We have made a multimode waveguide of x rays having an air gap as the guiding medium. Individual transverse electric modes were found to propagate through the planar waveguide with essentially no attenuation and with negligible scattering losses to other modes. If different modes are excited simultaneously at the waveguide entrance, then the phase relation between these modes as given by their propagation constants is found to be preserved over the entire length of the waveguide.

Phys. Rev. Lett.

Zwanenburg, M. J, Peters, J. F, Bongaerts, J. H. H, de Vries, S. A, Abernathy, D. L, & van der Veen, J. F. (1999). Coherent propagation of X rays in a planar waveguide with a tunable air gap. Phys.Rev.Lett., 82, 1696–1699.