This PDF file contains the front matter associated with SPIE Proceedings Volume 11386, including the Title Page, Copyright information, and Table of Contents.

Silicon photonics is poised to bring photonic integrated circuit (PIC) technologies to a range of solutions in imaging and sensing, providing the advantages of increased power efficiency, size reductions, and lower-cost packaging. Compared to conventional silicon photonics architectures, our platform uses larger, multi-micron waveguides, and holds the key to optimizing PIC performance, power efficiency, manufacturability and versatility. Our technology brings performance advantages such as low propagation losses, efficient integration of III-V actives, and high power handling, and we have demonstrated photonic integrated circuits with high levels of integration with a full suite of devices including hybrid lasers, compact arrayed waveguide grating filters, high-speed modulators and detectors and efficient out-couplers. We will discuss applications ranging from consumer devices to healthcare to automotive.

LIDAR (Light Detection and Ranging) is emerging as a necessity for fully automated self-driving automotive applications. In order to sample the far field with sufficient resolution for this application the system must incorporate many optical elements, leading to challenges for manufacturability and size. Due to the density of optical components required, LIDAR is well suited for photonic integration in order to achieve miniaturization and scalable manufacturability. This talk will give an overview of LIDAR, the components required for a chip-scale solution, and silicon photonics progress with respect to this goal.

Recently, there has been a growing research interest towards multichannel Time Correlated Single Photon Counting systems. To break the tradeoff between number of channels and performance that currently affects available systems, we have proposed an approach based on resource sharing and on the exploitation of different technologies. Here, we present a new standalone high-performance Time-to-Amplitude Converter featuring a timing jitter of 4.3ps rms and a DNL lower than 1% rms. The area occupation is 0.2mm2 , the power dissipation 70mW and the conversion rate 13.7MHz, making this circuit suitable to be the building block of a densely integrated, high-performance system.

Time-to-digital converters (TDCs) and time correlated single photon counters (TCSPC) are instruments commonly used in LiDAR systems, quantum optics experiments and many other applications. This work presents a new time-to-digital converter architecture to improve dead time, converter linearity and precision. The priority encoder is a large combinatorial logic circuit and is often the bottleneck in field programmable gate array (FPGA) TDC designs, as the conversion must complete within the TDC’s clock period. This work utilizes a new dual clock domain architecture which has allowed for the TDC clock rate to increase by 38.1% from previous work and potentially double for more modern FPGA devices. This reduces the required delay line length and allows for more precise and linear converters as both integral non-linearity and measurement uncertainty scale according to the square root of the number of delay elements used in the delay line. Single shot precision has improved by 12.9% and converter differential non-linearity and integral non-linearity has reduced by 1.27 and 1.57 least significant bits respectively. This work demonstrates a significant improvement to the performance of FPGA based TDCs at the expense of using slightly more block random access memory.

Fast and efficient single-photon detectors are of paramount importance for a broad range of applications, e.g. in optical quantum information technologies. A variety of novel and refined photon counting devices has resulted from the need of ever better performance. One such device is the resonant tunneling diode (RTD) single-photon detector. Compared to conventional single-photon detectors, RTDs provide several advantageous characteristics. Since in RTDs a tunneling current is gated by photogenerated charge carriers, they are inherently photon-number resolving, and the RTD currentvoltage characteristic’s negative differential conductance region allows for higher functionality, e.g. the operation as an optically triggered bistable switch. Here, we present an overview on RTD single-photon detectors with a focus on the stateof- the-art, its underlying physical mechanisms as well the device limitations, and recent developments. We compare different RTD device geometries and operation schemes for enhanced detection efficiencies and operation frequencies.

Some III-V digital alloy avalanche photodiodes demonstrate very low excess noise making them suitable for single photon detection applications. This behavior is attributed to the presence of minigaps in the valence band and high hole effective mass which reduce hole impact ionization. In this work, we present a physics based SPICE compatible compact model for these low noise avalanche photodiodes built from parameters extracted from Environment-Dependent Tight Binding model, that is calibrated to ab-initio Density Functional Theory, and Monte Carlo methods. Using this approach, we can accurately capture the physical characteristics of APDs in integrated photonics circuit simulation.

Single-photon detection impacts a broad range of applications ranging from the secure transfer of information for banking and communication, to quantum radar for national defense, assistance in search and rescue missions, to biomedical applications such as in dose monitoring for cancer treatment as well as in non-invasive imaging of the eye, to diagnose potentially blinding diseases. In these applications, high efficiency detection at the single-photon level with precise timing resolution is required. In this talk I will present a new type of quantum sensor based on a semiconductor nanowire array that detects light over an unprecedented wavelength range, from the UV to near-infrared with high speed and timing resolution, all while operating at room temperature. I will illuminate how our nanostructure is uniquely shaped to achieve near-unity efficiency over the entire wavelength range, and show how we can extend the wavelength detection range to the infrared and beyond.

Time-correlated single-photon counting (TCSPC) has emerged as a key detection technology for lidar and depth profiling in a number of emerging application areas due to its high optical sensitivity and excellent surface to surface resolution. We have applied this technique to measure three-dimensional scenes of stationary and moving targets in several underwater environments. The presentation will show the results of laboratory-based experiments obtained using several different optical transceiver configurations. Particular attention will be given to underwater depth imaging using silicon single photon avalanche diode (Si-SPAD) arrays with in-pixel picosecond timing electronics.

Photon counting signals are often complex and can have multiple signature of different of types. The origin of these signatures may be, for example, objects that are not in the line-of-sight, light pulses that are traveling through the scene, and uncorrelated ambient light. Using computational imaging approaches, we are able to distinguish different signatures and we can reconstruct their origin. Non-line-of-sight imaging use back projection approaches to estimate the shape of hidden reflecting surfaces. Further, light-in-flight sensing is capable to track the path of light pulse moving in air and to trace back the laser pulse to its origin. Finally, photon flux imaging with ambient light illumination uses the temporal information of uncorrelated light to estimate the photon flux.

Single-photon detectors time-stamp incident photon events with picosecond accuracy. When combined with pulsed light sources, these emerging detectors record transient measurements of a scene containing the time of flight information of the direct light reflecting off of visible objects, and also the indirectly scattered light from objects outside the line of sight. The latter information has recently been demonstrated to enable non-line-of-sight (NLOS) imaging, where advanced inverse methods process time-resolved indirect light transport of a scene to estimate the 3D shape of objects hidden around corners. In this article, we review computationally efficient NLOS approaches that build on confocally scanned data, where the light pulses used to probe a scene are optically aligned with the detection path. This specific scanning procedure has given rise to computationally efficient inverse methods that enable real-time NLOS image reconstruction.

Following on the success of the NASA-CNES CALIPSO/CALIOP space-based cloud-aerosol lidar, which is now approaching its 14th year of continuous operation, NASA Langley Research Center has been advancing space-based lidar mission concepts and related technologies to address future cloud, aerosol and ocean science objectives identified in the 2017 Decadal Survey for Earth Science and Applications from Space. Recently, scientists at NASA Langley have been able to use data from CALIOP to retrieve global ocean biomass, which is a measurement that was never originally envisioned for this instrument, and represents a new scientific frontier for space-based lidar. CALIOP’s measurements complement ocean color records, extending ocean retrievals into nighttime and to high latitudes, and creating a more complete picture of global ocean biomass. Due to detector bandwidth limitations, however, CALIOP, as with ocean color, only obtains column-averaged measurements that are biased towards the surface. To address this limitation, Politecnico di Milano is developing a lidar detector based on a single photon avalanche diode (SPAD) array. This detector is expected to achieve a depth resolution of <1 m in the ocean, and have a detection efficiency and dynamic range that far exceeds that of the analog-readout, PMT-based receiver on CALIOP. Naturally, these same characteristics will also benefit cloud and aerosol retrievals, making SPAD arrays an excellent detector candidate for future space-based atmosphere and ocean lidar missions. Here, we describe potential advantages of fast (~THz) photon counting SPAD arrays, and how they are being developed for this application.

In this work we design, fabricate and characterize superconducting nanowire single photon detectors (SNSPDs) optimized for mid infrared operation. The mid infrared is of interest for free space applications due to lower solar background than at shorter wavelengths as well as low atmospheric absorption. We show a proof-of-principle LIDAR imaging experiment at 2.3µm showing the viability of using SNSPDs for a variety of applications in the mid infrared.

Ever since superconducting nanowire single-photon detectors (SNSPDs or SSPDs) were demonstrated, they have attracted a considerable attention due to the detectors’ promising performance that includes high detection efficiency (DE), low dark count rate (DCR), broadband sensitivity, and low timing jitter. The system detection efficiency (SDE) has been improved to be over 90% with a wavelength of 1550 nm in a fiber-coupled system by embedding the SNSPD in optical cavities. However, the implementation of optical cavity usually reduces the bandwidth of SNSPDs. As a result, there are some recent works on increasing the bandwidth of SNSPD. Furthermore, the size of the active area of meandered nanowires has to be compatible with the core size of an optical fiber such that good optical coupling can be achieved; this setup requires a long nanowire with a large kinetic inductance, thus resulting in a small counting rate and a large timing jitter, particularly for multi-mode fiber-coupled devices. Nevertheless, a particularly impressive waveguide-coupled SNSPD has been developed in recent years. By placing an SNSPD atop an optical waveguide that has been fabricated on various substrate materials, photons propagating in the waveguide have been found to be evanescently absorbed by nanowires that are only a few tens of microns long. The on-chip DE of this device has been estimated to be over 90%, thus indicating that it has both high absorptance and high intrinsic DE. If the waveguide and the nanowire are correctly dimensioned, the absorptance of this device can reach over 90% for a wavelength range of more than 700 nm. Furthermore, the length of the nanowire is shorter than that of the fiber-coupled SNSPD, thus leading to a higher count rate and lower timing jitter (~18 ps). However, unsatisfactory coupling losses between the optical fiber and the waveguide in such devices can decrease its SDE (~10%), thus limiting its applicability to quantum photonic integrated circuits (QPIC). Recently, we proposed a type of SNSPD that was coupled with microfibers (MFs), and simulations for this SNSPD indicated that its absorption would be as high as 90%. Microfibers and nanofibers, tapered adiabatically from standard fibers, with diameters close to or smaller than the wavelength of the guided light were demonstrated to have low transmittance losses by Tong et al. in 2003. MFs have been shown to have many interesting properties, such as strong evanescent fields, tight optical confinement, wideband abilities, and small masses; these properties are appealing for applications such as optical sensing and coupling. MFs also provide seamless connections with high transmittance from standard fibers; therefore, MF-coupled SNSPDs can achieve both high SDEs and wideband compatibility. We proposed and demonstrated the MF-coupled SNSPDs successfully. The simulation results indicate that with optimal device structure, the optical absorption with efficiency < 90% can be realized over a wavelength range of 350 nm to 2150 nm. The fabricated MF-coupled SNSPD, shows unparalleled broadband system detection efficiencies (SDEs) of more than 50% from 630 nm to 1500 nm. The SDEs reach 66% at 785 nm and 45% at 1550 nm. These results pave the way for ultra-broadband weak light detection with quantum-limit sensitivity.

Beyond its obvious use for radiation monitoring, low-energy (thermal) neutron detection finds applications in road paving, hydrocarbon prospecting, hydrology, agronomy and planetary exploration.All known techniques are based on amplification of exothermic neutron absorption reactions with particular isotopes, especially helium-3. We have invented compact, low-voltage/power detectors based on optical scintillation of noble gases induced by energetic fragments of neutron absorption by boron-10. The primary scintillations occur in the ultraviolet (UV) and far-UV spectral regions, and wavelength-shifting materials are used to transform them into light that can be detected by silicon photomultipliers (SiPMs). SiPMs' light footprint facilitates cellular networks of diverse compact directors for momentum and energy resolution of neutron fields. We have built a thumb-size submersible detector and used it for in-situ neutron dosimetry in a water phantom at a proton therapy facility.

Time-resolved multispectral imaging has recently found many applications ranging from biomedical to environmental field. Multidimensional approach measuring spectral and ultrafast temporal dynamics of fluorescence signal combined with spatial information (imaging) allows one to characterize biological processes at both microscopic and macroscopic level, representing a fundamental step towards development of diagnostic strategies. Long acquisition time is the main drawback of multidimensional approach because it is not compatible with biological system dynamics. In order to reduce the measurement times, it is necessary to parallelize the acquisition (hardware level) and to optimize the acquisition strategy to reduce the measurements number while preserving the information content. In this work we have developed a time-resolved multispectral fluorescence imaging system based on a spad array combined with compression techniques which allows to reduce the number of time-resolved acquisitions by a factor < 70%. The system is based on a double DMD configuration (excitation and detection) coupled to a 32x1 SPAD array, each one with its own TCSPC circuit, placed after an imaging spectrometer. This allows one to use the spatial modulation of the excitation/detection light to acquire images at different wavelengths following the single pixel camera (SPC) scheme. In order to compress the number of acquisitions, a CW fluorescence image is acquired through a CCD and Hadamard transform is applied to select most significative coefficients. The patterns related to these coefficients are subsequently used for SPC acquisition for time and spectral resolution. A Total-Variation based algorithm is used for the reconstruction of the 4D images.

Diffuse optical imaging techniques offer the unique advantage of high sensitivity as well as imaging of multiple biomarkers to enable the noninvasive functional and molecular investigation of deep biotissues. In this talk, we will present our efforts at RPI in establishing novel instrumental and theoretical approaches for fluorescence lifetime-based 2D and 3D imaging at the mesoscopic and macroscopic scale within the context of bioprinted tissues and in vivo drug delivery assessment.

Since 2005, Fondazione Bruno Kessler (FBK, Trento) has been improving and customizing its SiPM technologies for a wide variety of applications, ranging from medical imaging to big science experiments and industrial applications. Current-generation Near Ultra Violet, High Density (NUV-HD) SiPM technology is based on a p-on-n junction in which microcells are separated by deep trench isolation. The technology features peak photon-detection efficiency (PDE) higher than 60% at 410 nm, low primary (80 kcps/mm2) and correlated noise (less than 15% optical crosstalk probability at 50% PDE) and is very well suited for medical imaging applications, such as time-of-flight positron emission tomography (TOF-PET). Indeed, using an advanced high-frequency readout, a research group at CERN demonstrated state-of-the-art coincidence resolving time of 58 ps FWHM, employing 4x4 mm2 NUV-HD SiPMs coupled to small LSO:Ce:Ca crystals. With a similar, optimized setup, a single-photon time resolution (SPTR) of 90 ps FWHM was also demonstrated, which is even more important when faint Cherenkov emission is used to improve timing performance in relatively slow scintillators such as BGO. --- On the other hand, different applications require specific optimizations of SiPM parameters, especially in the field of big science experiments. To this end, several improvements of the NUV-HD technology are ongoing. Reduction of optical crosstalk probability by a factor of ~2.5 is achieved by introducing light absorbing/reflecting material inside trenches, providing CT probability of 5% at 50% PDE. This technology, called NUV-HD-LowCT, will be used for the CTA experiment (Cherenkov Telescope Array). Optimization of SiPM performance for operation at cryogenic temperatures, for the readout of very large areas, was carried out for experiments such as DarkSide-20k, in which approximately 15 m2 of SiPMs will be used for the readout of a dual-phase, liquid argon TPC operated at 87 K. Achievement of excellent performance at this temperature required three different changes to NUV-HD technology and fabrication process, the main challenges being the reduction of band to band tunneling and afterpulsing at low temperatures. The resulting NUV-HD-Cryo technology features an exceptionally low DCR of less than 10 mHz/mm2 and AP probability of less than 15% at 87 K. Using a single electronic channel for the readout of 24 cm2 photo-sensitive active area, composed of 24 SiPMs connected in a peculiar series-parallel configuration, it was possible to measure a few-photon spectrum with a remarkable SNR larger than 24 for the single photon peak at 77 K. Direct detection of vacuum ultra-violet (VUV) light, below 200 nm, is required by the nEXO experiment. In this context, FBK demonstrated the beyond state-of-the-art performance, with a PDE larger than 23% at 190 nm, measured at 170 K. Finally, recent interest in using SiPMs in harsh radiation environment, such as in space missions or in accelerator experiments, poses additional challenges in detector optimization and partially redefines the typical design trade-offs. --- At the lower-energy end of the sensitivity spectrum, we observe a growing interest from industry in using SiPMs for detection of near infrared light. Among different applications, the most important one is LIDAR, especially for advanced driver-assistance systems in the automotive field. NIR-sensitive SiPMs (NIR-HD) fabricated at FBK employ a thicker epitaxial layer to enhance detection efficiency at longer wavelengths, achieving high PDE of 18% at 850 nm and 12 % at 905 nm, without the use micro lenses. Ongoing research is aimed at understanding the ultimate limits to the design of small, fast cells with high NIR sensitivity, such as the effect of the virtual guard region on the dead area at the microcell borders. Experimental variations of the technology, based on this research, show improved detection efficiency by careful engineering of the SPAD structure.

Thick reach-through silicon single-photon avalanche diodes (SPADs) are popular low-noise high efficiency single-photon detectors commonly used in actively quenched detector modules, and are useful at count rates < 10^7 /s. We demonstrate that these devices can achieve significantly higher count rates in a high-speed gating system. With gate durations of approximately 500 ps produced by a 1 GHz sinusoidal bias, we measure a reduction in the average charge per avalanche of more than 100x compared to an actively quenched device, enabling count rates above 10^8 /s without damage. We use a multi-harmonic interferometric readout to detect the minute avalanches at the SPAD anode. In our current setup we observe detection efficiencies above 0.55 (2) at 850 nm at count rates below 10^7 /s, and a count-rate dependent reduction in efficiency at higher rates; at 10^8 /s the efficiency is 0.35 (2). We discuss how to mitigate this effect to achieve higher efficiency at high rates.

This paper presents the performance of 26 μm and 50 μm diameter planar Ge-on-Si single-photon avalanche diode (SPAD) detectors. The addition of germanium in these detectors extends the spectral range into the short-wave infrared (SWIR) region, beyond the capability of already well-established Si SPAD devices. There are several advantages for extending the spectral range into the SWIR region including: reduced eye-safety laser threshold, greater attainable ranges, and increased depth resolution in range finding applications, in addition to the enhanced capability to image through obscurants such as fog and smoke. The time correlated single-photon counting (TCSPC) technique has been utilized to observe record low dark count rates, below 100 kHz at a temperature of 125 K for up to a 6.6 % excess bias, for the 26 μm diameter devices. Under identical experimental conditions, in terms of excess bias and temperature, the 50 μm diameter device consistently demonstrates dark count rates a factor of 4 times greater than 26 μm diameter devices, indicating that the dark count rate is proportional to the device volume. Single-photon detection efficiencies of up to ~ 29 % were measured at a wavelength of 1310 nm at 125 K. Noise equivalent powers (NEP) down to 9.8 × 10^-17 WHz^-1/2 and jitters < 160 ps are obtainable, both significantly lower than previous 100 μm diameter planar geometry devices, demonstrating the potential of these devices for highly sensitive and high-speed imaging in the SWIR.

Time-correlated measurements by means of single photon counting have been historically subject to a speed-distortion tradeoff, mainly due to the so-called pile-up effect. Recently, a new theoretical solution has been proposed showing that such tradeoff can be overcome using a detector dead time matched to the excitation period. This result opens the way to unprecedented speed operation in Time Correlated Single Photon Counting combined with low distortion. To achieve this goal, dedicated electronics has to be designed on purpose. In this paper, we report on the design of the first module that implements the proposed solution. Based on the exploitation of a high-performance Single Photon Avalanche Diode, the module features a fast full-integrated Active Quenching Circuit able to drive the sensor and a dummy cell with a finely tunable dead time as short as 12.5ns, a fully-differential avalanche current readout circuit providing the timing information with a jitter lower than 54ps and the logic circuitry that is needed to prevent distortion at high rates. The timing information is converted by a Fast-TAC providing picosecond timing and high linearity along with a count rate as high as 80Mcps..

In this paper, we present an overview of the devolvement of niobium nitride (NbN)-based superconducting nanowire single-photon detector (SNSPD) array for applications in SIMIT. These applications include high-speed interleaved nanowire arrays for optical quantum communication, large-active-area arrays for light detection and ranging (LIDAR), and free-space-coupled arrays for LIDAR and imaging. A recent development in fiber-array-coupled SNSPD array for quantum information processing is also presented.

Superconducting nanowires are currently a key technology for photon sensing, with key performance metrics exceeding those of most competing technologies. For example, recently detector timing resolution of just 3 ps for single visible photons was demonstrated. In addition, infrared sensitivity has long been a key advantage of these techniques. But the field continues to change rapidly, with interesting architectures emerging that promise increased functionality in the same form factor. For example, imaging, photon-number-resolution, and spectroscopy are all promising directions the field is now pursuing. In this review, I will explain the technology, applications, and background device physics, but also touch on some of the latest new developments.