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

4.2. OPTICAL PROPERTIES OF AU NANOPARTICLES ARRAYS 69ppssbefore annealingafter annealingR P0.040.02R P||R S0.150.12R S┴0.090.00200 600 1000 1400 1800200a. [nm]b.6001000 [nm]140018000.16R S||0.03R p┴R S0.12R P0.020.010.08200600100014001800c. [nm]d.0.002006001000 [nm]14001800Figure 4.9: Reflectivities, at an angle of incidence θ ≈ 50 ◦ , measured before (red curves)and after (black curves) annealing at T = 400 ◦ C of gold nanowires on LiF(110) (Λ =25 nm). Plane of incidence parallel (left panels) and perpendicular (right panels) to theripples. EM excitation along (panels a, b) and across (panels c, d) the ripples.A direct comparison between the reflectivities before and after the annealing is shownin fig. 4.9 for the sample of fig. 3.8(c), characterized by Λ ≈ 25 nm and t Au ≈ 5 nm; thecurveswereacquiredex-situ, theredonespriortotheannealing, theblackonesafterwards.In panels (c) and (d) we report the reflectivities R ||S and R⊥ P , similar to the geometry offig. 4.8 except at an angle of incidence of 50 ◦ . According to the observations made forfig. 4.8 following the annealing, the LSP peak, in the direction perpendicular the ripples,slightly blue shifts and the resonance become sharper. The main effects of annealing,however, are visible in panels (a) and (b) of fig. 4.9, in R || P and R⊥ S geometries, wherethe electric field is mostly oriented along the ripples. Here, the very broad peak observedfor the nanowires is replaced, after the annealing, by a much narrower peak, shifted ofλ ≈ 400 nm towards lower wavelengths.The corresponding comparison, concerning the same sample, between the transmissivitiesbeforeandaftertheannealingisinsteadreportedinfig.4.7;thespectraweremeasuredat normal incidence, with the electric field oriented either parallel (T || , red curve) or perpendicular(T ⊥ , black curve) to the LiF ridges. Similarly to what observed in reflection,the broad minimum observed in T || after the Au deposition has now evolved into a sharp