The short-wavelength/high-energy region of the electromagnetic spectrum spanning the extreme ultraviolet (EUV) to the hard X-ray presents a particular challenge to the development of efficient reflective optical elements such as mirrors and gratings: at normal incidence, the reflectance of even the best materials begins to plummet in the EUV, and continues to fall rapidly with increasing photon energy (like E4, in fact), as illustrated, for example, in the next figure. Consequently, conventional front-surface mirrors and single-layer reflective coatings are far too inefficient to be useful for the construction of optical instruments such as telescopes and spectrometers that are meant to operate at normal incidence with high sensitivity at these short wavelengths.
Figure 1. Reflectance of aluminum, silver, and gold, as a function of photon energy at normal incidence. These materials (and others) can be used to produce conventional front-surface mirrors that have very high reflectance in the UV, Visible, and IR bands. But such mirrors are far too inefficient for the construction of high-performance optical instruments operating near normal incidence in the EUV and X-ray bands.
Multilayer Coatings, on the other hand, make use of the principle of optical interference to achieve high reflectance at normal incidence in the EUV and soft X-ray bands, and in the hard X-ray band at grazing incidence angles that are several times larger than those possible using front-surface mirrors. Their high reflection efficiency has enabled remarkable advancements in a variety of scientific and technological disciplines over the past three decades, particularly solar physics and astronomy, but also microlithography, ultrafast science, plasma diagnostics, synchrotron optics, and many other areas as well.