Reflective coating for near-infrared immersion gratings

摘要

Achieving high reflectivity from an immersed grating facet can be challenging in the near infrared. The reflectivity ofmetallic coatings in common use, such as Al and Cr/Au, decrease with decreasing wavelength in the near IR. A layer ofcopper on ZnSe or ZnS should have a high, immersed reflectivity based on tabulated values of refractive index, but infact performs poorly. We attribute this to a chemical reaction between the copper and the selenium or sulfur.A non-reactive intermediate layer can prevent this problem. Since reflectivity at an interface increases with increasingdifference in refractive index, it is beneficial to choose an intermediate layer of low index. A further improvement isgained by adjusting the layer thickness so that reflections from the two interfaces of the intermediate layer addconstructively.We sputtered 130 nm of SiOsub2/sub onto ZnSe and ZnS substrates followed by 200 nm of Cu. The copper was then coatedwith 5 nm of SiC as a protective capping layer. Immersed reflectivity measured shortly after coating exceeded 95%between 1500 and 1100 nm and exceeded 90% down to 850 nm. A repeat measurement after long term exposure to highhumidity conditions showed no changes.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.