Fluorescence microscopy and derived techniques are continuously looking for photodetectors able to guarantee increased sensitivity, high spatial and temporal resolution and ease of integration into modern microscopy architectures. Recent advances in single-photon avalanche diodes (SPADs) fabricated with industry-standard microelectronic processes allow the development of new detection systems tailored to address the requirements of advanced imaging techniques (such as image-scanning microscopy). To this aim, we present the complete design and characterization of two bidimensional SPAD arrays composed of 25 fully independent and asynchronously-operated pixels, both having fill-factor of about 50% and specifically designed for being integrated into existing laser scanning microscopes. We used two different microelectronics technologies to fabricate our detectors: the first technology exhibiting very low noise (roughly 200 dark counts per second at room temperature), and the second one showing enhanced detection efficiency (more than 60% at a wavelength of 500 nm). Starting from the silicon-level device structures and moving towards the in-pixel and readout electronics description, we present performance assessments and comparisons between the two detectors. Images of a biological sample acquired after their integration into our custom imagescanning microscope finally demonstrate their exquisite on-field performance in terms of spatial resolution and contrast enhancement. We envisage that this work can trigger the development of a new class of SPAD-based detector arrays able to substitute the typical singleelement sensor used in fluorescence laser scanning microscopy.

We present the design and simulations of a singlephoton sensitive imager based on Single Photon Avalanche Diodes (SPADs) with an innovative pixel architecture that includes 4 separate SPADs with independent active time-gating and quenching circuit, a shared Time-to-Digital Converter (TDC) with 50 ps resolution, 4 independent photon counters, and multiple operation modes. The TDC is driven by smart arbitration logic, which preserves spatial information among the 4 detectors; furthermore, an alternative operation mode exploits photoncoincidence on multiple detectors to reduce the effect of high background levels, e.g. in LIDAR applications with strong ambient light.Key features are the ability to operate in simultaneous photon counting and timing modes for capturing 2D and 3D images of the scene in a single shot (frame), the option of a counting-only mode, reducing power consumption and increasing achievable framerate when timing information is not needed, and the ability to individually shut down noisy detectors or to enable just some regions of interests.

Traditional paradigms for imaging rely on the use of a spatial structure, either in the detector (pixels arrays) or in the illumination (patterned light). Removal of the spatial structure in the detector or illumination, i.e., imaging with just a single-point sensor, would require solving a very strongly ill-posed inverse retrieval problem that to date has not been solved. Here, we demonstrate a data-driven approach in which full 3D information is obtained with just a single-point, single-photon avalanche diode that records the arrival time of photons reflected from a scene that is illuminated with short pulses of light. Imaging with single-point time-of-flight (temporal) data opens new routes in terms of speed, size, and functionality. As an example, we show how the training based on an optical time-of-flight camera enables a compact radio-frequency impulse radio detection and ranging transceiver to provide 3D images. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Real time, high resolution 3D reconstruction of scenes hidden from the direct field of view is a challenging field of research with applications in real-life situations related e.g. to surveillance, self-driving cars and rescue missions. Most current techniques recover the 3D structure of a non-lineof-sight (NLOS) static scene by detecting the return signal from the hidden object on a scattering observation area. Here, we demonstrate the full colour retrieval of the 3D shape of a hidden scene by coupling back-projection imaging algorithms with the high-resolution time-of-flight information provided by a single-pixel camera. By using a high e ciency Single-Photon Avalanche Diode (SPAD) detector, this technique provides the advantage of imaging with no mechanical scanning parts, with acquisition times down to sub-seconds.

We present the design and characterization of a large-area, fast-gated, all-digital single-photon detector with programmable active area, internal gate generator, and time-todigital converter (TDC) with a built-in histogram builder circuit, suitable for performing high-sensitivity time-domain nearinfrared spectroscopy (TD-NIRS) measurements when coupled with pulsed laser sources. We used a novel low-power differential sensing technique that optimizes area occupation. The photodetector is a time-gated digital silicon photomultiplier (dSiPM) with an 8.6-mm 2 photosensitive area, 37% fill-factor, and ∼300 ps (20%-80%) gate rising edge, based on low-noise single-photon avalanche diodes (SPADs) and fabricated in 0.35-µm CMOS technology. The built-in TDC with a histogram builder has a least-significant-bit (LSB) of 78 ps and 128 time-bins, and the integrated circuit can be interfaced directly with a low-cost microcontroller with a serial interface for programming and readout. Experimental characterization demonstrated a temporal response as good as 300-ps full-width at half-maximum (FWHM) and a dynamic range >100 dB (thanks to the programmable active area size). This microelectronic detector paves the way for a miniaturized, stand-alone, multi-wavelength TD-NIRS system with an unprecedented level of integration and responsivity, suitable for portable and wearable systems. Index Terms-Digital silicon photomultiplier (dSiPM), fastgated single-photon avalanche diode (SPAD) array, photon counting, time-to-digital converter (TDC), time-domain near-infrared spectroscopy (TD-NIRS). I. INTRODUCTION T IME-DOMAIN near-infrared spectroscopy (TD-NIRS) is a powerful technique for obtaining non-invasive, in vivo measurements of tissue constituents and structure [1]. This can be exploited in many scientific fields, from a clinical Manuscript