Phase characterization of attosecond multilayer mirrors: from EUV to soft x-rays

Phase controlled multilayer mirrors provide an efficient solution to transport, focus and/or compress attosecond pulses in the extreme ultraviolet (EUV) domain (30 - 100 eV). In this spectral range, one can access the spectral phase of the multilayer stack by measuring the photocurrent generated at the mirror surface as a function of the incoming photon energy. It has been already demonstrated that one can extract the spectral phase from such measurements under specific hypotheses. In this paper, we present the experimental protocol for such measurements and discuss the validity of this technique in the EUV and in the soft x-ray domains. In the EUV spectral range, our experimental results are in good agreement with simulations. However, the previous hypotheses are no longer valid at shorter wavelengths, in the soft xrays domain. This is mainly due to the fact that the electron mean free path becomes comparable to the individual layer thickness in the multilayer mirror. Here we propose a new method that enables one to extend the validity of phase characterization using photocurrent measurements in the soft x-ray domain (100 - 1000 eV). We present the first experimental results concerning the phase characterization of Cr/Sc multilayer mirrors in the water window and compare these results with simulation.