Phototunable chip-scale topological photonics: 160 Gbps waveguide and demultiplexer for THz 6G communication

The work addresses the need for ultra-high-speed, low-latency on-chip communication for envisioned 6G applications such as industrial automation, intelligent healthcare, and massive internet-of-everything. Moving into the THz band (>300 GHz) offers large bandwidth but existing on-chip THz interconnects (metallic lines, dielectric strip waveguides, photonic crystal waveguides, THz fibers) face crosstalk, scattering/reflection losses, dispersion, bending losses, and lack active tunability, limiting data rates to a few tens of Gbit/s. The authors propose silicon Valley Photonic Crystal (VPC)-based topological photonic devices to provide robust, single-mode, linear-dispersion edge-state transport immune to sharp bends and defects. They aim to realize broadband, phototunable, CMOS-compatible topological THz waveguides and a demultiplexer achieving record on-chip single-channel data rates and active channel control, thereby enabling practical on-chip THz interconnects for 6G.

Conventional THz interconnects (metallic transmission lines, dielectric strip waveguides, THz fibers, and conventional photonic crystal waveguides) are susceptible to defects, scattering/reflection losses, dispersion, bending losses, and lack of active tunability, constraining on-chip data transmission to tens of Gbit/s. Prior on-chip THz demonstrations using VPCs showed promise but suffered from limited bandwidth and no tunability. Earlier THz topological photonics reports primarily focused on high-Q topological cavity modes and optical modulation without integrating broadband waveguiding, active switching, and demultiplexing on-chip. This work advances the field by expanding usable bandwidth (~30 GHz transmission band), demonstrating phototunable operation, achieving record single-channel data rates (up to 160 Gbit/s), and realizing an active topological demultiplexer with excellent channel isolation.

Design and simulation: A silicon Valley Photonic Crystal (VPC) is formed by a honeycomb lattice of inverted equilateral triangular air holes (lattice constant a = 242.5 µm). Breaking inversion symmetry by setting unequal triangle sides (l1 ≠ l2) reduces symmetry from C6v to C3, opening topological bandgaps at K/K' valleys. Type A (Δl = l1 − l2 > 0) and Type B (Δl < 0) unit cells have opposite valley-Chern numbers. Domain walls between Type A and Type B host valley-polarized helical edge states with opposite group velocities. The chip targets TE modes. To maximize bandwidth, l1 = 0.7a and l2 = 0.3a (Δl = 0.4a) are selected based on COMSOL Multiphysics band-structure calculations, yielding an expected wide bandgap and edge-state transmission band. Two device categories were fabricated: Device 1 includes straight (VPC-S) and bend (VPC-B) waveguides; Device 2 integrates a high-Q topological cavity forming a closed-loop domain wall critically coupled to the VPC waveguide for demultiplexing. Fabrication: Devices were fabricated on 8-inch high-resistivity silicon (ρ > 10 kΩ·cm). After cleaning, a 4 µm SiO2 layer was deposited. Triangular holes were defined by photolithography; SiO2 was selectively etched, followed by deep reactive ion etching (>200 µm) of Si. Photoresist and SiO2 were removed, and the wafer was back-ground to 200 µm thickness. Characterization: THz transmission was measured with a vector network analyzer (VNA)-based setup across 300–360 GHz. Phototunability was implemented by photoexciting the domain wall with a 780 nm laser (E = 1.59 eV), generating free carriers in Si (Eg ~ 1.1 eV) to attenuate THz propagation. Pump powers up to 12 mW (∼1 mm spot) were used. Switching speed was assessed (up to ~1 MHz, limited by carrier lifetime). Communication experiments at 340 GHz evaluated error vector magnitude (EVM) and bit error rate (BER) for QAM-16 and QAM-32 at various symbol rates; performance was compared against a reference link without the chip. BER versus transmitted power was measured with and without photoexcitation to quantify power penalty. Demultiplexer design and tests: A topological cavity (closed-loop domain wall) was placed adjacent to the VPC waveguide with optimized spacing for coupling. AB and BA domain walls produce edge states of opposite group velocity per valley, enabling coupling. Transmission spectra at output ports (S21, S31) reveal cavity eigenmodes (e.g., at 330.2 and 334.4 GHz) with high Q. Critical coupling (equality of intrinsic and coupling losses) was achieved dynamically by photoexciting the cavity’s domain wall, enhancing resonance contrast. Dual-channel THz communication was demonstrated by injecting two carriers (∼331.6 GHz for CH1 with OOK HD video, and 344 GHz for CH2 with QAM-16 at 10 GBaud) at the input and detecting at two outputs with separate receivers: CH1 via direct detection and CH2 via subharmonic mixing. Due to limited spacing between couplers, a parabolic mirror was used to collect CH1, causing a slight operating frequency shift to 331.6 GHz (~0.4%).

- Broadband topological THz waveguiding: Measured transmission band of ~30 GHz (∼325–355 GHz bandgap region with edge-state transmission) in Si-VPC; flat transmission preserved under photoexcitation indicating maintained topological protection. - Phototunability and switching: Photoexcitation (780 nm) at the domain wall attenuates THz signals by >25 dB at 12 mW pump across the band; ON/OFF switching demonstrated up to ~1 MHz (limited by Si carrier relaxation), with potential for GHz via ion implantation. - High-speed on-chip data transmission: At 340 GHz, achieved 160 Gbit/s through the straight VPC-S using QAM-32 at 32 GBaud with BER ≈ 1.3×10^-2; 125 Gbit/s through the bend VPC-B using QAM-32 at 25 GBaud with BER ≈ 1.0×10^-3. Clear I–Q constellations indicate robust transport and effective DSP equalization. - Bend robustness and performance: For data rates <100 Gbit/s, VPC-B outperformed VPC-S due to bend-induced mitigation of reflections, reducing noise. At very high data rates, VPC-S slightly outperformed VPC-B due to an additional transmission dip near 325 GHz in VPC-B, effectively narrowing its usable bandwidth. - BER versus transmit power with/without pump: Photoexcitation (8 mW) induces a power penalty (horizontal shift) while preserving the BER slope, indicating added attenuation without added distortion or bandwidth reduction—consistent with preserved topological protection under pumping. - Topological demultiplexer: Cavity-waveguide system exhibits high-Q resonances at ~330.2 and 334.4 GHz; S21 dips and S31 peaks confirm coupling. CH2 bandwidth near 344 GHz is ~14 GHz (blue-shaded region in S21). Dual-channel demultiplexing achieved: CH1 at 331.6 GHz (OOK, 1.5 Gbit/s HD video) and CH2 at 344 GHz (QAM-16, 10 GBaud, 40 Gbit/s). Photoexciting CH2 attenuates its signal, degrading the CH2 constellation while CH1 HD video streaming remains uninterrupted, confirming excellent channel isolation. Standing-wave effects from the parabolic mirror shift CH1 optimum to 331.6 GHz (~0.4% shift).

The study demonstrates that silicon VPC-based topological photonics can deliver robust, broadband, and phototunable THz on-chip interconnects and channel management suitable for 6G-era requirements. Topological edge states enable low-scattering transport through sharp bends with single-mode, linear dispersion, reducing crosstalk and dispersion-induced delays. Integrating a high-Q topological cavity provides narrowband selection with strong isolation between channels, enabling demultiplexing of simultaneous data streams. Phototuning via carrier injection in Si offers dynamic control over attenuation and coupling, including movement into critical coupling for enhanced filtering and switching. The achieved 160 Gbit/s single-channel rate and 40 Gbit/s demultiplexed channel alongside real-time 1.5 Gbit/s HD video exemplify the feasibility of high-capacity, reconfigurable, CMOS-compatible THz photonic links. These results directly address the limitations of conventional THz interconnects, showing a path toward scalable, low-loss, and actively controllable THz integrated circuits for future wireless systems and on-chip networks.

The authors present a CMOS-compatible, phototunable silicon topological platform delivering: (i) broadband topological THz waveguides with active ON/OFF switching; (ii) record on-chip single-channel data transmission up to 160 Gbit/s; and (iii) an on-chip topological demultiplexer with excellent channel isolation supporting simultaneous 40 Gbit/s data and 1.5 Gbit/s HD video. Dynamic critical coupling via photoexcitation introduces a powerful modality for switching and filtering. The work paves the way for compact, low-loss, and reconfigurable THz integrated photonic devices essential for 6G/7G communications and related applications. Future directions include: exploiting GHz-speed photomodulation (e.g., via ion implantation), optimizing couplers to remove residual spectral dips and expand bandwidth, integrating active control for add-drop filtering and channel routing, and exploring applications in nonlinear/topological photonics, quantum circuits, and low-threshold topological lasers.

- Fabrication-induced inhomogeneity resulted in measured bandwidth somewhat lower than simulations. - The VPC-B device showed an additional transmission dip near 325 GHz, effectively reducing its usable bandwidth and limiting maximum data rate compared with VPC-S; improved coupler design could mitigate this. - Phototuning speed was limited to ~1 MHz by Si carrier recombination; higher speeds may require material/process optimization (e.g., ion implantation). - Critical coupling effects on full communication performance (e.g., add–drop filtering metrics) were not comprehensively characterized and are deferred to future work. - Experimental constraints fixed the central operating frequency near 340 GHz and required a parabolic mirror for CH1 collection, introducing a ~0.4% frequency shift for optimal HD video streaming.