Light-Like Group Velocities and Long Lifetimes for Leaky Surface Plasmon Polaritons in Noble Metal Nanostripes

Propagating surface plasmon polaritons (PSPPs) are electron motions at the surfaces of conductors that have a definite momentum. For metal nanostructures on a glass substrate, two types of PSPP modes exist: bound modes where the field is localized at the metal-glass interface and leaky modes at the metal-air interface. In this paper, the leaky PSPP modes of Ag and Au nanostripes created by photolithography were examined by a combination of real space and Fourier space (back focal plane) imaging measurements and finite element simulations. The back focal plane images provide information about the wavevector k_SPP of the leaky mode, and the real space images yield the propagation length L_SPP. The measured wavevectors and propagation lengths are in good agreement with the finite element simulations for the Au nanostripes, but not for Ag. This is attributed to surface contamination of the Ag nanostructures. Performing the back focal plane experiments over a range of excitation frequencies generates ω versus k dispersion curves, which can be used to determine the PSPP group velocities via v_g = âω/âk. The group velocities are very high, â90% of the speed of light. Combining the propagation length and group velocity measurements allows an estimate of the lifetime of the PSPP modes by L_SPP/v_g. The results of this analysis show that the leaky PSPP modes have lifetimes that are an order of magnitude longer than those for the localized surface plasmon resonances of metal nanoparticles. The experiments show that the lifetimes decrease as the width of the stripes decreases. This is attributed to increased radiation damping for the thinner nanostripes.