Faint signals from the most distant reaches of space can tell us things about the structure of galaxies and the nature of the early universe that can't be learned in any other way. Most of the light in the universe is too dark to see, its wavelength ranging from the far infrared to microwaves, but one way to detect it is to use a bolometer, a device which changes temperature when struck by a long-wavelength photon.

Paul Richards and his colleagues have constructed superconducting bolometers that are able to detect weak cosmic microwave background radiation. The bolometers are nearly immune to spurious signals from energetic cosmic rays, which can swamp measurements from experiments carried onboard balloons and spacecraft.

bolometer is a heat detector whose sensitivity is limited, in principal, only by the quantum noise fluctuations inherent in the arrival of small numbers of photons. Until now, bolometers have been intricate, finicky, and slow, but physicist Paul Richards and his colleagues have come up with a novel design that is easier to make, more sensitive, and faster by orders of magnitude than its predecessors.

Bolometers used on balloons, aircraft, or spacecraft to detect millimeter and submillimeter waves, such as the cosmic microwave background, make use of semiconductors, including doped germanium crystals developed at Berkeley Lab. For adequate sensitivity these have to be cooled to a few tenths of a degree above absolute zero.

(Click on image to enlarge)

A balloon of the type used in experiments that measure weak cosmic microwave background radiation.

Richards, an infrared physicist in the Materials Sciences Division and a professor of physics at the University of California at Berkeley, says, "In order to make lots of detectors, we wanted to get away from germanium crystals"-which are difficult to fabricate and attach-"and we naturally thought of superconductors, which can be fabricated by thin-film deposition and optical lithography."

In most bolometers the heat deposited by incoming photons serves to decrease electrical resistance. The result is a voltage change proportional to the photon power.

Although superconducting bolometers were proposed as early as 1943, they have rarely been used, because it is so difficult to maintain the temperature at the transition edge in the face of fluctuating environmental conditions. "The advantages of easy fabrication weren't sufficient to justify the complexity of the system," Richards says.

"The trick turned out to be not to put a fixed current through the superconductor, but to apply a fixed voltage," says Richards. "The current/ voltage/resistance relation produces a negative feedback: the incoming photons increase the resistance, which decreases the current, so the total power and therefore the temperature stay essentially the same." Because this self-regulating system maintains itself at the transition edge, it is stable and reliable.

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Prototypes built by Richards, his collaborator Adrian Lee of the Center for Particle Astrophysics, and their co-workers incorporate superconducting quantum-interference devices, or squids, capable of measuring tiny changes in current. Strong negative feedback makes their bolometer orders of magnitude faster-250 times faster than conventional current-biased bolometers-and up to twice as sensitive.

The clever solution seems simple in retrospect, and indeed Richards had briefly considered voltage-biased superconducting bolometers years ago. More recently a Stanford graduate student, Kent Irwin, now of the National Institute for Standards and Technology, suggested that voltage bias would improve the performance of superconducting calorimeters used for detecting x-rays. Then Adrian Lee pointed out the advantages for infrared and millimeter-wave bolometers.

The new instruments will find many uses this side of the outer limits of the cosmos. The principles applied to supercooled bolometers for astronomy can be used to make fast, linear, low and high-temperature superconducting infrared sensors which will greatly improve the performance of infrared spectroscopy in the laboratory.

Most of the elements that make up our world were formed in ancient stellar explosions. Fixed-voltage superconducting bolometers designed to gather data about the early universe are destined to tell us much about the everyday world we live in as well.

- Paul Preuss
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