Metal heterowaveguides for nanometric focusing of light

Based on the different propagation characteristics of surface plasmon polaritons (SPPs) excited on different metallic surfaces and on the tendency of SPPs to the region in which SPPs possess lower phase velocity, we design a kind of metal heterowaveguide for nanometric focusing of light. Finite-difference time-domain simulations demonstrate that, by converting light into SPPs, such heterowaveguides constructed with both Ag and Al can focus incident light into about a $20\mathrm{nm}\times 30\mathrm{nm}$ $(\lambda ∕27\times \lambda ∕18\approx 0.002{\lambda }^2)$ domain with higher than 15% coupling efficiency.