Gap and Mie Plasmons in Individual Silver Nanospheres near a Silver Surface

We study the plasmons confined at the gap between silver nanospheres and silver planar surfaces by means of angle-and space resolved spectral cathodoluminescence. Plasmons in individual nanoparticles are excited by an electron beam, giving rise to light emission that is analyzed as a function of photon energy emission direction, and position of the beam spot. Gap plasmons are significantly red shifted due to the interaction between the particles and the metal substrate, and they are preferentially excited by positioning the beam close to the sphere centers, which results in an angular emission pattern similar to that of a dipole oriented along the surface normal. In contrast, weaker emission features are observed at higher energies when the beam is grazing to the spheres, corresponding to the excitation of Mie plasmons like those of isolated particles, which display an angular pattern approximately mimicking a dipole parallel to the surface. Our measurements are in excellent agreement with simulations, thus providing useful insight into gap plasmons arising from the interaction between metal particles and metal substrates that are relevant for molecular sensing applications.