The XUV optics play a central role in experimental studies of ultrafast dynamics. They should perform multiple functions [Bourassin13, Diveki14] : 

•  Select a given spectral bandwidth in the overall range 10-500 eV (124nm-2,5nm) and reflect it with high reflectivity. To accommodate several types of time-resolved studies, the bandwidth selection should vary from the narrow band mode (ΔE ~ 1-100 meV) and corresponding pulse duration (Δτ ~ 100 fs-1 ps), to the broadband (ΔE ~ 0,1-1 eV; Δτ ~ 1 fs-10fs) and very broadband mode(ΔE ~ 1-100 eV; Δτ ~ 10 as-1 fs), the latter corresponding to the most specific attosecond time scale ;
• Especially in the very broadband mode, introduce a controlled spectral phase, in order either to let the attosecond pulses unchanged after reflection, or rather to shape the spectral phase and therefore the temporal structure of the attosecond pulses ;
• Focus the pulse as close to the diffraction limit as possible ; 
• Superimpose the XUV and IR-vis-UV pulses in space and in time with controllable delay ;
• Allow manipulation of the XUV light polarization.

The LCF group develops the design and fabrication of optics which can perform one or several above functions. In particular, multilayer optics are well adapted and offer a large range of characteristics which can be optimized for a specific function. A multilayer mirror is made of a periodical/aperiodical stack of thin metallic layers, alternating low and high refractive indexes in the XUV spectral range, deposited on a high quality substrate. Spectral selectivity and reflectivity build up as the result of multiple reflections at interfaces of the stack, involving constructive or destructive interferences.