T-Rays from Superconductors

A device from Argonne National Lab takes a fresh approach to generating t-rays.

Researchers have fashioned a high-temperature-superconductor crystal into a structure that generates t-rays, electromagnetic waves with a frequency near one terahertz. Although the superconductor-based technique is not yet ready for commercial use, it offers a new option for exploiting this region of the spectrum for a variety of applications, including airport security and medical monitoring.

Super-emitter: A mesa built on a superconducting crystal acts as an electromagnetic cavity to promote emission of terahertz radiation (red waves) from the side faces. Credit: A. E. Koshelev, Argonne National Laboratory

Because terahertz radiation penetrates many millimeters into tissue, it could enable new medical-imaging techniques. T-rays have also been used in prototype security systems, where they pass readily through fabrics and packaging to reveal concealed weapons and the spectral fingerprints of toxins and explosives.

In spite of its potential, however, the terahertz region hasn't been widely exploited, partly due to limitations of current sources. "All the different sources that are available have nuances that make them not quite exactly what people want," says John Federici, a professor of physics at the New Jersey Institute of Technology, who works on generation schemes based on large pulsed lasers.

Superconductors could provide a solution, says Ulrich Welp, of Argonne National Laboratory, who led the new research. In particular, Josephson junctions--two superconducting slabs separated by a thin insulator--emit high frequencies and occur naturally in the atomic structure of high-temperature superconductors.

The problem is that a single junction emits only a tiny amount of terahertz radiation. "People have worked for a long time to get high-frequency radiation from these intrinsic Josephson junctions," says Welp, but "the power was limited to the picowatt range". He says that his team's new devices, described in the November 23 issue of Science, emit much more power: about half a microwatt.

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Researchers have long recognized that combining the output from many junctions could boost the power. "The problem was always to synchronize this large number of junctions," says Welp. To solve this problem, the group started with sin­gle crystals of bismuth strontium calcium copper oxide, better known as BSCCO, grown by team members in Japan. The group then used standard processing techniques to etch away parts of the crystal, leaving a flat-topped plateau about a micron high and tens of microns wide.

The etched edges of this mesa reflect radiation traveling along the surface, turning the raised structure into a "cavity" that traps the radiation, Welp says. "We synchronize these junctions through a cavity resonance, just like in a [semiconductor] laser--a standing electromagnetic wave inside a sample, which forces all these junctions to oscillate in phase, coherently." As a signature of coherence, the team found that the power output rose not in proportion to the number of active junctions, but as the square of that number, suggesting that their electric fields are summing together, not just their emission.