Titanium oxide/nickel oxide multilayer mirror for attosecond soft x rays

A novel TiO/NiO multilayer mirror based on an atomic layer epitaxy (ALE) technique has been designed to realize a high reflective mirror and an attosecond chirped mirror in soft x-rays "water-window" wavelengths region. The layer thickness should be controlled on an atomic scale, but in ALE, according to a self-limiting mechanism which is found in the fabrication of oxide thin films, the epitaxial growth is automatically stopped at strictly 1 monolayer (ML) in one alternative precursor dosing. An oxide multilayer is also expected to overcome serious problems such as scattering loss at interfaces of the multilayer. TiO and NiO are an exquisite combination which has the potential to make their superlattice on a MgO substrate because each oxide has the NaCl structure of nearly equal lattice constant (TiO = 0.41766 nm, NiO = 0.41684 nm, MgO = 0.42112 nm). The theoretical calculation of a periodic multilayer mirror showed the high reflectivity of over 50% at a wavelength of 2.73 nm and the incident angle of 18.9° from the normal incidence. Additionary, reflective properties of ultrashort pulses induced to design a chirped structure stacked by blocks of several multi-periodic structures, which must have a spectral bandwidth and a controlled phase to compress a temporal broadening of the pulses down to a few hundreds of attoseconds. In this paper, we report the details of structures between titanium oxide and nickel oxide, and properties both as a high reflection mirror and as a chirped mirror.