Terahertz metamaterial : surface lattice management with bound states in the continuum

• Main Author: Tan, Thomas Caiwei
• Other Authors: Ranjan Singh
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Physics::Optics and light
• Online Access: https://hdl.handle.net/10356/145519
• Institution: Nanyang Technological University

This thesis will discuss and highlight the importance of the lattice mode in minimising the radiative losses and enhancing the quality factor in metamaterial resonances. The study of lattice modes has been done extensively in plasmonic arrays, and we would like to extend it into the field of terahertz metamaterials. It is particularly important to understand the role of lattices modes when they are coupled to the eigen-structural modes of the metamaterial systems that could be engineered to support, either uncoupled eigen-resonances or strongly coupled modes including Fano resonance at terahertz frequencies. Fano resonance is known for its asymmetric feature and high-quality factor that arises from breaking the symmetry of the constituent resonator in a unit cell. Subsequently, Fano resonance also known to be a consequence of a symmetry broken bound state in the continuum, which at low loss supports ultrahigh quality factors that are difficult to be measured. The lattice modes that are inherent in periodic structures play a pivotal role in mediating the coupling between constituent resonators and can also be utilized to resonantly couple to metamaterial resonance for quality factor enhancement. This mediative coupling is observed in an avoided crossing and can be modelled as two or more coupled oscillators. Likewise, resonant coupling with metamaterial resonance enhances the resonant fields with the help of the trapped surface fields of the lattice mode. Lattice mode coupling to narrow split resonances and Fano resonance was demonstrated and we achieved enhancement of quality factor up an order of magnitude. This proves that radiative losses can be further minimized through optimization of the lattice modes but still is significantly insufficient to achieve an ultrahigh quality factor known to quasi-bound state in the continuum. Quasi-bound state in the continuum known to be excited through structural symmetry breaking have piqued the interest of many, due to its ultrahigh quality factor resonant states. A new concept and model were developed to show that a subwavelength thin dielectric can be used to support and modulate such resonance in a symmetric and metallic split ring resonator. Additionally, we have shown that it could be used as an active ultrafast filter or a micro-fluidic sensor. The lattice mode could also directly excite and support a resonant mode and such modes requiring phase matching conditions, e.g. spoof surface plasmons and guided-mode resonances. These modes are very narrow and can be exploited for low loss application. Therefore, we hope that the lattice mode can be used as an optimising strategy for minimising the radiative loss in metamaterials and that the new proposed device could be used to support an active high quality resonance on any existing symmetric and metallic metamaterial. The development of low-loss THz devices is beneficial for the support of upcoming and future communication devices and technologies.