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. 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.