Modelling and characterization of the spoof surface plasmon polariton waveguides and reconfigurable components for terahertz integrated systems

The demand in ultrawideband (UWB) communications, high-resolution imaging, and high-mobility systems attracted interest of researchers in the last decades. The novel terahertz technologies have the potential for solving UWB communications and high-resolution imaging problems providing a bandwidth wider than a few Giga-Bit-per-Second and a resolution in milli-meter scale. In spite of the fact that the researchers have presented a high number of on-chip terahertz circuit solutions, which enable the development of highly-mobile terahertz systems, the severe terahertz attenuation that the integrated circuits experience due to the lossy and large scale conventional planar terahertz waveguides, the state-of-the-art terahertz integrated circuit technology is far behind the industrial standards which are crucial for demonstration of a high-performance wireless network with high bit-error-ratio (BER), signal-to-noise ratio (SNR), and detectable power in long-range wireless data transmission for communications or radar imaging applications. In the last two decades, the Spoof Surface Plasmon Polaritons waveguides (SSPP WGs), which are single-conductor waveguides attracted interest of researchers with their high electromagnetic wave confinement and low loss properties as a potential solution for overcoming signal integrity, interchannel interference, and high radiation loss challenges that tackle the high-performance terahertz integrated circuits. This thesis work is dedicated to examining the planar terahertz SSPP WGs (SSPP WGs), which are easy to fabricate, in different aspects to investigate their potential for high-performance terahertz integrated circuits, that may enable novel, high-mobility terahertz systems. First, the first example of a closed-form mathematical formulation of the effective dielectric constant for the planar SSPP WGs in the literature is presented. The proposed mathematical model reaches an error rate lower than 20 different devices at three different frequencies at the frequency band of 0.25 to 0.3 THz with respect to the measurement results. The formulation is also employed for a rapid demonstration of a 2-bit terahertz phase-shifter at 0.3 THz and 1 THz. The measurement results at 0.3 THz not only reaches a near zero error rate between the measurements, simulations, and the ideal phase-states of the proposed phase-shifter, but also can be considered to be the first measurement-based evidence for the SSPP WGs providing high lateral confinement and hence occupation of smaller chip area. The experimental verification of the 2-bit SSPP phase-shifter at 1 THz will be presented in a near future. The thesis also examines the performance of the proposed idea that employs the closed form mathematical formulation in the design in comparison with the performance of eigenmode simulations at 7.15 GHz. The proposed designs show that the proposed mathematical calculation-based design approach and eigenmode analysis result in a very close error rate with each other verifying that the proposed idea is superior over the time consuming eigenmode simulations. The thesis work also examines the SSPP WGs to investigate their real-potential for minimizing the insertion-loss-per-unit-length for a planar terahertz waveguide. The proposed results not only achieved the record-low-insertion-loss-per-unit-length but also the record-high-figure-of-merit, i.e., (°/dB) performance in accordance with the measurement data. Above all, the proposed results are also employed for developing a systematic methodology without needing any preliminary simulation for selecting the SSPP WG dimensions for optimum performance. The algorithm is based on entirely mathematical calculations of the dispersion diagrams of different SSPP WGs and comparison with each other. The thesis also introduces the first-time mathematical calculation-based design approach of the mode-converters, namely the transitions between the conventional waveguides, the coplanar waveguide for measurement purposes for this case. The proposed results show that the calculation of the guided-wavenumbers at the input and the output of the transition enables the determining the corrugation dimensions by using the effective dielectric constant formulation for a tapered conversion of the guided-wavenumber from input to the output of the transition. Furthermore, the thesis work also investigates the relationship between the dimensions of an SSPP antenna and the guided-wavelength at the frequency of operation and preliminary results are presented paving way to the further analysis on the SSPP antennas with higher performance. Additionally, a group of fundamental circuits that are commonly employed in photonic integrated circuits is presented. The proposed designs include SSPP WG bending circuits, power splitters, and a Mach-Zehnder-Interferometer at 1 THz, whose measurement results are expected to be presented in a near future. In addition to all the above-mentioned theory and applications, the nano-electro-mechanical-system (NEMS) reconfigurable SSPP delay lines are designed and fabricated for demonstrating a 1-bit 90° phase-shifter at 0.3 THz, which is real-time controllable for demonstrating adaptive systems. The measurement results of the NEMS reconfigurable SSPP delay lines are expected to present in a near future.