Reflection / Suppression Coatings For 900 - 1200 A Radiation

Abstract

We describe the design and performance of multiple-layer, selective-reflection, selective-suppression coatings for the 900 - 1200 Å band. These coatings are designed to optimize both high reflectivity at a desirable wavelength and low reflectivity at an undesirable wavelength. The minimum structure for a selective coating consists of a thin metal or metal oxide layer (50 - 150 Å thickness) over an aluminum substrate protected with a semi-transparent dielectric (100 - 1000 Å. thickness). Predicted coating performance is strongly effected by varying the layer combination and thickness. Selec-tivity may be enhanced or augmented with further dielectric / metal layers. A graphical method of optimizing the coat-ing layer structure is developed. Aluminum, silicon, their oxides, and gold have been investigated as coating layer materials. We have fabricated a very simple coating with a 1026 to 1216 Å reflectivity ratio. greater than 100. Such reflection / suppression coatings may be of great utility to spaceborne EUV spectrographs. Strong background geocoronal emission lines could be rejected by use of properly coated collecting optics. By eliminating a diffraction element to reject background lines, efficiency losses and scattering may be avoided.