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

4.2. OPTICAL PROPERTIES OF GOLD NANOPARTICLES ARRAYS 63as a function of the amount of deposited LiF. In fig. 4.2 we show Ψ and ∆ spectra for asample with periodicity Λ ≈ 35 nm (fig. 3.4); like in fig. 3.4, the curves were measuredex-situ at an angle of incidence θ = 50 ◦ , at steps of 60 nm of deposited LiF during thedeposition of a 240 nm thick film. Increasing the thickness of the deposited LiF, we noticethat the average values of the Ψ and ∆ spectra remain almost unchanged, while the mainvariationsconsistintheappearanceofweakoscillationsinthespectra, whichbecomemoreclosely spaced at increasing LiF thickness. These oscillations are due to the interferencebetween the multiple reflections inside the deposited film; in particular, their periodicity isdirectly related to the thickness of the film, making spectroscopic ellipsometry an effectivetechnique to monitor ex-situ the LiF homoepitaxy and to calibrate with a precision of fewnanometers the deposition rate of the effusion cell.8.07.5||184180 [deg]7.0 [deg]1766.5172||6.01.02.03.04.01681.02.03.04.0E [eV]E [eV]Figure 4.3: Ψ (left panel) and ∆ (right panel) spectra, measured at an angle of incidenceθ = 50 ◦ , of a nanopatterned LiF(110) sample, with periodicity Λ = 35 nm, for differentorientations of the sample with respect to the optical plane. Red line: plane of incidenceparallel (||) to the ripples direction. Black line: plane of incidence transverse (⊥) to theLiF ridges.Another characteristic feature induced by the deposition of LiF is a weak but apparentoptical anisotropy between measurements performed with the optical plane parallel to the[001] and [1¯10] directions. This is shown in fig. 4.3, where we report Ψ and ∆ spectrameasured at θ = 50 ◦ with the plane of incidence oriented either parallel (red lines) orperpendicular (black lines) to the ripple direction; the anisotropy is more pronounced for∆, where it is as great as 3 ◦ at the highest energies, while it is barely noticeable for Ψ.Since it is not observed for the bare substrate, we conclude that it must be induced bythe deposited film, and in particular by the intrinsic morphological uniaxial asymmetryof the ripple structure. As is often the case in ellipsometry, the direct interpretation ofthe spectra, and of the corresponding anisotropy, is not a straightforward task, and ananalytical modelling is typically required. We will discuss these effects in the next chapter(§5.1).4.2 Optical properties of gold nanoparticles arraysWe will now address the optical response of the arrays of gold nanostructures. As seenin §1.3, metallic structures with a typical size of few tens of nanometers can sustaincharacteristic plasmonic resonances, whose properties depend on the specific shape andenvironment of the nanoparticles. Here, by tuning the fabrication of the arrays, we explore