Silicon chip passes terahertz waves through barriers at 100 Gbit/s

Researchers in India have demonstrated 100 Gbit/s data transmission across an engineered barrier on a silicon chip operating at terahertz frequencies, a result they say points toward compact terahertz circuits for future 6G networks.

The team at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), an autonomous institute of the Department of Science and Technology, used topological photonics to guide terahertz waves through obstacles that would badly degrade a conventional waveguide. The work appeared in the journal Advanced Materials (DOI: 10.1002/adma.75322) and was released by the Ministry of Science and Technology on 4 November.

Why terahertz is hard to route

Future 6G networks are expected to use terahertz frequencies, which sit between microwaves and infrared light and offer wide bandwidth. Guiding these waves through compact circuits is the difficult part. Bends, junctions and abrupt changes in a waveguide scatter or reflect part of the signal, weakening it as it travels.

The JNCASR group addressed the problem with a silicon photonic crystal: triangular holes arranged in a carefully designed pattern in a silicon slab guide terahertz waves along the boundary between two regions. The crystal's structure mimics the honeycomb lattice of graphene. The researchers created a barrier by changing the sizes of the triangular holes over a short section of the waveguide.

The physics resembles what the team calls photonic Klein tunnelling. Transmission stayed close to its barrier-free level as long as the barrier strength remained below a characteristic coupling strength of the crystal. Because the barrier is defined purely by hole geometry, this gives chip designers a practical rule.

What do the numbers show?

A single barrier added only 1–2% loss. Two barriers in sequence added 2–3%. In a conventional, non-topological waveguide, a comparable obstacle markedly reduces transmission.

The team fabricated three devices on high-resistivity silicon, a material compatible with established semiconductor manufacturing:

• a device with two barriers in sequence, which carried 48 Gbit/s in high-speed data tests;
• an omega-shaped route containing four sharp bends and a central barrier, which carried 60 Gbit/s;
• a power splitter with a barrier in one of its two branches, which supported 100 Gbit/s using QAM-32 modulation, encoding information in the signal's amplitude and phase.

At 100 Gbit/s, a 5-gigabyte high-definition film could in principle transfer in under half a second. Measurements across the 310–360 GHz band agreed closely with simulations. In the splitter, the two output signals were nearly identical, showing the barrier had little effect on transmission.

"Showing that terahertz waves can cross a barrier created simply by changing the hole geometry, while carrying 100 gigabits per second, suggests that this effect can become a useful design tool for terahertz chips," said Dr Abhishek Kumar, Assistant Professor at JNCASR, who led the study and heads the Meta Optics Laboratory there.

Who did the work

Samina Mulla and Sangram Kumar Sadhu of JNCASR were joint first authors. Collaborators included Asif Iqbal M and N. S. Vidhyadhiraja of JNCASR; Siddhartha Lal of IISER Kolkata; Anshuman Kumar of IIT Bombay; Zhonghua Gu and Prakash Pitchappa of the Institute of Microelectronics, A*STAR, Singapore; and Pascal Szriftgiser and Guillaume Ducournau of the University of Lille and CNRS in France.

What does this change for 6G?

The results are laboratory-scale, and no operator deployment follows from them. What they offer is an engineering path: terahertz circuits that route and split signals on a single chip with low loss, built on silicon processes the semiconductor industry already uses.

The team next plans to tune the barriers electrically, thermally or mechanically, which would let a device redirect signals between paths on demand, moving the concept one step closer to reconfigurable terahertz chipsets for 6G.