Light can couple to subwavelength nanostructures via the excitation of surface plasmon polaritions. The rapid development of nanofabrication techniques has enabled significant advances in nanophotonics with plasmonics playing a key role. Combining extreme confinement of light in plasmonic nanostructures with active elements has opened up attractive prospects for controllable nanophotonic devices. Magneto-optically active materials are a potential candidate for such active elements as they enable non-reciprocal manipulation of the polarization characteristics of optical excitations and are easily and reversibly controlled by external magnetic fields. This thesis presents results on the magneto-optics and plasmonics of ferromagnetic nanostructures. The use of ferromagnetic nickel in subwavelength nanostructures offers a relatively simple geometry for an in-depth study on the relationship between plasmon excitations and magneto-optical activity. Experiments on circular nickel nanodots show that plasmon resonances correspond with a resonant enhancement of magneto-optically induced polarization rotation. Breaking the rotational symmetry in elliptical nanostructures demonstrates that the magneto-optical response can be described by excitation of two orthogonal electric dipole oscillations. Arranging the nanoparticles into a periodic lattice gives rise to more narrow and intense surface lattice resonances that, in turn, further enhance the magneto-optical activity at the resonance frequency. These properties arise from radiative coupling between the dipole oscillations of the individual nanoparticles. However, the optical and magneto-optical dipole resonances couple along orthogonal directions in the lattice, which enables active tuning of the magneto-optical response. Finally, it is shown that hybrid arrays of noble metal and ferromagnetic nanoparticles combine high optical reflectivity and strong magneto-optical activity. Radiative coupling between the sub-lattices of the hybrid array induces a collective magneto-optical response that involves both the magnetic and non-magnetic nanoparticles. The experimental results on single nickel nanoparticles are reproduced by an analytical model based on modified long wavelength approximation (MLWA) while the optical and magneto-optical response of periodic particle arrays are reproduced by a numerical model based on the discrete dipole approximation (DDA) that was expanded to include magneto-optical effects. The results on plasmonic nanostructures with integrated magneto-optical activity open up new avenues towards integrating magneto-optical elements into nanophotonic devices such as photonic crystals and metasurfaces.
|Translated title of the contribution||Magneto-optiset ilmiöt nikkeliplasmonirakenteissa|
|Publication status||Published - 2016|
|MoE publication type||G5 Doctoral dissertation (article)|
- photonic crystals
- surface plasmons