Novel Materials Could Help Terahertz Chips Deliver Data at Terabits-Per-Second Rates

Novel materials known as photonic topological insulators could one day help terahertz waves send data across chips at unprecedented speeds of a trillion bits per second, a new study finds.

Terahertz waves fall between optical waves and microwaves on the electromagnetic spectrum. Ranging in frequency from 0.1 to 10 terahertz, terahertz waves could be key to future 6G wireless networks. With those networks, engineers aim to transmit data at terabits (trillions of bits) per second.

Such data links could also greatly boost intra-chip and inter-chip communication to support artificial intelligence (AI) and cloud-based technologies, such as autonomous driving.

"Artificial intelligence and cloud-based applications require high volumes of data to be transmitted to a connected device with ultra-high-speed and low latency," says Ranjan Singh, a photonics researcher at Nanyang Technological University in Singapore and coauthor of the new work. "Take for example, an autonomous vehicle that uses AI to make decisions. In order to increase the efficiency of decision-making tasks, the AI-sensors need to receive data from neighboring vehicles at ultra-high speed to perform the actions in real time."

Conventional terahertz waveguides are vulnerable to fabrication defects and considerable signal loss at sharp bends. Now, researchers find the burgeoning field of topological photonics may help solve these problems.

Topology is the branch of mathematics that explores what features of shapes can survive deformation. For instance, an object shaped like a doughnut can get pushed and pulled into the shape of a mug, with the doughnut's hole forming the hole in the cup's handle, but it could not get deformed into a shape that lacked a hole without ripping the item apart.

Using insights from topology, researchers developed the first electronic topological insulators in 2007. Electrons traveling along the edges or surfaces of these materials strongly resist any disturbances that might hamper their flow, much as a doughnut might resist any change that would remove its hole.

Recently, scientists have designed photonic topological insulators in which photons of light are similarly "topologically protected." These materials possess regular variations within their structures that lead specific wavelengths of light to flow within them without scattering or losses, even around corners and imperfections.

Image: Nanyang Technological University/Osaka University/Nature Photonics An optical image of the silicon chip. The white dashed line represents the interface between the two different sets of triangular holes.

Prior work on photonic topological insulators was largely focused on microwave and optical frequencies. Now researchers say they have for the first time experimentally achieved topological protection of terahertz waves.

Scientists fabricated a silicon chip that was 190 microns thick and measuring 8 millimeters by 26 millimeters. They perforated it with rows of triangular holes that alternated in size between 84.9 microns and 157.6 microns, with the smaller triangles pointing the opposite direction of the larger ones. These rows of holes were arranged in clusters where all the larger triangles either pointed up or down. Light entering this chip flowed topologically protected along the interface between the different sets of holes.

Photos: Nanyang Technological University/Osaka University/Nature Photonics An experimental demonstration of uncompressed 4K high-definition video transmission using the new chip (right). The transmitted 4K high-definition video is shown on the monitor in the background. The terahertz-signal transmitter is on the left side; the receiver is on the right side.

In experiments, the researchers found terahertz waves could also travel smoothly with virtually no losses even when routed around 10 sharp corners, including five 120-degree turns and five 60-degree turns. They achieved data transfer rates of 11 gigabits per second at a frequency of 0.335 terahertz with a bit error rate of less than 1 in 100 billion. They also showed they could transmit uncompressed 4K high-definition video in real-time through their chip across those 10 sharp bends at a rate of 6 gigabits per second.

Previous research achieved data rates of 1.5 gigabits per second with terahertz waves and photonic crystals (structures possessing features smaller than the wavelengths of light they're designed to deal with). Not only does the photonic topological insulator in the new work display higher data transfer rates, but traditional photonic crystals experience huge signal loss at bends, whereas such losses are negligible in the new material. "This is important when we consider miniaturization of devices in designing on-chip multiplexers and splitters, which normally require bending of waveguides," says Masayuki Fujita, a coauthor and photonics researcher at Osaka University in Japan.

The researchers note there are a number of ways to boost the data rates of their setup to achieve terabit-per-second speeds, though they haven't yet demonstrated those rates in an experiment. These techniques include using higher frequencies, more bandwidth, and more complex data-encoding schemes.

The scientists detailed their findings on 13 April in the journal Nature Photonics.