Gap surface plasmon

A gap surface plasmon (GSP) can arise when electromagnetic radiation of the right wavelength meets a solid metasurface built up from a subwavelength thin dielectric spacer sandwiched between an optically thick metal film and an array of metal sub-wavelength elements arranged in a strictly periodic or quasi-periodic fashion. 
Surface plasmons (SPs) are coherent delocalized electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface. The term gap here refers to plasmons in the gap between the layers in the metamaterial, called the metasurface.
Metasurfaces are two-dimensionally arranged metamaterials which are uniquely engineered at the nanoscale to have properties for the spectral and spatial manipulation of electromagnetic waves. They are ultra-thin planar structures and offer extraordinary phase shift, amplitude modulation or polarization transition by the careful patterning of nanoscale dense antenna arrays or slits. 
Plasmons arise in and travel across the plane of the metasurface. A plasmon is a quasiparticle which arises from the quantization of the electromagnetic field oscillations and these are confined within the geometry of the material by the carefully engineered gap.