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. This fascinating technology has progressed at a very fast pace in the past 15 years, since its inception in standard CMOS technology in 2003. 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. 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. 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.

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A 252 × 144 single-photon avalanche diode (SPAD) pixel sensor, called Ocelot, is reported for light detection and ranging (LiDAR). The sensor, fabricated in the 180-nm CMOS technology, features 1728 12-bit time-to-digital converters (TDCs) with 48.8-ps resolution (LSB). Each 126 pixels in a half-column are connected to six TDCs through a collision detection bus, which enables effective sharing of resources, and consequently a fill factor of 28% with a pixel pitch of 28.5 μ text{m}. The column-parallel TDCs, based on dual-clock architecture, exhibit a DNL of +0.48/-0.48 LSB and an INL of +0.89/-1.67 LSB; they are dynamically reallocated in a scalable daisy chain approach that enables a maximum of five photon detections per illumination cycle per half-column. The sensor can operate in time-correlated single-photon counting (TCSPC) and single-photon counting (SPC) modes, while peak detection (PD) and partial histogramming (PH) are included in the operation of the sensor. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via 3.32-Mb SRAM-based PH readout (PHR) scheme providing a 14.9-to-1 compression. Telemetry measurements up to 50 m achieve an accuracy of 8.8 cm and worst-case precision of 1.4 mm ( σ ). A flash LiDAR using direct time of flight (dTOF) and based on Ocelot is demonstrated, achieving depth imaging at short distances with a frame rate of 30 frames/s, employing an ultra-low power laser with an average power of 2 mW and peak power of 0.5 W.

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Per-pixel time-to-digital converter (TDC) architectures have been exploited by single-photon avalanche diode (SPAD) sensors to achieve high photon throughput, but at the expense of fill factor, pixel pitch and readout efficiency. In contrast, TDC sharing architecture usually features high fill factor at small pixel pitch and energy efficient event-driven readout. While the photon throughput is not necessarily lower than that of per-pixel TDC architectures, since the throughput is not only decided by the TDC number but also the readout bandwidth. In this paper, a SPAD sensor with 32 × 32 pixels fabricated with a 180 nm CMOS image sensor technology is presented, where dynamically reallocating TDCs were implemented to achieve the same photon throughput as that of per-pixel TDCs. Each 4 TDCs are shared by 32 pixels via a collision detection bus, which enables a fill factor of 28% with a pixel pitch of 28.5 μm. The TDCs were characterized, obtaining the peak-to-peak differential and integral non-linearity of -0.07/+0.08 LSB and -0.38/+0.75 LSB, respectively. The sensor was demonstrated in a scanning light-detection-and-ranging (LiDAR) system equipped with an ultra-low power laser, achieving depth imaging up to 10 m at 6 frames/s with a resolution of 64 × 64 with 50 lux background light.

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SPAD (single-photon avalanche diode) arrays are single-photon sensors, which enable photon counting and unparalleled time-resolved imaging. In this paper, we will detail the architecture and characteristics of three representative SPAD arrays, implemented in standard CMOS technologies and targeted to a range of applications such as biophotonics, basic sciences, and engineering. Two sensors feature a high-performance pixel and enable time-gated and TCSPC operation, respectively. The third sensor is a line array whose pixels are coupled on a one-To-one basis with an FPGA for flexible data acquisition and processing.

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A 252 × 144 single-photon avalanche diode (SPAD) pixel FLASH LiDAR is implemented in 180nm CMOS with 28.5μm pixel pitch and 28% fill factor. The sensor includes a collision detection bus with dynamic reallocation of 48.8 ps dual-clock time-to-digital converters (TDCs). It can operate in time-correlated single-photon counting (TCSPC), single-photon counting (SPC), peak-detection (PD) and partial-histogramming (PH) modes. The PD and PH modes are enabled by the first implementation of integrated histogramming for a full array via an SRAM based partial histogramming readout (PHR) scheme. This provides 16 5-bit bins for each pixel to enable a 14.9-to-l compression ratio.

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Confocal microscopes use photomultiplier tubes and hybrid detectors due to their large dynamic range, which typically exceeds the one of single-photon avalanche diodes (SPADs). The latter, due to their photon counting operation, are usually limited to an output count rate to 1/T_dead. In this paper, we present a thorough analysis, which can actually be applied to any photon counting detector, on how to extend the SPAD dynamic range by exploiting the nonlinear photon response at high count rates and for different recharge mechanisms. We applied passive, active event-driven and clock-driven (i.e. clocked, following quanta image sensor response) recharge directly to the SPADs. The photon response, photon count standard deviation, signal-to-noise ratio and dynamic range were measured and compared to models. Measurements were performed with a CMOS SPAD array targeted for image scanning microscopy, featuring best-in-class 11 V excess bias, 55% peak photon detection probability at 520 nm and >40% from 440 to 640 nm. The array features an extremely low median dark count rate below 0.05 cps/μm² at 9 V of excess bias and 0°C. We show that active event-driven recharge provides ×75 dynamic range extension and offers novel ways for high dynamic range imaging. When compared to the clock-driven recharge and the quanta image sensor approach, the dynamic range is extended by a factor of ×12.7-26.4. Additionally, for the first time, we evaluate the influence of clock-driven recharge on the SPAD afterpulsing.

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The aim of this research is to explore the potential advantages of SPAD imagers used in microscopy. An ideal microscopy detector requires high sensitivity (high quantum efficiency QE or photon detection probability PDP), photon counting operation, low noise (dark current or dark count rate), timing resolution in the order of 100 ps, frame rate higher than 10 fps, a large enough pixel resolution and wavelength resolvability. @en

sCMOS imagers are currently utilized (replacing EMCCD imagers) to increase the acquisition speed in super resolution localization microscopy. Single-photon avalanche diode (SPAD) imagers feature frame rates per bit depth comparable to or higher than sCMOS imagers, while generating microsecond 1-bit-frames without readout noise, thus paving the way to in-depth time-resolved image analysis. High timing resolution can also be exploited to explore fluorescent dye blinking and other photophysical properties, which can be used for dye optimization. We present the methodology for the blinking analysis of fluorescent dyes on experimental data. Furthermore, the recent use of microlenses has enabled a substantial increase of SPAD imager overall sensitivity (12-fold in our case), reaching satisfactory values for sensitivity-critical applications. This has allowed us to record the first super resolution localization microscopy results obtained with a SPAD imager, with a localization uncertainty of 20 nm and a resolution of 80 nm.

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Summary form only given. Single photon detectors allow us to work with the weakest signals such as auto-fluorescent biological sources. In combination with time gated operation mode, an array of detectors can be used as Fluorescence Lifetime Imaging system with extremely high sensitivity.Here we present fluorescence lifetime imaging using the `SwissSPAD' [1] sensor, an extensive 512-by-128pixel array of time-gated single photon avalanche diodes (SPADs). By taking a series of gated measurements and changing the gate delay, fluorescent lifetimes can be retrieved for each individual pixels. Over 65 thousand independent SPAD pixels creates a detailed spatial map of fluorescence lifetimes in the field of view of the sensor. In this work, we are presenting two milestones of our research. Firstly, we present proof of principle imaging lifetime measurements of Rhodamine B and Fluorescein, with lifetimes of 1.68ns and 4ns ns respectively, excited using a 532 nm 10 picosecond laser. Secondly, we show that using 266 nm excitation, we can distinguish between free(0.5ns) and bound(2.2ns) NADH using unlabelled auto-fluorescence. Freeand bound-NADH are known to be markers for the cancer tissues [2].@en

The paper presents a camera comprising 512 × 128 pixels capable of single-photon detection and gating with a maximum frame rate of 156 kfps. The photon capture is performed through a gated single-photon avalanche diode that generates a digital pulse upon photon detection and through a digital one-bit counter. Gray levels are obtained through multiple counting and accumulation, while time-resolved imaging is achieved through a 4-ns gating window controlled with subnanosecond accuracy by a field-programmable gate array. The sensor, which is equipped with microlenses to enhance its effective fill factor, was electro-optically characterized in terms of sensitivity and uniformity. Several examples of capture of fast events are shown to demonstrate the suitability of the approach. @en

For many scientific applications, electron multiplying charge coupled devices (EMCCDs) have been the sensor of choice because of their high quantum efficiency and built-in electron amplification. Lately, many researchers introduced scientific complementary metal-oxide semiconductor (sCMOS) imagers in their instrumentation, so as to take advantage of faster readout and the absence of excess noise. Alternatively, single-photon avalanche diode (SPAD) imagers can provide even faster frame rates and zero readout noise. SwissSPAD is a 1-bit 512×128 SPAD imager, one of the largest of its kind, featuring a frame duration of 6.4 μs. Additionally, a gating mechanism enables photosensitive windows as short as 5 ns with a skew better than 150 ps across the entire array. The SwissSPAD photon detection efficiency (PDE) uniformity is very high, thanks on one side to a photon-to-digital conversion and on the other to a reduced fraction of "hot pixels" or "screamers", which would pollute the image with noise. A low native fill factor was recovered to a large extent using a microlens array, leading to a maximum PDE increase of 12×. This enabled us to detect single fluorophores, as required by ground state depletion followed by individual molecule return imaging microscopy (GSDIM). We show the first super resolution results obtained with a SPAD imager, with an estimated localization uncertainty of 30 nm and resolution of 100 nm. The high time resolution of 6.4 μs can be utilized to explore the dye's photophysics or for dye optimization. We also present the methodology for the blinking analysis on experimental data.@en

While CMOS single-photon avalanche diode (SPAD) technology has steadily advanced, improving noise, timing resolution, and sensitivity, spatial resolution has been increasing as well. The increase in the number of pixels has made a comprehensive analysis of nonuniformity and its effects meaningful, allowing a more accurate comparison of SPAD imagers with other high-end scientific imagers, such as electron multiplying charge-coupled device and scientific CMOS. A comprehensive nonuniformity analysis was conducted on a 512 × 128 pixel gated SPAD imager, where dark noise, afterpulsing, crosstalk, signal response, and shot noise were measured. This analysis has led to a variety of postprocessing algorithms to improve the linearity of the response as for example required by ground state depletion microscopy-based superresolution microscopy and other techniques. We derived a new correction formula for the count rate applicable to 1-b SPAD imagers, and we measured a significant improvement of photon detection efficiency using microlenses. These techniques were used to validate the suitability of the imager in fluorescence microscopy examples.@en