What is "slot-waveguide"

A slot-waveguide is an optical waveguide that guides strongly confined light in a sub wavelength-scale low refractive index region by total internal reflection.

A slot-waveguide consists of two strips or slabs of high-refractive-index (n) materials separated by a subwavelength-scale low-refractive-index (n) slot region and surrounded by low-refractive-index (n) cladding materials.