With the advent and fast-paced development of ultrahigh-speed digital interconnects for next-generation computing systems, low-latency broadband wireless communications for fifth generation and beyond, high-resolution imaging techniques and emerging parametric sensing systems for medical/biomedical and security applications, material and structure analyses and evaluations, and deep-space radio astrophysics, and so on, the search for system solutions with larger bandwidth has sparked a growing research in expanding the electromagnetic spectrum into the terahertz (THz) regime. Generally, the THz frequency covers 300 GHz through 10 THz, which presents the most fundamental spectrum gap between electronics and photonics.