Analysis of Plasmonic Structures by Spectroscopic Ellipsometry

Introduction

Ellipsometry is a non-perturbing optical technique that enables the characterization of bulk materials, thin films and surfaces by studying the change in the state of polarization of a light beam that it is reflected on the sample under analysis or transmitted through it. The morphology and optical properties of materials and nanostructures can be investigated through the variations of the state of polarization of the reflected beam compared to the state of polarization of the incident beam. In reflection mode, ellipsometry has demonstrated capabilities to sense fractions of a monolayer of organic and inorganic materials. Thus, Aspnes reported that in clean air, the surface sensitivity can make possible the determination of oxide thickness in systems like SiO₂/Si down to the 0.1 pm range (Aspnes, 2004). Furthermore, aside being an effective tool for the study of the optical properties of solid-state materials, ellipsometric analysis can be employed as a transduction method in the development of gas and biological sensors (Arwin, Poksinski, & Johansen, 2004). Metal nanoparticles (NPs) and metal thin films contribute to increase the sensitivity of ellipsometry through the excitation of localized surface plasmons and surface polaritons. However, the traditional ellipsometric fitting models face new challenges in those scenarios due to the optical anisotropies introduced by the morphology and surface distribution of the nanostructures. These anisotropies limit the use of a single dielectric function to account for the optical properties of the material, hindering the application of effective medium theories.

In metal thin films, the excitation of surface polaritons requires the use of Krestchmann configuration, in which the film is directly deposited on the coupling prism, or alternatively, on a transparent substrate that is attached to the prism by means of an index matching fluid. Under the proper incidence angle and radiation energy, internal total reflection occurs and the condition for the excitation of the interface polariton at the air-film interface can be satisfied. If the metal thickness is thin enough, part of the radiation can tunnel through the film producing the collective oscillation of surface charges. Compared to regular surface plasmon resonance (SPR) sensing techniques, total internal reflection ellipsometry presents larger sensitivity to the chemical environment due to the phase information obtained from the parallel and transverse components of the reflected beam.

In metal NPs, the resonant absorption of radiation gives rise to oscillatory processes of the electrons confined in the particle whose characteristic frequencies depend on the size and morphology of the NP. Part of that absorbed energy is reemitted as scattered light (Rayleigh scattering) and part is delivered as thermal energy to the surrounding environment. Both energy losses contribute to reduce the specular reflection and modify the amplitude and phase of the polarization components in the reflected beam. Furthermore, when those oscillatory processes occur in a particle deposited on a surface, they experience interactions with the substrate and the neighboring particles through the formation of mirror image charges. Those interactions can be interpreted through a depolarization factor that leads to obtain different polarizabilities of the nanoparticle for directions parallel and perpendicular to the substrate.

This chapter reviews the plasmonic effects on the ellipsometric functions and the pseudodielectric function obtained from spectroscopic ellipsometry in reflection mode. Reflection mode is generally more sensitive to surface changes than transmission mode, and more versatile, since it allows the measurement of samples with absorbing substrates. The chapter will include references to recent works on the subject and an overview of the technique to characterize plasmonic structures of application in nanotechnology, biochemistry, and information technology. The analysis focuses on the visible, near infrared and ultraviolet regions of the electromagnetic spectrum where metal plasmon resonances are typically found.