Multipixel single-photon avalanche diode array for parallel photon counting applications

In life science, optical techniques for the characterization of biological processes are well established and widely used. In most of them, to obtain the best performances, detectors with single-photon detection capability are required. Moreover, the growing demand for this type of information is pushing the technology towards a parallelization of the analysis. These requirements make it very challenging to develop new detection heads, because state-of-the-art detectors, such as photomultiplier tubes (PMTs) and charged coupled devices (CCDs), have some drawbacks. For example, in fluorescence correlation spectroscopy (FCS) fluorescence fluctuations must be monitored on a short time scale because typical time constants range from hundreds of nanoseconds to milliseconds. In this case, developing parallel modules for this application is very challenging. In fact, PMTs are bulky and they cannot be integrated, while the use of imaging detectors, such as CCDs and electron multiplying CCDs, is strongly limited by the read time that determines the minimum correlation time detectable by FCS analysis. We present here a multichannel photon counting module that exploits a monolithic array of single-photon avalanche diodes (SPADs). The detector array consists of eight 50 m diameter SPADs featuring low dark counting rate and high photon detection efficiency (50% at 550 nm); inter-pixel crosstalk probability is as low as 2 10-3. The use of highly integrated active quenching circuits makes it possible to design a very compact read-out circuit, yet providing eight fully independent counting channels operating at high count rate (up to 30 million counts per second). Parallel-mode operation, high count rate and low-cost make this module a potential breakthrough for a widespread diffusion of FCS analysis.