September 2, 1998
When applied to light coming from celestial objects, the device's ability to directly measure the location, arrival time, and energy of individual photons could have a revolutionary impact on optical astronomy, say its inventors, Stanford physics Professor Blas Cabrera and his research team.
Not only can this detector measure all of an individual photon's important attributes, but it can do so throughout the infrared, optical and ultraviolet portions of the spectrum, the physicists report in the Aug. 10 issue of the journal Applied Physics Letters.
The basic sensor, called a superconducting transition edge sensor (TES), was invented with Department of Energy support as part of a physics experiment called the Cryogenic Dark Matter Search and patented by Stanford in 1997. The experiment is being operated on campus and involves more than 40 scientists from eight institutions, Stanford, University of California-Berkeley, University of California-Santa Barbara, Case Western Reserve University, University of Santa Clara, San Francisco State University, Lawrence Berkeley National Laboratory and Fermilab.
The sensor is a critical element in a new detector designed to detect elementary particles called WIMPs. These Weakly Interacting Massive Particles have been proposed as one possible explanation for the missing mass in the universe. Analyses of the rotation of visible galaxies have convinced scientists that as much as 50 percent of the matter that galaxies contain must be invisible to telescopes. Although WIMPS should be virtually invisible, scientists calculate that they should occasionally shake up the nuclei in crystalline material, and TES sensors have been developed to detect the heat produced by such interactions.
The new optical version of TES, developed with support from the National Aeronautics and Space Administration, consists of squares of tungsten film that are 20 microns (about a human hair width) on a side. When the sheets are cooled down to a temperature of 80 thousandths of a degree above absolute zero, the tungsten becomes superconducting, able to carry electric current without resistance. Tungsten's transition between ordinary metal and superconductor is exceptionally sharp, so extremely small changes in the material's temperature give rise to large changes in its electrical properties.
"The sharp resistive transition made it potentially an extremely sensitive calorimeter," says Cabrera, "but it was very difficult to keep it within the narrow temperature range required."
In 1994, Cabrera and Kent Irwin, who is now at the National Institute for Standards and Technology in Boulder, solved the control problem by borrowing a technique that is widely used in the design of stereo amplifiers: negative feedback. They placed the sensor in a special circuit that produces a weak electrical current that automatically keeps the material at its critical transition temperature. The sensor is cooled slightly below the transition temperature and the electrical current raises its temperature to the critical value. When the energy from an individual photon reaches the tungsten, it heats up the electrons in the material. This heating causes a slight increase in the electrical resistance of the film. The greater resistance, in turn, causes a decrease in the electrical heating that exactly equals the amount of energy that the photon deposited. Not only does this keep the film at the right temperature but it also gives the scientists a precise measurement of the photon's energy and its arrival time.
The new sensors have a number of potential uses. Irwin and his colleagues at NIST have customized TES detectors for use in an X-ray spectrometer. Using this technology, they have created the highest resolution, high-energy spectrometer in the world. The semiconductor industry is very interested in using this instrument to locate small-scale surface contamination that is a barrier to the continued miniaturization of integrated circuitry. According to current plans, the next generation X-ray satellite, called Constellation-X, will include a TES spectrometer to aid in the identification of the chemical compounds that make up the gas clouds that float between stars and galaxies.
One of the most exciting applications for the sensors could come from mounting them on existing optical telescopes. "By providing us with information about the energy of each photon and the time when it arrives, these detectors can provide important information about some of the key questions in astronomy," says physics Professor Roger Romani. He is working with Cabrera and graduate students Aaron Miller, Tali Figueroa and Sae Woo Nam on a trial application of the system on the 24-inch student telescope at Stanford this fall.
Over the last 25 years, astronomers have converted their telescopes from photographic film to electronic CCD detectors similar to those used in camcorders. This conversion has increased the power of the telescopes by 30 to 100 times. But, like film, CCDs only provide information about the position of photons. As with the human eye or a camcorder, many photons passing through various filters are needed to get a crude estimate of the color or average energy. More complicated electronic systems, called microchanneltrons, can obtain information about photon arrival times but not their energies.
Currently, the physicists can only make TES detectors with a few pixels. Even with this limitation, however, they should be able to make meaningful new measurements of time-varying cosmic phenomena such as pulsars and gas-eating black holes, Romani says.
Once they have a rudimentary TES array attached to Stanford's small student telescope, the scientists will make trial observations of the powerful pulsar in the Crab Nebula. A pulsar is a rapidly spinning neutron star that emits radio waves with clock-like regularity. By recording the way that the energy of the visible light from the pulsar varies on time scales as short as a thousandth of a second, the physicists hope to gain new insights into the outstanding question of how spinning neutron stars produce optical light. By examining how the distortion of the light pulses vary at different energies, it might also be possible to see evidence of the relativistic twisting of space that should take place in the neutron star's vicinity, Romani speculates.
If the experiment with the small telescope is a success, the scientists hope to put a larger array of optical TES sensors on the 10-meter Hobby Eberly telescope in Texas. In addition to studies of faint black holes and neutron stars, the team also hopes to demonstrate that the device will be a powerful tool for measuring cosmic distances. Because the universe is expanding, the farther away objects are the faster they are receding. This motion causes redshift, the apparent reddening of light coming from receding objects. The larger an object's redshift the further away it must be. Because the speed of light is constant, objects with the highest redshifts are also the oldest objects in the visible universe. An array of TES devices could in principle obtain the redshift of every object in each image that a telescope makes. Currently, astronomers must follow up their initial observations of a new object with a lengthy spectrographic analysis to measure its redshift.
An ultimate application of this new technology would be to equip the next generation of space telescope with a thousand-by-thousand element array of TES sensors. Such a system would allow astronomers the measure the redshift of even the most distant objects, those too faint for even the biggest telescopes on Earth to resolve. In its deep field mode, for example, the Hubble space telescope has produced images of objects that are a thousand times fainter than the glow of the dark night sky and are invisible to Earth- based telescopes. Redshift information about these and other similar objects could provide astronomers with a more complete picture of the size and shape of the universe, the distribution of galaxies within it, and how this has changed over time.
Two small white cylinders sitting on the horizontal shelf are new optical detectors so sensitive that they can measure the heat from a single photon that have been developed in Blas Cabrera's physics lab. The detectors are connected to a light source by optical fibers. Because they require an extremely low temperature to operate, they are immersed in a bath of liquid helium. Source: Cabrera laboratory.
One of the first uses of the new detector will be to study the pulsar that sits at the heart of the Crab Nebula. The image on the left is a view of the Crab taken with the Palomar telescope and the image on the right is a blow up of the center of the Crab where the pulsar resides taken by the Hubble Space Telescope. Source: NASA.