Tuesday, November 12, 2013

Superconducting Image Sensor Measures Energy of Each Photon, with Timestamp

IEEE Spectrum publishes an article on the Array Camera for Optical to Near IR Spectrophotometry (ARCONS) developed by a group led by Ben Mazin, a physics professor at the University of California, Santa Barbara with colleagues and collaborators from NASA’s Jet Propulsion Laboratory, Oxford University, and Fermilab.

"The heart of ARCONS is a 60-nanometer-thick layer of titanium nitride (TiN) carried on a silicon base. Depending on the ratio of nitrogen to titanium, the layer becomes superconducting at about 1 Kelvin. (As the proportion of nitrogen decreases, the superconducting transition temperature and band-gap energies get lower; consequently, the imager's sensitivity to incoming photons increases. At its tiniest, the band gap of the superconducting TiN is about three orders of magnitude smaller than in a typical semiconductor.)

The TiN layer is etched into a 44 x 46 pixel array, and each pixel gets its own individually tuned microwave resonator and a microlens. The ensemble is enclosed in a lens-topped Dewar jar cooled to 0.1 K. When a photon strikes the sensor surface, is sends a ripple through the superconductor, breaking up the paired electrons—the Cooper pairs—that carry superconducting currents. The more energetic the photon, the more Cooper pairs are divided. Disrupting these pairs alters the impedance of the pixel. This electrical change, in turn, shifts the amplitude and phase of the pixel’s resonance in proportion to the number of Cooper-pair disruptions.

The researchers use a microwave frequency comb to interrogate and read out all 2024 pixels over a single microwave channel. Each pixel can be read about 2500 times per second, accurately seeing colors that range from the ultraviolet (100 nm) through the visible spectrum and into the infrared (longer than 5000 nm).
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