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Photodiode anode cathode9/18/2023 ![]() ![]() Therefore, it is not surprising that recent efforts have been focused, at the device level, on increasing the SPADs’ key figures of merit 7 and improving the contact with foundries, to fully profit from possible process optimisations. On the technological side, the design of high performance, low-noise SPADs is challenging the same is true at the system level for data handling, leading to important firmware development efforts. The pixel sizes are typically larger, limiting so far the manufacturing of megapixel arrays. This is at first glance surprising, given the aforementioned potential for unrivalled photon counting and time-resolved performance however, it can be partly traced back to some performance parameters that still lag behind those of established CCDs and sCMOS imagers, such as the quantum efficiency over the whole spectrum and the fill factor, which are of importance for several light-starved applications. However, it is true that SPAD imagers are still mostly used in specialised research settings, apart from some notable non-imaging exceptions, such as SPAD arrays in the form of silicon photomultipliers (SiPMs), which are readily available from a number of manufacturers. In particular, we will discuss (endoscopic) fluorescence lifetime imaging (FLIM), (multibeam multiphoton) FLIM-FRET (Förster resonance energy transfer), single-plane illumination fluorescence correlation spectroscopy (SPIM-FCS), localisation- and entangled photons-based super-resolution microscopy (SRM), time-resolved Raman spectroscopy, near-infra-red optical tomography (NIROT) and positron emission tomography (PET). Furthermore, basically all implementations rely on FPGA-based host boards combined with the natively digital SPAD data output, this opens the door to real-time algorithmic implementations in close proximity to the sensor, such as FPGA-based autocorrelation and lifetime calculations.Īs SPAD technology matured, a range of applications have been explored in very diverse fields, such as consumer and robotics imaging, data and telecom security, advanced driver-assistance systems and biophotonics, which is the main subject of this review. Modular setups have also been designed, either through the combination of SPAD arrays with FPGAs (“reconfigurable pixels”), or by means of very recent 3D developments. This was soon followed by the first integrated SPAD array 6 and a host of architectures, ranging from simpler implementations of the early days, based solely on off-chip data processing, to progressively “smarter” sensors including on-chip, or even pixel level, time-stamping and processing capabilities. The breakthrough implementation of the first SPADs in standard complementary-metal-oxide semiconductor (CMOS) technology 5 triggered the exploration and design of large digital SPAD imagers, potentially manufactured in volume at affordable prices. Individual single-photon avalanche diodes (SPADs) have long been the detector of choice when deep sub-nanosecond timing performance is required, due to their excellent single-photon detection and time-stamping capability 1, 2, 3, 4. Finally, we will provide an outlook on the future of this fascinating technology. We will review some representative sensors and their corresponding applications, including the most relevant challenges faced by chip designers and end-users. ![]() As the technology has matured, a range of biophotonics applications have been explored, including (endoscopic) FLIM, (multibeam multiphoton) FLIM-FRET, SPIM-FCS, super-resolution microscopy, time-resolved Raman spectroscopy, NIROT and PET. A host of architectures have been investigated, ranging from simpler implementations, based solely on off-chip data processing, to progressively “smarter” sensors including on-chip, or even pixel level, time-stamping and processing capabilities. This fascinating technology has progressed at a very fast pace in the past 15 years, since its inception in standard CMOS technology in 2003. Single-photon avalanche diode (SPAD) arrays are solid-state detectors that offer imaging capabilities at the level of individual photons, with unparalleled photon counting and time-resolved performance. ![]()
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