Morphology and plasmonic properties of self-organized arrays of ...

34 CHAPTER 1. THEORYposition, has the same orientation like E ext and acts against the restoring forces of p.Recalling that the particle can be modelled as an oscillator, the lowering of the restoringforce is reflected by a reduction of the resonance frequency, i.e. a red-shift of the LSP. Thesame effect occurs when the E ext is oriented in-plane (fig. 1.18(d)): in this case p andp ind are in the same direction of the field but antiparallel with respect to each other, sothat E ind is again directed like E ext .E extp indpE extpp inda.b.E extpE indpE extE indp indp indc.d.Figure 1.18: Top panels: metallic sphere in proximity of an interface between two differentmedia, and electric field applied normal to the interface. When the sphere is nearlytouching the interface, the electric field generated by the image charge is highly inhomogeneouswithin the particle’s volume (panel (a)), and becomes more uniform as theparticle moves away from the interface (panel (b)). Bottom panels: image charge dipole,and corresponding dipolar field E ind generated at the particle center, for excitation E extnormal (panel (c)) and parallel (panel (d)) the interface. In both cases E ind has the samedirection of E ext .