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Sensor Detects Single Photons

Photonics Spectra
Dec 1998
Daniel C. McCarthy

A new detector, developed at Stanford University, has the sensitivity to measure the location, arrival time and energy of individual photons in the UV to IR wavelength ranges. The Stanford device is based on superconducting transition-edge detecting technology until now used for sensing in the x-ray region of the spectrum. The researchers were able to extend the technology's efficacy into the ultraviolet, visible and infrared regions by using miniaturized sensors with cryogenic cooling techniques.
"The device's significance comes from the idea that at these very low temperatures you can precisely measure the temperature rise of a metal system resulting from the very tiny energy from single photons," said Blas Cabrera, a Stanford physics professor and one of the detector's inventors.
The sensor is based on the resistive transition occurring in a square of tungsten film supercooled in a liquid helium bath. A weak electrical current warms the tungsten slightly to maintain it at a temperature of 80/1000° above absolute zero, the precise degree at which the material becomes a superconducting material.
When the energy from an individual photon reaches the tungsten, it heats up the electrons in the material, causing an increase in the film's electrical resistance. As electrical resistance rises, the electrical current automatically decreases to readjust the film's temperature. From this adjustment, researchers can calculate the exact amount of energy that the photon deposited and its exact arrival time.
Kent Irwin, now a physicist at the National Institute of Standards and Technology in Gaithersburg, Md., was among the original development team at Stanford. He explained that the detector combines information that was formerly available only via separate technologies. "It gathers imaging, wavelength and timing information in one device which has high quantum efficiency," Irwin said. "This is especially interesting in astronomical research, where you want to wring all the information you can out of every photon."
The sensor has potential applications in x-ray spectrometry or in locating small-scale surface contamination in semiconductors that has hindered the continued miniaturization of integrated circuitry.
"Our immediate interests are exploring fast time-domain phenomena. Eventually, we hope to create larger-format arrays using these sensors," said Cabrera. "But even at one pixel we can get an improved time and energy plot to investigate fast time evolution systems such as pulsars and neutron stars. That sort of detection offers us two more dimensions of information over a conventional CCD."

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