Tunable two-color hard x-ray multilayer Bragg mirrors

A tunable two-color multilayer Bragg coating capable of simultaneously reflecting the fundamental and the third harmonic of an x-ray free-electron laser at the same angle and with high reflectance R > 0.70 is presented. The novel coating will enable two-color x-ray pump/x-ray probe experiments. This mirror consists of a Si substrate that is coated with two different types of multilayer systems, Mo/B4C layers with a periodicity of d = 3.2 nm directly on the substrate and Ni/B4C layers with a periodicity of d = 11.85 nm on top. Fundamental radiation with photon energies between 3 and 9 keV is reflected by a Ni/B4C multilayer system while the third harmonic (9 keV < h nu < 27 keV) passes this system and is reflected by the Mo/B4C multilayers. The principle has successfully been proven at the beamline BM05 at ESRF.