The complex-valued index of refraction of germanium in the extreme ultraviolet (XUV) is measured by multi-angle reflectance of synchrotron radiation. The resulting index of refraction is higher resolution than previously measured values. It reveals new structures attributed to transitions from the 3d-core orbitals to the Σ5,2 and the 5,2 conduction bands. Additionally, it is shown that the problem of total external reflection, which renders multi-angle reflectance measurements insensitive to the complex-valued refractive index at grazing incidence, can be overcome by employing measurements at angles of incidence away from the critical angle.

J. Opt. Soc. Am. B
High-Harmonic Generation and EUV Science

Kaplan, C., Kraus, P., Gullikson, E., Borja, L. J., Cushing, S., Zuerch, M., … Leone, S. (2019). Retrieval of the complex-valued refractive index of germanium near the M-4,M-5 absorption edge. J. Opt. Soc. Am. B, 36(6), 1716–1720. doi:10.1364/JOSAB.36.001716