Reconfigurable optically-controlled waveguide for terahertz applications

The development of tunable waveguide components for systems that require multifunctionality, at terahertz frequencies is investigated using the photoconductivity e ect. Specifically, by the photo-generation of free charged carriers highly conducting plasma regions are created and by changing the light pattern in real time, various tunable components can be implemented. The aim of this thesis is to present a novel reconfigurable optically-controlled terahertz waveguide switch as an illustrative example of this approach, addressing the challenges and limitations involved in simulation, implementation and measurement of such devices and is organised in the following chapters. Chapter 1 gives the background theory of the fundamental principles of optoelectronic devices and presents a literature survey of existing optically-controlled structures across a wide frequency spectrum. Chapter 2 presents a comparative study of four commercial software packages with the aim to show that it is not always straightforward to select the most appropriate boundary conditions and define a material's parameter within a software when terahertz structures are modelled. A study of various modelling approaches using commercially-available software packages has been undertaken; a number of approaches have been identified and the most appropriate solutions are indicated. Chapter 3 presents a microwave plasma switch as a proof-of-concept scaled demonstrator. In this preliminary experiment a metal pipe rectangular waveguide (similar to WR-650 standard) has been implemented, which can be reconfigured as an ON-OFF switch using a plasma column formed by commercially available discharge tubes. This provides a good starting point for more sophisticated devices as presented in the following chapters. In Chapter 4 a novel optically-controlled waveguide plasma switch for terahertz applications is presented. The switch is excited by a continuous wave (CW) laser source and the photoconductivity profile, due to the laser illumination, is described in detail. The performance of the switch is studied by means of full-wave numerical simulations and various parametric studies are undertaken to provide physical insight in the device performance. The thermal characteristics of the device are also investigated. Chapter 5 gives in detail the processing steps for the microfabrication of various prototypes with the assembly of the prototypes being also discussed. The waveguide experimental setup is described in detail and the measurement results obtained are presented. In particular, emphasis has been given on the alignment of the devices with the Vector Network Analyser waveguide heads. Finally, Chapter 6 gives a summary of the work presented in this thesis and potentially new research directions are indicated as future work.Open Acces