Optical effects of metallic mirror surface modification

Abstract

Abstract The optical effects of radiation damage on metal mirrors of MeV- and keV-range ions have been examined. Al, Cu and Cr ion beams of 1–3 MeV energy were used to bombard Al, Cu and stainless steel mirrors respectively with doses spanning 10–25 dpa. The effects following long-term sputtering with keV-range ions of hydrogen plasma (or deuterium) on mirror reflectivity R in the UV and visible spectral regions were investigated. The modifications of two stainless steel mirrors' optical properties induced by the deposition of different thicknesses of B layers have also been studied. The ex situ ellipsometric data for multiple angles of incidence (within the interval from 64 to 80°) were recorded at a wavelength of λ =632.8 nm, before both ion irradiation and boronization and after the treatment. It was found that even short keV-range ion bombardment of Al mirrors drastically changed their reflectivity at any given wavelength, in contrast to a stainless steel mirror. The boronization resulted in noticeable changes of the optical constants in the UV. The optical constants of the boron film on the stainless steel substrate ( n =2.53, k ≈0), with thicknesses d ( d =7.3 and 16.6 nm), were calculated from the ellipsometric data. The IR ellipsometric measurements within the frequency range 400–5000 cm −1 were performed for stainless steel mirrors pre-irradiated by Cr ions and then coated by B to clarify the damaged layer formation mechanism. The ellipsometric changes have been attributed to both modification of the metallic mirrors' surface mirror relief and the formation of a skin layer with better conductivity after ion treatment.