The ARCONS spectophotometer captures single photons to capture wavelength and time data from ultiraviolet, visible, and infrared light in high-speed video.
Almost every imaging device on the planet (or in orbit, for that matter) sees the world in black and white: incoming photons hit the sensor, knock electrons loose, and generate a current. If the incoming photon’s energy is anywhere in the detector’s sensitivity range, the result is the same: the pixel is white.
To see color, imagers (including the human eye) integrate multiple black-and-white images made with defined parts of the spectrum. They either split the sensor field, using overlapping arrays of sensors with different filters to simultaneously make separate images—from red, green, and blue, for example—or they split the spectrum to project successive single-wavelength images on a single sensor field.
The Array Camera for Optical to Near IR Spectrophotometry (ARCONS) approaches the problem from a different angle, simultaneously capturing time and energy (and so wavelength) information from a single photon.
"What we have made is essentially a hyperspectral video camera with no intrinsic noise," says Ben Mazin, a physics professor at the University of California, Santa Barbara. Mazin—with UCSB colleagues and collaborators at NASA’s Jet Propulsion Laboratory, Oxford University, and Fermilab—is developing the ARCONS device for astronomical observation. "On a pixel-per-pixel basis, it's a quantum leap from semiconductor detectors; it's as big a leap going from film to semiconductors as it is going from semiconductors to these superconductors. This allows all kinds of really interesting instruments based on this technology."
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). CCD sensors, by contrast, typically detect light from 300 to 1000 nm—and only in a black and white.