Miniature rabbit ears for infrared sensors

Science News, Jan 4, 1992 by Ivars Peterson

From the telescoping rabbit ears of television sets to giant dishes capable of receiving faint radio signals emanating from distant galaxies, antennas play key roles in the detection of long-wavelength electromagnetic radiation. Scientists at the National Institute of Standards and Technology (NIST) in Boulder, Colo., have now fabricated tiny antennas that effectively capture infrared radiation, thereby extending antenna technology to shorter wavelengths.

"Our work on small antennas is part of a larger effort to move microwave-like technology toward shorter wavelengths, and this work represents the shortest wavelength to which it has been pushed so far," says Donald G. McDonald, a member of the NIST team. He and his co-workers describe their novel infrared antennas in the Dec. 16 APPLIED PHYSICS LETTERS.

Each "microantenna," about the size of a grain of sand, consists of a thin gold film deposited in a spiral pattern on a niobium surface. The gold layer captures and, in effect, channels infrared radiation toward the underlying niobium detector. NIST tests show that such an antenna efficiently responds at room temperature to infrared radiation at wavelengths from 3 to 30 microns.

Conventional infrared detectors operate without antennas. These detectors consist of specially fabricated materials that simply absorb infrared radiation at their surfaces to generate an electrical signal. The addition of microantennas permits the use of detectors much smaller in size than the wavelength of the detected radiation, McDonald says. In principle, arrays of such antennas coupled to tiny detectors could produce finely detailed infrared images of objects.

The NIST antenna work is part of a larger program aimed at the development of sensitive infrared detectors based on superconducting materials. Although the researchers initially made all their measurements on the new antennas at room temperature, they chose niobium as their detector because it becomes a superconductor at temperatures below 9 kelvins.

"Our work emphasizes low-temperature operation," McDonald says. "We've already built [low-temperature detectors], and they're working."

However, even at room temperature, a device consisting of a gold antenna coupled to a niobium detector already serves as a sensitive detector of blackbody radiation. "With some refinement, we believe that [our device] would be better than any other present-day, commercial detector," McDonald says.

The NIST work also supports the notion that certain miniature structures found in insects may similarly act as nonmetallic antennas, or waveguides, for infrared radiation. "This is fascinating but highly speculative," McDonald admits. "With insects, it's very difficult to tell precisely the function of any such structure."