Single-photon SPAD imagers in biophotonics: Review and Outlook

Single-photon avalanche diode (SPAD) arrays are solid-state detectors offering imaging capabilities at the level of individual photons, with unparalleled photon counting and time-resolved performance. This fascinating technology has progressed at very high pace in the past 15~years, since its inception in standard CMOS technology in 2003. A host of architectures has been explored, ranging from simpler implementations, based solely on off-chip data processing, to progressively ``smarter" sensors including on-chip, or even pixel-level, timestamping and processing capabilities. As the technology matured, a range of biophotonics applications has been explored, including (endoscopic) FLIM, (multi-beam multiphoton) FLIM-FRET, SPIM-FCS, super-resolution microscopy, time-resolved Raman, NIROT, and PET. We will review some representative sensors and their corresponding applications, including the most relevant challenges faced by chip designers and end-users. Finally, we will provide an outlook on the future of this fascinating technology.