for frequencies ω around the resonance frequency ω 0 far exceeding the linewidth γ. Here, ϵ ∞ describes the influence of transitions with higher energy. Figure presents the extinction spectra for the simple example of a sphere consisting of a Drude metal described by ϵ 1 = ϵ ∞,D + ω 2 p /(ω 2 -iΓω) with its plasma frequency ω p and damping Γ. The core diameter is set to 25 nm. The shell with a thickness of 3 nm is described by eq 2 with a resonance at λ 0 = 495 nm, close to the plasmon resonance. The linewidth γ is set to 50 meV and the oscillator strength f varies between 0 and 0.2. Details on the parameters are presented in Figure together with a sketch of the model system. We included the modified long wavelength approximation to extend the validity of the dipole approximation in eq 1 to larger particle sizes.