Ultrafast Grating Instruments in the Extreme Ultraviolet

Optical grating instruments enjoy several advantages in the optimal conditioning of subpicosecond, or ultrafast, pulses in the extreme ultraviolet (XUV) spectral region. The main application of such instruments is the spectral selection of high-order laser harmonics and free-electron-laser pulses in the femtosecond time scale. Broadband XUV monochromators require the use of diffraction gratings at grazing incidence. We discuss here an innovative configuration, that is the off-plane geometry, to realize single-grating XUV monochromators with high efficiency and ultrafast time response. Furthermore, the off-plane design has been modified to realize a double-grating XUV monochromator with time-delay compensated (TDC) response. Two examples of realizations applied to the spectral selection of high-order laser harmonics are presented. In particular, XUV pulses as short as 8 fs have been measured at the output of the TDC monochromator. Finally, the problem of temporal compression of broadband XUV attosecond pulses by means of a double-grating compressor will be addressed. The TDC design has been modified to realize an XUV attosecond compressor that introduces a variable group-delay dispersion to compensate for the intrinsic chirp of the attosecond pulse.