Design of time-preserving grating monochromators for ultrashort extreme-ultraviolet pulses

The design of grating monochromators to increase the spectral purity of extreme-ultraviolet (XUV) ultrashort femtosecond sources, such as high-order laser harmonics (HHs) or free-electron lasers, is discussed. The use of gratings for the spectral selection intrinsically introduces a temporal broadening of the output pulse because of diffraction. In the case of a single-grating monochromator, once the resolution λ/Δλ at the output has been defined, the minimum number of grooves that have to be involved in the diffraction to support such a resolution is N_min = λ/Δλ. It can be shown that the corresponding time broadening is close to the Fourier limit for a given bandwidth Δλ. Therefore the single-grating design can be adopted for ultrashort pulses without altering in a significant way the pulse duration, provided that the number of illuminated grooves is equal to the resolution. A grating monochromator is defined to be time-preserving if the temporal duration at the output is close to the Fourier limit for a given spectral bandwidth.