Topological Photonic Devices and Circuits at Terahertz Frequencies

ABSTRACT Topological photonics has emerged as a transformative paradigm for terahertz (THz) science and technology, offering unprecedented control over electromagnetic waves through modes that are inherently robust against disorder, defects, and fabrication imperfections. 3D topological insulators (TIs), characterized by spin‑polarized Dirac surface states, provide a powerful materials platform for realizing tunable, low‑loss THz components with functionalities that surpass those of conventional photonic systems. Operating across a spectral range central to next‑generation imaging, sensing, and high‑capacity wireless communications, TI‑based devices promise enhanced performance, stability, and energy efficiency. This paper covers recent advances in THz photonic devices and circuits enabled by TIs and topological design principles. We first outline the fundamental electronic and optical properties of 3D TIs and evaluate their integration into THz modulators, switches, detectors, and emitters. We then discuss the progression from traditional TI‑based photonics to fully topological photonic platforms, highlighting the emergence of structures that support unidirectional, backscattering‑immune light transport. Finally, we examine how advanced topological phases, including quantum spin Hall, quantum valley Hall, and higher‑order topologies, unlock defect‑immune propagation, reflectionless interfaces, and reconfigurable THz circuitry. Together, these developments position topological photonic materials and architectures as a foundational technology for scalable, resilient, and high‑performance THz and quantum photonic systems.