- Surface plasmon (SP) waves on the interface of a dielectric (such as water) and a metallic columnar thin film (CTF) of porosity as high as 0.55 were experimentally and theoretically investigated. The CTFs were made of Al, Au, Ag, or Cr. As the porosity increases, the SP resonance (SPR) dip was found to widen, shift to higher wave numbers, and become asymmetric due to increasing scattering losses. With further increase of porosity, the SPR dip was found to disappear, leaving behind only a peak near the onset to the total internal reflection regime. The shape of the nanoislands constituting the CTF is better described as ellipsoidal than as spherical or spheroidal, indicating thereby the existence of orientational biaxial anisotropy even for CTFs thinner than 60 nm. For a best fit between the theoretical calculations and the experimental data, the CTF was divided into two layers having different porosity and nanoisland shape, particularly for the Ag- and Au-CTFs. The sensitivity of the CTF-based SPR signal to refractive index variations of an analyte infiltrating the nanopores of and in the region adjoining the metallic CTF was found to be doubly enhanced compared to that for the SPR signal from a nonporous metallic film.