Progress in nanotechnology has enabled systematic development of artificial nanomaterials with extraordinary optical properties, such as zeroth and negative index of refraction, perfect absorption, and enhanced nonlinearity, anisotropy and temporal dispersion. Another property that is common for designed nanomaterials is spatial dispersion. It makes the refractive index and wave impedance depend on light propagation direction, which complicates the description, but also makes it possible to obtain previously unreachable capabilities for the materials. For example, the material can be made to reflect or absorb light differently by its different facets. In spite of a high potential of spatially dispersive nanomaterials in science and technology, only few theoretical models and design tools for these materials can be found in the literature.
The results presented in this thesis can pave the way to comprehensive characterization and more efficient design of spatially dispersive nanomaterials, including metamaterials and nanostructured optical waveguides. We develop novel theoretical methods and numerical calculation techniques to characterize and design nanostructured materials and devices made of them. Furthermore, we put forward a novel approach to characterize nanostructured optical media, for which electromagnetic modes cannot be introduced due to the effect of polarization conversion by spatial dispersion. We find that a large variety of designed optical nanomaterials belong to this class of optical media. In addition, the thesis contains an efficient semi-analytical technique to model the interaction of optical beams with spatially dispersive materials. Making use of the tools of Fourier optics, the method allows treating the beam-propagation phenomenon in large-sized nanomaterial samples. Compared to conventional numerical approaches, the technique is computationally light and provides a deeper understanding of the light-matter interaction picture.
Using the developed methods, we design novel optical nanoscatterers and nanomaterials composed of them. As an example, we demonstrate both theoretically and experimentally a spatially dispersive metasurface that shows primarily quadrupole optical response when illuminated from one side, and primarily dipole response when illuminated from the other side. Furthermore, we design novel diffraction-compensating optical metamaterials, whose anisotropy and spatial dispersion prevent focused light beams from spreading upon propagation. We also demonstrate a diffraction-compensating metamaterial slab waveguide with low reflection and absorption losses and a broadband operation. The designed materials and optical components based on them may find applications in efficient solar cells, novel laser resonators, and high-speed photonic integrated circuits, and together with the presented theoretical tools, contribute to the progress of nanostructured optical media towards real-life applications.
|Translated title of the contribution||Valon ja spatiaalisesti dispersiivisten nanomateriaalien vuorovaikutus|
|Publication status||Published - 2019|
|MoE publication type||G5 Doctoral dissertation (article)|
- optical nanostructures, metamaterials, wave propagation, diffraction compensation