Terahertz radiation (THz) is a band of electromagnetic spectrum that’s been getting a lot of attention lately due to its use in airport security to augment metal detectors. The same properties of terahertz radiation that make it useful in airports make it promising for cancer detection. Because THz can penetrate tissue and then bounce back, it can provide information on the tissue’s density and water content.
The problem is generating a tuned THz signal without using large pieces of equipment, something researchers at Cornell may have just overcome. In a study published in Physical Review Letters a new inexpensive method is described that uses a CMOS chip composed of coupled electronic oscillators to emit terahertz radiation.
From Cornell’s announcement:
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 — signals that naturally appear at multiples of the fundamental frequency of an oscillator. That fundamental frequency is usually set by a circuit that uses a variable capacitor called a varactor, but at terahertz frequencies varactors don’t tune sharply. Afshari has come up with a new way of tuning by coupling several oscillators in a ring, producing what engineers call 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 — channeling 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.
Cornell announcement: Solid-state terahertz devices could scan for cancer…
Abstract in Physical Review Letters: Delay Coupled Oscillators for Frequency Tuning of Solid-State Terahertz Sources