Superconductors See the Light At Shorter Wavelengths
28 January 1999
A University of Rochester scientist and his Russian colleagues from Moscow State Pedagogical University have developed a superconducting device capable of detecting light at wavelengths that were previously off-limits to the materials, with remarkable speed and sensitivity.
The structure detects light in a portion of the infrared spectrum that is important for telecommunications and infrared astronomy, from 3 to 10 micrometers. The superconducting material, niobium nitride, is capable of detecting just a single photon, and it can recognize changes in light signals as fast as 25 billion times each second (25 gigahertz). Details of the device, along with the ultra-fast measurements of its capability, were published in the December 28 issue of Applied Physics Letters.
"Detecting single photons is amazing, and ours is one of a few detectors that can do so," says electrical engineer Roman Sobolewski of the University. "But what really distinguishes our device is its speed -- 25 gigahertz is very fast for an infrared detector." Sobolewski says conventional infrared detectors are typically either much less sensitive or slower.
In some ways the instrument, known as a hot-electron photodetector (HEP), is "a very sensitive electron thermometer," Sobolewski says. When infrared light hits it, the temperature of its electrons goes up. At an atomic level, when a photon hits the niobium nitride, an electron absorbs it and becomes extremely energetic or "hot." This rogue electron goes on to collide with other electrons, which in turn run into still others, causing a cascade, rather like a snowball rolling down a hillside and gaining in size. The temperature of these excited electrons quickly rises enough that the material itself temporarily loses its ability to be a superconductor, or carry an electric current with no resistance. The result is an electrical signal that engineers can readily detect.
The type of light the detector captures is particularly important in telecommunications. Signals sent from Earth to satellites and back again travel in the range of 3 to 5 or 8 to 12 micrometers, in wavelengths that allow them to pass through Earth's atmosphere unscathed. Another possible application down the road: detectors for optical systems whose fibers would carry such light pulses. In astronomy such wavelengths capture tales of stellar birth and of the existence of planet-like objects outside our solar system.
Engineers have long used superconducting materials in other configurations to detect energy at longer wavelengths; this work marks one of the first times a superconducting material has been used to detect energy at shorter wavelengths, in the infrared. Light at these energies is currently detected by other methods, including semiconductors, which must be carefully grown and are expensive to make.
The team's device is simply a single thin layer of niobium nitride less than one-thousandth the thickness of a human hair that works at temperatures below about 15 degrees Kelvin. After absorbing a photon the material bounces back almost immediately, returning to its superconducting state within 40 trillionths of a second, or 40 picoseconds. The device works so fast because only electrons are heated up; the material's temperature remains very low. Such speed, combined with its small size and its ability to detect infrared light, gives the material potential as one component of a new type of computer known as a superconducting computer. The University of Rochester is one of three academic institutions in the country working on such technology.
The U.S. and Russian scientists involved in this project owe their collaboration to the U.S. Office of Naval Research, which sponsored the work in an effort to promote international cooperation among scientists in the post-Cold War era. The films were made and tested in Moscow, and the speed of the detector was measured at the University, whose engineers have long been known for their expertise in ultra-fast measurements.
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