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

70 CHAPTER 4. SELF-ORGANIZED NPS ARRAYS: OPTICAL PROP.absorption at λ = 600 nm, while the minimum in T ⊥ blue shifts from λ ≈ 650 nm toλ ≈ 560 nm and gets narrower. We notice that the positions of the resonances whenobserved in transmission or in reflection are slightly different, the peaks of R being systematicallyred shifted with respect to the minima of T; we will discuss these effects in§5.2.2.We can interpret the variations in the optical properties induced by the annealing procedureusing the same arguments of the previous sections. We saw in §3.2 that, followingthe annealing, the layer of gold reorganizes in ordered arrays of nanoparticles; however,due to the convolution of the AFM tip, we could not determine from the AFM data wheneverthe particles were disconnected or not. In this respect, reflectivity and transmissivityprovide complementary tools to characterize the NP arrays. In the previous section weassociated the presence of a sharp LSP resonance, in the direction perpendicular the ripples,to the formation of disconnected wires of gold. In the same way, we can explain thesharp resonance, observed when the electric field is applied along the ripples, as the resultof the formation of isolated metallic nanostructures within each groove: comparing theseconsiderations with the AFM data, we can then confirm that the nanoparticles of goldare disconnected both along and perpendicular to the ripples.4.2.3 Gold nanoparticlesWe now focus on the analysis of the optical properties of the 2D arrays of nanoparticles.In particular, we separately consider the dependence of the plasmonic resonances on theripples periodicity and on the gold coverage. For simplicity, we identify 3 distinct plasmonmodes, related tothe geometry of the system (fig. 4.10): alongitudinal (L) mode is excitedwhen the electric field oscillates along the ripples; a transverse (T) mode is excited whenthe electric field is applied across the ripples in the plane of the sample; a normal (N)mode is excited when the electric field has a component normal the sample surface. Wewilldiscussinthenextchapterthevalidityofthisassumption. Consideringthereflectivitygeometries introduced in the previous sections, we can deduce that R ||S and R⊥ S geometriesallow to excite the T and L modes only, respectively, while in R || P / R⊥ P geometries theN mode is activated beside the L/T mode. However, as we will see in the next sections,the intensity of the N mode is typically much weaker than the L and T ones, thereforethe corresponding contribution to the reflectivity or absorption peaks remains always“hidden” by the other optical features, and only the L and T modes are clearly observedin reflection or transmission.TNLFigure 4.10: Schematic representation of the three different LSP modes for the goldnanoparticles. Longitudinal mode (L): EM excitation along the ripples. Transverse mode(T): excitation transverse to the LiF ridges. Normal mode (N): excitation normal theplane of the sample.Following the same notation of the plasmonic modes, we define the length, the width