Nanostructured electromagnetic metasurfaces at optical frequencies

Electromagnetic metasurfaces are broadly defined as optically thin layersthat are structured on the subwavelength scale. In general, metasurfacesthus consist of nanoparticles, or other kinds of ”meta-atom”, arranged insome pattern where both the individual particle sizes and the inter-particledistances are much smaller than the wavelength. With advances in nanofabrication,it has become feasible to precisely engineer metasurface constituentelements to fulfil certain functions.This thesis, focuses on the properties of metasurfaces assembled by colloidallithography. 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 appendedpapers I and II, the individual elements are plasmonic gold nanoparticleswhile appended paper III deals with the geometric resonances supportedin silicon nanoparticles. The investigated systems are able to convertpropagating 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 respectto the substrate normal, resulting in an overall directional response.This directionality was manifested in enhanced fluorescence emission inparticular directions.In paper II, the ability of anisotropic individual particles to alter thepolarization of the incoming light beam was utilized to develop a sensingscheme based on the detection of rotation of polarization. The change inrotation and ellipticity of the light was shown to be sensitive to the localrefractive index around the particles. Refractometric biosensing was performedby tracking these changes in real time.The interaction between the incident light and geometric electric andmagnetic resonances was studied in paper III. At certain illumination conditions,it was shown that the interference between interface reflection andthe coherent scattering from the electric and magnetic dipole resonancesgave rise to almost complete light absorption independent of polarization.The ability to design metasurfaces with specific properties is of importancefor future applications. The results presented in this thesis contributeto the understanding of the properties of the individual particles that composea metasurface and how structuring of these particles affects its overallproperties