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

4.2. OPTICAL PROPERTIES OF AU NANOPARTICLES ARRAYS 65Although spectroscopic ellipsometry is probably the most powerful optical method forthe characterization of materials, in this particular case it might not represent the moststraightforward manner to achieve a clear and simple physical picture of the phenomenaoccurring within the Au nanostructures. The onset of plasmonic response in a system is infact typically manifested with the appearance of a sharp reflectivity or absorption featureinthespectra. Whenmeasuringaratioofreflectivities, asdoneinellipsometry, suchsharpfeatures typically tend to compensate each other, thereby effectively cancelling out themost remarkable manifestation of plasmonic response. For example, the peak observedin Ψ for the plane of incidence perpendicular to the ripples (lower panel in fig. 4.4) doesnot correspond to a “true” plasmonic resonance, but simply arises as the result of theratio between the p and s reflectivities (see (1.25)). In addition to this, we also noticethat the resonances are more easily interpreted as enhancements of light reflectivity orabsorption, rather than phase variations. A more direct approach to investigate theplasmonic properties is therefore to measure separately the plain p and s reflectivities ortransmissivity, and analyse the spectra features that appear there.[110][110]spθ[110][001]sp[001] [110]a. b.Figure 4.5: Optical geometries for the reflectivity measurements, and corresponding orientationsof the electric fields for either s- or p-polarized incident light. Plane of incidenceparallel (a) and perpendicular (b) to the ripples ridges.For simplicity we keep the same configurations used in the ellipsometric measurements,and we define a parallel (||) and a perpendicular (⊥) geometry for the plane of incidencerespectively parallel and perpendicular the ripples direction, as sketched in fig. 4.5 forreflection. In parallel geometry (panel (a)), when the incident light is s-polarized theelectric field is purely transverse to the ripples, oscillating in the [1¯10] direction, while forp-polarized light, it has a component both along the ripples and normal to the surface.In perpendicular (⊥) geometry (fig. 4.5(b)), instead, the system is excited purely parallel(s-polarized light) or transverse (p-polarized light) to the ripples, in the latter case simultaneouslyalong the in-plane [1¯10] and out-of-plane [110] directions. We will use thefollowing notation when referring to such reflectivities: R || P , R|| S , R⊥ P , R|| P .In fig. 4.6 we report sets of reflectivities recorded at an angle of incidence of 50 ◦ ,measured on various samples after the deposition of t Au ≈ 5 nm of gold. The blue curveswere measured on the same sample described in fig. 3.7(b), having periodicity Λ ≈ 45 nm,while the black and red ones correspond to the samples in fig. 3.7(c) (with Λ ≈ 25 nm) andin fig. 3.7(d) (with Λ ≈ 40 nm), respectively. The reflectivity of bulk gold is also reportedin fig. 4.6(c) as a reference. Furthermore, in fig. 4.7(a) we show the transmission spectrafor the sample with Λ ≈ 25 nm, corresponding to the black curves of the previous graphs.The spectra were measured at normal incidence, with light polarized either parallel (red