# _electronics_
## 1 Introduction  
Terahertz radiation has been considered a promising frequency range that could support high-speed transmission applications, such as next-generation communication systems (also known as the 6G) and inter/intra-chip connections (i.e., last-centimeter communication) [1; 2; 3]. In contrast to microwave communication technologies, terahertz transmission devices feature a higher carrier frequency, a reduced size, and, the exciting potential to bridge state-of-the-art electric and photonic components [4]. In addition, terahertz waveguides with densely integrated functionalities are able to not only carry and manipulate massive data but also enhance the robustness and compactness of terahertz information systems [5; 6]. As a result, many terahertz waveguides inspired by both microwave and photonic regimes have been investigated and demonstrated [7; 8; 9]. Specifically, benefiting from the high transparency of high-resistivity silicon in the terahertz range and the well-established microfabrication techniques of silicon wafers, silicon-based planar waveguides are high-quality platforms for short-length (generally a few centimeters) on-chip terahertz communications [10]. A simple example is the silicon-on-insulator waveguide which is uncladded along the lateral direction; in other words, terahertz waves are confined by total internal reflection between the core-air interface, leading to weak mechanical stability and high scattering losses [11]. To solve these issues, microstructured photonic claddings are designed for achieving full-direction confined, self-supporting terahertz photonic integrated chips. In 2015, a terahertz silicon planar waveguide with photonic crystal cladding was reported followed by a great number of works on such a platform, for instance, the integration of passive and active components and on-chip communications [12; 13; 14; 15]. Nevertheless,photonic crystal waveguides suffer from significant losses at sharp corners and unexpected defects. Thanks to the recent advances in 2D topological photonics [16; 17], topological waveguides have unprecedented advantages of robust and reflection-free propagations, therefore, attracting huge attention in the terahertz field [18]. Current terahertz topological waveguides largely mimic the quantum spin Hall effect or the quantum valley Hall effect originating from the condensed matter physics field [19; 20]. For each class, two pioneering waveguides working at the terahertz regime have been demonstrated in the last two years. Similar to the previous silicon photonic crystal slab, topological propagation was achieved by etching a 2D hexagonal lattice of triangular air holes with two different sizes and opposite orientations in a silicon slab so as to introduce a bandgap of valley photonic crystals. The silicon topological waveguide has low losses below 0.1 dB for a 120\({}^{\circ}\) bend [21; 22]. Based on this platform, on-chip high-speed communications (up to 108 Gbps) assisted by 16-ary quadrature amplitude modulation were realized [23]. However, the narrow bandgap of the silicon topological waveguide (relative bandwidth of 7.8%) could be a limitation for broadband terahertz communications. To broaden the topological transmission window, our group proposes a hollow-core metallic topological waveguide with tilted pillars which mimics the quantum spin Hall effect [24]. Such a metallic waveguide is fabricated using a fast, cost-effective 3D printer and gold sputtering. Although the relative topological bandgap of the metallic waveguide has a 60% improvement compared to the silicon counterpart, the topological transmission bandwidth is still limited (relative value of 12.5%) [24]. Due to the remarkable properties of topologically protected propagations, numerous waveguide-integrated topological devices have emerged rapidly, including the topological on/off switch, beam splitter, cavity, and sensor amongst others [25; 26; 27; 28; 29; 30].