Single Photon Counting System Research Articles

Improved echo signal detection for pseudo-random single-photon counting laser ranging

In this paper, a new echo signal detection method, to the best of our knowledge, for pseudo-random single-photon counting ranging (PSPCR) LiDAR systems is proposed, which is applied for long distances, low repetition rates, and system cost reduction. First, in order to achieve a comparable temporal resolution as that in time-correlated single-photon counting (TCSPC) systems, we extend the pseudo-random code to discriminate the minimal time slot in time correlation. Second, we use the full width at half maxima (FWHM) in the duration of each pseudo-random code for correlation to reduce the impact of pulse width variation and timing jitters on ranging accuracy. Third, we study the bias and errors caused by using synchronous signals as the “START” signal, and propose to use the time of flight (ToF) at half energy to reduce the walk error. Simulation results show that, compared with existing PSPCR methods, the proposed method improves ranging accuracy with a lower repetition rate and lower peak and average power—centimeter-level ranging accuracy over tens of kilometers can be achieved using a laser with a repetition rate of 400 kHz, peak power of up to 1 kW, and average power of up to 1 W.

64 picosecond time resolved time-correlated single photon counting imaging.

High-speed imaging of dynamic scenes is a challenging and important task in many applications. However, conventional imaging methods based on charge coupled devices or complementary metal oxide semiconductors have limitations in temporal resolution and photon sensitivity. To address this problem, we propose a novel high-speed imaging scheme that combines single-pixel imaging with single photon detection and time-correlated single photon counting. Our scheme can achieve high-speed imaging with 64ps resolution by repeating the motion scenes and using binary outputs from single photon detectors. We demonstrate our scheme by reconstructing the switching process of a digital micro-mirror device and a liquid crystal spatial light modulator. Our scheme can be further improved to 1ps resolution by using a more accurate time-correlated single photon counting system. Moreover, our scheme can adapt to different speed scenes by adjusting the temporal resolution and reducing the sampling time. Our high temporal resolution imaging scheme further expands the application areas of single-pixel imaging and provides solutions for scenes requiring single photon detection and higher temporal resolution, such as reproducible chemical reaction processes imaging, cellular or sub-cellular bio imaging, single-molecule imaging of rotary motors, high-speed equipment inspection, and other periodic high-speed scenes imaging.

Time-Resolved Photoluminescence Spectroscopy of Semiconductor Nanocrystals and Other Fluorophores.

Time-resolved photoluminescence (TRPL) is a key technique for understanding the photophysics of semiconductor nanocrystals and light-emitting materials in general. This work is a primer for setting up and conducting TRPL on nanocrystals and related materials using single-photon-counting (SPC) systems. Basic sources of error in the measurement can be avoided by consideration of the experimental setup and calibration. The detector properties, count rate, the spectral response, reflections in optical setups, and the specific instrumentation settings for single photon counting will be discussed. Attention to these details helps ensure reproducibility and is necessary for obtaining the best possible data from an SPC system. The main aim of the protocol is to help a student of TRPL understand the experimental setup and the key hardware parameters one must generally comprehend in order to gain useful TRPL data in many common single-photon-counting setups. The secondary purpose is to serve as a condensed primer for the student of experimental time-resolved luminescence spectroscopy.

Absolute spectral backscatter measurements of large-core multimode PMMA polymer optical fibers.

To our knowledge, we are the first to measure the absolute value of the backscattering coefficient of a standard 1 mm core-diameter, multimode (MM) step-index (SI) polymethylmethacrylate (PMMA) polymer optical fiber (POF) for the spectral range of 450 nm to 700 nm. Our optical time domain reflectometer (OTDR) setup consists of a femtosecond supercontinuum laser with an acousto-optical filter as a tunable light source with short pulses and a time-correlated single-photon counting system as a receiver with a high dynamic range. The backscattering coefficient is calculated from the ratio between the energy within the fiber end reflex and the distributed backscattering level. We also measured the spectral attenuation with our OTDR setup and compared it with a standardized measurement method. At the attenuation minima within the measured spectral range the backscattering level of a 1 ns pulse is about -46dB at 520 nm, -48dB at 570 nm, and -51 dB at 650 nm. We were also able to show by the observed wavelength dependence that Rayleigh scattering causes a majority of the scattering.

Label-free multimodal nonlinear optical microscopy for biomedical applications

This paper addresses the application of multimodal nonlinear optical (MNLO) microscopy to clinical research within the context of label-free non-invasive molecular imaging. Here, a compact MNLO microscope based on a laser scanning microscope, a femtosecond laser, a time-correlated single-photon counting system, and a photonic crystal fiber are introduced for biomedical applications. By integrating two-photon fluorescence, two-photon fluorescence lifetime imaging, second-harmonic generation, and coherent anti-Stokes Raman scattering microscopy, the proposed scheme provides profound insights into the physicochemical properties related to 3D molecular orientation distribution, inter- and intra-molecular interactions, and disease progression in biological systems and organs. The high peak power and the low average intensity of near-infrared laser pulses allow for deep-penetration imaging without compromising sample vitality. Linking nonlinear optical phenomena with time/spectral/polarization-resolved imaging also makes it possible to obtain multidimensional information to address complex biomedical questions.

Lead-halide Cs4PbBr6 single crystals for high-sensitivity radiation detection

Low-dimensional perovskite materials and their derivatives with excellent optical performance are promising candidates for light-emission applications. Herein, centimeter lead-halide Cs4PbBr6 single crystals (SCs), which have been used for radiation detection with the indirect conversion method, were synthesized by a facile solution process. The Cs4PbBr6 scintillator exhibits bright green emission peaking at 525 nm and a high photoluminescence quantum yield (up to 86.7%) under 375 nm laser excitation. The Cs4PbBr6 SCs exhibit high sensitivity to 40 keV X-rays, with a favorable linearity with the X-ray exposure dose rate, and the detection limit is as low as 64.4 nGyair/s. The scintillation time-response performance of the Cs4PbBr6 SCs was acquired by a time-correlated single-photon counting system under alpha-particle excitation. The Cs4PbBr6 SCs exhibit a very fast time response (τav = 1.46 ns) to alpha particles from a 241Am radiation source. This value is comparable to that of the commercial plastic scintillator EJ-228 (τav = 1.31 ns) and much faster than that of the LYSO(Ce) scintillator (τav = 36.17 ns). Conceptual X-ray imaging and alpha-particle pulse height spectroscopy experiments were also performed. These results demonstrated the potential of Cs4PbBr6 SCs for radiation detection applications, including X-ray imaging and charged particle detection with fast scintillation decay time and high sensitivity.

Achieving a high photon count rate in digital time-correlated single photon counting using a hybrid photodetector.

We report an enhanced photon count rate in a digitally implemented time-correlated single-photon counting (TCSPC) system by utilizing a hybrid photodetector (HPD). In our digital TCSPC scheme, the photoelectronic responses from a single photon-sensitive photodetector are digitally analyzed through a high-speed analog-to-digital convertor (ADC). By virtue of the HPD which provides nearly a constant signal gain, the single-photon pulses can be effectively distinguished from pulses of simultaneously detected multiple photons by the pulse heights. Consequently, our digital TCSPC system can selectively collect single-photon signals even in the presence of intense multi-photon detections with its temporal accuracy not to be compromised. In our experiment of fluorescence lifetime measurement, the maximum count rate of single photons nearly reached the theoretical limit given by the Poisson statistics. This demonstrated that the digital TCSPC combined with the HPD provides an ultimate solution for the TCSPC implementation for high photon count rates.

FLIM data analysis based on Laguerre polynomial decomposition and machine-learning.

Significance: The potential of fluorescence lifetime imaging microscopy (FLIM) is recently being recognized, especially in biological studies. However, FLIM does not directly measure the lifetimes, rather it records the fluorescence decay traces. The lifetimes and/or abundances have to be estimated from these traces during the phase of data processing. To precisely estimate these parameters is challenging and requires a well-designed computer program. Conventionally employed methods, which are based on curve fitting, are computationally expensive and limited in performance especially for highly noisy FLIM data. The graphical analysis, while free of fit, requires calibration samples for a quantitative analysis.Aim: We propose to extract the lifetimes and abundances directly from the decay traces through machine learning (ML).Approach: The ML-based approach was verified with simulated testing data in which the lifetimes and abundances were known exactly. Thereafter, we compared its performance with the commercial software SPCImage based on datasets measured from biological samples on a time-correlated single photon counting system. We reconstructed the decay traces using the lifetime and abundance values estimated by ML and SPCImage methods and utilized the root-mean-squared-error (RMSE) as marker.Results: The RMSE, which represents the difference between the reconstructed and measured decay traces, was observed to be lower for ML than for SPCImage. In addition, we could demonstrate with a three-component analysis the high potential and flexibility of the ML method to deal with more than two lifetime components.Conclusions: The ML-based approach shows great performance in FLIM data analysis.

Single-Photon Counting Laser Ranging With Optical Frequency Combs

Time-of-flight single-photon counting laser ranging offers an important tool for remote sensing and imaging. However, limited by the temporal properties of a single-photon detection and counting system, it encounters a challenge for high-resolution and high-precision distance metrology. Here, we incorporated dual-comb based laser ranging, an enabling technique for rapid distance measurements with high precision and large range ambiguity, with time-correlated single-photon counting (TCSPC) technology and demonstrated its feasibility for precise laser ranging under a photon-starved condition. Based on interferometric measurements of two asynchronous 2.8-pW, 25-GHz electro-optic laser combs with TCSPC, micrometer-level ranging precision was achieved for consistent distance measurements (a stand-off distance of 15.6 m) over several days. New opportunities might open up for high-resolution and high-precision single-photon counting laser ranging.

Structural, Optical and Decay Properties of Zinc(II) 8-Hydroxyquinoline and Its Thin Film

Zinc(II) 8-hydroxyquinoline (Znq2) green luminescent material and its blended thin film in a poly(methyl methacrylate) (PMMA) matrix have been prepared. XRD analysis confirms the formation of the compound and its presence in the PMMA blended thin film. The orbital molecular geometry, quantum chemical calculations and band gap energy of Znq2 phosphor have been confirmed from the DFT study. The morphology of the phosphor was shown by the SEM images. UV–Vis absorption, photoluminescence and decay analysis of zinc(II) 8-hydroxyquinoline phosphor confirms its suitability as a green emitter for organic light-emitting diodes. Uniform distribution of Znq2 powder in the PMMA matrix and its roughness was estimated by AFM analysis. Luminescence decay study was utilized to demonstrate its lifetime analysis by the time-correlated single-photon counting (TCSPC) system.

Resolution and contrast enhancement of laser-scanning multiphoton microscopy using thulium-doped upconversion nanoparticles

High-contrast optical imaging is achievable using phosphorescent labels to suppress the short-lived background due to the optical backscatter and autofluorescence. However, the long-lived phosphorescence is generally incompatible with high-speed laser-scanning imaging modalities. Here, we show that upconversion nanoparticles of structure NaYF4:Yb co-doped with 8% Tm (8T-UCNP) in combination with a commercial laser-scanning multiphoton microscopy are uniquely suited for labeling biological systems to acquire high-resolution images with the enhanced contrast. In comparison with many phosphorescent labels, the 8T-UCNP emission lifetime of ∼ 15 µs affords rapid image acquisition. The high-order optical nonlinearity of the 8T-UCNP (n ≈ 4, as confirmed experimentally and theoretically) afforded pushing the resolution limit attainable with UCNPs to the diffraction-limit. The contrast enhancement was achieved by suppressing the background using (i) bandpass spectral filtering of the narrow emission peak of 8T-UCNP at 455-nm, and (ii) time-gating implemented with a time-correlated single-photon counting system that demonstrated the contrast enhancement of > 2.5-fold of polyethyleneimine-coated 8T-UCNPs taken up by human breast adenocarcinoma cells SK-BR-3. As a result, discrete 8T-UCNP nanoparticles became clearly observable in the freshly excised spleen tissue of laboratory mice 15-min post intravenous injection of an 8T-UCNP solution. The demonstrated approach paves the way for high-contrast, high-resolution, and high-speed multiphoton microscopy in challenging environments of intense autofluorescence, exogenous staining, and turbidity, as typically occur in intravital imaging.

Fast single-cell biochemistry: theory, open source microscopy and applications

Fluorescence lifetime sensing enables researchers to probe the physicochemical environment of a fluorophore providing a window through which we can observe the complex molecular make-up of the cell. Fluorescence lifetime imaging microscopy (FLIM) quantifies and maps cell biochemistry, a complex ensemble of dynamic processes. Unfortunately, typical high-resolution FLIM systems exhibit rather limited acquisition speeds, often insufficient to capture the time evolution of biochemical processes in living cells. Here, we describe the theoretical background that justifies the developments of high-speed single photon counting systems. We show that systems with low dead-times not only result in faster acquisition throughputs but also improved dynamic range and spatial resolution. We also share the implementation of hardware and software as an open platform, show applications of fast FLIM biochemical imaging on living cells and discuss strategies to balance precision and accuracy in FLIM. The recent innovations and commercialisation of fast time-domain FLIM systems are likely to popularise FLIM within the biomedical community, to impact biomedical research positively and to foster the adoption of other FLIM techniques as well. While supporting and indeed pursuing these developments, with this work we also aim to warn the community about the possible shortcomings of fast single photon counting techniques and to highlight strategies to acquire data of high quality.

In situ measurement of temperature dependent picosecond resolved carrier dynamics in near infrared (NIR) sensitive device on action.

The carrier dynamics study of emerging near infrared (NIR) absorbing materials is an essential need to develop device technology toward enhanced NIR light harvesting. In this study, we have documented the design of an indigenously developed time correlated single photoncounting (TCSPC) system working in the NIR (900 nm-1700 nm) spectral region. The system is compatible to study transient photoluminescence of device samples under tunable bias voltages. The liquid nitrogen cooling and electrical heating of the sample chamber provides additional flexibility of temperature dependent study starting from -196 °C to 400 °C. As a model system to study, we have chosen a multilayer InAs/InGaAs/GaAs/AlGaAs dot in the dual well device sample as the thin film quantum dot heterostructures are of huge relevance in various NIR harvesting devices. We have investigated the detail carrier dynamics of the device sample using the transient photoluminescence upon varying temperature (80 K-300 K), varying emission energy and different bias voltages (0 V-15 V). The critical temperature (160 K) and critical bias (12 V) of achieving longest excited state lifetime has been mechanistically explained using various competing photophysical phenomena such as hole diffusion, energy relaxation, etc. The emission wavelength dependent study at below and above critical temperature further provides an insight into the dominance of carrier capture and thermal escape at the two different temperature zones. Along with the detail understanding of the carrier dynamics, the results can be helpful to get an idea of the electrical stability of the device and the operability temperature as well. The reasonable good resolution of the NIR TCSPC system and considerable good results ensure the future application of the same for other devices also.

Influence of thermal treatment on the structural and optical properties of methoxy-substituted 2, 4-diphenyl quinoline

Herein, we report blue-emitting methoxy-substituted 2, 4-diphenyl quinoline phosphor prepared using acid-catalyzed Friedlander reaction along with the enhancement in the phase purity due to employed thermal treatment in Ar atmosphere. Improvement in the phase purity of the phosphor leading to a change in their optical properties is reported. XRD and FTIR analyses confirm the formation and improvement in the phase purity of the phosphor. TG/DTA analysis endorses the change in their behavior and thermal stability. The modification in optical band gap of the phosphors due to thermal treatment has been confirmed from the UV–vis spectroscopy. Enhancement in the fluorescence intensity of blue-emitting phosphor due to thermal treatment has been described through photoluminescence analysis. Luminescence decay profile has also been recorded using time-correlated single-photon counting (TCSPC) system for the lifetime analysis of the phosphors which supports the obtained PL analysis. The overall study indicates that methoxy-substituted 2, 4-diphenyl quinoline phosphor which was annealed at 90 °C in Ar atmosphere serves better fluorescence intensity, thermal stability and lifetime profile, and may be utilized for the organic display applications.

Pattern Recognition algorithm for charge sharing compensation in single photon counting pixel detectors

Thanks to the detector technology development based on high Z materials (GaAs, CdTe, CZT, etc.), the hybrid pixel detectors with direct photon-to-charge conversion become more and more popular, even in medical applications. Single photon counting systems aim at good position resolution, so making a pixel size smaller is a general tendency in such systems. However, for the detectors with pixel pitch of about 100 μm charge division effect becomes visible and it affects up to 20–30% of all interactions, depending on detector thickness and bias voltage. For the smaller pixel size this effect is even more pronounced. In this article we propose a new algorithm for charge sharing compensation—the Multithreshold Pattern Recognition algorithm. The algorithm was implemented in a readout circuit of pixel architecture with four energy thresholds, designed for operation with wide energy-range X-ray radiation. The simulation results and implementation details are presented in this paper.

Experimental research on noninvasive reconstruction of optical parameter fields based on transient radiative transfer equation for diagnosis applications

The noninvasive reconstruction of optical parameter fields in participating phantom is investigated experimentally based on the measured time-resolved radiative signals, which are detected by the time-correlated single-photon counting system. The discrete ordinate method is employed to solve the forward transient radiative transfer equation to simulate the time-resolved radiative transfer in the physical phantom exposed to ultra-short pulse laser irradiation. On top of that, the sequential quadratic programming algorithm based on the generalized Gaussian Markov random field is applied to solve the inverse problem. To improve the computational efficiency, an adjoint equation technique is adopted to determine the gradient of objective function. Good agreement was obtained between the predicted optical parameter fields and realistic ones. All the reconstruction results demonstrate that the proposed reconstruction methods are proved to be accurate and efficient experimentally.

Non-Poisson variations in photomultipliers and implications for luminescence dating

Previous studies have suggested that excess variations from single-photon counting systems used in luminescence dating may result in underestimation of errors and profoundly influence age models. In this study ten different photon counting systems have been investigated to explore this effect with a greater number of photomultiplier types and instrumental architectures. It is shown that radiation induced phosphorescence from F1 feldspar produces a controllable low-level light source whose local variance approximates Poisson expectations. However excess variation in dark counts was observed to varying extents from all systems. The excess variance is slightly anti-correlated with the age of the system, with older devices conforming more closely to Poisson behaviour. This observation does not seem to fit the hypothesis that enhanced levels of helium diffused into older tubes increase non-Poisson components. It was noted that a significant part of the non-Poisson behaviour was associated with multi-event pulse streams within time series. Work was also undertaken to develop mitigation methods for data analysis and to examine the implications for dating uncertainties in a test case. A Poisson-filtering algorithm was developed to identify and remove improbable multi-event streams. Application to data from signal-limited single grains of sediments from a Neolithic chambered tomb in Corsica has shown that, for this case, removing non-Poisson components improves the robustness of retained data, but has less influence on overall dating precision or accuracy. In signal limited applications use of this algorithm to remove one source of excess variation is beneficial. The algorithm and test data are appended to facilitate this.

Cascaded systems analysis of charge sharing in cadmium telluride photon-counting x-ray detectors.

Single-photon-counting (SPC) and spectroscopic x-ray detectors are under development in academic and industry laboratories for medical imaging applications. The spatial resolution of SPC and spectroscopic x-ray detectors is an important design criterion. The purpose of this article was to extend the cascaded systems approach to include a description of the spatial resolution of SPC and spectroscopic x-ray imaging detectors. A cascaded systems approach was used to model reabsorption of characteristic x rays, Coulomb repulsion, and diffusion in SPC and spectroscopic x-ray detectors. In addition to reabsorption, diffusion, and Coulomb repulsion, the model accounted for x-ray conversion to electron-hole (e-h) pairs, integration of e-h pairs in detector elements, electronic noise, and energy thresholding. The probability density function (PDF) describing the number of e-h pairs was propagated through each stage of the model and was used to derive new theoretical expressions for the large-area gain and modulation transfer function (MTF) of CdTe SPC x-ray detectors, and the energy bin sensitivity functions and MTFs of CdTe spectroscopic detectors. Theoretical predictions were compared with the results of MATLAB-based Monte Carlo (MC) simulations and published data. Comparisons were also made with the MTF of energy-integrating systems. Under general radiographic conditions, reabsorption, diffusion, and Coulomb repulsion together artificially inflate count rates by 20% to 50%. For thicker converters (e.g. 1000 μm) and larger detector elements (e.g. 500 μm pixel pitch) these processes result in modest inflation (i.e. ∼10%) in apparent count rates. Our theoretical and MC analyses predict that SPC MTFs will be degraded relative to those of energy-integrating systems for fluoroscopic, general radiographic, and CT imaging conditions. In most cases, this degradation is modest (i.e., ∼10% at the Nyquist frequency). However, for thicker converters, the SPC MTF can be degraded by up to 25% at the Nyquist frequency relative to EI systems. Additionally, unlike EI systems, the MTF of spectroscopic systems is strongly dependent on photon energy, which results in energy-bin-dependent spatial resolution in spectroscopic systems. The PDF-transfer approach to modeling signal transfer through SPC and spectroscopic x-ray imaging systems provides a framework for understanding system performance. Application of this approach demonstrated that charge sharing artificially inflates the SPC image signal and degrades the MTF of SPC and spectroscopic systems relative to energy-integrating systems. These results further motivate the need for anticharge-sharing circuits to mitigate the effects of charge sharing on SPC and spectroscopic x-ray image quality.

A Low-Cost Time-Correlated Single Photon Counting System for Multiview Time-Domain Diffuse Optical Tomography

Diffuse optical tomography resorting to time-domain measurements (TD-DOT) provides information-rich data that have not yet been fully exploited for image reconstruction, notably to increase imaging spatial resolution. Current TD-DOT scanners suffer from a very low sensitivity owing to their small number of detection channels. This leads to excessively long acquisition times for in vivo imaging. To obtain a higher number of detection channels, thus increasing detection density, a low-cost time-correlated single photon counting (TCSPC) system dedicated to TD-DOT was designed, resorting solely to off-the-shelf electronic components to reduce costs, in distinction to custom application-specific integrated circuit solutions. The system features four input channels, each of them including a leading-edge discriminator for direct interfacing with off-the-shelf photodetector modules and a programmable delay line to compensate for undesired propagation delays, thus also avoiding manual adjustment of cable lengths. The system supports a large number of TCSPC channels using a daisy-chain configuration through an onboard Ethernet switch. It also features a 13.02 ps bin width with a 12.5 ns dynamic range. Results show excellent linearity, with a $\sigma _{\mathrm{ DNL}}$ of 1.7% least significant bit (LSB), peak differential nonlinearity of 5% LSB, and peak integral nonlinearity of 10% LSB and an average accuracy of 19 ps full-width at half-maximum (FWHM) throughout two separate four-channel systems. The system was tested in a TD-DOT acquisition setup, and 37 ps FWHM instrument response functions were obtained using a single-photon avalanche diode (SPAD) detector. Diffuse intrinsic measurements were also acquired and compared with a reference TCSPC system showing similar results.

Characterization of the Timing Homogeneity in a CMOS SPAD Array Designed for Time-Gated Raman Spectroscopy

A characterization environment was built to verify the timing characteristics of a single photon avalanche diode (SPAD) array designed for time-gated Raman spectroscopy. The characterization was applied to a $256 \times 16$ SPAD array that employed an on-chip time-to-digital converter (TDC) with a 50–100-ps resolution for time resolving. The timing skew and the time window homogeneity across the array were resolved, moving the time-resolving windows over an optical pulse by picosecond-level delay steps. A typical one 160-ps skew across the array was measured. The TDC time bins had average sizes of 33–144 ps while their deviation across the array was 8–12 ps. The method is applicable to multidetector time-correlated single photon counting systems that can finely adjust the delay between the optical pulse and the reference signal.