Broadband Terahertz Metasurfaces

The terahertz frequency range spans from 0.1 THz to 10 THz and presents unique potential for medical imaging, material characterisation, non-destructive evaluation, and wireless communications. In recent years, various functional devices have been developed to harness the potential of terahertz waves, however mostly with limited bandwidth and efficiency. The thesis presents diverse broadband terahertz metasurfaces for wavefront control and polarisation manipulation. The concept of metasurfaces along with rigorous design approaches yields terahertz components with bandwidth and efficiency superior to existing devices. Such properties are much needed for terahertz technology to leverage vast available bandwidth with moderate source power. This thesis comprises nine chapters in total that are divided into four major parts. Part I introduces relevant research background and fundamental theories. Specifically, Chapter 1 presents the definitions of terahertz technology and metasurfaces, while Chapter 2 details the fundamental theories involved in this doctoral research. Part II concerns terahertz reflectarray bandwidth enhancement. A single-layer stubloaded resonator is proposed in Chapter 3 for constructing broadband reflectarrays. As a proof-of-concept, a terahertz reflectarray is designed, fabricated, and experimentally characterised to focus an incident plane wave to a focal spot at a finite distance. Part III involves terahertz transmitarrays for antireflection and polarisation manipulation. Each transmitarray employs three metallic layers to realise a complete control over the electric and magnetic responses, so that an arbitrary transmission phase together with high transmittance can be produced. Chapter 4 introduces a broadband semi-analytical approach that is developed on the basis of an existing narrowband method. The broadband approach involves network analysis combined with a genetic algorithm to determine the optimal frequency-independent circuit parameters, so that the desired transmission coefficients can be achieved over a wide bandwidth. In order to illustrate and verify the functionality of this broadband design approach, a wideband and highly efficient antireflection transmitarray is systematically designed based on this approach. Chapters 5 and 6 detail a quarter- and half-wave transmitarray, respectively, which are designed using the same procedure. The measured results confirm the functionality of the proposed quarter- and half-wave transmitarrays and suggest that they provide superior bandwidth and efficiency over the notable existing designs. Lastly, Chapter 7 shows a terahertz circular polariser with enhanced bandwidth, which is designed with the assistance of the broadband semi-analytical approach. The circular polariser is capable of transmitting circularly polarised waves of one handedness while blocking the other. Importantly, the three-layer circular polariser possesses frequency tunability obtained by adjusting the air spacers. Moreover, by rotating the top or bottom metallic layer, the circular polariser can be reconfigured to function as a transmissive quasi-half-wave plate. Part IV focuses on ultra-wideband absorber design. Chapter 8 presents a non-resonant absorber that is developed by etching air cavities into moderately doped silicon. In the realised absorber, inverted pyramidal air cavities are etched into doped silicon using a wet-etching technique, so as to realise impedance matching between the lossy silicon and free-space. The measured results demonstrate that a high absorption can be maintained over an ultra-wide bandwidth that spans nearly the entire far-infrared spectrum. The presented absorber far outperforms the existing resonance-based perfect absorbers in terms of achieved fractional bandwidth. Lastly, Chapter 9 concludes the thesis and gives an outlook of terahertz metasurfaces for practical applications. This thesis introduces technical advancements to metasurfacebased terahertz devices for wavefront control and polarisation manipulation. The developed and experimentally validated functional devices can be incorporated into compact terahertz systems, and they address the bandwidth and efficiency limitations associated with the existing designs.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 202