Solving Surface Plasmons: Understanding Components & Charge Density

[geo_alchemist]

I'm still trying (yet unsuccessfully) to deal with surface plasmons, and I still hope on your help.
let me start like this:
I find in the review that:

We consider an interface in the xy-plane between two half-infinite spaces, 1 and 2, of materials the optical properties of which are described by their complex frequency-dependent dielectric functions $$\epsilon$$₁($$\omega$$) and $$\epsilon$$₂($$\omega$$), respectively. We ignore magnetic materials. Surface polaritons can only be excited at such an interface if the dielectric displacement $$\stackrel{\rightarrow}{D}$$ of the electromagnetic mode has a component normal to the surface which can induce a surface charge density $$\sigma$$,
(D2-D1)z=4$$\pi$$$$\sigma$$

and here I found that I don't quite understand why there must be a component, normal to the surface, and what is the connection between surface charge density and surfacce plasmons.

Any help will be greatfully appreciated.

 

[Gokul43201]

About the normal components: think Gauss' Law.

For the connection between surface charge density and surface plasmons, go back to square one, and start at the definition of a surface plasmon.