A gap-plasmon is a guided mode, a solution of Maxwell's equations without source. It is the form under which light propagates inside an extremely thin gap between two metals (having the same nature or not). As a gap plasmon, the electromagnetic wave can propagate up to four to five times slower than in vacuum. Such a guided mode only exists for parallel to the interface magnetic fields (p polarization). The distance between the metallic area has to be typically smaller than 50 nm in order to noticeably slow the guided mode. Actually, the GAP plasmon propagates partially inside the metal : the field of the GAP plasmon penetrates the metal to a depth of typically 25 nm, called the skin depth. A slow guided mode presents a short effective wavelength and so a very large wave vector (noted kx when the wave propagates along an Ox axis). As the thickness of the dielectric region decreases, the gap-plasmon is slowed by the metal and its effective index (as well as its wavevector) increases, while its effective wavelength shrinks.