A light emitting device for generating infrared light includes a substrate, a first metal layer, a dielectric layer and a second metal layer. The substrate has a first surface. The first metal layer is formed on the first surface of the substrate. The dielectric layer is formed on the first metal layer. A thickness of the dielectric layer is greater than a particular value. The second metal layer is formed on the dielectric layer. When the light emitting device is heated, the dielectric layer has a waveguide mode such that the infrared light generated by the light emitting device can be transmitted in the dielectric layer. A wavelength of the infrared light generated in the waveguide mode relates to the thickness of the dielectric layer.