We pursued the extremely low loss of photonic-crystal waveguides composed of a silicon slab with high resistivity (20 kΩ-cm) in the terahertz region. Propagation and bending losses as small as <0.1 dB/cm (0.326–0.331 THz) and 0.2 dB/bend (0.323–0.331 THz), respectively, were achieved in the 0.3-THz band. We also developed 1.5-Gbit/s terahertz links and demonstrated an error-free uncompressed high-definition video transmission by using a photonic-crystal waveguide with a length of as long as 50 cm and up to 28 bends thanks to the low-loss properties. Our results show the potential of photonic crystals for application as terahertz integration platforms.

The development of terahertz integrated circuits is vital for realizing sixth-generation (6G) wireless communication, high-speed on-chip interconnects, high-resolution imaging, on-chip biosensors, and fingerprint chemical detection. Nonetheless, the existing terahertz on-chip devices suffer from reflection, and scattering losses at sharp bends or defects. Recently discovered topological phases of light endow the photonics devices with extraordinary properties, such as reflectionless propagation and robustness against impurities or defects, which is vital for terahertz integrated devices. Leveraging the robustness of topological edge states combined with a low-loss silicon platform is poised to offer a remarkable performance of the terahertz devices providing a breakthrough in the field of terahertz integrated circuits and high-speed interconnects. In this Perspective, we present a brief outlook of various terahertz functional devices enabled by a photonic topological insulator that will pave the path for augmentation of complementary metal oxide semiconductor compatible terahertz technologies, essential for accelerating the vision of 6G communication and beyond to enable ubiquitous connectivity and massive digital cloning of physical and biological worlds.

The terahertz frequency range locates between 0.1 and 10 THz. This range accommodates atmospheric windows with staggering absolute bandwidth. It holds a potential for point‐to‐point wireless communications with an aggregate capacity reaching terabit per second in a range up to a kilometer. This unique capability is envisaged for backhauls between base stations and for local area networks. To this end, efficiency and compactness of the transceivers are crucial for successful large‐scale adoption. However, state‐of‐the‐art terahertz front ends are based on radio‐frequency or photomixing technologies that are inefficient, bulky, or complicated. In principle, as a neighbor of the microwave and optics domains, the terahertz band can leverage technologies from both sides to overcome those challenges. Recently, low‐loss integrated circuits based on photonic crystal waveguides are developed for routing terahertz waves. Here, a progress report on core components, including waveguides and diplexers, is presented. Additionally, the interfacing of the platform with electronic sources and detectors on one end, and with antennas for free‐space coupling on the other end, is discussed. Currently, the platform can support terahertz communications at a data rate over 10 Gbit s⁻¹. Challenges and opportunities are discussed in the light of future development in this area.

In this paper, we report on a terahertz (THz) sensing system. Compared to previously reported systems, it has increased system sensitivity and reduced size. Both are achieved by using a photonic crystal (PC) cavity as a resonator and compact resonant tunneling diodes (RTDs) as signal source and as detector. The measured quality factor of the PC cavity is higher than 10,000, and its resonant frequency is 318 GHz. To demonstrate the operation of the refractive index sensing system, dielectric tapes of various thicknesses are attached to the PC cavity and the change in the resonator’s refractive index is measured. The figure of merit of refractive index sensing using the developed system is one order higher than that of previous studies, which used metallic metamaterial resonators. The frequency of the RTD-based source can be swept from 316 to 321 GHz by varying the RTD direct current voltage. This effect is used to realize a compact frequency tunable signal source. Measurements using a commercial signal source and detector are carried out to verify the accuracy of the data obtained using RTDs as a signal source and as a detector.