14 Dec 2012

Silicon chips radiate terahertz waves

Electrical engineers at the California Institute of Technology have developed silicon chips that generate and radiate terahertz waves, and can penetrate a host of materials without the ionizing damage of X-rays.

When incorporated into handheld devices, the new microchips could enable a broad range of applications in fields ranging from homeland security and wireless communications to health care and touchless gaming.

"Using the same low-cost, integrated-circuit technology that's used to make the microchips found in our cell phones and notepads today, we have made a silicon chip that can operate at nearly 300 times their speed," says Ali Hajimiri, the Thomas G. Myers Professor of Electrical Engineering at Caltech. "These chips will enable a new generation of extremely versatile sensors."

To finally realize the promise of terahertz waves, Hajimiri and colleague Dr Sengupta used CMOS technology to design integrated circuits with fully integrated functionalities and that operate at terahertz frequencies—but fit on a fingertip.

The new chips boast signals more than a thousand times stronger than existing approaches, and emanate terahertz signals that can be dynamically programmed to point in a specified direction, making them the world's first integrated terahertz scanning arrays.

Hajimiri and Sengupta had to overcome multiple hurdles to translate CMOS technology into workable terahertz chips, including the fact that silicon chips are simply not designed to operate at terahertz frequencies. Every transistor has a cut-off frequency, above which it fails to amplify a signal; standard transistors can't amplify signals in the terahertz range.

To work around the cut-off-frequency problem, the researchers operated multiple transistors, combining the power to boost the strength of the collective signal.

"We came up with a way of operating transistors above their cut-off frequencies," explains Sengupta. "We are about 40 or 50 percent above the cut-off frequencies, and yet we are able to generate a lot of power and detect it because of our novel methodologies."

"Traditionally, people have tried to make these technologies work at very high frequencies, with large elements producing the power," says Hajimiri. "Nowadays we can make a very large number of transistors that individually are not very powerful, but when combined and working in unison, can do a lot more. If these transistors are synchronized they can do everything that the element does and then some."

The researchers also figured out how to radiate, or transmit, the terahertz signal once it has been produced.

At such high frequencies, a wire cannot be used, and traditional antennas at the microchip scale are inefficient. What they came up with instead was a way to turn the whole silicon chip into an antenna. Again, they went with a distributed approach, incorporating many small metal segments onto the chip that can all be operated at a certain time and strength to radiate the signal en masse.

"We had to take a step back and ask, 'Can we do this in a different way?'" says Sengupta. "Our chips are an example of the kind of innovations that can be unearthed if we blur the partitions between traditional ways of thinking about integrated circuits, electromagnetics, antennae, and the applied sciences. It is a holistic solution."

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