Near-Field Optics of Localized Surface Plasmons

PrefaceThe study of the electromagnetic neareld of metal nanostructures is currently a subject of majorinterest in optics [1,2,3,4], for fundamental reasons as well as in the lightofthedevelopmentof applications in the hot eld of nanotechnology. Indeed, ensembles of metal nanoparticles arealready used in optical lters, and their utility has also been demonstrated recently in the eldof biomedical analysis [5], signal enhancement in molecular spectroscopy [6], and data storage [7].Noble metals in particular attracted the attention because of their ability to sustain excitation oflocalized surface plasmons in nanostructures [8]. This phenomenon gives rise to remarkable opticalproperties, namely spectral sensitivity and high local eld enhancement in close proximity of thestructures. Both these properties could be exploited in future nano-optical devices [9], to provideoptical circuit elements ideally analogous to the microelectronic transistors, basically consisting ofacombination of a switch and an amplier, respectively.Since a long time now it has been shown that these plasmon resonances can be tuned in thevisible and near infrared spectrum bycontrolling geometry and arrangement of the structures, inaddition to the surrounding medium [10,11]. Therefore, by means of tailored plasmon resonantmetal nanostructures the tasks of optical signal exchange on the nanoscale [12] and its transportand interfacing on the microscale [3, 13] can in principle be fullled. However, for an ecientdesign of the whole nano-optical device the electromagnetic response of the elementary componentshas rst to be known. For this reason in the present work we deal with the optical neareldcharacterization of plasmon resonant gold nanoparticles, in dierent geometrical arrangements.In Chapter 1 a brief introduction to the optical neareld theory and the related techniques isprovided, showing the possibilitytoovercome the resolution limit aecting conventional fareldoptical microscopy. The focus is placed on the Photon Scanning Tunneling Microscope (PSTM),whichisthe instrument of choice for us in order to study metallic samples without introducingmajor perturbations, due to the use of non-metalized ber tips as the optical probes. For thetheoretical aspects, in particular the neareld (NF) and fareld (FF) prole of an oscillatingdipole is shortly discussed, which in the quasistatic approximation represents a realistic model forour samples, consisting of plasmon resonant gold nanoparticles.The plasmon resonances in noble metals are the subject introduced in Chapter 2, starting fromthe fundamental Lorentz and Drude theories for the optical behavior of dielectrics and metals,respectively. Some basic characteristics of the dierent plasmon resonances, i.e. volume, surfaceand localized surface (particle) plasmons, are also pointed out. For the latter type, which is theone of major interest, the dependence of the main property, namely the resonance frequency,onboth material media and particle geometry, is shown. A short description of the method used forcalculations of the optical response of our samples of metal nanostructures, namely the Green'sdyadic technique (GDT), is also included.The practical aspects of both elds previously introduced, namely the fabrication of the samplesconsisting of gold nanoparticles, and of the ber tips used as the probes in the optical NF inves-tigations, are faced with in Chapter 3. Here also a general description of our implementation ofthe PSTM is presented. Particular attention has been put on the operational denitions of theexperimental procedures involved in these techniques.v