Design and performances of two channel EUV multilayer mirrors with enhanced spectral selectivity

In this paper, we present a study on two-channel multilayer mirrors which can operate at two wavelengths in Extreme Ultraviolet (EUV) spectral range. We propose a new method to design two-channel EUV multilayer mirrors with enhanced spectral selectivity. The mirror structure is a stack of two periodic multilayers separated by a buffer layer. We have defined the main parameters which allow adjustment of the distance between different order Bragg's peak and of wavelength positions of reflectivity minima. Two mirrors have been designed and deposited for solar EUV telescope applications by using this method. The first mirror reflects Fe IX-X line (17.1 nm) and Fe XVI (33.5 nm) lines with attenuation of the He II line (30.4 nm). The second mirror reflects Fe IX-X and He II lines with attenuation of Fe XV (28.4 nm) and Fe XVI lines. Measurements with synchrotron radiation source confirm that, in both cases, for these mirrors, we are able to adjust reflectivity maxima (Bragg peak position) and minima. Such multilayers offer new possibilities for compact design of multi-wavelength EUV telescopes and/or for high spectral selectivity.