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

64 CHAPTER 4. SELF-ORGANIZED NPS ARRAYS: OPTICAL PROP.the variations in the plasmonic response as a function of the systems’ characteristics.4.2.1 Gold nanowiresWe start by considering the optical properties of gold nanostructures realized directlyfollowing the grazing-angle deposition onto the rippled LiF substrates without any postdepositiontreatment. In fig. 4.4 we report Ψ and ∆ spectra measured on the samesample described in fig. 3.7(d). In analogy with the previous case, the plane of incidencewas set either parallel or perpendicular to the ripples direction. In the inset of the figure,the corresponding Ψ and ∆ spectra measured for a reference bulk gold sample are alsoreported.403020018016020180140 [deg]10 [deg]4030400800 1200 [nm]1600140100 [deg]120240 [deg]302202020010180400800 [nm]12001600160Figure 4.4: Ellipsometric spectra acquired at an angle of incidence θ = 50 ◦ measured ona nanopatterned LiF(110) sample after the deposition of t Au = 5 nm Au at T ≈ 100 ◦ C,with the plane of incidence oriented either along (top panel) or across (bottom panel) theripples. Inset of the figure: Ψ and ∆ curves (θ = 50 ◦ ) for a bulk gold sample.Comparingfig.4.4withfig.4.3,wenoticethattheellipsometricspectrahavedrasticallychanged after the deposition of Au, and that the contribution of gold to the optical spectrahas clearly become dominant. When the plane of incidence is along the ripples (toppanel of fig. 4.4) Ψ has a very broad maximum at ≈ 1300 nm, while ∆ monotonicallyincreases, almost linearly with increasing wavelength; on the contrary, when the plane ofincidence is rotated by 90 ◦ , Ψ is sharply peaked around 600 nm and ∆ shows a broadmaximum at ≈ 1000 nm. The main feature is now a strong in-plane optical birefringence,that becomes most pronounced in the wavelength range between 600 nm and 1700 nm.Furthermore, despite some similarities at the lowest wavelengths, these curves are alsomarkedly different compared to the bulk reference spectra (inset of fig. 4.4). From purelyqualitative considerations, we can assign the main features of the optical response of thesystem to the occurrence of electron confinement within the gold nanostructures.