A new tunable source of terahertz radiation that delivers 10,000 times the power of previous devices has been created by scientists in the US. Unlike most other systems for generating terahertz radiation, the new tuner is made using the same CMOS technologies used in commercial integrated circuits. The researchers hope that their new approach could allow terahertz radiation to become more widely used outside the laboratory in applications such as security scanning and telecommunications.
Because it can penetrate several millimetres of tissue but is non-ionizing, terahertz radiation could have potential as a safe, non-ionizing alternative to medical X-rays. It could also be used for security screening because it can pass through clothes and because many illicit materials such as explosives and drugs have unique spectral fingerprints in the terahertz range. In addition, researchers have demonstrated that it could be used for much higher-bandwidth wireless internet technology than the present radio-wave-based system.
However, it has proved very difficult to create practical, tunable and low-cost terahertz sources that can easily be integrated with standard electronic devices. Current tunable terahertz devices are expensive and involve laboratory-based equipment such as free-electron lasers, quantum cascade lasers or stacks of Josephson junctions. Generating terahertz radiation using conventional solid-state electronic circuitry – the same way that radio waves are generated – has not proved feasible because the transistor in an electric circuit cannot sustain a current oscillating at terahertz frequencies. It is possible to produce an electrical output oscillating above the frequency limit of a circuit by using a frequency multiplier, but today’s technologies drastically limit the output power at terahertz frequencies. As a result, it had not been possible to produce a solid-state terahertz source with an output of even a microwatt, which itself is far too low to be useful.
Playing a new tune
Now, Ehsan Afshari and colleagues Cornell University in Ithaca, New York, have designed a new type of tunable frequency multiplier that exploits a fundamentally different tuning methodology than previous devices. Moreover, the team has built a CMOS-based terahertz source with an output power 10,000 times that its predecessors. The theory behind the device is based on the self-synchronization of coupled oscillators, which allows the circuit to produce an output frequency at a series of harmonics of a circuit’s fundamental frequency.
While other terahertz researchers are impressed by the original thinking that went into the new device, some point out that a milliwatt-output terahertz source is not likely to be powerful enough for practical applications. “The idea presented is new and might pave the way for commercial implementation. At present, however, the tuning range and output power do not make a revolution,” Martin Dressel of the University of Stuttgart in Germany told physicsworld.com.
Mark Rodwell of University of California, Santa Barbara agrees that the device is more interesting in theory than in practice. “The work is reporting a little less than 1 mW at a little below 300 GHz,” he says. “Radios operating between 100–1000 GHz, if they are to have reasonable range or interesting data rate, will need to radiate considerably more than 1 W.” He concludes that “The techniques reported in this work are intellectually attractive. The power levels being produced, and the energy efficiency, are not.”
However, Afshari told physicsworld.com that some applications should be possible. “Terahertz radiation has many applications in biosensing, imaging, and spectroscopy and for these applications 1 mW is more than enough,” he explained.
He also said that future implementations of the device using indium phosphide or gallium nitride instead of CMOS should deliver around 1 W. He also believes that the power of CMOS devices could be further boosted
“While this [1 mW] power might not be enough for some terahertz applications, the method can be used to generate even higher power.”
The research is published in Physical Review Letters.
