Detected Photon Counts Research Articles

Two-layer reconstruction of Raman spectra in diffusive media based on an analytical model in the time domain.

We derive and validate an analytical model that describes the migration of Raman scattered photons in two-layer diffusive media, based on the diffusion equation in the time domain. The model is derived under a heuristic approximation that background optical properties are identical on the excitation and Raman emission wavelengths. Methods for the reconstruction of two-layer Raman spectra have been developed, tested in computer simulations and validated on tissue-mimicking phantom measurements data. Effects of different parameters were studied in simulations, showing that the thickness of the top layer and number of detected photon counts have the most significant impact on the reconstruction. The concept of quantitative, mathematically rigorous reconstruction using the proposed model was finally proven on experimental measurements, by successfully separating the spectra of silicone and calcium carbonate (calcite) layers, showing the potential for further development and eventual application in clinical diagnostics.

Analyzing the Effect of the Intra-Pixel Position of Small PSFs for Optimizing the PL of Optical Subpixel Localization

Subpixel localization techniques for estimating the positions of point-like images captured by pixelated image sensors have been widely used in diverse optical measurement fields. With unavoidable imaging noise, there is a precision limit (PL) when estimating the target positions on image sensors, which depends on the detected photon count, noise, point spread function (PSF) radius, and PSF’s intra-pixel position. Previous studies have clearly reported the effects of the first three parameters on the PL but have neglected the intra-pixel position information. Here, we develop a localization PL analysis framework for revealing the effect of the intra-pixel position of small PSFs. To accurately estimate the PL in practical applications, we provide effective PSF (ePSF) modeling approaches and apply the Cramér–Rao lower bound. Based on the characteristics of small PSFs, we first derive simplified equations for finding the best PL and the best intra-pixel region for an arbitrary small PSF; we then verify these equations on real PSFs. Next, we use the typical Gaussian PSF to perform a further analysis and find that the final optimum of the PL is achieved at the pixel boundaries when the Gaussian radius is as small as possible, indicating that the optimum is ultimately limited by light diffraction. Finally, we apply the maximum likelihood method. Its combination with ePSF modeling allows us to successfully reach the PL in experiments, making the above theoretical analysis effective. This work provides a new perspective on combining image sensor position control with PSF engineering to make full use of information theory, thereby paving the way for thoroughly understanding and achieving the final optimum of the PL in optical localization.

Deep-learning-based denoising of X-ray differential phase and dark-field images

PurposeStatistical photon noise has always been a common problem in X-ray multi-contrast imaging and significantly influenced the quality of retrieved differential phase and dark-field images. We intend to develop a deep learning-based denoising algorithm to reduce the noise of retrieved X-ray differential phase and dark-field images. MethodsA novel deep learning based image noise suppression algorithm (named DnCNN-P) is presented. We proposed two different denoising modes: Retrieval-Denoising mode (R-D mode) and Denoising-Retrieval mode (D-R mode). While the R-D mode denoises the retrieved images, the D-R mode denoises the raw phase stepping data. The two denoising modes are evaluated under different photon counts and visibilities. ResultsExperimental results show that with the algorithm DnCNN-P used, the D-R mode always exhibits a better noise reduction under diverse experimental conditions, even in the case of a low photon count and/or a low visibility. With a detected photon count of 1800 and a visibility of 0.3, compared to the differential phase images without denoising, the standard deviation is reduced by 89.1% and 16.4% in the D-R and R-D modes. Compared to the dark-field images without denoising, the standard deviation is reduced by 83.7% and 12.6% in the D-R and R-D modes, respectively.Conclusions.The novel supervised DnCNN-P algorithm can significantly reduce the noise in retrieved X-ray differential phase and dark-field images. We believe this novel algorithm can be a promising approach to improve the quality of X-ray differential phase and dark-field images, and therefore dose efficiency in future biomedical applications.

Time-Gated Photon Counting Receivers for Optical Wireless Communication

Photon counting detectors such as single-photon avalanche diode (SPAD) arrays are commonly considered for reliable optical wireless communication at power limited regimes. However, SPAD-based receivers suffer from significant dead time induced intersymbol interference (ISI) especially when the incident photon rate is relatively high and the dead time is comparable or even larger than the symbol duration, i.e., sub-dead-time regime. In this work, we propose a novel time-gated SPAD receiver to mitigate such ISI effects and improve the communication performance. When operated in the gated mode, the SPAD can be activated and deactivated in well-defined time intervals. We investigate the statistics of the detected photon count for the proposed time-gated SPAD receiver. It is demonstrated that the gate-ON time interval can be optimized to achieve the best bit error rate (BER) performance. Our extensive performance analysis illustrates the superiority of the time-gated SPAD receiver over the traditional free-running receiver in terms of the BER performance and the tolerance to background light.

Hyperbolic metamaterial-based metal–dielectric resonator-antenna designs for GHz photon collection rates from wide-range solid-state single-photon sources

Solid-state single-photon sources (SPS) based on quantum dots as well as color centers in diamonds and silicon-carbide have promise for application in emerging quantum technologies. Many of these technologies, however, demand photon rates in the GHz range, thereby hindering the use of these SPS, for which the maximum observed count rates are limited to a few tens of MHz. Here we first study the performance of hyperbolic metamaterial-based 5-layered metal–dielectric resonator antenna structures with metallic as well as hybrid metal–dielectric antennas in the wavelength range of 600 to 1000 nm. The performance of these resonator-antenna structures was analyzed for the Purcell enhancement, quantum efficiency (QE), collection efficiency (CE), and normalized collected photon counts (NCPC). The hybrid metal–dielectric antenna helps in providing the directivity to the dipole emission, thereby significantly improving the collection efficiency. We then present the novel design of a 5-layered metal–dielectric pillar resonator. This resonator structure with a metallic cylindrical antenna over the top showed significantly large fluorescence enhancement values. The Purcell factor was observed to reach close to 1600 at 680 nm corresponding to the central peak of the nitrogen vacancy center spectrum. The NCPC value reached close to 550 at 680 nm. The maximum CE from the structure was observed to be around 60%, with the maximum QE reaching close to 80%. With the above performance, the detected photon count rates for a solid-state SPS is expected to be well into the GHz range. Our designs show a state-of-the-art improvement in the antenna performance for SPS with properties very close to a practical SPS.

A 3D Polymeric Platform for Photonic Quantum Technologies

AbstractThe successful development of future photonic quantum technologies will much depend on the possibility of realizing robust and scalable nanophotonic devices. These should include quantum emitters like on‐demand single‐photon sources and non‐linear elements, provided their transition linewidth is broadened only by spontaneous emission. However, conventional strategies to on‐chip integration, based on lithographic processes in semiconductors, are typically detrimental to the coherence properties of the emitter. Moreover, such approaches are difficult to scale and bear limitations in terms of geometries. Here an alternative platform is discussed, based on molecules that preserve near‐Fourier‐limited fluorescence even when embedded in polymeric photonic structures. 3D patterns are achieved via direct laser writing around selected molecular emitters, with a fast, inexpensive, and scalable fabrication process. By using an integrated polymeric design, detected photon counts of about 2.4 Mcps from a single cold molecule are reported. The proposed technology will allow for competitive organic quantum devices, including integrated multi‐photon interferometers, arrays of indistinguishable single‐photon sources, and hybrid electro‐optical nanophotonic chips.

Short-time fractal analysis of biological autoluminescence.

Biological systems manifest continuous weak autoluminescence, which is present even in the absence of external stimuli. Since this autoluminescence arises from internal metabolic and physiological processes, several works suggested that it could carry information in the time series of the detected photon counts. However, there is little experimental work which would show any difference of this signal from random Poisson noise and some works were prone to artifacts due to lacking or improper reference signals. Here we apply rigorous statistical methods and advanced reference signals to test the hypothesis whether time series of autoluminescence from germinating mung beans display any intrinsic correlations. Utilizing the fractional Brownian bridge that employs short samples of time series in the method kernel, we suggest that the detected autoluminescence signal from mung beans is not totally random, but it seems to involve a process with a negative memory. Our results contribute to the development of the rigorous methodology of signal analysis of photonic biosignals.

High-resolution two-photon spectroscopy of a 5p56p←5p6 transition of xenon

We report high-resolution Doppler-free two-photon excitation spectroscopy of Xe from the ground state to the $5{p}^{5}(^{2}P_{3/2})6p\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}^{2}[3/2]_{2}$ electronic excited state. This is a first step to developing a comagnetometer using polarized $^{129}\mathrm{Xe}$ atoms for planned neutron electric dipole moment measurements at TRIUMF. Narrow linewidth radiation at 252.5 nm produced by a continuous wave laser was built up in an optical cavity to excite the two-photon transition, and the near-infrared emission from the $5{p}^{5}6p$ excited state to the $5{p}^{5}6s$ intermediate electronic state was used to detect the two-photon transition. Hyperfine constants and isotope shift parameters were evaluated and compared with previously reported values. In addition, the detected photon count rate was estimated from the observed intensities.

Scattered hard X-ray and γ-ray generation from a chromatic electron beam

An array of photon diagnostics has been deployed on a high power relativistic electron beam diode. Electrons are extracted through a 17.8 cm diode from the surface discharge of a carbon fiber velvet cathode with a nominal diode voltage of 3.8 MV. <10% of the 100 ns electron pulse is composed of off energy electrons (1–3 MeV) accelerated during the rise and fall of the pulse that impact the stainless steel beam pipe and generate a Bremsstrahlung spectrum of 0.1–3 MeV photons with a total count of 1011. The principal objective of these experiments is to quantify the electron beam dynamics and spatial dynamics of the hard X-ray and γ-ray flux generated in the diode region. A qualitative comparison of experimental and calculated results are presented, including time and energy resolved electron beam propagation and scattered photon measurements with X-ray PIN diodes and a photomultiplier tube indicating a dose dependence on the diode voltage >V4 and detected photon counts of nearly 106 at a radial distance of 1 m which corresponds to dose ∼40 μrad at 1 m.

Performance analysis of digital silicon photomultipliers for PET

A silicon photomultiplier (SiPM) with electronics integrated on cell level has been developed by Philips Digital Photon Counting. The device delivers a digital signal of the detected photon counts and their time stamp, making it a potential candidate for positron emission tomography (PET) applications. Several operational parameters of the specifically developed acquisition protocol can be adjusted to optimize the photon detection. In this work, the combination of five different parameters (trigger scheme, validation scheme, cell inhibition, temperature and excess bias voltage) is analyzed. Their impact on both the intrinsic as well as the PET-oriented sensor's performance is studied when coupled to two different PET candidate scintillators, GAGG and LYSO (2 × 2 × 6 mm3). The results show that SiPM intrinsic properties such as breakdown voltage temperature coefficient (20 mV/K) and optical crosstalk (20%) are similar to state-of-the-art analog devices. The main differences are induced by the logic of the acquisition sequence and its parameters. The sensor's dark-count-rate (DCR) is 770 kHz/mm2 at 24°C and 100% active cells. It can be reduced through cell inhibition and lower temperatures (ca. 2 orders of magnitude at 0°C and 20% cell inhibition). DCR reduction is necessary to avoid acquiring dark-count-triggered and validated events, causing loss of detection sensitivity. The typical time fraction spent with these events is 42.5% (GAGG) and 35.5% (LYSO). Increasing percentages of cell inhibition affect the photodetection efficiency and with it the energy resolution and the coincidence time resolution (CTR). At 5.6 °C and 10% cell inhibition, the measured energy resolution is 11.9% and 13.5% (FWHM, saturation corrected) and a FWHM CTR of 458 ps and 177 ps is achieved, for GAGG and LYSO respectively. With the implemented setup, the optimum configuration for PET, in terms of sensitivity, energy resolution and CTR, is trigger scheme 1, validation scheme 8, 10% cell inhibition and 3.0 V excess bias. Cooling the sensor to 0°C considerably improves the device's performance.

Improvement of fluorescence-enhanced optical tomography with improved optical filtering and accurate model-based reconstruction algorithms

The goal of preclinical fluorescence-enhanced optical tomography (FEOT) is to provide three-dimensional fluorophore distribution for a myriad of drug and disease discovery studies in small animals. Effective measurements, as well as fast and robust image reconstruction, are necessary for extensive applications. Compared to bioluminescence tomography (BLT), FEOT may result in improved image quality through higher detected photon count rates. However, background signals that arise from excitation illumination affect the reconstruction quality, especially when tissue fluorophore concentration is low and/or fluorescent target is located deeply in tissues. We show that near-infrared fluorescence (NIRF) imaging with an optimized filter configuration significantly reduces the background noise. Model-based reconstruction with a high-order approximation to the radiative transfer equation further improves the reconstruction quality compared to the diffusion approximation. Improvements in FEOT are demonstrated experimentally using a mouse-shaped phantom with targets of pico- and subpico-mole NIR fluorescent dye.

Real-time monitoring of the Bragg-peak position in ion therapy by means of single photon detection

For real-time monitoring of the longitudinal position of the Bragg-peak during an ion therapy treatment, a novel non-invasive technique has been recently proposed that exploits the detection of prompt gamma-rays issued from nuclear fragmentation. Two series of experiments have been performed at the GANIL and GSI facilities with 95 and 305 MeV/u (12)C(6+) ion beams stopped in PMMA and water phantoms. In both experiments, a clear correlation was obtained between the carbon ion range and the prompt photon profile. Additionally, an extensive study has been performed to investigate whether a prompt neutron component may be correlated with the carbon ion range. No such correlation was found. The present paper demonstrates that a collimated set-up can be used to detect single photons by means of time-of-flight measurements, at those high energies typical for ion therapy. Moreover, the applicability of the technique both at cyclotron and at synchrotron facilities is shown. It is concluded that the detected photon count rates provide sufficiently high statistics to allow real-time control of the longitudinal position of the Bragg-peak under clinical conditions.

The Photon Counting Histogram in Fluorescence Fluctuation Spectroscopy

Fluorescence correlation spectroscopy (FCS) is generally used to obtain information about the number of fluorescent particles in a small volume and the diffusion coefficient from the autocorrelation function of the fluorescence signal. Here we demonstrate that photon counting histogram (PCH) analysis constitutes a novel tool for extracting quantities from fluorescence fluctuation data, i.e., the measured photon counts per molecule and the average number of molecules within the observation volume. The photon counting histogram of fluorescence fluctuation experiments, in which few molecules are present in the excitation volume, exhibits a super-Poissonian behavior. The additional broadening of the PCH compared to a Poisson distribution is due to fluorescence intensity fluctuations. For diffusing particles these intensity fluctuations are caused by an inhomogeneous excitation profile and the fluctuations in the number of particles in the observation volume N¯. The quantitative relationship between the detected photon counts and the fluorescence intensity reaching the detector is given by Mandel's formula. Based on this equation and considering the fluorescence intensity distribution in the two-photon excitation volume, a theoretical expression for the PCH as a function of the number of molecules in the excitation volume is derived. For a single molecular species two parameters are sufficient to characterize the histogram completely, namely the average number of molecules within the observation volume and the detected photon counts per molecule per sampling time ϵ. The PCH for multiple molecular species, on the other hand, is generated by successively convoluting the photon counting distribution of each species with the others. The influence of the excitation profile upon the photon counting statistics for two relevant point spread functions (PSFs), the three-dimensional Gaussian PSF conventionally employed in confocal detection and the square of the Gaussian-Lorentzian PSF for two photon excitation, is explicitly treated. Measured photon counting distributions obtained with a two-photon excitation source agree, within experimental error with the theoretical PCHs calculated for the square of a Gaussian-Lorentzian beam profile. We demonstrate and discuss the influence of the average number of particles within the observation volume and the detected photon counts per molecule per sampling interval upon the super-Poissonian character of the photon counting distribution.

Simulated annealing image reconstruction method for a pinhole aperture single photon emission computed tomograph (SPECT)

A series of computer experiments was performed to determine the relative performance of simulated annealing, quenched annealing, and a least-squares iterative technique for image reconstruction for single photon emission computed tomography (SPECT). The simulated SPECT geometry was of the pinhole aperture type, with 32 pinholes and 128 or 512 detectors. To test the robustness of the reconstruction techniques upon arbitrary geometries, a 360-detector geometry with a random pixel-detector-factor matrix was tested. Eight computer-simulated, 10-cm-diameter planar phantoms were used with 1961 2-mm(2) reconstruction bins and a range of 3000 to 50,000,000 detected photon counts. Reconstruction quality was measured by a normalized, squared error picture distance measure. Over a wide range of noise, the simulated annealing method had slightly better reconstruction quality than the iterative method, although requiring greater reconstruction time. Quenched annealing was faster than simulated annealing, with comparable reconstruction quality. Methods of efficiently controlling the simulated annealing algorithm are presented.