Peculiar features of confinement factors in a metal-semiconductor waveguide

The concept of confinement factor (CF) is analyzed for a metal-semiconductor-metal (MSM) waveguide near the surface plasmon resonance. We show that the CF is inversely proportional to the average energy velocity, and thus can be enhanced by the slowing down of energy propagation. This slowing down in the MSM waveguide results in a value of CF as high as 10⁵, as opposed to a value of smaller than 1 in a typically dielectric structure. Contrary to the CF in pure dielectric waveguide where it is independent of material gain, the CF in MSM waveguide shows a resonancelike dependence on material gain near the surface plasmon resonance.