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The reduction of detector dead time represents an enabling factor in several photon counting applications. In this work, we investigate the free-running operation of reach-through single-photon avalanche diodes (SPADs) at ultra-low dead times. By employing a fast active quenching circuit with direct bonding to the detector, we are able to achieve a 10 ns dead time with a thick SPAD by Excelitas, still maintaining extremely low afterpulsing probabilities (below 1.5%). [ABSTRACT FROM AUTHOR]

Time-Correlated Single-Photon Counting (TCSPC) is an excellent technique used in a great variety of scientific experiments to acquire exceptionally fast and faint light signals. Above all, in Fluorescence Lifetime Imaging (FLIM), it is widely recognized as the gold standard to record sub-nanosecond transient phenomena with picosecond precision. Unfortunately, TCSPC has an intrinsic limitation: to avoid the so-called pile-up distortion, the experiments have been historically carried out, limiting the acquisition rate below 5% of the excitation frequency. In 2017, we demonstrated that such a limitation can be overcome if the detector dead time is exactly matched with the excitation period, thus paving the way to unprecedented speedup of FLIM measurements. In this paper, we present the first single-channel system that implements the novel proposed methodology to be used in modern TCSPC experimental setups. To achieve this goal, we designed a compact detection head, including a custom single-photon avalanche diode externally driven by a fully integrated Active Quenching Circuit (AQC), featuring a finely tunable dead time and a short reset time. The output timing signal is extracted by using a picosecond precision Pick-Up Circuit (PUC) and fed to a newly developed timing module consisting of a mixed-architecture Fast Time to Amplitude Converter (F-TAC) followed by high-performance Analog-to-Digital Converters (ADCs). Data are transmitted in real-time to a Personal Computer (PC) at USB 3.0 rate for specific and custom elaboration. Preliminary experimental results show that the new TCSPC system is suitable for implementing the proposed technique, achieving, indeed, high timing precision along with a count rate as high as 40 Mcps. [ABSTRACT FROM AUTHOR]

Recently developed Active Quenching Circuits (AQCs) with fast-gating capabilities allow us to control a single photon avalanche diode with gate windows in the nanosecond and sub-nanosecond range, thus paving the way to advanced applications, especially in the field of time-correlated single photon counting. In this scenario, an accurate measurement of the time needed by the AQC to turn-on the detector is of utmost importance. Indeed, it permits us to evaluate the impact of the system in specific applications and provides a tool to designers to understand AQC limitations and to enhance its performance. Here we propose a simple non-invasive technique to accurately measure the time needed by a gated system to turn on the detector. The effectiveness of the measure has been proved on a gated system, and results have been compared to those obtained starting from the distribution of recorded photons under constant illumination, which is a widely used approach in the literature. The great advantage of the proposed approach is that it avoids typical artifacts that affect other kinds of measurements. [ABSTRACT FROM AUTHOR]

In this paper, we describe a novel solution to increase the speed of Time-Correlated Single Photon Counting (TCSPC) measurements by almost an order of magnitude while providing, in principle, zero distortion regardless of the experimental conditions. Typically, the relatively long dead time associated with the conversion electronics requires a proper tune of the excitation power in order to avoid distortions of the reconstructed waveform due to pileup and counting loss. As a result, the maximum operating rate of a TCSPC channel is now limited between 1% and 5% of the excitation frequency, thus leading to relatively long acquisition times. We show that negligible distortion (below 1%) is guaranteed if the dead time associated with the converter is kept below the dead time of the detector, and at the same time the detector dead time is matched to the duration of the excitation period. In this way, unprecedented high-speed operation is possible. In this paper, we provide a theoretical analysis of the technique, including the main non-idealities which are introduced by a generic physical implementation. The results are supported by both numerical simulations and analytical calculations. [ABSTRACT FROM AUTHOR]

Time-Correlated Single Photon Counting (TCSPC) is a very efficient technique for measuring weak and fast optical signals, but it is mainly limited by the relatively "long" measurement time. Multichannel systems have been developed in recent years aiming to overcome this limitation by managing several detectors or TCSPC devices in parallel. Nevertheless, if we look at state-of-the-art systems, there is still a strong trade-off between the parallelism level and performance: the higher the number of channels, the poorer the performance. In 2013, we presented a complete and compact 32 x 1 TCSPC system, composed of an array of 32 single-photon avalanche diodes connected to 32 time-to-amplitude converters, which showed that it was possible to overcome the existing trade-off. In this paper, we present an evolution of the previous work that is conceived for high-throughput fluorescence lifetime imaging microscopy. This application can be addressed by the new system thanks to a centralized logic, fast data management and an interface to a microscope. The new conceived hardware structure is presented, as well as the firmware developed to manage the operation of the module. Finally, preliminary results, obtained from the practical application of the technology, are shown to validate the developed system. [ABSTRACT FROM AUTHOR]

In recent years, lifetime measurements by means of the Time Correlated Single Photon Counting (TCSPC) technique have led to a significant breakthrough in medical and biological fields. Unfortunately, the many advantages of TCSPC-based approaches come along with the major drawback of a relatively long acquisition time. The exploitation of multiple channels in parallel could in principle mitigate this issue, and at the same time it opens the way to a multi-parameter analysis of the optical signals, e.g., as a function of wavelength or spatial coordinates. The TCSPC multichannel solutions proposed so far, though, suffer from a tradeoff between number of channels and performance, and the overall measurement speed has not been increased according to the number of channels, thus reducing the advantages of having a multichannel system. In this paper, we present a novel readout architecture for bi-dimensional, high-density Single Photon Avalanche Diode (SPAD) arrays, specifically designed to maximize the throughput of the whole system and able to guarantee an efficient use of resources. The core of the system is a routing logic that can provide a dynamic connection between a large number of SPAD detectors and a much lower number of high-performance acquisition channels. A key feature of our smart router is its ability to guarantee high efficiency under any operating condition.q [ABSTRACT FROM AUTHOR]

Time-Correlated Single Photon Counting (TCSPC) has been long recognized as the most sensitive method for fluorescence lifetime measurements, but often requiring "long" data acquisition times. This drawback is related to the limited counting capability of the TCSPC technique, due to pile-up and counting loss effects. In recent years, multi-module TCSPC systems have been introduced to overcome this issue. Splitting the light into several detectors connected to independent TCSPC modules proportionally increases the counting capability. Of course, multi-module operation also increases the system cost and can cause space and power supply problems. In this paper, we propose an alternative approach based on a new detector and processing electronics designed to reduce the overall system dead time, thus enabling efficient photon collection at high excitation rate. We present a fast active quenching circuit for single-photon avalanche diodes which features a minimum dead time of 12.4 ns. We also introduce a new Time-to-Amplitude Converter (TAC) able to attain extra-short dead time thanks to the combination of a scalable array of monolithically integrated TACs and a sequential router. The fast TAC (F-TAC) makes it possible to operate the system towards the upper limit of detector count rate capability (~80 Mcps) with reduced pile-up losses, addressing one of the historic criticisms of TCSPC. Preliminary measurements on the F-TAC are presented and discussed. [ABSTRACT FROM AUTHOR]

A new algorithm for controlling the temperature of a thermoelectric cooler is proposed. Unlike a classic proportional-integral-derivative (PID) control, which computes the bias voltage from the temperature error, the proposed algorithm exploits the linear relation that exists between the cold side’s temperature and the amount of heat that is removed per unit time. Since this control is based on an existing linear relation, it is insensitive to changes in the operating point that are instead crucial in classic PID control of a non-linear system. [ABSTRACT FROM AUTHOR]

The minimization of Single Photon Avalanche Diodes (SPADs) dead time is a key factor to speed up photon counting and timing measurements. We present a fully integrated Active Quenching Circuit (AQC) able to provide a count rate as high as 100 MHz with custom technology SPAD detectors. The AQC can also operate the new red enhanced SPAD and provide the timing information with a timing jitter Full Width at Half Maximum (FWHM) as low as 160 ps. [ABSTRACT FROM AUTHOR]

Fluorescence correlation spectroscopy (FCS) is a well-established technique to study binding interactions or the diffusion of fluorescently labeled biomolecules in vitro and in vivo. Fast FCS experiments require parallel data acquisition and analysis which can be achieved by exploiting a multi-channel Single Photon Avalanche Diode (SPAD) array and a corresponding multi-input correlator. This paper reports a 32-channel FPGA based correlator able to perform 32 auto/cross-correlations simultaneously over a lag-time ranging from 10 ns up to 150 ms. The correlator is included in a 32 x 1S P A D array module, providing a compact and flexible instrument for high throughput FCS experiments. However, some inherent features of SPAD arrays, namely afterpulsing and optical crosstalk effects, may introduce distortions in the measurement of auto- and cross-correlation functions. We investigated these limitations to assess their impact on the module and evaluate possible workarounds. [ABSTRACT FROM AUTHOR]

Nowadays, an increasing number of applications require high-performance analytical instruments capable to detect the temporal trend of weak and fast light signals with picosecond time resolution. The Time-Correlated Single-Photon Counting (TCSPC) technique is currently one of the preferable solutions when such critical optical signals have to be analyzed and it is fully exploited in biomedical and chemical research fields, as well as in security and space applications. Recent progress in the field of single-photon detector arrays is pushing research towards the development of high performance multichannel TCSPC systems, opening the way to modern time-resolved multi-dimensional optical analysis. In this paper we describe a new 8-channel high-performance TCSPC acquisition system, designed to be compact and versatile, to be used in modern TCSPC measurement setups. We designed a novel integrated circuit including a multichannel Time-to-Amplitude Converter with variable full-scale range, a D/A converter, and a parallel adder stage. The latter is used to adapt each converter output to the input dynamic range of a commercial 8-channel Analog-to-Digital Converter, while the integrated DAC implements the dithering technique with as small as possible area occupation. The use of this monolithic circuit made the design of a scalable system of very small dimensions (95 × 40 mm) and low power consumption (6 W) possible. Data acquired from the TCSPC measurement are digitally processed and stored inside an FPGA (Field-Programmable Gate Array), while a USB transceiver allows real-time transmission of up to eight TCSPC histograms to a remote PC. Eventually, the experimental results demonstrate that the acquisition system performs TCSPC measurements with high conversion rate (up to 5 MHz/channel), extremely low differential nonlinearity (<0.04 peak-to-peak of the time bin width), high time resolution (down to 20 ps Full-Width Half-Maximum), and very low crosstalk between channels. [ABSTRACT FROM AUTHOR]

Emerged as a solid state alternative to photo multiplier tubes (PMTs), single-photon avalanche diodes (SPADs) are nowadays widely used in the field of single-photon timing applications. Custom technology SPADs assure remarkable performance, in particular a 10 counts/s dark count rate (DCR) at low temperature, a high photon detection efficiency (PDE) with a 50% peak at 550 nm and a 30 ps (full width at half maximum, FWHM) temporal resolution, even with large area devices, have been obtained. Over the past few years, the birth of novel techniques of analysis has led to the parallelization of the measurement systems and to a consequent increasing demand for the development of monolithic arrays of detectors. Unfortunately, the implementation of a multidimensional system is a challenging task from the electrical point of view; in particular, the avalanche current pick-up circuit, used to obtain the previously reported performance, has to be modified in order to enable high parallel temporal resolution, while minimizing the electrical crosstalk probability between channels. In the past, the problem has been solved by integrating the front-end electronics next to the photodetector, in order to reduce the parasitic capacitances and consequently the filtering action on the current signal of the SPAD, leading to an improvement of the timing jitter at higher threshold. This solution has been implemented by using standard complementary metal-oxide-semiconductor (CMOS) technologies, which, however, do not allow a complete control on the SPAD structure; for this reason the intrinsic performance of CMOS SPADs, such as DCR, PDE, and afterpulsing probability, are worse than those attainable with custom detectors. In this paper, we propose a pixel architecture, which enables the development of custom SPAD arrays in which every channel maintains the performance of the best single photodetector. The system relies on the integration of the timing signal pick-up circuit next to the photodiode, achieved by modifying the technological process flow used for the fabrication of the custom SPAD. The pixel is completed by an external standard CMOS active quenching circuit, which assures stable timing performance at quite high count rate (>1 MHz). [ABSTRACT FROM AUTHOR]

A modification of a standard Perkin Elmer SPCM-AQR photon detector module that remarkably improves the photon timing performance is presented here. The modification consists of an additional timing circuit board, which is inserted in the module without modifying the original circuit board. The essential feature is a pulse pickup linear network, connected to the high-voltage terminal of the photodetector, which extracts a short pulse signal with fast rise, coincident with the rise of the avalanche current. The information about the photon arrival time is obtained by sensing the onset of the rise. At low counting rates (<105 counts/s) time-correlated photon counting tests show that the instrumental resolution function (IRF) thus obtained has full width at half maximum (FWHM) narrower by about 40% with respect to the original module. At higher counting rate, up to few Mcounts/s, the advantage is even more remarkable: The timing circuit practically eliminates the drawbacks that plague the original module, namely, a progressive increase of the FWHM and a progressive shift of the peak position of the IRF with increasing counting rate. The modified SPCM-AQR module is therefore suitable also for applications requiring subnanosecond time resolution at high and/or variable counting rate, such as fluorescent decay measurements, fluorescent lifetime imaging, single molecule detection and spectroscopy, and optical radar techniques. [ABSTRACT FROM AUTHOR]

Describes a true constant fraction trigger circuit for picosecond photon-timing with ultrafast microchannel plate photomultipliers. Operation principle of the circuit; Instrument structure and key features; Circuit description; Reduction of residual amplitude dependent time walk.

Focuses on a compact active quenching circuit for fast photon counting with avalanche photodiodes in Geiger mode. Features of the single photon avalanche diodes (SPAD) detector; Circuit design; Photo detection efficiency; Dark counts.

Discusses performance optimization of active quenching circuits (AQC) for picosecond timing with single photon avalanche diodes. Electronic problems experienced with single photon avalanche diodes; Minimization of the noise of the input stage; Electronic time jitter of the AQC; Improvement of the circuits.

Since single-photon pulses of photomultiplier tubes (PMTs) have statistically fluctuating amplitude, constant-fraction trigger circuits (CFTs) are normally employed for accurate timing. However, with the subnanosecond signals of microchannel-plate types (MCPs) nonideal CFT behavior is observed. A residual amplitude-dependent time walk sets the ultimate resolution in time-correlated photon counting (TCPC). We present a quantitative analysis of the problem and discuss published results. We assess the effect of pulse shaping filters and derive criteria for selecting the fast preamplifier. For MCP models with 40-ps intrinsic resolution, we show that by changing the pulse delay and attenuation ratio in the available CFTs, with minor circuit modifications, the residual time walk is strongly reduced and the detector performance can be fully exploited. For faster MCP types, with 20-ps resolution or better, the improvement is also remarkable, but we show that there is margin for further improvement and we point out criteria for designing new CFTs. [ABSTRACT FROM AUTHOR]

Ultrafast microchannel-plate photomultipliers offer picosecond resolution in time-correlated photon counting, provided all additional causes of timing jitter be minimized. Here we analyze the contribution of the electronic circuit noise, mainly dependent on the fast preamplifier and no more negligible at picosecond level. Criteria are derived for minimizing this contribution by proper design or selection of the amplifier. We highlight that it does not make sense to select amplifiers with bandwidths of 3 GHz or more, looking mainly to the rise time of the fast microchannel-plate pulse and paying minor attention to the noise. We demonstrate that (i) the noise spectral amplitude has chief importance, (ii) the bandwidth providing minimum jitter is around 1 GHz, and (iii) satisfactory results are obtained with lower bandwidths, in some cases down to about 500 MHz. [ABSTRACT FROM AUTHOR]

The resolution of Geiger-mode avalanche photodiodes in single photon timing is shown to be dependent on the diameter of the illuminated area. For the RCA C30902S device at room temperature, an improvement in the resolution from 460 to 320 ps FWHM (full width at half maximum) is demonstrated by concentrating the light in a 50-μm-diam spot in the center of the active area. The physical reasons that can justify such a result are dealt with. The resolution curve is affected by a tail due to diffusion effects. The tail shape and amplitude are investigated for different positions of the light spot on the active area and shown to be strongly position dependent. An analysis of the device structure accounts for this effect. [ABSTRACT FROM AUTHOR]

Single photon avalanche diodes (SPADs) are avalanche photodiodes specifically designed for reverse bias operation above the breakdown voltage and used for detecting single optical photons. A new silicon epitaxial device structure was designed to give improved timing performance with respect to previous SPADs. Extensive tests were carried out in order to establish the timing resolution of the device in time correlated photon counting (TCPC). The timing resolution of the SPAD in terms of its full-width at half-maximum (FWHM) contribution to the overall instrumental response width is 20 ps with the detector cooled to -65 °C, and 28 ps at room temperature. This is the highest resolution so far reported for solid-state single-photon detectors. In vacuum tubes, comparable results are obtained only with special microchannel-plate photomultipliers (MCP-PMT). Results from time-resolved photoluminescence measurements in GaAs demonstrate the power of the TCPC technique when used with the new SPAD detector. With the excellent timing resolution of the SPAD and the well-known advantages of TCPC systems (high sensitivity, linearity, etc.), various applications are foreseen in areas so far dominated by streak cameras. [ABSTRACT FROM AUTHOR]

Avalanche photodiodes biased above the breakdown voltage are an interesting alternative to photomultiplier tubes in time-correlated single-photon counting. The characteristics and performance of a commercially available device (RCA C30921S) have been investigated. The time resolution is found to improve as the excess bias above the breakdown voltage is increased. Full width at half-maximum values down to 400 ps have been measured with the detector cooled at - 40°C, and down to 460 ps at room temperature. The best results were obtained with an active quenching circuit, suitable for operation with excess bias voltage up to 40 V; at room temperature, fast gated operation was used for attaining optimum performance. Experimental data on the statistical behavior of the avalanche current pulses in these devices are reported and discussed. [ABSTRACT FROM AUTHOR]

Over the past years an always growing interest has arisen about the measurement technique of time-correlated single photon counting TCSPC), since it allows the analysis of extremely fast and weak light waveforms with a picoseconds resolution. Consequently, many applications exploiting TCSPC have been developed in several fields such as medicine and chemistry. Moreover, the development of multianode PMT and of single photon avalanche diode arrays led to the realization of acquisition systems with several parallel channels to employ the TCSPC technique in even more applications. Since TCSPC basically consists of the measurement of the arrival time of a photon, the most important part of an acquisition chain is the time measurement block, which must have high resolution and low differential nonlinearity, and in order to realize multidimensional systems, it has to be integrated to reduce both cost and area. In this paper we present a fully integrated time-to-amplitude converter, built in 0.35 μm Si-Ge technology, characterized by a good time resolution (60 ps), low differential nonlinearity (better than 3% peak to peak), high counting rate (16 MHz), low and constant power dissipation (40 mW), and low area occupation (1.38×1.28 mm2). [ABSTRACT FROM AUTHOR]

Time correlated single photon counting techniques allow in depth examinations of very important chemical and biological processes involving complex interactions of biomolecules. Understanding of these processes is of the utmost importance to address vital medical issues such as the origin and growth of tumors. Modern developments of fluorescence analysis techniques require compact, low cost, and high performance instrumentation, and a major path to these goals is the successful integration of the electronics. In this paper we present a fully monolithic time to amplitude converter, built in standard 0.35 μm CMOS technology, characterized by good time resolution (60 ps), low differential nonlinearity (better than 0.5% rms), short dead time (80 ns), low power dissipation (60 mW), and low area occupation (1.8×1.4 mm2). [ABSTRACT FROM AUTHOR]

We present a new photon timing circuit that achieves a time resolution of 35 ps full width at half maximum with single photon avalanche diodes having active area diameters up to 200 μm. The timing circuit is based on a double avalanche current sensing network that makes it particularly suited to operation at high photon counting rates. Thanks to its self-adjusting capabilities, no trimming is needed even when changing the photodetector operating conditions over a wide range. [ABSTRACT FROM AUTHOR]

We have investigated for the first time the propagation of the avalanche multiplication over the area of p-n junctions reverse biased above the breakdown voltage. The multiplication process spreads from the point where the avalanche is triggered to the whole junction area with a speed proportional to the final steady-state value of the avalanche current. The values of the propagation speed suggest that the phenomenon is due to diffusion of carriers assisted by avalanche multiplication. This effect strongly affects the rise of the avalanche current and turns out to limit the performance of single photon avalanche diodes. [ABSTRACT FROM AUTHOR]

Low noise single-photon sources are a critical element for quantum technologies. We present a heralded single-photon source with an extremely low level of residual background photons, by implementing low-jitter detectors and electronics and a fast custom-made pulse generator controlling an optical shutter (a LiNbO3 waveguide optical switch) on the output of the source. This source has a second-order autocorrelation g(2)(0)=0.005(7), and an output noise factor (defined as the ratio of the number of noise photons to total photons at the source output channel) of 0.25(1)%. These are the best performance characteristics reported to date. [ABSTRACT FROM AUTHOR]

When an electric field is applied across an interface, a dielectric will acquire a polarization charge layer, assumed infinitely thin in the theory of macroscopic dielectrics and also in most treatments of electrokinetic phenomena in nanoscale structures. In this work we explore the polarization charge layer in molecular detail. Various formal relations and a linear response theory for the polarization charge are presented. Properties of the polarization charge layer are studied for three aqueous interfaces: air-water, a crystalline silica surface with water, and an amorphous silica surface with water. The polarization charge is calculated from equilibrium simulations via linear response theory and from non-equilibrium simulations, and the results are within statistical error. The polarization charge is found to be distributed within a region whose width is on the order of a nanometer. [ABSTRACT FROM AUTHOR]

We present an experimental realization and characterization of artificial spiking neuron based on an optoelectronic pair "microcavity laser-single photon avalanche diode" operating in few photon regime. We show that basic properties of biological neurons, such as an existence of the threshold and the refractory period, the insensitivity to the effect of the stimuli strength above the threshold, and the dependence of the neuron fire rate of the stimuli strength, can be realized with such a type of artificial neuron. To compare, we present corresponding results of the numerical simulation in the framework of the FitzHugh–Nagumo neuron model. [ABSTRACT FROM AUTHOR]

We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors. [ABSTRACT FROM AUTHOR]

The authors describe the integration of low-loss optical waveguides with lab-on-a-chip structures to produce an integrated optical-microfluidic platform for time-correlated single-photon counting of fluorescent molecules. Waveguides were fabricated using electron beam densification of planar silica on silicon, eliminating any requirement for depositing upper cladding silica layers. Microfluidic channels were dry etched directly through the waveguides and the device was sealed using a poly(dimethylsiloxane) gasket. Time-resolved fluorescence lifetime measurements of the fluorophore nile blue were used as a model system to demonstrate the operation of the microfluidic device, with dye concentrations as low as 1.5 nM (equivalent to <6000 molecules) being measured. [ABSTRACT FROM AUTHOR]