Novel approaches in manipulating, guiding, and generating THz and sub-THz fields

This thesis serves to address the main challenges of the terahertz technology, providing new efficient ways of actively manipulating, guiding, and generating THz and sub-THz fields. This is accomplished by taking a truly interdisciplinary approach and exploiting the physics of superconductors, and the electrodynamics of metamaterial and plasmonic structures. Metamaterial arrays made of superconducting films are suggested for manipulating the THz radiations, while superconducting plasmonic waveguides are considered for achieving efficient propagation of THz waves. In addition, metamaterial arrays composed of bimetallic rings that exhibit both plasmonic and thermoelectric properties are investigated as a possible new source of THz radiation and strong magnetic fields.

I have demonstrated experimentally, for the first time, that high- and low-critical temperature superconducting metamaterials are able to show sub-radiant resonances of Fano type that can be controlled with temperature. Such metamaterial resonances show vanishing radiation losses, while superconductors have very low Ohmic losses. Thus, these structures offer an efficient way to actively manipulate sub-THz (and THz) fields.

I have reported on the first experimental realisation of the extraordinary transmission effect in periodically perforated superconducting films. I have shown that the level of transmission of sub-THz waves through these structures could be controlled with temperature near the superconducting transition point. The latter enabled to identify the role of the plasmonic excitations in the mechanism of extraordinary transmission. I have shown that superconductors below their gap-frequency (several THz for high-temperature superconductors) are similar in behaviour to plasmonic metals at optical frequencies. Geometries of superconducting structures have been identified that support almost dispersionless propagation of plasmonic-like modes with frequencies up to several THz, exhibiting both extreme localisation and very low propagation losses. Finally, I have theoretically demonstrated that metamaterial arrays composed of bimetallic gold-nickel nanorings, when illuminated by ultrafast optical pulses, support transient thermoelectric currents that lead to the generation of magnetic pulses of subpicosecond duration, nanoscale localisation and peak amplitudes of the order of one Tesla. These results could facilitate the study of ultrafast nanoscale magnetic phenomena and have potential use in such applications as material characterisation and magnetic recording.