Coherent propagation of white X-rays in a planar waveguide

The far‐field diffraction pattern of a front‐coupled planar waveguide supporting two guided modes has been measured using a white X‐ray beam. Interference of the guided modes leads to a characteristic variation of the far‐field diffraction pattern for different photon energies. The experiment verifies the predicted properties of the guided modes, shows that these modes superpose coherently, and demonstrates that the electromagnetic field downstream of the waveguide is significantly different from that expected for a hypothetical small slit of the same size.