Diffractive optical elements for the extreme ultraviolet regime

Low-loss diffractive optics are of crucial importance wherever weak signals need to be detected, e.g. in astronomy. Unlike reflective components, lightweight normal-incidence transmission gratings maximize the sensitive area per unit mass. Recently realized true free-standing binary phase gratings made of an ultra-nanocrystalline diamond membrane with a thickness of a few 100 nm reach an efficiency up to 28% in the (±1)st order in the spectral range between 5 nm and 7.5 nm - in contrast, the 0th order is reduced to 1% near 6.8 nm. Higher order contributions up to the (±3)rd one sum up to ~ 7%. So far, the residual compressive stress of the diamond film limits the practical grating diameter to ~ 400 µm. The future goal is an extension to dimensions of several (10-3-10-2) m (Fig.1).

Optics for u-s wavelengths_Braig
Fig.1: Diffraction by a stand-alone phase transmission grating made of an ultrathin diamond membrane
       (left) and by a reflective EUV beamsplitter in conical grazing incidence (right). In each case,
       prominent propagating orders are drawn in sunset color code.(rights: IAP)

In laboratory plasma physics, the complex emission from ionized atoms is usually investigated by means of interferometry. The classical grating beamsplitter, required for the Mach-Zehnder setup, often suffers from unbalanced diffraction efficiencies. Our alternative design exploits conical grazing incidence diffraction from binary lamellar structures: Their symmetry naturally permits an equalization of the (±1)st order efficiency, experimentally found as 33% in each of them at a wavelength of 25 nm for the "proof-of-concept" sample made of Si and SiO2. Simultaneously, destructive interference almost eliminates the 0th order to 0.12%. Higher orders beyond the (±1)st one are associated with evanescent fields and cannot propagate. An absolute intensity difference of 0.01% between the (-1)st and the (+1)st order was measured at BESSY II by the Physikalisch-Technische Bundesanstalt (PTB), which implies an interference contrast close to 1 (Fig.1). Forthcoming developments focus on a monolithic design made of carbon, to improve the reflectivity even at soft X-ray wavelengths.

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