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Single-photon array aids astronomy

29 Nov 2005

A high-speed camera developed in Italy could help scientists study fast-evolving events such as gamma ray bursts.

Italian researchers have come up with a 2D imaging array of 60 single-photon avalanche diodes (SPADs) that operates at 20,000 fps with a detection efficiency of more than 40% in the visible range. The monolithic design is set to make its debut at the European Southern Observatory (ESO) and at the Astrophysics Observatory of Catania (INAF), Italy, where scientists hope to capture fast moving events such as supernovae and high-energy gamma ray bursts. (OPTICS LETTERS 30 3024)

Monolithic arrays of SPADs have been fabricated before, but unfortunately the integrated devices are often noisy and suffer from low detection efficiency.

"We had to design the chip geometry and layout to reject spurious optical coupling among pixels," Franco Zappa of Politecnico di Milano told Optics.org. "When a pixel is hit by a photon, the electrons flowing through the pixel emit secondary photons that can ignite neighbouring pixels."

As Zappa explains, a single photon hitting the diode can trigger an avalanche current of tens or even hundreds of milliamps. Multiplexing the output signal is one way to reduce the impact of cross talk between pixels, but in this case a parallel readout scheme was required to give the device high-speed performance.

The group's solution to managing current flow across the detector was to attach so-called "quenching" circuitry to each pixel. Dubbed iAQC, the technology temporarily lowers the bias voltage across an activated pixel within just a few nanoseconds of photon impact.

Following successful trials in the lab, the researchers now plan to assemble their imaging system at the ESO facility in Chile. The Italian team, which also includes scientists from Università di Pisa and research agency CNR-IMM Bologna, will characterize its detector in both adaptive optics and fast transient imaging applications.

Meanwhile, Zappa and his colleague, Sergio Cova, are already busy commercializing the technology and have formed a University spin-off company dubbed Micro Photon Devices to market a range of single-photon counting modules.

"[Although] we are working with silicon SPADs, we are also developing InGaAs/InP devices to extend the sensitivity into the near infrared at 1.3 µm and 1.55 µm wavelengths," added Zappa.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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