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

42 CHAPTER 2. EXPERIMENTAL METHODStilt/xyz manip.sampleLens60°LensViewportViewportSpectrometerXe lampPolarizerMirrorFigure 2.3: Schematic diagram of the in-situ reflectivity setup. See text for details.and ∆ values (1.25) as a function of wavelength, and often at several angles of incidence;usually the measurements are carried out in the visible and near-UV region, but the rangecan also be extended to near-IR. As described in §1.1.2, Ψ and ∆ represent the amplituderatio and phase variations between the p and s components of the reflected light; byconvention, they are written asρ = tanΨ e i∆ = r pr s(1.25)where r p and r s are the complex Fresnel coefficients (1.23). Separating the amplitudesand the phases, we findtanΨ = |r p||r s |∆ = δ p −δ s(2.1a)(2.1b)Lookingatthesedefinitions, wecanexpectthatthedirectinterpretationofΨand∆spectrais quite difficult. While certain optical features can be determined by a fast inspectionof the spectra, the analysis typically requires the definition of an optical model, whichdescribes the optical constants and layer thickness of the sample, and then is employed toreproduce the experimental data. In extreme cases, one has to construct an optical modeleven when the sample structure is not clear at all. Another disadvantage, which howeverin most cases is not limiting, is the low spatial resolution of the measurement, due to thefinite spot size of the light beam, typically of few millimeters diameter.Since the first null ellipsometers [126] working at a single wavelength, several spectroscopicconfigurations have been developed, allowing the fast acquisitions of spectra over a