Researchers in the US have built an antenna so small that it can pick
up infrared radiation. The developers hope that arrays of the tiny antennas
will improve infrared detectors which produce heat images. Infrared detectors
have many uses, such as detecting where heat escapes from buildings or spotting
the body heat of people trapped beneath the rubble of fallen buildings.
Normal radio antennas are free-standing wires, rods, poles or dishes.
They do not directly detect radiation; they collect it and deliver it to
a detector. They improve sensitivity of the detector because they collect
radiation from a larger area than the detector on its own. Antennas work
best if their size is comparable to the wavelengths being detected – around
a metre for broadcast radio and television.
The infrared antenna was developed by researchers at the National Institute
of Standards and Technology in Boulder, Colorado. The antenna is a thin
layer of gold etched on a silicon substrate and is only 60 micrometres across,
making it sensitive at wavelengths of about 10 to 160 micrometres. The developers
say the antenna is about 50 per cent efficient at infrared wavelengths around
The antenna consists of two spirals, each making one full turn outward
from opposite ends of the detector. Erich Grossman, Joe Sauvageau, and Don
McDonald of NIST built it to improve the sensitivity of a superconducting
infrared detector called a bolometer which changes its electrical resistance
when heated up by the radiation. The bolometer, which sits at the centre
of the spirals, measures only 2 by 0.8 micrometres.
Curiously, the researchers say, nature may have equipped some insects
with their own infrared antennas. Phillip Callahan, an entomologist for
the Department of Agriculture in Gainesville, Florida, has found that some
insects have antenna-like structures, about the right size to detect infrared
wavelengths. However, he has not yet shown that the structures act as antennas.