Fast-gated single-photon detectors boost dynamic range in NIR spectroscopy

We present a novel instrumentation for wide dynamic range optical investigations based on a time-gated silicon Single- Photon Avalanche Diode (SPAD) in a Time-Correlated Single-Photon Counting (TCSPC) setup. The detector is gatedon and off in less than 200 ps and kept-on for detecting photons only within short time slots. Such technique is particularly useful in applications where a large amount of unnecessary photons precedes or follows the optical signal to detect, such as in time-resolved NIR spectroscopy, optical mammography, and optical molecular imaging. In particular, in time-resolved reflectance spectroscopy, when the source-detector separation is decreased, the detection electronics easily saturates, due to the huge amount of "early" photons, diffused by superficial layers. Instead, our setup is able to reject those photons and to detect only "late" photons, thus allowing to increase the injected power and to drastically widen the investigation dynamic range, while remarkably speeding up the acquisition. We acquired Instrument Response Functions (IRFs) at multiple wavelengths between 600 nm and 1000 nm and we achieved up to 108 dynamic range in a very short measurement time (few minutes). Moreover, we tested the instrumentation with SPADs of different active areas and we compared the performances.