One of the major goals of life science research is to investigate elementary processes in small biological samples, even down to the single molecule level.

Analyzing such small samples requires very sensitive techniques, and the most widely used are those based on fluorescence spectroscopy. But, in order to achieve the best possible performances, the light detectors must possess single photon sensitivities. Besides, the growing request for this kind of data is driving the technology to concurrent parallel analysis.

State-of-the art detectors, such as PMTs and CCDs, have drawbacks that limit their employability in the conditions imposed by the techniques, namely parallel detection of fast-changing signals in low light levels.

A different kind of detector is available, though. SPADs are compact, solid state single photon detectors the offer the advantages of both PMTs and CCDs: being solid state devices, they are easier to combine in compact parallel systems then PMTs, offer higher photon detection efficiencies and require less power to operate. On the other hand, SPADs present a faster response time than CCDs, so are more suitable when fast signal are to be detected, for example in FCS (Fluorescence Correlation Spectroscopy) measurements.

To meet all the challenges posed by the measurement techniques, we designed a complete parallel photon counting module that provides 8 independent channels and takes advantage of the characteristics of SPAD detectors.

The full module is very small, measuring just 8 x 5 x 4 cm, and requires a single, low voltage power supply to operate. It employs a linear array of 8 SPAD detectors.

Each SPAD has a 50-µm diameter active area, and the elements of the array are separated by 250 µm. The detectors are operated at 5 V excess bias and are cooled at a moderate -10 °C, thus achieving remarkable performances in term of photon detection efficiency, DCR, afterpulsing and optical crosstalk probability.

Each SPAD is connected to an integrated Active Quenching Circuit (AQC) for its proper operation. This way, the dead time is set to 100 ns, thus enabling a maximum counting rate of 10 million counts per second.

The large pixel area is particularly advantageous in FCS measurements relying on confocal detection. Indeed, each SPAD can directly be used as a confocal pinhole, since the spacing to radius ratio is high enough to guarantee good volume confinement in the axial direction. Thanks to this particular feature, no complex refocusing optics is needed to recollect the light passing through the pinhole array, thus further increasing the miniaturization of the whole apparatus.

Publications

• Colyer, R. A., Scalia, G., Kim, T., Rech, I., Resnati, D., Marangoni, S., et al. :
  "High-throughput multispot single-molecule spectroscopy."
  Paper presented at SPIE Photonics West, 2010, BIOS, Single Molecule Spectroscopy and Imaging III, San Francisco, California, USA.
  2010, January 23—28
• Rech, I., Marangoni, S., Resnati, D., Ghioni, M., Cova, S.:
  "Multipixel single-photon avalanche diode array for parallel photon counting applications."
  Journal of Modern Optics, 56(2)
  2009
  
• Rech, I., Resnati, D., Marangoni, S., Ghioni, M., Cova, S.:
  "Compact-eight channel photon counting module with monolithic array detector."
  Paper presented at SPIE Optics East 2007 - Advanced Photon Counting Techniques II, Boston.
  2007, September 9—12

Contacts

Massimo Ghioni - ghioni@elet.polimi.it
Ivan Rech - rech@elet.polimi.it
Angelo Gulinatti - gulinatt@elet.polimi.it