Design of a compact time-delay-compensated monochromator for femtosecond pulses in the extreme-ultraviolet

Abstract

Table-top beamlines based on high-order laser harmonics (HHs) are nowadays lab-based facilities commonly used both for ultrafast experiments on its own sake and in preparatory experiments conceived, ab initio, for large-scale facilities such as FELs. Differently from FELs, HHs are emitted in a broad spectral range, requiring for most experiments the selection of a single harmonic. The monochromatization should preserve the temporal structure of the femtosecond pulse in a so-called time-delay-compensated monochromator (TDCM), where a couple of gratings is used in a configuration to compensate for the pulse-front tilt. At present, TDCMs in the extreme ultraviolet (15–100 eV) are realized using six optics at grazing incidence: two plane gratings and four toroidal mirrors. The gratings are illuminated by a collimated beam and the mirrors are used to collimate and focus the beam in the two sections of the monochromator: intermediate slit and target area. Here we present the design of a TDCM with four optical elements: two gratings, a cylindrical(spherical) mirror and a toroidal mirror. The gratings are used in the off-plane geometry and are illuminated by a divergent beam. The optical design is discussed in detail giving all the parameters for the definition of the configuration. We present the design of a TDCM for the 15-60 eV region, being a physical realization in progress. The main topics discussed are the beam size at the target area, the residual temporal broadening, the error budget for the alignment and the expected throughput. The design has advantages in terms of costs, compactness, alignment stability and throughput.