Generating terahertz on silicon chips

A method of generating terahertz signals on an inexpensive silicon chip promises future applications in medical imaging, security, scanning and wireless data transfer.

Terahertz radiation is that portion of the electromagnetic spectrum between microwaves and infrared light that penetrates cloth and leather and just a few millimetres into the skin. But it does not have the potentially damaging effects of X-rays.

Terahertz scanning can identify skin cancers too small to see with the naked eye. Many of the complex organic chemicals used in explosives absorb terahertz radiation at particular frequencies, creating a ‘signature’ that detectors can read.

Because higher frequencies can carry more bandwidth, terahertz signals could make a sort of super-Bluetooth that could transfer an entire high-definition movie wirelessly in a few seconds.

Current methods of generating terahertz radiation involve lasers, thermionic valves and special circuits cooled near absolute zero, often in room-sized apparatuses costing thousands of dollars.

Cornell researchers with Ehsan Afshari, assistant professor of electrical and computer engineering, have developed a new method using the familiar and inexpensive CMOS chip technology, generating power levels high enough for some medical applications.

With further research, higher power will be possible, Afshari said, enabling devices as handheld scanners for law enforcement.

The ability of solid-state devices to generate high frequencies is limited by the characteristics of the material - basically, how fast electrons can move back and forth in a transistor. So circuit designers make use of harmonics.

That fundamental frequency is usually set by a circuit that uses a varactor, but at terahertz frequencies varactors don’t tune sharply.

Afshari has found a new way of tuning by coupling several oscillators in a ring, producing a high-quality signal, where all the power goes into a very narrow frequency band.

Connect two springs and set one vibrating and the other will begin to vibrate as well, and eventually they will settle to an equilibrium. A ring of electronic oscillators does the same, and the circuits coupling the oscillators can set the frequency at which they will lock in.

In Afshari’s device the couplers also shift the phase of the signals, that is, how the peaks and valleys of the waves line up. With the right adjustment, the peaks and valleys cancel each other out at several harmonics but reinforce each other at one - in this case the fourth - channelling most of the power there.

In early experiments, the researchers fabricated chips that generated signals with about 10,000 times the power level previously obtained at terahertz frequencies on a silicon chip.

The signal emerges along the axis of the ring and what the researchers called an intriguing possibility is that by adjusting the couplers separately they could aim the output, making it possible to scan large areas with a narrow, high-powered beam.

The power could be increased by adding more oscillators to the ring or using multiple rings, and Afshari is working with Cornell experts on gallium nitride, a chip material that can handle both higher frequencies and higher power.

But Afshari said he wants to focus on less expensive silicon. “The goal is to make a complete device on one CMOS chip,” he said. “I can envision a tiny thing you could put in a mobile phone.”

The research is funded by the National Science Foundation, the US Office of Naval Research and the Semiconductor Research Corp, a consortium supported by private industry and the Defense Advanced Projects Research Agency.