Metastructures enable THz chips for 6G ...

Rather than making devices smaller to get higher speeds, Elison Matioli of the Power and Wide-band-gap Electronics Research Lab (POWERlab) at EPFL’s School of Engineering has been using metastructures to achieve frequencies of 200GHz to 20THz.

“New papers come out describing smaller and smaller devices, but in the case of materials made from gallium nitride, the best devices in terms of frequency were already published a few years back,” says Matioli. “After that, there is really nothing better, because as device size is reduced, we face fundamental limitations. This is true regardless of the material used.”

The metastructures are etched and patterned at sub-wavelength distances onto a semiconductor made of gallium nitride and indium gallium nitride. These allow the electrical fields inside the device to be controlled, yielding extraordinary properties that do not occur in nature.

“We found that manipulating radiofrequency fields at microscopic scales can significantly boost the performance of electronic devices, without relying on aggressive downscaling,” said EPFL researcher Samizadeh Nikoo and first author of a paper in Nature (see below).

Because terahertz frequencies are too fast for current electronics to manage, and too slow for optics applications, this range is often referred to as the ‘terahertz gap’. Using sub-wavelength metastructures to modulate terahertz waves is a technique that comes from the world of optics. But the POWERlab’s method allows electronic control, unlike the optics approach.

“In our electronics-based approach, the ability to control induced radiofrequencies comes from the combination of the sub-wavelength patterned contacts, plus the control of the electronic channel with applied voltage. This means that we can change the collective effect inside the metadevice by inducing electrons (or not),” says Matioli.

The POWERlab’s metadevices can reach 20 THz with a breakdown voltage of over 20 volts. This enables the transmission and modulation of terahertz signals with much greater power and frequency than is currently possible.

“This new technology could change the future of ultra-high-speed communications, as it is compatible with existing processes in semiconductor manufacturing. We have demonstrated data transmission of up to 100 gigabits per second at terahertz frequencies, which is already 10 times higher than what we have today with 5G,” said Nikoo.

The next step is to develop other electronics components ready for integration into terahertz circuits.

The paper is at www.nature.com/articles/s41586-022-05595-z

Researchers at UCLA in California have also developed a fully integrated terahertz (THz) comb/pulse radiator and a broadband frequency-comb heterodyne receiver for sensing and imaging applications that they are offering to research labs.

The chipset is fabricated in the GlobalFoundries 90-nm SiGe BiCMOS process. The comb radiator utilizes p-i-n diode sharp reverse recovery to generate THz frequency comb/pulses. The repetition rate of the radiated pulses is locked to a stable off-chip source, which can be adjusted to as high as 15 GHz.

By using a low-phase noise off-chip source rather than an on-chip oscillator, low phase noise and high-frequency stability are achieved. The phase noise of 405-GHz tone is −82 dBc at a 10-kHz offset frequency while the radiated tones are characterized from 220 GHz up to 1.1 THz using VDI SAX modules with the measured EIRP of −11, −15, and −36 dBm at 405, 500, and 750 GHz, respectively.

A THz frequency comb detector uses a Schottky barrier diode passive mixers as the local oscillator for heterodyne detection of any arbitrary spectrum in mm-wave/THz band by adjusting the spacing of the comb from 100 s of MHz up to 15 GHz with a resolution of 2 Hz.

The receiver chip is characterized from 220 up to 500 GHz with the measured NF of 24.5, 36, and 44 dB at 270, 405, and 495 GHz, respectively. A dual-comb technique using the radiator and receiver chips provides a compact low-cost solution for dual-comb sensing applications as the radiator and receiver chips consume a DC power of 40 and 38 mW, respectively.

The paper is at ieeexplore.ieee.org/document/9525036