Direct measurement of index of refraction in the extreme-ultraviolet wavelength region with a novel interferometer.

To the best of our knowledge, the first direct measurement of the dispersive part of the refractive index is performed at extreme-ultraviolet (EUV) wavelengths, where absorption is higher as compared with hard-x-ray and visible wavelengths. A novel diffractive optical element that combines the functions of a grating and a zone plate is fabricated with Fourier optical techniques and employed here for the first time at EUV/soft-x-ray wavelengths. Both the real and the imaginary parts of the complex refractive indices are measured directly by this technique without recourse to Kramers-Kronig transformations. Data for Al and Ni in the vicinity of their L and M edges, respectively, are presented as first examples of this technique.