Excitation of Surface Plasmons on He-Filled Cavities in Al

Surface plasmon losses excited on spherical, He-filled cavities in Al are studied with use of transmission electron energy-loss spectroscopy. It is found that the excitation energy of the $l=1$ (dipole) surface plasmon mode decreases with increasing He concentration, in line with predictions of an effective-medium theory taking into account the dipole-dipole interactions between bubbles in dense populations, the density dependence of the He dielectric constant, and a density-dependent atomic polarizability of the He in the bubbles.