Public defence in Engineering Physics, M.Sc. (Tech) Rebecca Heilmann

Title of the doctoral thesis: Light-matter interaction in plasmonic and dielectric nanoparticle array

Doctoral student: Rebecca Heilmann
Opponent: Prof. William L. Barnes, University of Exeter, UK
Custos: Prof. Päivi Törmä, Aalto University School of Science, Department of Applied Physics

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Plasmonics is a field of research, where the interaction between light and small particles consisting of metals is studied. The particles, which usually are in the nanoscale, act as antennae for light as the electrons in the metal interact with the photons. This enables confining light into tiny spaces, smaller than the wavelength of light. The light is localized around the nanoparticles. In addition to metals, these nanoparticles can also consist of dielectrics, leading to similar phenomena. 

In this thesis, nanoparticles are arranged in periodic patterns forming lattices which give rise to optical modes called lattice resonances. These lattices (or arrays) are combined with organic dye molecules enabling coupling between photons from the molecules and the lattice resonances. Strong coupling between the lattice resonances of dielectric nanoparticle arrays and dye molecules has been realized. This serves as a basis for realizing quantum phenomena such as lasing and Bose-Einstein condensates in dielectric nanoparticle arrays. 

Furthermore, lasing was realized in gold nanoparticle arrays combined with organic dye molecules. The system is pumped with a laser, exciting the dye molecules which provide photons. The lattice resonances provide feedback and laser light is emitted. The lasing output depends strongly on the array geometry. In this thesis, complex array structures are studied. Arrays with complex unit cells with more than one particle per lattice site, enable lasing in bound states in continuum. Bound states in continuum are peculiar states that cannot be observed unless they are coupled to a leakage mechanism. In this work, the edges of the array act as a leakage mechanism. With polarization resolved measurements in real and momentum space different types of bound states in continuum are identified. 

Multimode lasing, which is simultaneous lasing at different wavelengths and angles, has been realized in supercell arrays. These consist of very large unit cells with tens of nanoparticles in an aperiodic structure. These unit cells are ordered periodically which leads to many more additional lattice modes which enable multimode lasing. This work adds new fundamental insights to the field of nanoplasmonics with possible future applications in for instance biosensors, solar cells, or lighting applications such as LEDs.

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Doctoral theses in the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52