A wavelength multiplexing device is disclosed. When light is irradiated on a first longitudinal end region of a metal nano-structure, surface plasmon polaritons are generated in the first longitudinal end region. The surface plasmon polaritons and the light are coupled with each other to form first coupled surface plasmon polaritons, wherein the first coupled surface plasmon polaritons propagate along and on a surface of the metal nano-structure. When the first coupled surface plasmon polaritons reach a two-dimensional material layer, excitons are induced in the two-dimensional material layer, wherein the induced excitons and the first coupled surface plasmon polaritons are coupled with each other to form second coupled surface plasmon polaritons. The second coupled surface plasmon polaritons propagate along and on a surface of the metal nano-structure toward a second longitudinal end thereof.