The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in the medium. It has no units, therefore. This is true also in the context of surface plasmon resonance. RIU is sometimes used to distinguish a number as referring to a refractive index. It also is useful for expressing quantities that vary with refractive index. For instance you might want to express the change in scattering cross-section of a small dielectric particle due to changes in its refractive index. In that case, the appropriate units are m^2/RIU.
It is a non-dimensional (adimensional or dimensionless, sorry but I do not know which one is the most adequate english term) value (like for example the Q-value of aspheric surfaces).
The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in the medium.
RIU (refractive index unit) is a convenient unit to quantify the sensitivity of a surface plasmon resonance instrument. I believe that this is different from the conventional refractive index.
As far as I know, an RIU is just defined as a change of 1 of the refractive index of a given material - therefore it also should not have any additional units.
look for example here: Shape-controlled synthesis and surface plasmonic properties of metallic nanostructures, Y Xia, NJ Halas - MRS bulletin, 2005 - Cambridge Univ Press
If what I understand is correct, does this means that the refractive index of air is ~ 1 RIU?? In this article http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-39-1-26 the authors report minimum detectable refractive index change of the order of 10^-11 RIU, which seems unimaginable low to me... Is this correct ?
RIU: Refractive Units Index. It is used in optical biosensing, for evanescent wave sensors (optical waveguides, ring resonators, interferometers, surface plasmon resonance). It is the minimun detectable change in the refractive index of the surface where the evanescent wave is travelling.