Partial spatial coherence and polarization in purely evanescent electromagnetic fields

We study the partial spatial coherence and partial three-dimensional polarization in electromagnetic fields consisting of a superposition of evanescent plane waves generated in total internal reflection. In particular, we investigate the coherence length in such fields, and demonstrate that it can be shorter than the wavelength of the propagating field above the dielectric surface. The higher the refractive-index contrast at the interface is, and the closer to the surface the field is considered, the shorter is the coherence length. Physical explanation for this behavior analogous to the generation of incoherent light in a multimode gas laser is provided. We also assess the degree of polarization in evanescent fields by using a recent threedimensional formulation.