Coherent x-ray scattering from ultrathin probe layers

We have studied x-ray grazing incidence reflection from ultrathin layers that are embedded in thin film systems. This technique can be applied to selectively probe the properties of stratified media as a function of depth. A remarkable coherent enhancement of the reflected intensity renders this technique sensitive to ultrathin layers in the monolayer range. We investigate this enhancement theoretically and show that the coherently scattered signal is proportional to the square of the normalized field intensity at the position of the probe layer. This is in contrast to incoherent scattering from such probe layers where the signal scales linearly with the normalized field intensity at the position of the probe layer. The coherent enhancement is investigated experimentally by nuclear resonant scattering of synchrotron radiation from ultrathin isotopic layers of ⁵⁷Fe. The technique can be applied to all fields of coherent x-ray scattering from thin films.