Tiny Terahertz Modulator Packs a Big Data Punch

A close-up of a chip-sized terahertz modulator shows the path a wave travels through it.
Image: Nano Lab/Tufts University School of Engineering

Researchers covet terahertz waves for their ability to deliver data wirelessly at rates as high as 100 gigabits per second. That’s an unbelievably fast rate to achieve over the air, especially when you consider that: a) the average U.S. broadband speed is 55 megabits per second; and b) broadband service is piped into homes on fiber optic cables. For decades, academics have tried to develop terahertz-capable components to bring these exceedingly fast speeds to all of us.     

Last week, researchers at Tufts University showed off a new terahertz modulator that is the first such device to fit on a chip. This modulator, like any modulator found in any wireless device, can alter the amplitude of a wave during its transmission to encode data. But this new modulator can do so at the impossibly fast speeds required for waves found in the terahertz band.  

The Tufts group, led by Sameer Sonkusale, a computer engineering professor, note that larger terahertz modulators have been shown to work in open space, with a mechanical chopper that simply blocks a terahertz wave or permits it to pass by to indicate an “off” or “on” status, But device manufacturers would need a miniaturized, on-chip version to bring ultra-fast terahertz service to future smartphones and tablets.

On the electromagnetic spectrum, terahertz waves fall between the microwaves used by electronic devices and the optical waves that comprise visible light. Their frequencies are far above those of the signals used for cellular phones (which in the U.S. include 800 megahertz and 1.9 gigahertz), and for WiFi (which operates at 2.4 GHz). These lower, everyday frequencies are defined as “ultra high frequency” by the International Telecommunication Union. Lately, carriers have also been eyeing “extremely high frequency” millimeter waves, which fall roughly between 30 and 300 GHz, for their potential to bring data rates approaching 1 Gb/s to future 5G networks.

Research into the terahertz band, whose waves are classified as infrared, aims to move us even further along the electromagnetic spectrum. Terahertz waves are broadcast at a range of frequencies between 300 gigahertz and 3 THz. For their first demo, the Tufts researchers broadcast at 0.22 THz (220 GHz) to 0.325 THz (325 GHz), but say the modulator is capable of operating at frequencies up to 1 THz.

The new modulator, which is only 100 micrometers long, consists of two gold wires that serve as a waveguide. A bed of two-dimensional electron gas rests beneath the wires to dampen the wave as it travels by. By altering the number of electrons in this bed, the researchers can control the degree to which the terahertz waves (generated by a pair of lasers and a photon mixer) are absorbed by the electron gas. By absorbing more or less, they can modulate the waves as they travel across the wires.  

With this setup, the Tufts team achieved minimum data rates of 28 Gb/s in the terahertz band. At that rate, Sonkusale says a user could download a thousand high-definition videos in a fraction of a second. They reached this rate using on-off keying, one of the simplest types of signal modification. As the name implies, it communicates data through the presence or absence of a signal. More sophisticated modulation techniques could deliver even higher rates.

The group also reported an intensity modulation of 96 percent. This means the modulator could create two states (“on” and “off,” for example) with a separation of up to 96 percent to distinguish between them. A higher modulation index typically leads to fewer errors during transmission, because it makes it easier for a detector to tell the difference between signals.

“The modulation index is so high that it literally behaves as a terahertz switch,” Sonkusale says. “You can turn it on—and you get 96 percent transmission and you turn it off—and it completely blocks transmission.”

Of course, a modulator is only one component in a communications system—to build a new cell network that could handle terahertz waves, researchers would also need to build terahertz-capable antennas and receivers, as well as powerful sources to generate the waves in the first place.

One problem with terahertz waves: They are very easily absorbed by many materials as they propagate through the air, which makes it difficult to rely on them for long-distance communication. “Almost everything absorbs the terahertz wave,” says Sonkusale.

For that reason, Sonkusale thinks terahertz waves will first be used to send messages between devices within fairly short range of each other. Such a setup would be similar to Bluetooth, but with much faster data rates. “Imagine your Blu-Ray is connected to your TV without a wire,” he says.  

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