• Stenger, Nicolas
• Raza, Søren

Abstract

The optical properties of silver metal nanoparticles are dominated by the excitation of localized surface plasmons. Light incident on silver particles with spherical shape and diameters below 100 nanometers excites primarily the dipolar mode. The higher-order modes, i.e., modes with larger angular momentum l, such as the quadrupole mode, are strongly damped as their larger resonance energy tend to be positioned in the range of interband transitions in silver. By encapsulating silver nanospheres in a high permittivity dielectric medium, in this work silicon nitride, we redshift all of the plasmon modes and thereby get access to the higher-order modes of silver nanoparticles. We map the dipolar and higher-order modes of individual silver nanoparticles with electron energyloss spectroscopy in a state-of-the-art transmission electron microscope. For particle radii smaller than 4 nanometers, we observe a strong blueshift of the dipolar mode of 0.9 eV and a strong damping of the higherorder modes in qualitative agreement with a generalized nonlocal optical response theory. We discuss the influence of these higher-order modes on the optical properties of few-nanometer silver nanoparticles probed by electron beams as done routinely in electron energy-loss spectroscopy experiments and on single silver nanospheres coupled to dipole emitters such as single atoms or molecules.

Topics

• nanoparticle
• surface
• silver
• theory
• experiment
• nitride
• Silicon
• spectroscopy