Measurement of SEW phase velocity by heterodyne interferometry method

Abstract

The surface electromagnetic wave (SEW) attenuation coefficient determined by the imaginary part of the SEW wave vector kiilb is generally measured to obtain information concerning the optical properties of metal surface layers and thin films by SEWs. However, the essential information can be obtained by measuring the real part of kiilb, which determines the phase velocity v by the relation v = ω/ Re(kiilb), where ω is the SEW frequency. Thus, for example, Re(kiilb) is directly determined by plasma frequency ωP for the case when -γ, ω ≪ ωP, where γ is the collision frequency. In the middle infrared, where the use of SEWs is particularly attractive due to large propagation distance, Re(kiilb) differs insignificantly from a bulk wave number kiilb = ω/c, and therefore measuring it is a problem. Schemes have been suggested1,2 where the interference of SEW and bulk radiation launched by the SEW were considered for measuring Re(kiilb). Such schemes have some disadvantages. In the present method for measuring Re(kiilb) the interference of SEW and the bulk radiation propagating along the surface of a sample is also used. However, unlike the usual methods, the bulk radiation is directed along the surface of the sample in the SEW propagation path from a laser. This enables the bulk radiation directed along the sample surface to pass first through a modulator that shifts the laser radiation frequency by the value Δω ≪ ω, and the optical heterodyning method can be used to determine Re(kiilb) and corresponding SEW phase velocity.