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

5.1. LITHIUM FLUORIDE NANOSTRUCTURES 77the possible presence of a surface layer, with optical constants slightly different from thesubstrate, whose presence does not change the intensity of the reflected fields (relatedto Ψ), but modifies their phases (hence ∆). This is a reasonable assumption, especiallysince the hygroscopicity of LiF might favour the adsorption of water molecules; furthermore,the polishing procedure could have left some residuals contaminating the surface,or have induced some morphological disorder or defects like colour centers. Accountingfor these contributions requires a more detailed inspection of the surfaces, which is farbeyond the scope of the current analysis. Nevertheless, fitting Ψ already provides accurateoptical constants for the substrate, so any further improvement of the model is notstrictly necessary.5.1.2 Nanostructured LiF templatesWe proceed by considering the optical properties of the substrates following the furtherdeposition of LiF. In §3.1 we saw that elongated ripples spontaneously form after thedeposition of ≈250 nm of LiF. The Ψ and ∆ spectra corresponding to this situation,reported in fig. 4.2 and fig. 4.3, show, as expected, several differences with respect tothe bare crystals. Ψ and ∆ are both characterized by evident oscillations, and theyacquire a slight but clear anisotropy when the plane of incidence is oriented parallel orperpendicular the ripples. Such an optical birefringence can be directly related to theintrinsic morphological anisotropy of the ripple structure, while the oscillations are mostlikely due to the interference between multiple reflections inside the deposited film (see§1.1.3). In particular, the latter observation implies that the molecular-beam-depositedlayer of LiF has a refractive index different from the bulk crystal, probably due to a notperfect recombination of the Li and F ions during the desublimation of the gaseous phase,and by the consequent proliferation of structural defects; some contaminants can also getadsorbed and incorporated, since the typical base pressure during deposition is in the10 −7 mbar range.aAirbAθ 0θ 1 dN 0BDN 1CThin filmN 2SubstrateFigure 5.3: Schematic representation for the reflection of an EM plane wave from a thinfilm on a substrate. The rays a and b are in phase in A and B, and sum up in D,respectively after a direct reflection from the surface and a first reflection from inside thefilm.We can exploit the optical interference to finely estimate the thickness of depositedLiF film. This can be easily seen for the ideal situation (sketch in fig. 5.3) in which thesystem is composed of a flat homogeneous thin film (N 1 ) in air (N 0 ), supported on a