Development of novel on-chip optical components via advanced design methods for time-domain terahertz radio-on-fiber applications

The terahertz (THz) band, located between the microwave and optical frequency ranges in the electromagnetic spectrum, typically spans frequencies from 0.1 to 10 THz. In particular, the wide bandwidths available at THz frequencies above 100 GHz have attracted significant interest for future wireless communication systems, enabling data rates on the order of terabits per second—approximately 100 times higher than those achievable with millimeter-wave systems. However, to fully harness the advantages of THz frequencies, the development of high-performance THz sources and detectors with ultrawide operational bandwidths and high-power levels remains a critical research challenge. In THz generation, the photoconduction method offers considerably higher power efficiency compared to nonlinear optical processes, with a theoretical potential of achieving 100% efficiency. Nevertheless, effective implementation of this approach requires precise and ultra efficient control of essential parameters—such as phase, amplitude, far-field profile, and beam steering angle—of the generated THz signals. This control is expected to be realized with maximum performance and scalability through an on-chip photonic architecture that manipulates ultrashort optical pulses to perform the intended operations prior to reaching the active regions of the photoconductive antennas. In this context, the ability of ultrashort optical pulses (particularly those shorter than 100 femtoseconds) to propagate without degradation through a multi-channel photonic architecture while enabling precise time-delay control across different channels necessitates the design of photonic integrated circuits optimized for ultrafast time-domain dynamics. To this end, it is critical to redesign key components—such as on-chip optical couplers, true time delay lines, power splitters, reflectors, optical bends, transition regions, mode converters, and optical switches—in a way that preserves the spectral and temporal integrity of ultrashort pulses. Moreover, the development of advanced optimization methods is essential to achieving these objectives. In this thesis, a hybrid THz-photonic architecture is designed for a THz time-domain beam-steering photoconductive antenna array system through the identification and integration of appropriate on-chip photonic components. The design of these components, their hybrid integration on various material platforms alongside THz photoconductive antennas, and their characterization in both the time and frequency domains are investigated. The first chapter of the thesis provides a general introduction to terahertz integrated photonic systems, followed by a second chapter that addresses ultrafast pulse propagation and time-domain dynamics in dispersive photonic waveguides. To efficiently realize true time delay lines and beam-steering operations for ultrafast applications, the influence of structural parameters on key performance metrics—such as high peak power, enhanced temporal resolution, and extended pulse storage duration—is analyzed. In the third part, the design of optical power splitters, group-index-engineered transition regions, optical bends, and reflectors is examined to ensure the scalable operation of dispersive photonic waveguides within multi-channel photonic architectures for beam-steering applications. Advanced design methods have been employed to achieve low-loss, wideband performance while preserving Bloch mode crystal lattice profiles. Furthermore, similar advanced design approaches have been applied to overcome challenges in silicon nitride true time delay lines. In the fourth part of the thesis, the fabrication steps of silicon- and silicon nitride-based photonic architecture with nanometer-scale precision (down to 100 and 150 nm, respectively) are presented in detail, along with their hybrid integration with terahertz photoconductive antennas. Subsequently, the detail of the frequency- and time-domain measurement processes are examined. First, the frequency-domain characterization of the photonic components discussed in the previous chapters is presented, along with the methodologies used to analyze the measurement data. Then, the time-domain measurement steps are presented based on the fundamentals of the Finite Impulse Response (FIR) model, using data obtained from conventional continuous-wave (CW) photonic measurements to demonstrate how the performance of the designed structures is experimentally validated under ultrafast pulse excitation. The proposed methods and designs are not limited to THz waveform generation; they also hold significant potential for advanced applications, including photonic neuromorphic computing, photonic neural networks, photonic LiDAR, and quantum photonics.