Time-correlated Single Photon Counting Electronics Research Articles

Scaling waveguide-integrated superconducting nanowire single-photon detector solutions to large numbers of independent optical channels.

Superconducting nanowire single-photon detectors are an enabling technology for modern quantum information science and are gaining attractiveness for the most demanding photon counting tasks in other fields. Embedding such detectors in photonic integrated circuits enables additional counting capabilities through nanophotonic functionalization. Here, we show how a scalable number of waveguide-integrated superconducting nanowire single-photon detectors can be interfaced with independent fiber optic channels on the same chip. Our plug-and-play detector package is hosted inside a compact and portable closed-cycle cryostat providing cryogenic signal amplification for up to 64 channels. We demonstrate state-of-the-art multi-channel photon counting performance with average system detection efficiency of (40.5 ± 9.4)% and dark count rate of (123 ± 34) Hz for 32 individually addressable detectors at minimal noise-equivalent power of (5.1 ± 1.2) · 10-18 W/Hz. Our detectors achieve timing jitter as low as 26ps, which increases to (114 ± 17) ps for high-speed multi-channel operation using dedicated time-correlated single photon counting electronics. Our multi-channel single photon receiver offers exciting measurement capabilities for future quantum communication, remote sensing, and imaging applications.

Photon arrival time tagging with many channels, sub-nanosecond deadtime, very high throughput, and fiber optic remote synchronization.

Time-Correlated Single Photon Counting (TCSPC) and time tagging of individual photon detections are powerful tools in many quantum optical experiments and other areas of applied physics. Using TCSPC, e.g., for the purpose of fluorescence lifetime measurements, is often limited in speed due to dead-time losses and pileup. We show that this limitation can be lifted by reducing the dead-time of the timing electronics to the absolute minimum imposed by the speed of the detector signals while maintaining high temporal resolution. A complementing approach to speedy data acquisition is parallelization by means of simultaneous readout of many detector channels. This puts high demands on the data throughput of the TCSPC system, especially in time tagging of individual photon arrivals. Here, we present a new design approach, supporting up to 16 input channels, an extremely short dead-time of 650 ps, very high time tagging throughput, and a timing resolution of 80 ps. In order to facilitate remote synchronization of multiple such instruments with highest precision, the new TCSPC electronics provide an interface for White Rabbit fiber optic networks. Beside fundamental research in the field of astronomy, such remote synchronization tasks arise routinely in quantum communication networks with node to node distances on the order of tens of kilometers. In addition to showing design features and benchmark results of new TCSPC electronics, we present application results from spectrally resolved and high-speed fluorescence lifetime imaging in medical research. We furthermore show how pulse-pileup occurring in the detector signals at high photon flux can be corrected for and how this data acquisition scheme performs in terms of accuracy and efficiency.

A 16 Channel Spad Array for High-Throughput Tcspc Measurements of Single-Molecule FRET of Freely Diffusing Molecules

Single-molecule techniques probe the spectroscopic properties of biomolecules one molecule a time. These approaches allow detecting and quantifying subpopulations which would be masked in ensemble measurements. Single-molecule FRET (smFRET), which indirectly measures the distance between fluorophores, has been extensively used to characterize the conformation of biomolecules or molecular interactions in complex biomolecular assemblies among many applications. FRET efficiency is typically computed ratiometrically from photon counts detected in two spectral channels. However, this approach requires careful correction and calibrations to account for background and differences in quantum yield and detection efficiency between the two channels. Alternatively, time-correlated single-photon counting (TCSPC) measurements can provide the same result from the donor fluorescence lifetime only, which is much less sensitive to these experimental aspects.In both approaches, a fundamental limitation remains the long acquisition time needed to accumulate sufficient statistics from freely-diffusing molecules, severely limiting the throughput of these techniques and preventing the study of rapidly evolving systems. To circumvent these limitations, we have developed a multi-spot setup using sensitive SPAD arrays, achieving high-throughput data acquisition for FCS and smFRET. These systems had so far been limited to counting applications.In this work, we present the first proof-of-principle TCSPC multispot system employing a simple line-excitation, a new dedicated SPAD array with low jitter and integrated multichannel TCSPC electronics[1]. This system allows single-molecule detection on all 16 channels using a total excitation power of 100 mW. We report measurements on short doubly-labeled dsDNA molecules and demonstrate uniform performance across the 16 detection channels. These results are a first step toward extending current single-spot TCSPC spectroscopic techniques (ns-ALEX or PIE) to a multispot geometry for high-throughput time-resolved smFRET measurements.[1] Cuccato, A et al. IEEE Phot. J. 5-5, p6801514, 2013.

Sensitive Time-Correlated Single Photon Counting Enables Efficient Singlet Oxygen Detection

Single photon counting based data acquisition has proven to yield a major sensitivity increase in the optical evaluation of pharmaceuticals and biotechnology products. We will show for the first time that a state of the art time-correlated single photon counting (TCSPC) based fluorescence lifetime spectrometer is able to quantify singlet oxygen generation and to characterize the singlet oxygen phosphorescence decay. This makes TCSPC based fluorescence lifetime spectrometers a valuable tool for studying photosensitizers widely used for example in photodynamic therapy (PDT). The detection of the faint singlet oxygen phosphorescence signal has been made possible by using a special burst mode for the pulsed laser excitation and a new generation of TCSPC electronics with a significantly reduced dead-time which enables efficient multi-stop photon detection.Thanks to a recently developed integrating sphere add-on we are also able to measure fluorescence quantum yields with the same instrument. Leveraging the possibility to measure fluorescence lifetime in conjunction with quantum yield, we performed a systematic investigation of the relation between reduced fluorescence emission and different contributions of dynamic and static quenching processes.Furthermore, since quenching normally does not affect the radiative rate constant, this combined set-up allows to verify the accuracy of the extracted lifetime. Especially for very short lifetimes in the range of the instrument response function the presented method allows to assess whether the proper fluorescence lifetime was extracted.

Quantitative Analysis of Tat Peptide Binding to Import Carriers Reveals Unconventional Nuclear Transport Properties

A detailed study of nuclear import mediated by the HIV-1 Tat peptide (47YGRKKRRQRRR57, TatRRR) is reported. Fluorescence-based measurements, calibration of protein concentrations, and binding assays are exploited to address the physicochemical mechanisms of Tat peptide recognition by the classical importin α (Impα) and importin β (Impβ) receptors both in vitro and in intact cells. We show that TatRRR is an unconventional nuclear localization sequence that binds directly to both Impα and Impβ carriers in the absence of competitors (in vitro), whereas this property is silenced in the actual cellular environment. In the latter case, Impα/β-dependent nuclear import can be successfully restored by replacing the "RRR" stretch with "GGG". We apply a recently developed method to determine quantitatively TatGGG affinity for each receptor. Based on these results, we can rationalize previous controversial reports on the Tat peptide and provide coherent guidelines for the design of novel intracellular targeting sequences.

Probing Nuclear Localization Signal-Importin α Binding Equilibria in Living Cells

The regulated process of protein import into the nucleus of a eukaryotic cell is mediated by specific nuclear localization signals (NLSs) that are recognized by protein-import receptors. In this study, we present fluorescence-based methods to quantitatively address the physicochemical details of NLS recognition by the receptor protein importin alpha (Impalpha) in living cells. First, by combining fluorescence recovery after photobleaching measurements and protein-concentration calibration, we quantitatively define nuclear import saturability and afford an affinity value for NLS-Impalpha binding. Second, by fluorescence lifetime imaging microscopy, we directly monitor the occurrence of NLS-Impalpha interaction and measure its effective dissociation constant (K(D)) in the actual cellular environment. Our kinetic and thermodynamic analyses independently indicate that the subsaturation of Impalpha with the expressed NLS cargo regulates nuclear import rates in living cells, in contrast to what can be predicted on the basis of available in vitro data. Finally, our experiments also provide evidence for the regulation of nuclear import mediated by the intrasteric importin beta-binding domain of Impalpha and yield the first estimate of its autoinhibition energy in living cells.

Time-resolved optical imager for assessment of cerebral oxygenation

A time-resolved optical instrument allowing for noninvasive assessment of cerebral oxygenation is presented. The instrument is equipped with picosecond diode lasers, fast photodetectors, and time-correlated single photon counting electronics. This technology enables depth-resolved estimation of changes in absorption and, in consequence, assessment of changes in hemoglobin concentrations in the brain cortex. Changes in oxyhemoglobin (HbO(2)) and deoxyhemoglobin (Hb) can be evaluated selectively in extra- and intracerebral tissue compartments using the moments of distributions of times of flight of photons measured at two wavelengths in the near-infrared region. The combination of the data acquired from multiple sources and detectors located on the surface of the head with the depth-resolved analysis, based on the moments, enables imaging of cortex oxygenation. Results of the tests on physical phantoms as well as in vivo validation of the instrument during the motor stimulation experiment are presented.

Near‐infrared time‐resolved fluorescence lifetime determinations in poly(methylmethacrylate) microchip electrophoresis devices

High aspect-ratio microstructures were hot-embossed in polymer substrates with a molding tool fabricated using lithography/electroplating/forming (LIGA). The resulting devices were used for the electrophoretic separation of oligonucleotides labeled with near-infrared (near-IR) dyes. Near-IR time-resolved fluorescence was used as an identification method for the labeling dyes. The detection apparatus consisted of a pulsed laser diode operating at 680 nm, a single-photon avalanche diode, an integrated microscope, and a PC-board incorporating time-correlated single photon counting electronics. Investigation of the optical quality and amount of autofluorescence generated from different polymer substrates was carried out in the near-IR region for determining compatibility with time-resolved fluorescence. Our results revealed that of several poly(methylmethacrylate)(PMMA) substrates, brand Plexiglas offered minimal replication errors in the embossed features using appropriate embossing conditions with low background fluorescence contributions to the observed decay. Near-IR dye-labeled oligonucleotides were separated to determine the applicability of fluorescence lifetime discrimination between Cy5.5 (tauf = 930 ps) and IRD700 (tauf = 851 ps) labeling dyes during the microchip separation. These dyes were used to label T-fragments (thymine) of an M13mp18 ssDNA template. The DNA ladders were electrophoresed at 130 V/cm in a 4% linear polyacrylamide gel (LPA) gel matrix in a 9.5 cm long serpentine channel heated to 50 degrees C. The electropherogram revealed that the lifetimes could be accurately read well beyond 450 bases, although single-base pair resolution in the electropherogram was difficult to achieve due to potential solute-wall interactions in the polymer microdevice or the electroosmotic flow (EOF) properties of the device. The relative standard deviations secured for individual bands in the electropherogram were similar to those obtained using capillary gel electrophoresis, in spite of the lower load volume.