ON THE COUPLING OF THE LOCALIZED PLASMON AND INTERBAND TRANSITIONS IN NICKEL NANOANTENNAS

Licentiate thesis, 2013

Interaction of light with metallic nanostructures smaller than the wavelength leads to excitation of collective oscillations of “free” conduction electrons. This effect leads to locally enhanced electric fields inside and outside the nanoparticle and is called localized surface plasmon resonance (LSPR). The history of this phenomenon goes back to 4th century where metallic nanoparticles were used to stain glass windows in medieval church windows. It was not until hundred years ago, however, that the physics behind the vibrant colors of noble metal nanoparticles was studied and understood - a development that ultimately resulted in the foundation of a new field in nanoscience, nanoplasmonics. Nickel (Ni) is a transition metal, which is mainly used in the preparation of alloys due to its strength, ductility and resistance to corrosion and heat. In nanoplasmonics, this metal has so far received much less attention than the “classic” plasmonic metals such as gold or silver. This is mainly due to its high ohmic losses and electronic interband transitions, which induces large damping to its LSPR. However, since most metals feature local or broad interband transitions, it is crucial to gain deeper fundamental understanding of their interaction with the plasmonic excitations. As Ni features a spectrally localized interband transition at 4.7 eV is a very good “model system” to scrutinize the LSPR-interband transition interaction. The latter is crucial in view of an increasing number of metals being considered for nanoplasmonic applications. In this thesis, the interaction between a spectrally localized interband transition and LSPR in nickel nanodisks is studied as a model system. This was done by spectrally tuning the LSPR by engineering of the nickel nanoantenna geometry or by tuning the refractive index of the surrounding medium. Using this approach, we show both experimentally and theoretically that the plasmon-interband interaction can be understood in the classical picture of two coupled harmonic oscillators, which approach a strong coupling regime. That is characterized by characteristic “energy anticrossing” behavior. These results can be generalized to all plasmonic metals, which feature spectrally localized interband transitions, and contribute to the general fundamental understanding of the role of interband transitions in plasmonic systems.