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

88 CHAPTER 5. MODELLING AND ANALYSIS OF THE OPT. PROP.[001]30Counts [%]2010ab01.02.0Aspect ratio1510Counts [%]105Counts [%]5c0510 15NP lenght [nm]20d0510 15NP width [nm]20Figure 5.10: Panel a: AFM image of a nanopatterned LiF(110) sample with Λ ≈ 30 nm,following the grazing deposition of ≈ 3 nm of gold at T = 100 ◦ C and annealing atT = 400 ◦ C (“rectangular” configuration). Panels b, c, d: statistical distributions of theNP in-plane aspect ratio and semiaxes along and across the LiF ridges, respectively. Thecontinuous lines are best-fit lognormal probability density functions. See text for details.along and across the LiF ridges, were characterized by very similar parameters, exhibitinga mean and a standard deviation of a x = a y = (8.5 ± 3.0) nm, corresponding to anaspect ratio of 1.0 ± 0.3. Mean NP spacings of d y = (20 ± 5) nm along the chains andd x = (30±5) nm across the ripples were found, indicating that the particles can be statisticallythought as laying on a rectangular mesh. We will refer to this class of samplesas the “rectangular” arrays.Optical characterizationIn analogy with the previous cases, the optical response of the samples under scrutinyhas been investigated by means of polarized light reflectivity, with the plane of incidenceeither along (||) or across (⊥) the LiF ridges, in order to selectively discriminate thecontributions of the individual L and T plasmonic modes (see §4.2.3).In fig. 5.11 and fig. 5.12 we report, as open circles, R S (panels (b)) and R P (panels (c))spectra measured for the “square” and the “rectangular” samples, respectively, at θ = 50 ◦of incidence. For each polarization, the longitudinal and transverse LSP modes have beenexcited by fixing the plane of incidence in parallel or perpendicular configuration: L mode(black lines) excited in RS ⊥ , T mode (red lines) excited in R||S , and vice versa for R P.The in-plane optical anisotropy of the system is particularly accentuated for the“square” configuration, reported in fig. 5.11: looking at R S , the L and T modes areexcited by setting the plane of incidence perpendicular (⊥, fig. 5.11(c)) and parallel (||,fig. 5.11(b)) to the LiF ridges, respectively, and are found at λ S L = (597 ± 3) nm and= (542 ± 3) nm, separated by ≈ 55 nm; the full widths at half maximum (Γ) ofλ S T