Failure of local Mie theory: optical spectra of colloidal aggregates

Abstract

Fully retarded calculations of the optical properties of aggregates of metallic nanoparticles have been performed. Within a local theory, the optical spectra of particles in contact with each other are dominated by a series of resonances below the Fröhlich resonance of isolated spheres. The resonances are shown to be related to surface plasmon polaritons of extremely short wavelength, which are localised at the points of contact of the particles, and are connected with strong field enhancements in the vicinity of the contact point. Standard local Mie theory of the optical properties of interacting spheres does not describe this excitation correctly at finite cut-off multipole orders ⩽30. Intensity distributions and spectra were therefore calculated by means of the semianalytical multiple multipole technique. For small particles, however, a nonlocal refinement of aggregate Mie theory predicts, that the localised surface excitations are strongly damped by the excitation of volume plasmons at the surface. Local Mie theory should therefore not be applied to colloidal aggregates of small metallic particles in contact with each other. The localised surface plasmon polaritons are important, on the other hand, for aggregates of somewhat larger particles, even if nonlocality is taken into account. Employing aggregate Mie theory with a nonlocal dielectric function, we show in addition, that, even for relatively small particles, retardation effects and high-order multipoles must be taken into account.