New T-ray source would improve airport security, cancer detection
especially those of the skin and breast, Welp said. Dentists could also use T-rays to image their patients’ teeth.
The new T-ray sources created at Argonne use high-temperature superconducting crystals grown at the University of Tsukuba in Japan. These crystals comprise stacks of so-called Josephson junctions which exhibit a unique electrical property: When an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage. This phenomenon is known as the Josephson effect (more formally: The Josephson effect is the phenomenon of current flow across two weakly coupled superconductors, separated by a thin insulating barrier. This arrangement — two superconductors linked by a non-conducting barrier — is known as a Josephson junction; the current that crosses the barrier is the Josephson current. The terms are named after British physicist Brian David Josephson, who discovered the phenomenon in 1962). These alternating currents then produce electromagnetic fields the frequency of which is tuned by the applied voltage. Even a small voltage — around two millivolts per junction — can induce frequencies in the terahertz range, according to Welp. Since each of these junctions is tiny — a human hair is roughly 10,000 times as thick — the researchers were able to stack approximately 1,000 of them on top of each other in order to generate a more powerful signal. Note that even though each junction would oscillate with the same frequency, the researchers needed to find a way to make them all radiate in phase. “That’s been the challenge all along,” Welp said. “If one junction oscillates up while another junction oscillates down, they’ll cancel each other out and you won’t get anything.”
In order to synchronize the signal, Argonne physicist Alexei Koshelev suggested that the stacks of Josephson junctions should be shaped into resonant cavities, which visiting scientist Lufti Ozyuzer of the Izmir Institute of Technology, Turkey, and graduate student Cihan Kurter then fashioned. When the width of the cavities was precisely tuned to the frequencies set by the voltage, the natural resonances of the structure synchronized the oscillations and thus amplified the T-ray output, in a method similar to the production of light in a laser. “Once you apply the voltage,” Welp said, “some junctions will start to oscillate. If those have the proper frequency, an oscillating electric field will grow in the cavity, which will pull in more and more and more of the other junctions, until in the end we have the entire stack synchronized.” By keeping the length and thickness of the cavities constant while varying their width between 40 and 100 micrometers, the researchers were able to generate frequencies from 0.4 to 0.85 terahertz at a signal power of up to 0.5 microwatts. Welp hopes to expand the range of available frequencies and to increase the strength of the signal by making the Josephson cavities longer or by linking them in arrays. “The more power you have, the easier it is to adopt this technology for all sorts of applications,” he said. “Our data indicate that the power stored in the resonant cavities is significantly larger than the detected values, though we need to improve the extraction efficiency. If we can get the signal strength up to 1 milliwatt, it will be a great success.”
The research was supported by DOE’s Office of Basic Energy Sciences and by Argonne’s Laboratory Directed Research and Development funds.
-read more in L. Ozyuzer et al. “Emission of Coherent THz Radiation from Superconductors,” Science 318. no. 5854 (23 November 2007): 1291-93 (sub. req.)
