Gérard Albrand

High-reflectivity HfO 2 /SiO 2 ultraviolet mirrors

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 ∼99% near 250 nm are reported.

Simple layer-by-layer photonic crystal for the control of thermal emission

We present a theoretical and experimental study of a simple layer-by-layer photonic crystal structure designed for the control of the thermal emission in the infrared wavelength domain. We show that a relatively simple structure made of alternated ZnSe homogenous layers and gold microstructured grids can act as a thermal source itself giving us the unique opportunity to tailor its emission spectra. Comparisons between computed and measured transmission and emissivity spectra illustrate the relevance of our approach.

Low-level scattering and localized defects

We investigate the origin of low-level scattering from high-quality coatings produced by ion-assisted deposition and ion plating. For this purpose we use the polarization ratio of light scattering to separate surface and bulk effects that characterize the intrinsic action of the thin-film materials. In the first step the method is tested and validated at scattering levels greater than 10(-5). In the second step it is applied at low levels, and the results reveal some anomalies. To conclude, we perform a detailed analysis of scattering resulting from the presence of a few localized defects in the coatings.

Absorption mapping for characterization of glass surfaces

The surface quality of bare substrates and preparation procedures take on an important role in optical coating performances. The most commonly used techniques of characterization generally give information about roughness and local defects. A photothermal deflection technique is used for mapping surface absorption of fused-silica and glass substrates. We show that absorption mapping gives specific information on surface contamination of bare substrates. We present experimental results concerning substrates prepared by different cleaning and polishing techniques. We show that highly polished surfaces lead to the lowest values of residual surface absorption. Moreover the cleaning behavior of surfaces of multicomponent glasses and their optical performance in terms of absorption are proved to be different from those of fused silica.

Substrate effects on absorption of coated surfaces

Photothermal deflection is used for mapping the absorption of bare and coated surfaces. The same area is mapped before and after coating and also after annealing. The great importance of the substrate with respect to the total losses of the coated component is emphasized. First the influence of surface contamination of the bare substrate on the total absorption of the coated substrate is studied for BK7 and fused-silica substrates. Then the mean value of the coated-substrate absorptance is shown to be strongly dependenton the type of substrate. Experimental results show that this effect is associated with a localization of the absorption at the near surface of the substrate and at the interfaces of the film.

Solarization of glass substrates during thin-film deposition

We demonstrate that solarization occurs in glass substrates during thin-film deposition and that it induces high absorption near the surface of the substrate. Solarization has been observed especially in ion-plating deposition. We show that the solarization of the substrate is caused by electromagnetic radiation emitted from the material to be evaporated. The radiation is due to the energy losses of the heating beam of electrons (bremsstrahlung radiation). Multicomponent glasses such as BK7 are much more sensitive to solarization than fused-silica substrates. The photoinduced high absorption can be partially reversed by thermal annealing.

Fabry–Perot multilayers for enhancing the diffraction efficiency of ion-implanted gratings

Enhancement of the free-space diffraction efficiency of gratings made by titanium-ion implantation is demonstrated both theoretically and experimentally. Indeed, by insertion of a grating into a multilayer dielectric Fabry-Perot cavity, the diffraction efficiency can be increased to as much as 24 times that of a single grating. The sensitivity of the diffraction efficiency to the optogeometrical parameters of the grating or of the Fabry-Perot cavity is discussed. Moreover, a process for performance of a phase grating inside a Fabry-Perot cavity is described, and experimental results concerning efficiency measurements are compared with computed values for various grating periods.

Manufacturing of linear variable filters with straight iso-thickness lines

Linear variables filters are band-pass multilayer coatings manufactured with a thickness gradient that allows a significant wavelength shift of the centring wavelength of the filter according to the point that is illuminated. In case such a filter is associated with a 2D matrix detector in order to form a compact spectrometer, iso-thickness lines must be as straight as possible, perpendicularly to the thickness gradient. To answer this problem, we developed a masking mechanism that combine the classical rotation movement of substrates and a translation movement for the mask, this last movement being induced during the rotation by the mean of a cam. Thickness gradient can be freely adjusted according to specifications, while transverse uniformity is 99.9%.

Manufacturing of four-quadrant phase mask for nulling interferometry in the thermal infrared

The Four Quadrant Phase Mask is a key component for the design of advanced coronagraphs that may be used to search exo-planets. The validity of this concept has been demonstrated in the visible and need now to be demonstrated in the mid infrared. For this purpose, two components are manufactured for wavelengths 4.75 and 16.25 μm. This manufacturing requires the deposition of ZnSe layers using Ion Assisted Deposition, followed by a lift off process.

Multiscale Degradations of Storage Ring FEL Optics

The advanced understanding of the complete degradation phenomena is crucial in order to develop robust optics for FEL. Under very harsh Synchrotron Radiation conditions, results show that multiscale wavelength damages could be observed, inducing local crystalline structure modifications of the high optical index material with a severe increase of the surface roughness.