Design of hybrid coatings composed of homogeneous layers and refractive index gradients Vesna Janicki a,b , Robert Leitel c , Steffen Wilbrandt c , Olaf Stenzel a , Dieter Gäbler a , Norbert Kaiser a a Fraunhofer Institut für Angewandte Optik und Feinmechanik, Optical Coatings Department, Albert-Einstein-Str. 7, 07745 Jena, Germany b Ruđer Bošković Institute, Division of Laser and Atomic Research and Development, Bijenička cesta 54, 10000 Zagreb, Croatia c Friedrich-Schiller-Universität Jena, Institut für Angewandte Physik, Max-Wien-Platz 1, 07743 Jena, Germany ABSTRACT Inhomogeneous coatings are promising for superior optical properties, e.g. broadband antireflection, in comparison to conventional HL-stack designs. Although a lot of excellent theoretical work on optical behaviour of rugates and gradient index films has been done during the last decades, there is no real breakthrough in industrial fabrication. The realization of such coatings leads to an extensive and time-consuming computer-aided control, because of complicated layer designs with continuously changing refractive index gradients. We describe the design and optical performance of an omnidirectional antireflection coating that essentially represents a hybrid coating composed from homogeneous layers and linear refractive index gradient layers. Keywords: antireflection, gradient index, mixing layers, omnidirectional 1. INTRODUCTION Antireflective films are among the most frequently used coatings. They are applied in almost each optical system to improve intensity of transmitted light and prevent appearance of ghost images due to multiple reflections on intrinsic surfaces. The requirements are set on the spectral region and range of incidence angles where a low reflectivity is expected. Typically, two material based HL-systems are used, but if performance at oblique incidence or, especially, broad angular range is necessary, the implementation of at least one additional material of intermediate refractive index is favourable. However, traditional dielectric multilayer antireflection stacks remain limited for this purposes by the choice of available materials having both, good mechanical and optical properties at the same time. In this case, the application of gradient index layers is preferable. The ideal antireflective coating for a given spectral and angular range would be a gradient index structure having refractive index varying from the value of substrate, at substrate side, to the value of incidence medium towards the surface 1-3 . This means that in the case of substrate/air interface, the coating should end with a refractive index of air or at least close to it. The problem is how to achieve such low values, since there are no suitable low index materials. Potential approaches to obtain gradients of low refractive index are moth’s eye structures 4 or increased porosity of the film towards the incidence medium 5 . However, the main drawback is the mechanical resistance and cleaning of such surfaces. This paper presents an alternative approach for antireflection designs, combining homogeneous and linear graded index layers. The introduced hybrid coatings 6 are optimised for the visible spectral region and for omnidirectional incidence up to 50 degrees. This method merges the advantages of gradient index layers, e.g. reduced scatter losses due to the absence of abrupt changes of refractive indices at interfaces, and the capability of fabrication inhomogeneous coatings on industrial deposition plants. Additional profits that can be expected from inhomogeneous coatings are their good mechanical properties, like low stress and better tribological resistance 7-9 than classical multilayer stacks. Fig.1. represents a general refractive index profile of a hybrid coating. Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission. 5963-49 V. 1 (p.1 of 8) / Color: No / Format: A4 / Date: 8/2/2005 1:13:08 AM SPIE USE: ____ DB Check, ____ Prod Check, Notes: