The demand for 6G communication is driven by the need for ultra-high-speed connectivity with sub-millisecond latency to support applications like industrial automation, advanced healthcare, and the Internet of Everything. Expanding the radiofrequency spectrum into the terahertz (THz) band (0.1-10 THz) is crucial for achieving this massive capacity and connectivity. However, current terahertz on-chip communication devices face challenges such as crosstalk, scattering losses, limited data speed, and insufficient tunability, hindering the development of high-speed on-chip communication technologies needed for handling large data volumes (e.g., real-time 8K video transmission). This necessitates a novel design paradigm for terahertz interconnects with advanced functionalities like on-demand active control and channel demultiplexing. Conventional approaches, including metallic transmission lines, dielectric strip waveguides, photonic crystal waveguides, and THz fibers, suffer from inherent limitations at THz frequencies. These limitations restrict on-chip data transmission speeds to a few tens of gigabits per second, falling short of the requirements for 6G communication devices. This research aims to address these challenges by introducing a new class of broadband phototunable silicon (Si) THz topological devices.

Existing research has explored on-chip THz communication using Valley Photonic Crystal (VPC) waveguides, demonstrating promising results but with limitations in bandwidth and active tunability. Previous work has shown the potential of topological photonics for high-quality (Q) modes in topological cavities and their optical modulation. However, these studies lacked a comprehensive theoretical framework for dynamic critical coupling and the demonstration of broadband THz topological waveguiding and switching at record single-channel data rates alongside active chip-scale topological demultiplexing.

The researchers fabricated a topological photonic chip using high-resistive silicon (HR-Si) with a resistivity >10 kΩ-cm, chosen for its low absorption loss, large refractive index, and CMOS compatibility. The chip design incorporates a Valley Photonic Crystal (VPC) structure with a honeycomb lattice of triangular holes, where the size of the holes is carefully controlled to break inversion symmetry and create topological bandgaps. This enables the creation of valley-polarized topological edge states, which exhibit robust transport of electromagnetic waves even at sharp bends or defects. Two types of devices were fabricated: Device 1, consisting of straight and bent VPC waveguide chips; and Device 2, an on-chip topological demultiplexer incorporating a high-Q topological cavity critically coupled to a VPC waveguide. The phototunability of the devices was achieved by photoexciting the Si-VPC chip using a 780 nm optical pump beam, modulating the THz waves by introducing attenuation without destroying the topological protection. Detailed characterization of the devices included THz transmission measurements using a vector-network analyzer (VNA) and THz communication experiments using various modulation formats (QAM-16, QAM-32) to assess data rates and error vector magnitude (EVM). The demultiplexing functionality was demonstrated by simultaneously sending two independently modulated data signals at different carrier frequencies, one modulated for HD video streaming, and the other for high-speed data transmission. The performance was quantified in terms of bit error rate (BER) and error vector magnitude (EVM). Numerical simulations using COMSOL Multiphysics were employed to support the experimental findings. The fabrication process involved depositing a SiO2 layer on a silicon wafer, photolithography to define the triangular etch holes, deep reactive ion etching to etch the silicon, and back grinding for wafer thinning.

The straight VPC waveguide chip achieved a record single-channel data rate of 160 Gbit/s (using QAM-32 with 32 GBaud symbol rate and BER of 1.3 × 10⁻²), while the bent waveguide chip reached 125 Gbit/s (using QAM-32 with 25 GBaud symbol rate and BER of 1.0 × 10⁻³). The photoexcitation demonstrated broadband THz topological switching capability, achieving >25 dB attenuation with 12 mW pump power. The on-chip topological demultiplexer successfully demultiplexed two channels: one carrying real-time uncompressed HD video (1.5 Gbit/s) and the other transmitting data at 40 Gbit/s. The high-Q topological cavity ensured excellent isolation between the channels, as demonstrated by the uninterrupted HD video streaming even after attenuating the 40 Gbit/s signal via photoexcitation. The experimentally observed bandwidth of -30 GHz was slightly less than the simulation prediction, attributed to minor inhomogeneities in the fabricated chip. The device operates in the 300 GHz band, aligning with the target frequency range for 6G communications. The research established a detailed theoretical framework for dynamic critical coupling in the topological waveguide-cavity chip.

The results demonstrate the successful integration of topological photonics with a low-loss silicon platform for high-speed THz communication. The achieved data rates significantly exceed previous results and pave the way for the development of chip-scale THz integrated circuits for 6G and beyond. The phototunability enables crucial functionalities like on-demand channel switching, filtering, and beam steering. The high-Q topological cavity provides a robust and unique platform for various future research directions including nonlinear topological photonics, topological quantum circuits, ultra-low threshold topological lasers, and topological light-matter interactions. The experimental validation of the theoretical framework for dynamic critical coupling opens avenues for exploring new applications in channel switching and filtering.

This research successfully demonstrated a phototunable, broadband topological waveguide and demultiplexer on a CMOS-compatible silicon platform, achieving record data rates and enabling on-chip THz channel manipulation. The topological protection of THz waves combined with low-loss silicon enables robust and energy-efficient communication. The integration of a high-Q topological cavity achieved effective channel demultiplexing with excellent isolation. The results pave the way for developing compact and low-loss THz integrated photonic devices for 6G and future generations of wireless communication. Future research could focus on improving the switching speed and exploring more complex device designs to improve functionality.

The experimentally observed bandwidth (-30 GHz) was slightly lower than the simulated bandwidth, potentially due to minor fabrication imperfections. The highest data rate for the bent waveguide chip (125 Gbit/s) was slightly lower than the straight waveguide chip (160 Gbit/s) due to an additional dip in the transmission spectrum which could be addressed with improved coupler design. The switching speed (1 MHz) was limited by the silicon carrier relaxation time, and further improvements could be achieved through ion implantation. The current study is a proof-of-concept demonstration, and further scalability and integration into practical communication systems require additional research.