T-Rays from Superconductors
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.
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.
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.
Federici agrees that the lack of a compact, solid-state terahertz source is “probably one of the major issues that’s preventing terahertz from progressing, in many different areas.” But to compete with other terahertz sources, such as quantum-cascade lasers and mixers, he says, the power will have to be even larger–closer to milliwatts. The low temperature needed by the superconductors is also a problem, Federici says, especially for portable equipment: “As soon as you say, I’ve got to cool it down to 50 degrees kelvin, a lot of people that are interested in the application of terahertz will say, ‘Well, forget it, then.’ “
Such cooling is also needed for quantum-cascade lasers, which have recently been used to produce frequencies in the range above one terahertz. (See “T-Rays Advance Toward Airport Screening.”) Although these devices are not likely to operate at much lower frequencies, they are well established and continually improving.
The superconducting emitters are “beautiful science, absolutely,” says Michael Pepper, a professor of physics at the University of Cambridge, in England, and scientific director for Cambridge-based TeraView, which employs different sources in terahertz equipment that it sells for medical and security uses. But Pepper sees little need for a constant-output, fixed-frequency terahertz source. Three-dimensional imaging needs pulses, whose delayed reflection indicates distance, he says. Spectroscopic identification of chemicals–the other major technique–requires a tunable source, whereas in the current work, the cavity that enhances the superconductor’s emission also locks its frequency.
Welp acknowledges that “to make a useful device, we need maybe a milliwatt.” But he says that the team has ideas for modifying the device’s geometry to change how well individual junctions contribute to the radiation. “I think we can gain a lot in power, just by improving the efficiency of coupling to the cavity mode,” he says.
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