Terahertz bound states in the continuum

• Main Author: Han, Song
• Other Authors: Singh Ranjan
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Physics::Optics and light
• Online Access: https://hdl.handle.net/10356/137303
• Institution: Nanyang Technological University

Localized wave confinement in electromagnetic devices is crucial in science and technology. Waves can be confined with diverse methods ranging from metallic reflectors, total internal reflection, photonic bandgap, to Anderson localization and symmetry separability, where the “resonator” is surrounded by a medium that does not support outgoing waves, which inevitably trapping the waves in it. Recently, it has been found that waves can sometimes be perfectly trapped without leakages through symmetry separability which is known as a bound state in continuum (BIC). It has a localized eigenstate, whose frequency (or energy) lies within the continuous plane waves. In this thesis, we numerically and experimentally investigate diverse resonant all-dielectric metamaterials and photonic crystal (PhC) slabs for observation of these non-radiating BICs. A 2D dielectric metasurface is proposed to demonstrate supercavity govern by the BICs under TE-polarized excitation. With silicon as the metasurface building blocks, the supercavity enables low-fluence (10.16 µJ/cm^2) switching at nanosecond time scales. Under TM-polarized excitations, terahertz guided mode resonances (GMRs) are also discovered in the same 2D dielectric metasurfaces. It is also observed multiple BICs at terahertz regime by employing a C4v symmetric metasurface and their diverse counterparts with in-plane symmetry breaking. The BICs, demonstrated by ultra-high-Q resonances enable extremely high field localization, which are desired for low-threshold terahertz quantum cascade lasers. Finally, we investigate, for the first time, the dynamic zero-index BICs in terahertz large-scale integrated photonic devices. The zero-index is realized by constructing an accidental degeneracy dispersion, also known as photonic Dirac-like cone, which enables non-radiating BICs such that a low-loss zero-index PhC slab is achieved. The active modulation of zero-index PhC is also demonstrated. This thesis opens up bright avenues for designing low-loss, on-chip, and dynamic terahertz resonant devices.