The development of multilayer dispersive mirrors for the short-wavelength XUV spectral range is hindered by the complex index of refraction of all materials in the periodic table of elements. While the dispersive part of the index of refraction of all materials in the XUV is close to 1, the absorptive part is limiting the attenuation length to tens to hundreds of nanometers only.
Thus, multilayer reflective coatings for the XUV require alternative arrangement of tens to hundreds of nanometer layers with atomically smooth interfaces and sub 0.1 nm thickness control and imposes very high demands on the multilayer deposition process.
Example a) displays the measured reflectivity of a Mo/Si multilayer mirror exhibiting 40 double layers of 6.8 nm period thickness.
The development of XUV mirrors for attosecond pulses requires optimization and realization of periodic or aperiodic broad bandwidth multilayer coatings with customized spectral phase characteristic (see example b).
This is accomplished by using dedicated XUV optics simulation tools (“Optilayer”, “IMD”) as well as using Dual Ion Beam Deposition (“IBDO”) as a state-of-the-art deposition tool.
Example c) shows an example of a Mo/Si multilayer mirror as simulated by “Optilayer” and as realized by the deposition process.
The spectral properties of the layers are characterized in the soft X-ray range (in collaboration with the Center for X-ray Optics, LBNL) and hard X-ray range.
The currently accessible multilayer wavelength range from 30 eV to 100 eV is currently extended to the 100-200 eV range by using new layer material combinations. A further extension to the 200-500 eV photon energy range, which also covers the “water window” spectral range (spectrum where thin water films are highly transparent), is under development.