A waveguide including a substrate, a plurality of cladding layers, a first dielectric layer, a second dielectric layer, and a third dielectric layer. The cladding layers are present on the substrate and define at least one tunnel therein, in which at least one of the cladding layers is made of metal. The first dielectric layer is disposed in the tunnel and has a first refractive index N1. The second dielectric layer is disposed in the tunnel and has a second refractive index N2. The third dielectric layer is disposed in the tunnel and has a third refractive index N3, and N2>N1 and N2>N3, in which the second dielectric layer is present between the first and third dielectric layers.