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CMOS counts single photons

13 Apr 2007

Researchers in Italy unveil what they believe to be the first CMOS-based single photon counter.

Researchers in Italy claim to have developed the first ultra-compact CMOS single photon counter. The team says that the device could have applications in adaptive optics and micro-array based biological analysis. (Optics Express 15 2873)

"We have squeezed a complete photon counting system into a chip measuring just 2x1.1 mm, which can be integrated with every kind of optics and mechanics very easily," Franco Zappa of Politecnico di Milano told optics.org.

Traditionally, single photon avalanche diodes (SPADs) are fabricated using dedicated processes that require additional technological steps. CMOS technology could not guarantee satisfactory performances, in particular concerning noise (dark-counting rate and after-pulsing effect). This fact limited the possibility of size reduction for a single-photon counting system, since different microelectronic chips have to be assembled together.

Now, Zappa and colleagues present SPADs that attain good performances, by using a standard high-voltage CMOS process. The complete chip comprises the SPAD detector, integrated active quenching circuitry (iAQC), counting and communication electronics.

"This is a major improvement over previously published works in which only a part of the photon counting setup was integrated, usually with poor detector performance and diameter" claimed Zappa. "The use of an integrated iAQC helps in mitigating after-pulsing effect, which in previous attempts was the limiting factor for on-field use of CMOS SPADs, due to the use of a simple passive quenching approach."

According to Zappa, the on-chip SPAD has a detection efficiency higher than 30% in the visible wavelength range and peaks at about 45% at 450 nm, timing performance better than 37 ps FWHM and a dark count rate of around a few kcounts/s

The team is currently pursuing two goals, firstly the development of a user friendly USB-enabled photon-counting module which can be plugged into most applications. Secondly, a new multi-pixel chip that integrates four single-photon counting channels and all related electronics. "This will focus on the development of the Shack Hartman quench-cells for tip-tilt compensation in adaptive optics, already employed in many European Southern Observatory telescopes," concluded Zappa.

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