Nanostructured electromagnetic metasurfaces at optical frequencies

Licentiatavhandling, 2016

Electromagnetic metasurfaces are broadly defined as optically thin layers that are structured on the subwavelength scale. In general, metasurfaces thus consist of nanoparticles, or other kinds of ”meta-atom”, arranged in some pattern where both the individual particle sizes and the inter-particle distances are much smaller than the wavelength. With advances in nanofabrication, it has become feasible to precisely engineer metasurface constituent elements to fulfil certain functions. This thesis, focuses on the properties of metasurfaces assembled by colloidal lithography. In contrast to most metasurfaces studied in the literature, samples produced by colloidal lithography lack long-range periodicity. Two different approaches to metasurface design are investigated. In appended papers I and II, the individual elements are plasmonic gold nanoparticles while appended paper III deals with the geometric resonances supported in silicon nanoparticles. The investigated systems are able to convert propagating electromagnetic fields into localized ones and vice-versa. Hence, we can measure information about local properties in the far-field. In paper I, the individual particles were progressively tilted with respect to the substrate normal, resulting in an overall directional response. This directionality was manifested in enhanced fluorescence emission in particular directions. In paper II, the ability of anisotropic individual particles to alter the polarization of the incoming light beam was utilized to develop a sensing scheme based on the detection of rotation of polarization. The change in rotation and ellipticity of the light was shown to be sensitive to the local refractive index around the particles. Refractometric biosensing was performed by tracking these changes in real time. The interaction between the incident light and geometric electric and magnetic resonances was studied in paper III. At certain illumination conditions, it was shown that the interference between interface reflection and the coherent scattering from the electric and magnetic dipole resonances gave rise to almost complete light absorption independent of polarization. The ability to design metasurfaces with specific properties is of importance for future applications. The results presented in this thesis contribute to the understanding of the properties of the individual particles that compose a metasurface and how structuring of these particles affects its overall properties.