Increasing the collection efficiency of time-correlated single-photon counting with single-photon avalanche diodes using immersion lenses

Single-photon avalanche diodes (SPADs) achieving high timing resolution (approximate to 20-50 ps) developed for timecorrelated single-photon counting (TCSPC) generally have very small photosensitive areas (25-100 mu m in diameter). This limits the achievable photon counting rate and signal-to-noise ratio and may lead to long counting times. This is detrimental in applications requiring several measurements, such as fluorescence lifetime imaging (FLIM) microscopy, which requires scanning, and time-domain diffuse optical tomography (TD-DOT). We show in this work that the use of an immersion lens directly affixed onto the photosensitive area of the SPAD helps alleviate this problem by allowing more light to be concentrated onto the detector. Following careful optical design and simulations, our experimental results show that it is actually possible to achieve the predicted theoretical increase in the photon counting rate (we achieve a factor of approximate to 4 here). This work is of high relevance in high timing resolution TCSPC with small photosensitive area detectors and should find widespread interest in FLIM and TD-DOT with SPADs. (C) 2016 Optical Society of America