A Single-Element Plane Grating Monochromator

Abstract

Concerted rotations of a self-focused varied line-space diffraction grating about its groove axis and surface normal define a new geometric class of monochromator. Defocusing is canceled, while the scanned wavelength is reinforced at fixed conjugate distances and horizontal deviation angle. This enables high spectral resolution over a wide band, and is of particular advantage at grazing reflection angles. A new, rigorous light-path formulation employs non-paraxial reference points to isolate the lateral ray aberrations, with those of power-sum ď 3 explicitly expanded for a plane grating. Each of these 14 Fermat equations agrees precisely with the value extracted from numerical raytrace simulations. An example soft X-ray design (6˝deviation angle and 2ˆ4 mrad aperture) attains a resolving power ą 25, 000 over a three octave scan range. The proposed rotation scheme is not limited to plane surfaces or monochromators, providing a new degree of freedom in optical design. Grating rotation about its third (meridional) axis may be employed to cancel vertical deflection of the diffracted beam while maintaining the above aberration correction. This enables a simpler (pure rotary) motion for the exit slit and a fixed beam direction both horizontally and vertically.

Compact undulator-based soft x-ray radiation source at ELI Beamlines: user-oriented program

“…It is possible that in the XUV spectral range we will use a time-preserving monochromator (TPM) 64 based on a pair of focusing mirrors and a rotative off-plane diffraction grating, while for the 'water window' soft X-ray spectral range it is better to employ a one-element, or at maximum two-element monochromator scheme. The candidates are a two-element Hettrick-Underwood monochromator 65,66 , a zone-plate-based monochromator 67 , or exotic single-element monochromators 68,69,70 .…”