engleski

Application of Active Elements in Artificial Electromagnetic Structures

The thesis focuses on artificial electromagnetic structures, called active metamaterials. Active metamaterials are a group of metamaterials that employ active electronic circuits. They are characterized by specific properties, such as amplification, non- reciprocal response, dynamic reconfigurability and dynamic tunability, in addition to properties associated with passive metamaterials. Instability is the most challenging problem facing active metamaterials, especially those incorporating negative elements. Thus, the thesis includes a detailed stability analysis of lumped and distributed networks commonly used in modeling of active metamaterials. Closed-form stability criteria are derived for the analyzed lumped networks. The detailed analysis of distributed networks revealed that closed-form stability criterion can be derived only for specific types of distributed networks. Some distributed networks are proven to be inherently unstable. Dispersion and band- limited operation of an active element are proposed as a solution to the stability problem. Furthermore, the proposed stability analysis is applied to the model of the recently proposed metasurface- based PT-symmetric system. A closed-form stability criterion is successfully derived for such a system. The analysis shows that every PT-symmetric system is necessarily marginally stable. Finally, the analysis, synthesis, fabrication and experimental verification of passive and active reflective polarization converting metasurfaces are presented. Stable operation of the active metasurface was ensured and verified. The measured results clearly demonstrate amplification, polarization conversion, polarization selectivity and non- reciprocal behavior.

negative elements, stability analysis, PT symmetry, metasurfaces

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