Metasurfaces are ultrathin artificial material structures. By engineering the sizes and shapes of constituent meta-atoms, it is possible to design metasurfaces which control the amplitude, phase, and polarization state of waves in a general manner. The electromagnetic properties of metasurfaces can be characterized by the effective surface impedance, susceptibility, or meta-atom polarizability. Systematic design of metasurfaces is a process of finding effective surface parameters that ensure the creation of desired scattered fields under specific excitations, and then physically implementing the metasurface using engineered meta-atoms. This dissertation focuses on metasurface engineering using the surface impedance homogenization model. Based on the equivalent circuit model, we have developed general analytical methods for calculation of scattering characteristics of an arbitrary impedance boundary. The surface impedance can be either uniform for the control of transmission or reflection (e.g., perfect absorption) at one specific incidence direction, or modulated in space and/or time with an arbitrary periodic function to control multiple spatial and/or temporal harmonics. Using the developed analytical models, extraordinary wave phenomena and new concepts such as extremely asymmetric absorption, nonreciprocal wave amplification, and multifunctional nonreciprocal wave propagation have been demonstrated. The developed analytical model, combined with mathematical optimization, further allows one to find a proper surface impedance, that simultaneously fulfills the desired scattering properties for illuminations from different directions. In general, the surface impedance of metasurfaces shows both resistive and reactive properties. In order to implement impedance boundaries with desired properties, we have introduced systematic methods for independent engineering the effective sheet resistance and reactance by properly patterning highly or poorly conductive material layers. The methods for impedance control are then applied to the synthesis of metasurface absorbers using highly conductive inks and frequency/angle tunable absorbers with poorly conductive graphene sheets, and further validated for space-modulated metasurfaces where the impedance of each sub-cell needs to be individually implemented.
|Julkaisun otsikon käännös||Surface-impedance engineering for advanced wave transformations|
|Tila||Julkaistu - 2020|
|OKM-julkaisutyyppi||G5 Tohtorinväitöskirja (artikkeli)|