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Title of the thesis: Topological effects in plasmonic lattices

Thesis defender: Kristian Arjas
Opponent: Researcher Paloma Arroyo Huidobro, Universidad Autónoma de Madrid, Spain
Custos: Prof. Päivi Törmä Aalto University School of Science

Metallic nanoparticles interact with light very differently from their macroscopic counterpart. Instead of reflecting light in the usual sense, the conductive electrons on the surface of the metal form a plasmonic resonance with the electromagnetic field of the light. These resonances are of high quality, and can be greatly enhanced and controlled by placing the nanoparticles in lattices. One application of particular interest are plasmonic nanolasers. By controlling the properties of the lattice, i.e., the sizes, shapes and positions of the nanoparticles, we can control the properties of the emitted laser beam. Conversely, we can study  how the nanoparticles affect the resonances from the emitted laser beam.

Topology, in the context of light, refers to a few different concepts. In the thesis, we are focusing on local topology, which is connected to the bound states in the continuum. These are special kinds of resonances, which are best understood as singularities and vortices in the electric field. When lasing, these states produce donut-shaped laser beams that are dark in the middle. This is because the electric field forms a vortex akin to a cyclone with a calm center. However, electric field is more easily manipulated, and we can form much more complex vortices.

A major line of research in my doctoral thesis considers the generation and manipulation of vortices of ever increasing complexity. By analysing a hexamer lattice (a triangular lattice of hexagons made from six nanoparticles), we were able to identify the symmetry of the system and Ohmic losses, i.e., the electromagnetic energy lost as heat in the nanoparticles, as primary factors determining the type of vortex in the produced laser beam. By using this information, we formulated a general recipe utilizing quasiperiodic structures for the design of plasmonic structures that lase with almost arbitrary vortices. This result is significant from the point-of-view of applications. In telecommunications, information is transferred by encoding it onto light, which is then read at the receiving end. Having access to these novel vortices would effectively unlock an additional set of alphabets to be used, further increasing the information density in the signals.

Keywords: Plasmonics, topology, lasing, bound states in the continuum, group theory

Contact information: kristian.arjas@aalto.fi 

Thesis available for public display 7 days prior to the defence at .