Effective Surface of Optical Coatings, 2006 TAPPI Advanced Coating Fundamentals Symposium

The response of a medium with respect to electromagnetic fields is commonly described in terms of its dielectric function. For optically homogeneous materials, the square root of the dielectric function determines the refractive index and the absorption properties at specific optical frequency. Optical coatings however are composite materials for which one cannot define an effective dielectric function. When describing the properties of such thin material layers, both optical and physical considerations must be taken into account. Using approaches specific to near-field optics, one can determine simultaneously the mechanical and optical properties of a surface. We have developed a statistical treatment that allows the surface and bulk contributions to the total scattering to be calculated from an optically diffusing layer. This approach permits then the determination of the surface variations of the local dielectric constant. Our results demonstrate that surface variations can be a driver of optical properties and that this is a degree of freedom which was not previously recognized in coating design. By understanding and leveraging the ability to design into the coating surface an inhomogeneous dielectric constant on the surface of the paper coating, optics can be controlled and enhanced to new levels. This finding coupled with new coating technology that will allow ultra thin films of coatings to be deposited on the surface could result in significantly new properties to be designed into any substrate coating with a thin film.