An IR grating on a clean W(100) surface is shown to generate both homogeneous and inhomogeneous surface electromagnetic waves. An observed interference between these two components, which can be described in terms of a two-beam interferometer with variable arm amplitude and fixed optical path, is used to measure the plasma frequency accurately in the IR.
L. M. Hanssen, D. M. Riffe, and A. J. Sievers, "Infrared surface-wave interferometry on W(100)," Opt. Lett. 11, 782-784 (1986) http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-11-12-782