Observation of coupling between surface plasmons in index-matched hole arrays

We measured the transmission of a large array of holes in an optically thick gold film, immersed in liquids of different refractive indices. For a large difference in refractive index between the substrate and the liquid (Delta n greater than or similar to 0.05), the transmission spectra contain separate resonances, due to surface plasmons propagating on each of the metal-to-dielectric interfaces. When the index difference is reduced we observe an avoided crossing between a strong low-energy mode and a weak high-energy mode. The low-energy mode becomes broader and gains amplitude at the expense of the high-energy mode. For an index-matched array, a single broad resonance remains. These observations provide direct evidence that the two surface plasmon modes on both sides of the interface are coupled.