Metal layer beamsplitters with one dielectric achromatisation layer

A high refractive index dielectric layer of thickness ?/4 on the backside of a metal layer can reduce the absorption losses of metallic beamsplitters considerably. This was published by H. Pohlack in 1953. The practical importance of this knowledge may even be greater nowadays, as the availability of sputtering techniques makes the combination of metallic and dielectric layers within an optical coating much easier. In this contribution it will be shown, that the inclusion of dielectric layers into a metallic beamsplitter can possibly have further benefit. For some applications, where achromatic transmission and/or reflectance are needed, an additional dielectric layer can enhance the performance of a metallic beamsplitter. As an example the coating of a beamsplitter cube will be shown. The projected application of this cube demands a high reflection and only a low transmission over a spectral range from 400 nm to 800 nm. Using a metallic beamsplitter coating enhanced by a high refractive index dielectric layer it is possible to fulfil this demand with a comparably simple coating and at the same time to obtain a constant transmission of about 5% over a large spectral range.