1.00 ± 0.02 pm/V, r33 = +1.60 ± 0.05 pm/V at 633 nm, and for the converse piezoelectric coefficient d33 = +4.59 ± 0.03 pm/V. The value measured for the electro-absorptive variation at 633 nm is Δko/ΔE = +0.77 ± 0.05 pm/V