Stephane Bruynooghe

Spectral measurement in reflection on steeply aspheric surfaces

The controls by optical mean of coatings deposited on optical components are generally made with flat witnesses. When the components are spherical or aspherical, like lenses or mirrors, the spectral response may vary because of the nonuniformity of thickness that is really linked to the deposition process. For large radius of curvature, control can be achieved even with classical spectrophotometers. However, control becomes more and more difficult when the radius of curvature decreases or when the optical device has a complex shape. Thus special devices are needed to perform this kind of measurement. ZEISS and CEA Le Ripault use spectral reflection as a mean of measurement, which enables to investigate optical coatings on curved parts. Two different devices have been implemented and used to measure the same antireflective coating deposited on an aspheric lens. In this work, we show the obtained results and we compare these results to theoretical simulation.

Broadband and wide-angle hybrid antireflection coatings prepared by combining interference multilayers with subwavelength structures

Abstract. To reduce the intensity of the Fresnel reflections of optical components, subwavelength structures prepared by reactive ion etching of SiO2 thin films were combined as the outermost layer with a multilayer system made of conventional thin-film materials. A hybrid coating was thus realized, with the nanoscaled structured outermost layer expected to further improve the antireflection properties of common interference stacks. The microscopic and optical spectroscopic analysis of the subwavelength structures revealed that pillar-shaped nanostructures formed during etching exhibit low-refractive-index properties and have a depth-dependent refractive index. To take into account the refractive-index gradient in the coating design, the optical properties of the nanostructures were modeled using the effective-medium approximation. The calculated average effective refractive index turned out to be 1.11 at 500-nm wavelength. A hybrid coating was designed to minimize the residual reflectance in the 400-nm to 900-nm spectral range for BK7 glass substrate. Experimental results demonstrated that the hybrid-coating approach yields a low residual reflectance with very good omnidirectional properties, owing to the properties of the nanostructured surface.

Optical properties of plasma ion-assisted deposition silicon coatings:application to the manufacture of blocking filters for the near-infrared region

I report on the preparation and characterization of optical constants of silicon coatings deposited by an electron beam gun with plasma ion-assisted deposition. With the fabrication of long-wave-pass filters the reliability of the optical constants is assured.

Wideband antireflection coatings combining interference multilayers and subwavelength structures prepared by reactive ion etching

To further reduce the intensity of the Fresnel reflections of optical components, subwavelength structures prepared by reactive ion etching of SiO2 thin films are combined as outermost layer with a multilayer system made of conventional thin film materials and prepared by magnetron sputtering. In this approach, a hybrid coating is realized in which the nanoscaled structured outermost layer is expected to further improve the antireflection properties of common interference stacks. The subwavelength structures are examined by spectroscopic ellipsometry, spectral photometry and scanning electron microscopy. The microscopic and optical spectroscopic analysis revealed that pillar-shaped nanostructures are formed during etching which exhibit low-index properties and have a depth-dependent refractive index. To take into account the index gradient in the coating design, the optical properties of the nanostructures are modeled using the effective medium approximation. The calculated average effective refractive index is 1.11 at 500 nm wavelength. A hybrid coating was designed to minimize the residual reflectance in the 400 – 900 nm spectral range for BK7 glass substrate. Experimental results showed that the hybrid coating achieves a low residual reflectance with very good omni-directional properties, owing to the properties of its nanostructured surface. The residual reflection of the hybrid coating is found to be two times smaller than the reflection obtained by applying a common interference multilayer system which demonstrates the benefit of the use of hybrid systems for the realization of broadband antireflective coatings with wide-angle properties.