Superconductor terahertz metamaterials
Srivastava, Yogesh Kumar
Date of Issue2018-12-31
School of Physical and Mathematical Sciences
Center for Disruptive Photonic Technologies, Division of Physics and Applied Physics
Terahertz metamaterials showcase tremendous potential for plethora of applications ranging from biochemical sensing, high-speed wireless communication, and efficient modulators. Conventional terahertz metamaterials are made of noble metals. Incorporating superconductors in terahertz metamaterials potentially provide unprecedented opportunities to control resonance features which has huge implications in the real-world technologies. This thesis reports on new applications of superconducting terahertz metamaterials for ultra-low loss operation, ultrafast manipulation of electromagnetic radiation and novel quantum effects. This work experimentally and numerically demonstrates that the quality factor of a low asymmetry Fano resonant asymmetric split ring metamaterial is extremely sensitive to the conductivity of the constituent materials. At low asymmetry regime even high conductivity noble metals fail to support Fano resonance excitation. The metamaterials designed using high-Tc superconductors provide access to the extremely low asymmetry regime and excites extremely sharp Fano resonances. Highest Q factor achieved with superconducting resonator (167) found to be two-fold in comparison to metallic resonators (84). There is a clear advantage of superconducting resonators sustaining high Q Fano resonances compared to conventional metals in designing low loss metamaterials at terahertz frequencies. Further, all-optical dual-channel ultrafast switching of sharp Fano resonances excited in superconducting terahertz asymmetric split ring resonators have been shown. Upon irradiation with optical pump, the ultrasensitive Cooper pairs in cuprate superconductor undergo dual dissociation-relaxation dynamics within a single superconductivity restoration cycle and lead to dual switching windows at picoseconds timescale. The high sensitivity of Cooper pairs to external perturbations combined with the strong field confinement in Fano resonators enables access to such unique dual switching features. Moreover, we discover that superconducting metamaterials of thickness 25 nm (λ/40,000, where λ is the Fano resonance wavelength) show well evolved Fano resonances while metallic samples of identical thickness do not show any resonance excitation. Ultrathin superconducting metamaterials provide solution for low threshold switching and inductance enhanced devices. Moreover, a niche approach to realize Meissner effect at terahertz frequencies in quantum metamaterial system to achieve the quantum level switching between flux exclusion and flux penetration in a metal-superconductor hybrid quantum metamaterial system is developed. The observed dynamic switching behavior is attributed to the reconfiguration of the total inductance of the system due to magnetic flux exclusion and penetration. Most importantly, upon irradiation with the external optical pump, the quantum metamaterial undergoes ultrafast switching between the flux penetration and flux exclusion states at very low pump fluences and thereby lead to low-loss, ultrasensitive, frequency agile, switchable quantum photonics devices. Overall, in this thesis we demonstrated ultra high-Q resonances in superconductor metamaterials, which could be actively tuned in the ultrafast timescale using extremely low threshold optical pump fluences. We further demonstrate the existence of the Meissner effect phenomenon at terahertz frequencies which lead to the realization of novel quantum metamaterials. Thus the superconductor metamaterials presented in this thesis manifests great prospects and pathways for new generation terahertz high-speed photonics and electronics. The field of terahertz quantum metamaterials could be further enriched by including Josephson junctions in the geometry of the metamaterials.