Surface plasmon polariton analogue to Young's double-slit experiment

When a light wave strikes a metal film it can, under appropriate conditions, excite a surface plasmon polariton (SPP)-a surface electromagnetic wave that is coupled to the free electrons in the metal. Such SPPs are involved in a wide range of phenomena(1), including nanoscale optical waveguiding(2-5), perfect lensing(6), extraordinary optical transmission(7), subwavelength lithography(8) and ultrahigh-sensitivity biosensing(9). However, before the full potential of technology based on SPPs (termed 'plasmonics') can be realized, many fundamental questions regarding the interaction between light and matter at the nanoscale need to be answered. For over 200 years, Young's double-slit experiment has been a valuable pedagogical tool for demonstrating the wave nature of light(10). Here, we perform a double-slit experiment with SPPs to reveal the strong analogy between SPP propagation along the surface of metallic structures and light propagation in conventional dielectric components (such as glass waveguides). This allows us to construct a general framework to describe the propagation, diffraction and interference of SPPs. It also suggests that there is an effective diffraction limit for the lateral confinement of SPPs on metal stripe waveguides, and justifies the use of well-developed concepts from conventional optics and photonics in the design of new plasmonic devices.