The revolutionary 5G cellular systems represent a breakthrough in the communication network design to provide a single platform for enabling enhanced broadband communications, virtual reality, autonomous driving, and the internet of everything. However, the ongoing massive deployment of 5G networks has unveiled inherent limitations that have stimulated the demand for innovative technologies with a vision toward 6G communications. Terahertz (0.1-10 THz) technology has been identified as a critical enabler for 6G communications with the prospect of massive capacity and connectivity. Nonetheless, existing terahertz on-chip communication devices suffer from crosstalk, scattering losses, limited data speed, and insufficient tunability. Here, we demonstrate a new class of phototunable, on-chip topological terahertz devices consisting of a broadband single-channel 160 Gbit/s communication link and a silicon Valley Photonic Crystal based demultiplexer. The optically controllable demultiplexing of two different carriers modulated signals without crosstalk is enabled by the topological protection and a critically coupled high-quality (Q) cavity. As a proof of concept, we demultiplexed high spectral efficiency 40 Gbit/s signals and demonstrated real-time streaming of uncompressed high-definition (HD) video (1.5 Gbit/s) using the topological photonic chip. Phototunable silicon topological photonics will augment complementary metal oxide semiconductor (CMOS) compatible terahertz technologies, vital for accelerating the development of futuristic 6G and 7G communication era driving the real-time terabits per second wireless connectivity for network sensing, holographic communication, and cognitive internet of everything.

Independent control of near‐field‐coupled bright and dark resonators for active modulation and frequency tuning of electromagnetically induced transparency analogs in the terahertz spectral region is experimentally demonstrated. An out‐of‐plane reconfigurable microcantilever is the active element for resonator control. The proposed microelectromechanical systems (MEMS) metamaterial is miniaturized, electrically controlled, and can achieve up to an order of magnitude change in THz group‐delay bandwidth product.

An ultrasensitive photonic sensor is vital for sensing matter with absolute specificity. High specificity terahertz photonic sensors are essential in many fields, including medical research, clinical diagnosis, security inspection, and probing molecular vibrations in all forms of matter. Widespread photonic sensing technology detects small frequency shifts due to the targeted specimen, thus requiring ultra-high quality (Q) factor resonance. However, the existing terahertz waveguide resonating structures are prone to defects, possess limited Q-factor, and lack the feature of chip-scale CMOS integration. Here, inspired by the topologically protected edge state of light, we demonstrate a silicon valley photonic crystal based ultrasensitive, robust on-chip terahertz topological insulator sensor that consists of a topological waveguide critically coupled to a topological cavity with an ultra-high quality (Q) factor of $Q=0.14\times {10}^6$ . Topologically protected cavity resonance exhibits strong resilience against disorder and multiple sharp bends. Leveraging on the extremely narrow linewidth (2.3 MHz) of topological cavity resonance, the terahertz sensor shows a record-high figure of merit of $4000\hspace{0.17em}\mathrm{RIU}\hspace{0.17em}{\mathrm{mm}}^{-1}$ . In addition to the spectral shift, the intensity modulation of cavity resonance offers an additional sensor metric through active tuning of critical coupling in the waveguide-cavity system. We envision that the ultra-high Q photonic terahertz topological sensor could have chip-scale biomedical applications such as differentiation between normal and cancerous tissues by monitoring the water content.

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 forthcoming sixth generation (6G) communication network is envisioned to provide ultra-fast data transmission and ubiquitous wireless connectivity. The terahertz (THz) spectrum, with higher frequency and wider bandwidth, offers great potential for 6G wireless technologies. However, the THz links suffers from high loss and line-of-sight connectivity. To overcome these challenges, a cost-effective method to dynamically optimize the transmission path using reconfigurable intelligent surfaces (RISs) is widely proposed. RIS is constructed by embedding active elements into passive metasurfaces, which is an artificially designed periodic structure. However, the active elements (e.g., PIN diodes) used for 5G RIS are impractical for 6G RIS due to the cutoff frequency limitation and higher loss at THz frequencies. As such, various tuning elements have been explored to fill this THz gap between radio waves and infrared light. The focus of this review is on THz RISs with the potential to assist 6G communication functionalities including pixel-level amplitude modulation and dynamic beam manipulation. By reviewing a wide range of tuning mechanisms, including electronic approaches (complementary metal-oxide-semiconductor (CMOS) transistors, Schottky diodes, high electron mobility transistors (HEMTs), and graphene), optical approaches (photoactive semiconductor materials), phase-change materials (vanadium dioxide, chalcogenides, and liquid crystals), as well as microelectromechanical systems (MEMS), this review summarizes recent developments in THz RISs in support of 6G communication links and discusses future research directions in this field.