Time-correlated Photon Counting Research Articles

Free-space coupled, large-active-area superconducting microstrip single-photon detector for photon-counting time-of-flight imaging

Numerous applications at the photon-starved regime require a free-space coupling single-photon detector with a large active area, low dark count rate (DCR), and superior time resolutions. Here, we developed a superconducting microstrip single-photon detector (SMSPD), with a large active area of 260 µm in diameter, a DCR of ∼5kcps, and a low time jitter of ∼171ps, operated at a near-infrared of 1550 nm and a temperature of ∼2.0K. As a demonstration, we applied the detector to a single-pixel galvanometer scanning system and successfully reconstructed the object information in depth and intensity using a time-correlated photon counting technology.

Photoinduced energy transfer in sulfur-based heterocyclic compounds and corrole dyads

We have designed and synthesized three donor-acceptor systems in which corrole macrocycle is an energy acceptor and sulfur-based heterocyclic compounds such as thianthrene (S1-Cor), dibenzothiophene (S2-Cor) and benzothiophene (S3-Cor) are the donors at its meso phenyl position. The molecular integrity of all three D-A systems has been characterized by various spectroscopic techniques that include elemental analysis, 1H NMR and MALDI-MS. Differential functional theory studies suggest that the electron density of the highest occupied molecular orbital (HOMO) spread on both corrole and sulfur-substituted heterocyclic units whereas the lowest occupied molecular orbital (LUMO) spreads on corrole macrocycle only. Singlet excited state properties indicate that there is an efficient intramolecular photoinduced energy transfer (PEnT) from donor heterocyclic unit to corrole, when excited at [Formula: see text]290 nm. Förster theory was adopted to explain the PEnT reaction in these dyads. The time-correlated singlet photon counting (TCSPC) studies suggest that energy transfer rates ([Formula: see text] are found to be in the range of 1.06 × 109 S[Formula: see text] to 6.67 × 109 S[Formula: see text] and are solvent-dependent.

Incremental peritoneal dialysis and survival outcomes: a propensity-matched cohort study.

The advantages of an incremental dialysis start are not fully clear.We aimed to evaluate the association of incremental initiation of peritoneal dialysis with mortality. Incident peritoneal dialysis patients with a catheter placed at our hospital between 2008 and 2017 were included. All patients were followed up until December 31, 2019. Patients were categorized into different groups according to the initial daily dialysis exchanges, and were matched at a ratio of 1:2 with propensity score matching. Multiple variables including age, sex, residual kidney function, urine volume, hemoglobin, serum albumin and other important variables were included for the matching. Primary outcomes were all-cause and cardiovascular mortality. A total of 1315 patients with a mean age of 45.9years were enrolled. The mean glomerular filtration rate was 4.32ml/min/1.73 m2 at start ofdialysis. Two hundred eighty-fivepatientsin the incremental group and 502 in the full dose group were matched for age, sex, residual kidney function, urine volume, hemoglobin, serum albumin and other important variables. Patient survival and cardiovascular event-free survival were similar between the two groups. However, during the first 6 years of peritoneal dialysis, patients inthe incremental group had bettersurvival (P = 0.011) and cardiovascular event-free survival (P = 0.044) than the full dose group, while such advantages disappeared when dialysis vintage became longer. Further analysis showed that the incremental group (vs full dose dialysis) had a 39% lower risk (95% CI 0.42-0.90, P = 0.012) of all-cause mortality and a 41% decreased risk (95% CI 0.35-0.99, P = 0.047) of cardiovascular mortality during the first 6 years of dialysis. Additionally, the cumulative hazard for anuria was significantly lower in the incremental group versus the full dose group (P = 0.006). Our study shows a time-related survival advantage for incremental peritoneal dialysis patients, suggesting that an incremental regimen for startingperitoneal dialysis is feasibleand is not associated with worse outcomes. Graphical Abstract presenting schematically the measurements of the solvation response function by processing the relevant streak camera images and the time-correlated photon counting (TCSPC) data and appropriately combining them together.

The luminescence photostability of carbon nanodots synthesized by the plasma method from a glucose solution after UV radiation

Carbon nanodots (CNDs) were synthesized by the microplasma method using a glucose solution as a precursor. The photoluminescence of CNDs before and after UV irradiation was studied. The spectral and kinetic properties of the synthesized CNDs were studied using a MicroTime 200 confocal scanning luminescence microscope. It was experimentally demonstrated that the short component (tau1) of luminescence refers to C=O bonds on the CND surface. A significant decrease in the luminescence intensity of CNDs after UV irradiation due to a decrease in oxygen-containing groups on the surface of carbon dots was found. Keywords: Luminescence, carbon nanodots, photostability, time-correlated photon counting.

BSA Interaction, Molecular Docking, and Antibacterial Activity of Zinc(II) Complexes Containing the Sterically Demanding Biomimetic N3S2 Ligand: The Effect of Structure Flexibility.

Two zinc(II) complexes, DBZ and DBZH4, that have (ZnN3S2) cores and differ in the bridging mode of the ligating backbone, effectively bind to BSA. The binding affinity varies as DBZ > DBZH4 and depends on the ligand structure. At low concentrations, both complexes exhibit dynamic quenching, whereas at higher concentrations they exhibit mixed (static and dynamic) quenching. The energy transfer mechanism from the BSA singlet excited state to DBZ and DBZH4, is highly likely according to steady-state fluorescence and time-correlated singlet photon counting. Molecular docking was used to support the mode of interaction of the complexes with BSA and showed that DBZ had more energy for binding. Furthermore, antibacterial testing revealed that both complexes were active but to a lesser extent than chloramphenicol. In comparison to DBZH4, DBZ has higher antibacterial activity, which is consistent with the binding constants, molecular docking, and particle size of adducts. These findings may have an impact on biomedicine.

Interrogating the mechanism of the solvation dynamics in BmimBF4/PC mixtures: A cooperative study employing time-resolved fluorescence and molecular dynamics

In this article, the solvation in ionic liquid/molecular solvent (IL/MS) mixtures is investigated. The steady-state and time-resolved measurements of the solvation of coumarin 153 (C153) in BmimBF4/propylene carbonate are presented. A slight bathochromic shift was observed in the steady-state spectra upon increasing the IL molar fraction xIL. The time correlated photon counting TCSPC and fluorescence upconversion techniques were used to obtain the experimental solvation response functions (SRFs). The SRFs were found to be multi-exponential and the average solvation times increased with xIL. The MD simulations were performed to help the interpretation of the experimental data. An unprecedentedly good agreement between the measured and the calculated SRFs was achieved. The partition of the coulombic energy of interaction between the atoms of C153 and cations, anions and PC molecules provides a deep understanding of the time signatures of the mixture components participation in the solvation response. Indeed, the slow portion of the SRFs was found to be dominated by the IL. These time signatures change in function of xIL and represent the previously established regimes of IL/MS mixtures: electrolyte solution (xIL< 0.2) and lubricated IL (xIL> 0.2). These results will be of crucial interest for the understanding of the molecular rearrangements occurring in IL/MS electrolyte models for dye sensitized solar cells (DSSCs) after photo-excitation.

Time-Correlated Photon Counting for Estimating the Duration of Na-Line and Continuum Emission Flashes in the Spectra of Multi-Bubble Sonoluminescence

The time-correlated photon counting technique using software is used to estimate the duration and succession of Na-line and continuum emission flashes upon irradiation of aqueous solutions of NaCl and Na dodecyl sulfate with ultrasound at a frequency of 20 kHz. The duration of Na flashes was determined as 5.1, 8.0, and 11.7 ns for 5 M NaCl, 0.5 M NaCl, and 5 mM Na dodecyl sulfate solutions, respectively. The duration of a continuum emission flash varied insignificantly within 2.1–2.9 ns for all solutions. The close correlation revealed between the Na-line and continuum emission flashes allows us to conclude that bubbles producing both types of flashes emit them simultaneously with a measured delay of ~0.5 ns for Na.

Realization of a time-correlated photon counting technique for fluorescence analysis.

An ultralow level light detection module, the time-correlated photon counter, is proposed and evaluated for fluorescence analysis. The time-correlated photon counter employs a silicon photomultiplier as a photon counting sensor in conjunction with a Poisson statistics algorithm and a double time windows technique, and therefore it can accurately count the photon number. The time-correlated photon counter is compatible with the time-correlated single photon counting technique and can record the arrival time of very faint light signals. This low-cost and compact instrument was used to analyze the intensity and lifetime of fluorescein isothiocyanate; a limit of detection of 16 pg/ml with a large linear dynamic range from 2.86 pg/ml to 0.5 µg/ml was obtained, and the lifetime of fluorescein isothiocyanate was measured to be 3.758 ns, which agrees well with the results of a sophisticated commercial fluorescence analysis instrument. The time-correlated photon counter may be useful in applications such as point-of-care testing.

Ultra-Low Level Light Detection Based on the Poisson Statistics Algorithm and a Double Time Windows Technique With Silicon Photomultiplier

In this paper, we present a method to detect ultra-low level light ranging from several mean photoelectrons (MPEs) down to 10−5 MPEs. It is based on the Poisson statistics algorithm and a double time windows technique with a silicon photomultiplier (SiPM) and a field programmable gate array (FPGA). It measures the mean incident photoelectron number of pulsed light with zero peak statistics of the Poisson distribution to reduce the influence of the correlation noises and the dark count rate (DCR) fluctuation. The linear measurement ranges from ~10−4 MPEs to ~10 MPEs (i.e., ~97 dB) were demonstrated with double 35-ns time windows at room temperature. Its upper detection limit is determined by the availability for the zero peak counts of the Poisson distribution, and the lower detection limit is mainly determined by the DCR of the SiPM. By narrowing the time window to ~550 ps or decreasing the operating temperature of SiPM to −30 °C, the detection limit can be further decreased to ~10−5 MPEs. This method, with capability to record the arrival time of incident photons, demonstrated an instrument response function (IRF) of ~214.9 ps (FWHM), showing its compatibility to the time correlated photon counting (TCPC) technique and the time of flight (TOF) measurement.

Methods toward simplification of time resolved fluorescence anisotropy in proteins labeled with NBD (4-chloro-7-nitrobenzofurazan) adducts

The analysis of time resolved fluorescence anisotropy for NBD tagged proteins is difficult when multiple exponential components arise from heterogeneous amino acid fluorescent adducts. Two approaches were taken toward simplification. First, N terminal selective labeling of tear lipocalin with NBD-Cl was attempted at pH 7.0. While lysines were predominantly labeled at pH 8.0, selective N terminal labeling was attained at neutral pH. Second, fluorescence anisotropic decay analysis was simplified to recover only the rotational correlation time of the protein not the side chain. The boundaries for analysis of anisotropic decays were limited to the longer lifetimes. A modified tail fit enabled fitting the anisotropic decay to a single exponential. The correlation time for tear lipocalin matched published values. Additionally, a method for normalization of acquisition times of vertically (VV) and horizontally (VH) polarized fluorescence emission decays is presented for time-resolved anisotropy. Here it is applied to Picoharp software (Picoquant, Berlin). Picoharp software is programmed with an automatic stop at unequal acquisition times if the fluorescent counts exceeds a default. The method adjusts the intensity decays to the same acquisition time and is applicable to all time-resolved anisotropic decay data collected with time-correlated photon counting.•NBD labeling at pH 7.0 was not selective for N terminus of LCN1.•Constraints for range simplifies fittings of anisotropic decays.•Different acquisition times for decays can be normalized to facilitate fitting in data obtained by Picoharp.

Time-resolved double-resonance spectroscopy: Lifetime measurement of the electronic state of molecular sodium.

We report on the lifetime measurement of the state of Na2 molecules, produced in a heat-pipe oven, using a time-resolved spectroscopic technique. The level was populated by two-step two-color double resonance excitation via the intermediate state. The excitation scheme was done using two synchronized pulsed dye lasers pumped by a Nd:YAG laser operating at the second harmonics. The fluorescence emitted upon decay to the final state was measured using a time-correlated photon counting technique, as a function of argon pressure. From this, the radiative lifetime was extracted by extrapolating the plot to collision-free zero pressure. We also report the calculated radiative lifetimes of the ro-vibrational levels in the range of = 0-200 with J = 1 and J = 31 using the LEVEL program for bound-bound and the BCONT program for bound-free transitions. Our calculations reveal the importance of the bound-free transitions on the lifetime calculations and a large difference of about a factor of three between the J = 1 and J = 31 for the = 40 and = 100, respectively, due to the wavefunction alternating between having predominantly inner and outer well amplitude.

Quantum description of timing jitter for single-photon ON-OFF detectors

In the context of ultra-fast quantum communication and random number generation, detection timing-jitters represent a strong limitation as they can introduce major time-tagging errors and affect the quality of time-correlated photon counting or quantum state engineering. Despite their importance in emerging photonic quantum technologies, no detector model including such effects has been developed so far. We propose here an operational theoretical model based on POVM density formalism able to explicitly quantify the effect of timing-jitter for a typical class of single photon detector. We apply our model to some common experimental situations.

Two-Color Pump-Probe Measurement of Photonic Quantum Correlations Mediated by a Single Phonon.

We propose and demonstrate a versatile technique to measure the lifetime of the one-phonon Fock state using two-color pump-probe Raman scattering and spectrally resolved, time-correlated photon counting. Following pulsed laser excitation, the n=1 phonon Fock state is probabilistically prepared by projective measurement of a single Stokes photon. The detection of an anti-Stokes photon generated by a second, time-delayed laser pulse probes the phonon population with subpicosecond time resolution. We observe strongly nonclassical Stokes-anti-Stokes correlations, whose decay maps the single phonon dynamics. Our scheme can be applied to any Raman-active vibrational mode. It can be modified to measure the lifetime of n≥1 Fock states or the phonon quantum coherences through the preparation and detection of two-mode entangled vibrational states.

A total internal reflection-fluorescence correlation spectroscopy setup with pulsed diode laser excitation.

Fluorescence correlation spectroscopy (FCS) measures fluctuations in a (sub-)femtoliter volume to analyze the diffusive behavior of fluorescent particles. This highly sensitive method has proven to be useful for the analysis of dynamic biological systems as well as in chemistry, physics, and material sciences. It is routinely performed with commercial fluorescence microscopes, which provide a confined observation volume by the confocal technique. The evanescent wave of total internal reflectance (TIR) is used in home-built systems to permit a surface sensitive FCS analysis. We present a combined confocal and TIR-FCS setup which uses economic low-power pulsed diode lasers for excitation. Excitation and detection are coupled to time-correlated photon counting hardware. This allows simultaneous fluorescence lifetime and FCS measurements in a surface-sensitive mode. Moreover, the setup supports fluorescence lifetime correlation spectroscopy at surfaces. The excitation can be easily switched between TIR and epi-illumination to compare the surface properties with those in liquid bulk. The capabilities of the presented setup are demonstrated by measuring the diffusion coefficients of a free dye molecule, a labeled polyethylene glycol, and a fluorescent nanoparticle in confocal as well as in TIR-FCS.

Temperature-dependent properties of single long-wavelength InGaAs quantum dots embedded in a strain reducing layer

We report on temperature-dependent investigations of single metal-organic vapor phase epitaxy-grown In(Ga)As/GaAs quantum dots at wavelengths above 1 μm. Here, two types of samples are compared, whereas the quantum dots differ in the material composition and are embedded in a strain reducing layer to achieve an emission redshift. The analysis is performed by standard micro-photoluminescence spectroscopy, time-correlated photon counting, and intensity second-order autocorrelation measurements. It is found that the long-wavelength quantum dots experience a high charge carrier confinement (∼200 meV), but the thermal emission of carriers into the barrier or the wetting layer is mainly dominated by the shell spacing of individual dots. Additionally, we demonstrate that the single-dot carrier dynamics is reservoir-dominated. The influence of the strain reducing layers seems to cause this effect, leading to changes in the effective dot filling rate and charge configuration. Single-photon emission is preserved up to 77 K for a sample spectrally reaching the telecom O-band and then allowing for quantum dot single-photon operation at liquid nitrogen temperatures.

Nanoscale volume confinement and fluorescence enhancement with double nanohole aperture.

Diffraction ultimately limits the fluorescence collected from a single molecule, and sets an upper limit to the maximum concentration to isolate a single molecule in the detection volume. To overcome these limitations, we introduce here the use of a double nanohole structure with 25 nm gap, and report enhanced detection of single fluorescent molecules in concentrated solutions exceeding 20 micromolar. The nanometer gap concentrates the light into an apex volume down to 70 zeptoliter (10−21 L), 7000-fold below the diffraction-limited confocal volume. Using fluorescence correlation spectroscopy and time-correlated photon counting, we measure fluorescence enhancement up to 100-fold, together with local density of optical states (LDOS) enhancement of 30-fold. The distinctive features of double nanoholes combining high local field enhancement, efficient background screening and relative nanofabrication simplicity offer new strategies for real time investigation of biochemical events with single molecule resolution at high concentrations.

Improving z-tracking accuracy in the two-photon single-particle tracking microscope.

Here, we present a method that can improve the z-tracking accuracy of the recently invented TSUNAMI (Tracking of Single particles Using Nonlinear And Multiplexed Illumination) microscope. This method utilizes a maximum likelihood estimator (MLE) to determine the particle's 3D position that maximizes the likelihood of the observed time-correlated photon count distribution. Our Monte Carlo simulations show that the MLE-based tracking scheme can improve the z-tracking accuracy of TSUNAMI microscope by 1.7 fold. In addition, MLE is also found to reduce the temporal correlation of the z-tracking error. Taking advantage of the smaller and less temporally correlated z-tracking error, we have precisely recovered the hybridization-melting kinetics of a DNA model system from thousands of short single-particle trajectories in silico. Our method can be generally applied to other 3D single-particle tracking techniques.

A flexible approach to assess fluorescence decay functions in complex energy transfer systems.

BackgroundTime-correlated Förster resonance energy transfer (FRET) probes molecular distances with greater accuracy than intensity-based calculation of FRET efficiency and provides a powerful tool to study biomolecular structure and dynamics. Moreover, time-correlated photon count measurements bear additional information on the variety of donor surroundings allowing more detailed differentiation between distinct structural geometries which are typically inaccessible to general fitting solutions.ResultsHere we develop a new approach based on Monte Carlo simulations of time-correlated FRET events to estimate the time-correlated single photon counts (TCSPC) histograms in complex systems. This simulation solution assesses the full statistics of time-correlated photon counts and distance distributions of fluorescently labeled biomolecules. The simulations are consistent with the theoretical predictions of the dye behavior in FRET systems with defined dye distances and measurements of randomly distributed dye solutions. We validate the simulation results using a highly heterogeneous aggregation system and explore the conditions to use this tool in complex systems.ConclusionThis approach is powerful in distinguishing distance distributions in a wide variety of experimental setups, thus providing a versatile tool to accurately distinguish between different structural assemblies in highly complex systems.Electronic supplementary materialThe online version of this article (doi:10.1186/s13628-015-0020-z) contains supplementary material, which is available to authorized users.

Note: Optical trigger device with sub-picosecond timing jitter and stability

We are presenting the design, construction, and overall performance of the optical trigger device. This device generates an electrical signal synchronously to the detected ultra-short optical pulse. The device was designed for application in satellite laser ranging and laser time transfer experiments, time correlated photon counting and similar experiments, where picosecond timing resolution and detection delay stability are required. It consists of the ultrafast optical detector, signal discriminator, output pulse forming circuit, and output driver circuits. It was constructed as a single compact device to optimize their matching and maintain stability. The detector consists of an avalanche photodiode--both silicon and germanium types may be used to cover the wavelength range of 350-1550 nm. The analogue signal of this photodiode is sensed by the ultrafast comparator with 8 GHz bandwidth. The ps clock distribution circuit is used to generate the fast rise/fall time output pulses of pre-set length. The trigger device timing performance is excellent: the random component of the timing jitter is typically 880 fs, the temperature dependence of the detection delay was measured to be 370 fs/K. The systematic error contribution depends on the laser used and its stability. The sub-ps values have been obtained for various laser sources.

Anomalous photo-induced spectral changes in CdSe/ZnS quantum dots

We study photo-induced static and dynamic spectral changes in self-assembled CdSe/ZnS quantum dot (QD) thin films with varying QD concentrations under ambient conditions. Using spatially resolved scanning photoluminescence microscopy in conjunction with spectrally resolved time-correlated photon counting, we measure the variations in spectral intensity, emission wavelength, and recombination lifetimes as functions of photo-exposure time. We find that at low concentrations photo-darkening and photo-oxidation rates slow down with increasing QD density, but in the high concentration limit these rates are strongly enhanced. Our measurements lead us to conclude that the interplay of photo-induced surface trap discharging with preferential photo-oxidation of smaller QDs is further modulated by resonant energy transfer driven by strong inter-dot interactions in highly concentrated samples. Our results would imply that the efficiency and longevity of semiconducting nanoparticle based opto-electronic devices will be limited by the concentration of the active material