Time-resolved Single Photon Counting Research Articles

Large active-area superconducting microwire detector array with single-photon sensitivity in the near-infrared

Superconducting nanowire single photon detectors (SNSPDs) are the highest-performing technology for time-resolved single-photon counting from the UV to the near-infrared. The recent discovery of single-photon sensitivity in micrometer-scale superconducting wires is a promising pathway to explore for large active area devices with application to dark matter searches and fundamental physics experiments. We present 8-pixel 1 mm2 superconducting microwire single photon detectors (SMSPDs) with 1 μm-wide wires fabricated from WSi and MoSi films of various stoichiometries using electron-beam and optical lithography. Devices made from all materials and fabrication techniques show saturated internal detection efficiency at 1064 nm in at least one pixel, and the best performing device made from silicon-rich WSi shows single-photon sensitivity in all eight pixels and saturated internal detection efficiency in 6/8 pixels. This detector is the largest reported active-area SMSPD or SNSPD with near-IR sensitivity, and it extends the SMSPD to an array format. By further optimizing the photolithography techniques presented in this work, a viable pathway exists to realize larger devices with cm2-scale active area and beyond.

Simultaneous single photon counting and ion channel recording from free-standing lipid bilayers on a microstructured array.

Fluorescence microscopy and, in particular, single molecule optical spectroscopy is of great potential value in characterizing the structural dynamics of membranes and membrane proteins. A particular challenge is to combine such high-resolution optical measurements with high-resolution voltage clamp electrical recordings that can provide direct information on single ion channel gating a block by drugs and analytes. Here, we report on the use of a novel chip-based, 2 × 2 arrangement of microelectrode cavities with borosilicate glass optical windows which facilitates optical access on an inverted microscope with water or oil-immersion objectives of high numerical aperture to horizontal free-standing lipid membranes, while controlling membrane voltage and recording currents using individual micropatterned, ring-shaped Ag/AgCl-electrodes to perform time-resolved single photon counting on free-standing membranes spanning sub-nanoliter cavities. Single channel activity induced by the fluorescently labelled pore-forming peptide ceratotoxin A was simultaneously acquired. This device allows for rapid formation of four membranes that are simultaneously monitored electrically using a four-channel amplifier and can be sequentially optically addressed, greatly reducing the time needed for successful experimentation. During our experiments, we noted autofluorescence of the borosilicate to be a limiting factor for wide-field low-intensity fluorescence recording, but this can likely be circumvented by using quartz glass instead. In summary, the novel device increases the likelihood of realizing the long standing ambition to correlate structural and functional dynamics of single membrane proteins on the single molecule level.

Quantum tomography of entangled qubits by time-resolved single-photon counting with time-continuous measurements

In this article, we introduce a framework for entanglement characterization by time-resolved single-photon counting with measurement operators defined in the time domain. For a quantum system with unitary dynamics, we generate time-continuous measurements by shifting from the Schrödinger picture to the Heisenberg representation. In particular, we discuss this approach in reference to photonic tomography. To make the measurement scheme realistic, we impose timing uncertainty on photon counts along with the Poisson noise. Then, the framework is tested numerically on quantum tomography of qubits. Next, we investigate the accuracy of the model for polarization-entangled photon pairs. Entanglement detection and precision of state reconstruction are quantified by figures of merit and presented on graphs versus the amount of time uncertainty.

High-resolution nanosecond spectroscopy of even-parity Rydberg excitons in Cu2O

We present a study of even parity Rydberg exciton states in cuprous oxide using time-resolved second harmonic generation (SHG). Excitonic states with principal quantum number n = 5 - 12 were excited by nanosecond pulses around 1143 nm. Using time-resolved single-photon counting, the coherently generated second harmonic was isolated both temporally and spectroscopically from inelastic emission due to lower-lying free and bound excitonic states, which included narrow resonances at 1.99 eV associated with an exceptional lifetime of 641 $\pm$ 7 $\mu$s. The near transform-limited excitation bandwidth enabled detailed measurements of the exciton lineshape and position, from which we obtained values for the quantum defects of the S and D excitonic states associated with the appropriate crystal symmetries. Odd parity P and F excitonic states were also observed, in accordance with predicted quadrupole-allowed two-photon excitation processes. We compared our measurements to conventional one-photon spectroscopy in the same sample, and find that the SHG spectrum is cut off at a lower principal quantum number (n = 12 vs n = 15). We attribute this effect to a combination of spatial inhomogeneities and local heating, and discuss the prospects for observing higher principal quantum number even parity states in future experiments.

A chip-based array for high-resolution fluorescence characterization of free-standing horizontal lipid membranes under voltage clamp.

Optical techniques, such as fluorescence microscopy, are of great value in characterizing the structural dynamics of membranes and membrane proteins. A particular challenge is to combine high-resolution optical measurements with high-resolution voltage clamp electrical recordings providing direct information on e.g. single ion channel gating and/or membrane capacitance. Here, we report on a novel chip-based array device which facilitates optical access with water or oil-immersion objectives of high numerical aperture to horizontal free-standing lipid membranes while controlling membrane voltage and recording currents using individual micropatterned Ag/AgCl-electrodes. Wide-field and confocal imaging, as well as time-resolved single photon counting on free-standing membranes spanning sub-nanoliter cavities are demonstrated while electrical signals, including single channel activity, are simultaneously acquired. This optically addressable microelectrode cavity array will allow combined electrical-optical studies of membranes and membrane proteins to be performed as a routine experiment.

Time-resolved spectral-domain optical coherence tomography with CMOS SPAD sensors.

We present a first spectral-domain optical coherence tomography (SD-OCT) system deploying a complementary metal-oxide-semiconductor (CMOS) single-photon avalanche diode (SPAD) based, time-resolved line sensor. The sensor with 1024 pixels achieves a sensitivity of 87 dB at an A-scan rate of 1 kHz using a supercontinuum laser source with a repetition rate of 20 MHz, 38 nm bandwidth, and 2 mW power at 850 nm centre wavelength. In the time-resolved mode of the sensor, the system combines low-coherence interferometry (LCI) and massively parallel time-resolved single-photon counting to control the detection of interference spectra on the single-photon level based on the time-of-arrival of photons. For proof of concept demonstration of the combined detection scheme we show the acquisition of time-resolved interference spectra and the reconstruction of OCT images from selected time bins. Then, we exemplify the temporal discrimination feature with 50 ps time resolution and 249 ps timing uncertainty by removing unwanted reflections from along the optical path at a 30 mm distance from the sample. The current limitations of the proposed technique in terms of sensor parameters are analysed and potential improvements are identified for advanced photonic applications.

Phase estimation of time-bin qudits by time-resolved single-photon counting

We present a comprehensive framework for quantum state tomography of time-bin qudits sent through a fiber. Starting from basic assumptions, we define a positive-operator valued measure which is then applied to the quantum state reconstruction problem. A realistic scenario is considered where the time uncertainty of the detector is treated as a source of experimental noise. The performance of the quantum tomography framework is examined through a series of numerical simulations conducted for different parameters describing the apparatus. The quality of state recovery, quantified by the notion of minimum fidelity, is depicted on graphs for a range of fiber lengths. Special attention is paid to relative phase reconstruction for qubits and qutrits. The results present relevant interdependence between the fiber length and the detector jitter.

Fast, large area multiphoton exoscope (FLAME) for macroscopic imaging with microscopic resolution of human skin

We introduce a compact, fast large area multiphoton exoscope (FLAME) system with enhanced molecular contrast for macroscopic imaging of human skin with microscopic resolution. A versatile imaging platform, FLAME combines optical and mechanical scanning mechanisms with deep learning image restoration to produce depth-resolved images that encompass sub-mm2 to cm2 scale areas of tissue within minutes and provide means for a comprehensive analysis of live or resected thick human skin tissue. The FLAME imaging platform, which expands on a design recently introduced by our group, also features time-resolved single photon counting detection to uniquely allow fast discrimination and 3D virtual staining of melanin. We demonstrate its performance and utility by fast ex vivo and in vivo imaging of human skin. With the ability to provide rapid access to depth resolved images of skin over cm2 area and to generate 3D distribution maps of key sub-cellular skin components such as melanocytic dendrites and melanin, FLAME is ready to be translated into a clinical imaging tool for enhancing diagnosis accuracy, guiding therapy and understanding skin biology.

Colorimetric and fluorescence sensing of Zn2+ ion and its bio-imaging applications based on macrocyclic “tet a” derivative

We report on the synthesis and characterization of trans N, N′-di-substituted macrocyclic “tet a” probe (L) for metal ion sensing. Both the colorimetric and fluorescent titration studies are performed with different metal ions. The results have suggested that the probe L is very selective and sensitive towards Zn2+ ions with significant changes in color. The pendant armed macrocyclic “tet a” probe has exhibited 1.28× 105 M−1 binding constant and virtuous selectivity for Zn2+ ion than other common metal ions. The detection limit of the probe towards Zn2+ ion is 0.027 nM. The selective sensing of Zn2+ ion is efficiently reversible with EDTA, which is demonstrated for five cycles without losing sensitivity. The time-resolved single-photon counting (TCSPC) studies have determined the average lifetime value for the probe L and L+ Zn2+ ion of 1.29 and 2.96 ns, respectively. The theoretical DFT studies have well supported the experimental outcomes. The practical application of the probe in visualizing intracellular Zn2+ ion distribution in live Artemia salina has proved the low cytotoxicity and cell membrane permeability of probe, which makes it capable of sensing Zn2+ ion in HeLa cells. Thus, the probe L can act as a selective recognition of Zn2+ ion in living cell applications.

A $192\times128$ Time Correlated SPAD Image Sensor in 40-nm CMOS Technology

A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$192 \times 128$ </tex-math></inline-formula> pixel single photon avalanche diode (SPAD) time-resolved single photon counting (TCSPC) image sensor is implemented in STMicroelectronics 40-nm CMOS technology. The 13% fill factor, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$18.4\,\,\mu \text {m} \times 9.2\,\,\mu \text{m}$ </tex-math></inline-formula> pixel contains a 33-ps resolution, 135-ns full scale, 12-bit time-to-digital converter (TDC) with 0.9-LSB differential and 5.64-LSB integral nonlinearity (DNL/INL). The sensor achieves a mean 219-ps full-width half-maximum (FWHM) impulse response function (IRF) and is operable at up to 18.6 kframes/s through 64 parallelized serial outputs. Cylindrical microlenses with a concentration factor of 3.25 increase the fill factor to 42%. The median dark count rate (DCR) is 25 Hz at 1.5-V excess bias. A digital calibration scheme integrated into a column of the imager allows off-chip digital process, voltage, and temperature (PVT) compensation of every frame on the fly. Fluorescence lifetime imaging microscopy (FLIM) results are presented.

Investigation on the interaction of Rutin with serum albumins: Insights from spectroscopic and molecular docking techniques

The binding interaction of Rutin, a flavonoid, with model transport proteins, bovine serum albumin (BSA) and human serum albumin (HSA), were investigated using different spectroscopic techniques, such as fluorescence, time-resolved single photon counting (TCSPC) and circular dichroism (CD) spectroscopy as well as molecular docking method. The emission studies revealed that the fluorescence quenching of BSA/HSA by Rutin occurred through a simultaneous static and dynamic quenching process, and we have evaluated both the quenching constants individually. The binding constants of Rutin-BSA and Rutin-HSA system were found to be 2.14 × 106 M−1 and 2.36 × 106 M−1 at 298 K respectively, which were quite high. Further, influence of some biologically significant metal ions (Ca2+, Zn2+ and Mg2+) on binding of Rutin to BSA and HSA were also investigated. Thermodynamic parameters justified the involvement of hydrogen bonding and weak van der Waals forces in the interaction of Rutin with both BSA and HSA. Further a site-marker competitive experiment was performed to evaluate Rutin binding site in the albumins. Additionally, the CD spectra of BSA and HSA revealed that the secondary structure of the proteins was perturbed in the presence of Rutin. Finally protein–ligand docking studies have also been performed to determine the probable location of the ligand molecule.

Fast recognition of single quantum dots from high multi-exciton emission and clustering effects.

Recognition of single quantum dots (QDs) from high multi-exciton emission and clustering effects is challenging using the conventional second-order correlation function method. Here we demonstrate a method for fast recognizing single QDs based on the probabilities of detecting single- and two-photon events. The time-tagged, time-resolved and time-correlated single-photon counting technique is applied to effectively remove multi-exciton emission and low-counting background. By this way, single QDs can be fastly recognized by the spatial coincidence-counting model. In addition, the fast recognition of single QDs by using the collected photons during the confocal scanning imaging process has been achieved synchronously.

Study of the Eosin-Y/PAMAM interactions in alkaline aqueous solution

The interactions between the xanthene dye Eosin-Y (Eos) and amino-terminated PAMAM dendrimers of low generations (G0–G3) were studied in alkaline water solution. The effect of concentration and generation of the dendrimer on the photophysics of Eos was evaluated by means of absorption and fluorescence spectroscopies. The observed spectral changes were ascribed to the association dye/dendrimer. From these data, the Eos/PAMAM binding constants (Kbind) were determined, which strongly increased with the size of the dendrimer. Stationary fluorescence anisotropy and time-resolved single photon counting were also used to characterize the association process. The restriction in the rotational diffusion of the Eos increased as a function of the concentration and generation of PAMAM, as determined by anisotropy measurements. Biexponential fluorescence decays were obtained in the presence of G3, and the respective lifetimes were ascribed to free and bound Eos species. These results correlate with Kbind values and suggest the formation of host/guest system with larger dendrimers. Therefore, this environmentally-friendly dye/dendrimer system would be appropriate for potential applications in fields such as drugs delivery and photopolymerization.

Studying of the photoluminescence characteristics of Au(0)@Au(I)-thiolate core–shell nanoclusters

The photoluminescence characteristics of Au(0)@Au(I)-thiolate Core–shell nanoclusters (Au-NCs), whose size is <2 nm, are investigated. We use the time-resolved single-photon counting technique and the transient absorption technique to distinguish the mechanism of the photoluminescence of Au-NCs. The dynamic spectral data show that the photoluminescence mechanism should be composed of the fluorescence and the phosphorescence. Meanwhile, Au-NCs have the aggregation induced photoluminescence enhancement property, which is attributed to the suppression of conversion from the singlet state to triplet state when they aggregated together. In addition, the photoluminescence characteristics of aggregated Au-NCs should be further improved through infiltrating them into photonic crystals.

Thermodynamic Characterization of Proteins with Electrically Actuated DNA Nanolevers

switchSENSE is a novel chip-based bioanalytical method that employs electrically switchable DNA nanolevers as molecular probes. While the nanolevers are driven to oscillate on the surface of microelectrodes by alternating electric fields, their orientation-switching is detected by time-resolved single photon counting. By measuring the molecular dynamics of the switching process it is possible to gauge the hydrodynamic properties of analyte molecules which are bound to the oscillating nanolevers.

Complete and Compact 32-Channel System for Time-Correlated Single-Photon Counting Measurements

Single-photon detectors play a key role in many research fields such as biology, chemistry, medicine, and space technology, and in recent years, single-photon avalanche diodes (SPADs) have become a valid alternative to photo multiplier tubes (PMTs). Moreover, scientific research has recently focused on single-photon detector arrays, pushed by a growing demand for multichannel systems. In this scenario, we developed a compact 32-channel system for time-resolved single-photon counting applications. The system is divided into two independent modules: a photon detection head including a 32 × 1 SPAD array built in custom technology, featuring high time resolution, high photon detection efficiency (44% at 550 nm), and low dark count rate (mean value <; 400 cps at -10 °C) at 6-V excess bias voltage and a 32-channel acquisition system able to perform time-correlated single-photon counting (TCSPC) measurements. The TCSPC module includes eight four-channel time-to-amplitude converter (TAC) arrays, built-in 0.35-μm Si-Ge BiCMOS technology, characterized by low differential non-linearity (rms value lower than 0.15% of the time bin width) and variable full-scale range. The system response function of this TCSPC instrumentation achieves a mean time resolution of 63 psFWHM, considering a mean count rate of 1 Mcps.

Understanding the Parameters Affecting the Photoluminescence of Silicon Nanoparticles

Fil: Llansola Portoles, Manuel Jose. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquimicas Teoricas y Aplicadas; Argentina

Dating bloodstains with fluorescence lifetime measurements.

Dating bloodstains with fluorescence lifetime measurements.

New Heteroleptic Bis-Phenanthroline Copper(I) Complexes with Dipyridophenazine or Imidazole Fused Phenanthroline Ligands: Spectral, Electrochemical, and Quantum Chemical Studies

Two new sterically challenged diimine ligands L(1) (2,9-dimesityl-2-(4'-bromophenyl)imidazo[4,5-f][1,10]phenanthroline) and L(2) (3,6-di-n-butyl-11-bromodipyrido[3,2-a:2',3'-c]phenazine) have been synthesized with the aim to build original heteroleptic copper(I) complexes, following the HETPHEN concept developed by Schmittel and co-workers. The structure of L(1) is based on a phen-imidazole molecular core, derivatized by two highly bulky mesityl groups in positions 2 and 9 of the phenanthroline cavity, preventing the formation of a homoleptic species, while L(2) is a dppz derivative, bearing n-butyl chains in α positions of the chelating nitrogen atoms. The unambiguous formation of six novel heteroleptic copper(I) complexes based on L(1), L(2), and complementary matching ligands (2,9-R(2)-1,10-phenanthroline, with R = H, methyl, n-butyl or mesityl) has been evidenced, and the resulting compounds were fully characterized. The electronic absorption spectra of all complexes fits well with DFT calculations allowing the assignment of the main transitions. The characteristics of the emissive excited state were investigated in different solvents using time-resolved single photon counting and transient absorption spectroscopy. The complexes with ligand L(2), bearing a characteristic dppz moiety, exhibit a very low energy excited-state which mainly leads to fast nonradiative relaxation, whereas the emission lifetime is higher for those containing the bulky ligand L(1). For example, a luminescence quantum yield of about 3 × 10(-4) is obtained with a decay time of about 50 ns for C2 ([Cu(I)(nBu-phen)(L(1))](+)) with a weak influence of strong coordinating solvent on the luminescence properties. Overall, the spectral features are those expected for a highly constrained coordination cage. Yet, the complexes are stable in solution, partly due to the beneficial π stacking between mesityl groups and vicinal phenanthroline aromatic rings, as evidenced by the X-ray structure of complex C3 ([Cu(I)(Mes-phen)(L(2))](+)). Electrochemistry of the copper(I) complexes revealed reversible anodic behavior, corresponding to a copper(I) to copper(II) transition. The half wave potentials increase with the steric bulk at the level of the copper(I) ion, reaching a value as high as 1 V vs SCE, with the assistance of ligand induced electronic effects. L(1) and L(2) are further end-capped by a bromo functionality. A Suzuki cross-coupling reaction was directly performed on the complexes, in spite of the handicapping lability of copper(I)-phenanthroline complexes.

Rotational Diffusion of the α2a Adrenergic Receptor Revealed by FlAsH Labeling in Living Cells

The fluorescein arsenical hairpin binder (FlAsH) shows much promise to determine the relative orientations of protein regions and structures even in living cells and in the plasma membrane. In this study, we characterized FlAsH's photophysical properties by steady-state anisotropy and time-resolved single photon counting for further applications with G-protein coupled receptors. We find that FlAsH has a relatively high initial anisotropy of 0.31 ± 0.01 and a three-component fluorescence lifetime with an average of 4.1 ± 0.1 ns. We characterized the FlAsH fluorophore orientation in the α 2A adrenergic receptor revealing rigid orientations of FlAsH in the membrane plane for rotational correlation times of ∼50 ns in living cells. To elucidate the fluorophore-membrane orientation and rotational correlation time, an anisotropy treatment similar to that of another researcher (Axelrod, D. 1979. Biophys. J. 26:557–573) was developed. The rotational correlation times were observed to increase by up to 16 ns after agonist addition. The rotational correlation time also allowed for a comparison to the theoretical relationship between translational and rotational diffusion (originally proposed by Saffman, P. G., and M. Delbrück. 1975. Proc. Natl. Acad. Sci. USA. 72:3111–3113) and revealed a discrepancy of a factor between 10 and 100.