The LinoSPAD2 camera combines a 512×1 linear single-photon avalanche diode (SPAD) array with an FPGA-based photon-counting and time-stamping platform, to create a reconfigurable sensing system capable of detecting single photons. The read-out is fully parallel, where each SPAD is connected to a different FPGA input. The hardware can be reconfigured to achieve different functionalities, such as photon counters, time-to-digital converter (TDC) arrays and histogramming units. Time stamping is performed by an array of 64 TDCs, with 20 ps resolution (LSB), serving 256 channels by means of 4:1 sharing. At sensor level, the pixel pitch is 26.2 μm with a fill factor of 25.1%. The median dark count rate of each SPAD at room temperature is below 100 cps at 6V excess bias, the single-photon timing resolution (SPTR) of each channel is 50 ps FWHM, and the peak photon detection probability reaches ~50% at 510 nm at the same excess bias. The fill factor can be increased by 2.3× by means of microlenses, with good spatial uniformity and flat spectral response above 400 nm. At system level, the average instrument response function (IRF) is 135 ps FWHM. The LinoSPAD2 camera enables a wide range of time-of-flight and time-resolved applications, including 3D imaging, fluorescence lifetime imaging microscopy (FLIM), heralded spectroscopy, and compressive Raman imaging, to name a few. Thanks to its features, LinoSPAD2 is a novel generation of reconfigurable single-photon image sensors capable of adapting their read-out and processing to match application-specific requirements, and combining SPAD arrays with advanced, massively-parallel computational functionalities.

Single-photon avalanche diode (SPAD) arrays can be used for single-molecule localization microscopy (SMLM) because of their high frame rate and lack of readout noise. SPAD arrays have a binary frame output, which means photon arrivals should be described as a binomial process rather than a Poissonian process. Consequentially, the theoretical minimum uncertainty of the localizations is not accurately predicted by the Poissonian Cramér-Rao lower bound (CRLB). Here, we derive a binomial CRLB and benchmark it using simulated and experimental data. We show that if the expected photon count is larger than one for all pixels within one standard deviation of a Gaussian point spread function, the binomial CRLB gives a 46% higher theoretical uncertainty than the Poissonian CRLB. For typical SMLM photon fluxes, where no saturation occurs, the binomial CRLB predicts the same uncertainty as the Poissonian CRLB. Therefore, the binomial CRLB can be used to predict and benchmark localization uncertainty for SMLM with SPAD arrays for all practical emitter intensities.

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.

We present the world's first backside-illuminated (BSI) single-photon avalanche diode (SPAD) based on standard silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology. This SPAD achieves a good dark count rate (DCR) after backside etching, comparable to DCRs of BSI SPADs fabricated on bulk wafers. Unlike bulk-wafer-based BSI SPADs, which typically suffer from poor violet and blue sensitivity, the proposed BSI SPAD features increased near-ultraviolet sensitivity as well as significant sensitivity in the violet and blue spectral ranges, thanks to the ultrathin-body SOI. To the best of our knowledge, this is the best result ever reported for any BSI SPAD in the standard CMOS technology. In addition, it also shows high sensitivity at long wavelengths thanks to the interface between silicon and silicon-dioxide layers. Therefore, it achieves a photon detection probability over 26% at 500 nm and 10% in the 400-875 nm wavelength range at 3 V excess bias voltage. The timing jitter is 119 ps full width at half maximum at the same operation condition at 637 nm wavelength. For the proposed BSI SPAD, the buried oxide layer in SOI wafers is used as an etching stop during the wafer backside-etching process, and therefore it ensures the excellent performance uniformity in large arrays.

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.

Silicon Photomultipliers (SiPMs), which emerged as all solid state, MRI compatible alternative to PMTs, can provide high miniaturization and increased timing performance. Large effort has been spent in improving the figures of merit of such devices (e.g. jitter, timing resolution, sensitivity, noise, etc.). In this paper we propose a novel approach relying on the combination of photonic crystals with microlens arrays, integrated on top of each SPAD cell in the SiPM, to collimate and focus the light generated by a scintillator, in order to improve the overall timing performance of the system so as to approach the 10ps target, while at the same time reducing the noise floor (Dark Count Rate).

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.

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.