Be/Al-based multilayer mirrors with improved reflection and spectral selectivity for solar astronomy above 17Â nm wavelength

It is shown that in the vicinity of 17 nm wavelength, Be can be used as an optically contrasting material with Al in periodic multilayer mirrors. At the same time, due to the low absorption of both Be and Al, we can expect a unique combination of reflective characteristics: record high peak reflectivity and spectral selectivity simultaneously. Although the theoretical reflectivity of a Be/Al multilayer is 78% at normal incidence and at the 17.14 nm wavelength, we recorded only 46% for a pure Be/Al structure. The introduction of 1 nm Si on top of the Be layers in each period increases the reflection up to 61% at normal incidence. The increase of the reflection coefficient of Be/Si/Al multilayer mirrors is conditioned by smoothing the interfaces from root-mean-square value of 1.3 nm in pure Be/Al structures to 0.6 nm in Si-containing samples. We suppose that the smoothing effect is connected with the amorphization of Al films in the presence of Si.