Hello

I advise you to look to the azimuthal mode index as a determinant of SPP refractive index

In short, we often calculate the skin depth of the plasmon mode and indeed this is in relation to the mode index as Yousef mentions above.

You can determine the distribution of light inside the MIM waveguide and then determine the confinement factor (Gamma) of a layer (available in all of text books).

For dielectric modes or dielectric waveguide, the cross section is at least a few times of wavelengths, otherwise because of the cut-off,    more lights or carrier information will be leaked for smaller cross section. However,  for plasmonic (metal) waveguide, the cross section can be far less than the wavelength, or in other words, the SPP modes can  largely confine and propagate light.  I would say the book from Stefan Maier is  a good one for such knowledge.  

Dear Juemin, the book from Stefan Maier is very useful, thanks.

Hello Frank the skip depth seems more straightforward to understand the modes confinement, thanks.

SPP modes is created by the discontinuity of electric field at the boundary or interface between dielectric and metallic layer with evanescent parts both in the metallic and dielectric layers. This means that the effective index of the surface plasmonic mode is lower than a standard waveguide mode that propagates is the same dielectric layer. With a mode solver, it is possible to calculate the total power of the mode with the poynting vector. The confinement factor is the part of the power only in the dielectric layer normalized by the total power. The confinement factor increase when the refractive index of metal tends to thus of a perfect metal (real part close to 0 and imaginary part close to infinity). The evanesent part of the mode in the metal is reduced and also the propagation losses.

The response of Regis is perfect.

Yes thanks Regis.