Low Photon Flux Research Articles

4H-SiC Schottky photodiodes for ultraviolet flame detection

In the last few years silicon carbide (SiC) has emerged as an appropriate material for the detection of very low ultraviolet photon fluxes even at elevated temperatures. In this paper we report on the electro-optical characteristics of large area interdigit Ni2Si/4H-SiC photodiodes in TO metal can package with a suitable molded cap quartz window with high transmission in the ultraviolet wavelength range. The detectors have been tested for the detection of the ultraviolet component of the yellow flame emitted by a small candle, showing good sensitivity for very weak photon fluxes notwithstanding the linear operation condition of the photodiodes.

Antarctic microbial mats: A modern analog for Archean lacustrine oxygen oases

The evolution of oxygenic photosynthesis was the most important geochemical event in Earth history, causing the Great Oxidation Event (GOE) ∼2.4 b.y. ago. However, evidence is mixed as to whether O2 production occurred locally as much as 2.8 b.y. ago, creating O2 oases, or initiated just prior to the GOE. The biogeochemical dynamics of possible O2 oases have been poorly constrained due to the absence of modern analogs. However, cyanobacteria in microbial mats in a perennially anoxic region of Lake Fryxell, Antarctica, create a 1–2 mm O2-containing layer in the upper mat during summer, providing the first known modern analog for formation of benthic O2 oases. In Lake Fryxell, benthic cyanobacteria are present below the oxycline in the lake. Mat photosynthesis rates were slow due to low photon flux rate (1–2 µmol m−2 s−1) under thick ice cover, but photosynthetic O2 production was sufficient to sustain up to 50 µmol O2 L−1, sandwiched between anoxic overlying water and anoxic sediments. We hypothesize that Archean cyanobacteria could have similarly created O2 oases in benthic mats prior to the GOE. Analogous mats may have been at least partly responsible for geological evidence of oxidative weathering prior to the GOE, and habitats such as Lake Fryxell provide natural laboratories where the impact of benthic O2 oases on biogeochemical signatures can be investigated.

Analysis of photon statistics with Silicon Photomultiplier

The Silicon Photomultiplier (SiPM) is a novel silicon-based photodetector, which represents the modern perspective of low photon flux detection. The aim of this paper is to provide an introduction on the statistical analysis methods needed to understand and estimate in quantitative way the correct features and description of the response of the SiPM to a coherent source of light.

Technical prerequisites and imaging protocols for CT perfusion imaging in oncology

The aim of this review article is to define the technical prerequisites of modern state-of-the-art CT perfusion imaging in oncology at reasonable dose levels. The focus is mainly on abdominal and thoracic tumor imaging, as they pose the largest challenges with respect to attenuation and patient motion. We will show that low kV dynamic scanning in conjunction with detection technology optimized for low photon fluxes has the highest impact on reducing dose independently of other choices made in the protocol selection. We discuss, derived from relatively simple first principles, on what appropriate temporal sampling and total scan duration depend on and why optimized contrast medium injection protocols are also essential in limiting dose. Finally we will examine the possibility of simultaneously extracting standard morphological and functional information from one single 4D examination as a potential enabler for a more widespread use of dynamic contrast enhanced CT in oncology.

Few-photon imaging at 1550 nm using a low-timing-jitter superconducting nanowire single-photon detector.

We demonstrated a laser depth imaging system based on the time-correlated single-photon counting technique, which was incorporated with a low-jitter superconducting nanowire single-photon detector (SNSPD), operated at the wavelength of 1550 nm. A sub-picosecond time-bin width was chosen for photon counting, resulting in a discrete noise of less than one/two counts for each time bin under indoor/outdoor daylight conditions, with a collection time of 50 ms. Because of the low-jitter SNSPD, the target signal histogram was significantly distinguishable, even for a fairly low retro-reflected photon flux. The depth information was determined directly by the highest bin counts, instead of using any data fitting combined with complex algorithms. Millimeter resolution depth imaging of a low-signature object was obtained, and more accurate data than that produced by the traditional Gaussian fitting method was generated. Combined with the intensity of the return photons, three-dimensional reconstruction overlaid with reflectivity data was realized.

Enhanced efficiencies in thin and semi-transparent dye-sensitized solar cells under low photon flux conditions using TiO2 nanotube photonic crystal

The photovoltaic output of dye-sensitized solar cells (DSSCs) are greatly dependent on the amount of absorbed photons, which is limited by the thickness of active layer of DSSCs and the illumination conditions. To improve the cell performance under low irradiance condition, a photoanode was designed by attaching a TiO2 nanotube photonic crystal (NTPC) onto the thin TiO2 nanoparticle (NP) layer for applications in thin and semi-transparent DSSCs. It is found that the introduction of the TiO2 NTPC significantly increases the light harvesting and hence the power conversion efficiency (PCE) of the respective DSSCs. The TiO2 NTPC provides multi-functionalities, such as Bragg reflection, light scatting and additional light harvesting from its nanotube structure, leading to more significant light harvesting enhancement in these thin and semi-transparent DSSCs. Compared with the single-layer TiO2 NP based reference DSSCs, the above-mentioned synergic effects in a cell incoporated with a ∼2.3-μm-thick TiO2 NTPC yield PCE enhancements up to 99.1% and 130%, under 1 and 0.5 Sun conditions, respectively. Meanwhile, an obvious compensation effect of TiO2 NTPC to reduce the output power drop of these cells under tilted incient light is also demonstrated. The work will boost the practical applications of PC in irradiance sensitive devices.

Spore Germination of Three Tree Fern Species in Response to Light, Water Potential, and Canopy Openness

The response to the canopy openness of species in tropical forests is a source of niche differentiation. The tree ferns Alsophila firma, Cyathea divergens and Lophosoria quadripinnata coexist in a cloud forest in Mexico; the first is abundant in creeks, the second occupies slightly open places, and the third prefers more open sites. We hypothesized that the distribution of these species would be correlated with their germination patterns. To test the hypothesis we evaluated the effect of light, water potential and canopy openness on spore germination. Spores were incubated under different light filters modifying red:far-red ratio or light intensity, water potential was modified with polyethylene glycol solutions and distinct canopy openness were selected in the field. No germination occurred in darkness, and it was low under red light and far-red light; however, all species germinated at a very low photon flux density (PFD, 0.04 mmol m 22 s 21 ). The decrease of water potential from 0 MPa to 20.7 MPa reduced germination in all species, from 98.6% to 72.2% in L. quadripinnata, 66.4% to 18.9% in A. firma and 74.2% to 4.1% in C. divergens. The increase of the canopy openness reduced the germination capacity of the three species, but L. quadripinnata was the least affected. The germination requirements of the three tree fern species were partially related to the distribution of the sporophytes in the cloud forest.

Impact of anomalous climates on carbon allocation to biomass production of leaves, woody components, and fine roots in a cool-temperate deciduous forest

We investigated carbon allocation in a cool-temperate forest in central Japan in years of contrasting climate anomalies: an early spring warming induced by the El Niño Southern Oscillation in 2002 and a low summer photosynthetic photon flux density (PPFD) induced by a stationary rain front in 2003. Observations of eddy flux, biometric variables, and chamber measurements from 1999 to 2006 and interannual variations in fine root net primary production (frNPP) were analyzed in conjunction with a terrestrial biosphere model simulation with multiple biometric constraints. Compared to the annual means (excluding 2002 and 2003), the low summer PPFD in 2003 reduced the annual gross primary productivity (GPP; −6%), soil respiration (SR; −11%), and ecosystem respiration (RE; −5%). Under the low summer PPFD, CO2 fluxes commonly decreased but components of the NPP were not affected. The low variation in NPP is explained by previous findings that NPP is more sensitive to climate conditions before or during the early stage of the growing season. The early spring warming in 2002 increased the GPP (8%) and woody tissue NPP (wNPP; 55%) and decreased the frNPP (−33%) and SR (−6%). Although early spring warming prolonged the growing season, the foliage NPP (fNPP) and litterfall were invariant. The increase in wNPP and the decrease in frNPP imply that the forest decreased frNPP in favor of wNPP under the high spring temperature. Although the frNPP was estimated by model-data integration, we argue that the decrease in frNPP is plausible because the decrease in SR cannot be explained without the contribution from fine root respiration. These results suggest that increasing or decreasing patterns of component fluxes cannot necessarily be inferred from the GPP. Factors such as the nature and duration of climate anomalies and allocation shift between components of the NPP may need to be considered when characterizing carbon allocation under anomalous climate events.

Heralding efficiency and correlated-mode coupling of near-IR fiber-coupled photon pairs

We report on a systematic experimental study of the heralding efficiency and generation rate of telecom-band infrared photon pairs generated by spontaneous parametric down-conversion and coupled to single-mode optical fibers. We define the correlated-mode coupling efficiency, an inherent source efficiency, and explain its relation to heralding efficiency. For our experiment, we developed a reconfigurable computer-controlled pump-beam and collection-mode optical apparatus which we used to measure the generation rate and correlated-mode coupling efficiency. The use of low-noise, high-efficiency superconducting nanowire single-photon detectors in this setup allowed us to explore focus configurations with low overall photon flux. The measured data agree well with theory, and we demonstrated a correlated-mode coupling efficiency of $97%\ifmmode\pm\else\textpm\fi{}2%$, which is the highest efficiency yet achieved for this type of system. These results confirm theoretical treatments and demonstrate that very high overall heralding efficiencies can, in principle, be achieved in quantum optical systems. It is expected that these results and techniques will be widely incorporated into future systems that require, or benefit from, a high heralding efficiency.

Absolute calibration of a charge-coupled device camera with twin beams

We report on the absolute calibration of a Charge-Coupled Device (CCD) camera by exploiting quantum correlation. This method exploits a certain number of spatial pairwise quantum correlated modes produced by spontaneous parametric-down-conversion. We develop a measurement model accounting for all the uncertainty contributions, and we reach the relative uncertainty of 0.3% in low photon flux regime. This represents a significant step forward for the characterization of (scientific) CCDs used in mesoscopic light regime.

Energy-Dispersive Residual Stress Analysis under Laboratory Conditions: Concept for a New Type of Diffractometer

In the past decade energy-dispersive (ED) synchrotron diffraction has evolved into a powerful tool for materials analysis. Recording complete diffraction patterns in rather few different measuring directions allows for depth-resolved analysis not only of the near-surface residual stress state, but also of composition and even texture gradients. However, since the number of synchrotron beamlines dedicated to ED-diffraction is restricted to very few instruments, alternatives have to be found which allow for ED residual stress analysis even under low flux laboratory conditions. In this project we start to establish the scientific basis for a measuring and evaluation method to make the transfer of the ED method to the laboratory dimensions possible, which is adapted to the conditions of much lower photon flux and larger beam divergences of laboratory X-ray sources. In this paper, we present the concept of an ED-diffractometer which is equipped with two detectors to enable simultaneous data acquisition for two orientations of the diffraction vector with respect to the sample reference system. The first constructive and experimental steps are presented and furthermore the possibilities and limitations of the new laboratory method and the advantages of the ED diffraction method to realize short measurement times in order to realize a high resolution of information depth are discussed.

LABORATORY INFRARED SPECTROSCOPY OF GASEOUS NEGATIVELY CHARGED POLYAROMATIC HYDROCARBONS

Based largely on infrared spectroscopic evidence, polycyclic aromatic hydrocarbon (PAH) molecules are now widely accepted to occur abundantly in the interstellar medium. Laboratory infrared spectra have been obtained for a large variety of neutral and cationic PAHs, but data for anionic PAHs are scarce. Nonetheless, in regions with relatively high electron densities and low UV photon fluxes, PAHs have been suggested to occur predominantly as negatively charged ions (anions), having substantial influence on cloud chemistry. While some matrix spectra have been reported for radical anion PAHs, no data is available for even-electron anions, which are more stable against electron detachment. Here we present the first laboratory infrared spectra of deprotonated PAHs ([PAH – H] − )i n the wavelength ranges between 6 and 16 μm and around 3 μm. Wavelength-dependent infrared multiple-photon electron detachment is employed to obtain spectra for deprotonated naphthalene, anthracene, and pyrene in the gas phase. Spectra are compared with theoretical spectra computed at the density functional theory level. We show that the relative band intensities in different ranges of the IR spectrum deviate significantly from those of neutral and positively charged PAHs, and moreover from those of radical anion PAHs. These relative band intensities are, however, well reproduced by theory. An analysis of the frontier molecular orbitals of the even- and odd-electron anions reveals a high degree of charge localization in the deprotonated systems, qualitatively explaining the observed differences and suggesting unusually high electric dipole moments for this class of PAH molecules.

Quantum Spectroscopy of Plasmonic Nanostructures

We use frequency entangled photons, generated via spontaneous parametric down conversion, to measure the broadband spectral response of an array of gold nanoparticles exhibiting Fano-type plasmon resonance. Refractive index sensing of a liquid is performed by measuring the shift of the array resonance. This method is robust in excessively noisy conditions compared with conventional broadband transmission spectroscopy. Detection of a refractive index change is demonstrated with a noise level 70 times higher than the signal, which is shown to be inaccessible with the conventional transmission spectroscopy. Use of low photon fluxes makes this method suitable for measurements of photosensitive bio-samples and chemical substances.

Enhancing solar cells with photochemical upconversion

Photovoltaics have the potential to become a major contributor to future sustainable energy generation. To this end, device efficiency needs to be brought beyond present physical limits. Solar cells cannot absorb light with less energy than the bandgap (absorption threshold of the photovoltaic absorber material), and consequently this energy source is usually uncaptured. Upconversion (UC) devices harvest those unused subthreshold photons behind the solar cell, create one higher-energy photon out of (at least) two transmitted photons, and radiate upconverted light back toward the solar cell, thus expanding the usable solar spectrum (see Figure 1). Key requirements for UC units are broad absorption and high UC quantum yield under even the low-intensity incoherent illumination that is available to solar energy conversion devices. Upconversion was proposed as a means to increase solar cell efficiency about a decade ago,1 and research has since then been primarily focused on UC processes occurring in lanthanide-doped glasses or nanoparticles.2 Although transitions between the rare earth 4f orbitals lie in an energy range promising for combination with common crystalline silicon solar cells, UC systems based on erbium (Er) or ytterbium (Yb) suffer from weak absorption by Laporte-forbidden transitions, a narrow absorption bandwidth, and the resulting low efficiency. Recently, triplet-triplet annihilation (TTA) in organic molecules has been demonstrated to be an alternative UC mechanism highly promising for solar energy application.3 It proceeds efficiently even for low photon flux and displays highly tunable absorption characteristics. The key to high quantum yield from TTA-UC lies in the combination of two different classes of organic molecules dissolved together in organic solvents or in a polymeric host matrix.4 A socalled sensitizer species, which in many cases is a metalated porphyrin, absorbs a low-energy photon (process 1 in Figure 2) and undergoes fast intersystem crossing (ISC, process 2), whereby Figure 1. Principle of a solar cell assisted by upconversion (UC). The creation of usable electron-hole pairs in the photovoltaic absorber material (left) is augmented by light, usually transmitted unused, which is converted to higher photon energies in the UC unit (right).

Limitations to soybean photosynthesis at elevated carbon dioxide in free-air enrichment and open top chamber systems

It has been suggested that the stimulation of soybean photosynthesis by elevated CO2 was less in free-air carbon dioxide enrichment (FACE) systems than in open top chambers (OTC), which might explain smaller yield increases at elevated CO2 in FACE systems. However, this has not been tested using the same cultivars grown in the same location. I tested whether soybean photosynthesis at high light and elevated CO2 (ambient+180μmolmol−1) was limited by electron transport (J) in FACE systems but by ribulose-bisphosphate carboxylation capacity (VCmax) in OTC. FACE systems with daytime and continuous CO2 enrichment were also compared. The results indicated that in both cultivars examined, midday photosynthesis at high light was always limited by VCmax, both in the FACE and in the OTC systems. Daytime only CO2 enrichment did not affect photosynthetic parameters or limitations, but did result in significantly smaller yields in both cultivars than continuous elevation. Photosynthesis measured at low photosynthetic photon flux density (PPFD) was not higher at elevated than at ambient CO2, because of an acclimation to elevated CO2 which was only evident at low measurement PPFDs.

Under low irradiation, the light regime modifies growth and metabolite production in various species of microalgae

The effects of continuous light exposure (24L:0D) and a 12 h:12 h light/dark regime (12L:12D) were compared on the growth and carotenoid, protein, sugar, lipid, and fatty acid contents in Chlorella vulgaris, Nannochloropsis sp., Isochrysis galbana, and Dunaliella salina cultured in a batchwise facility. These microalgae were grown axenically under a low photon flux density (PFD) of 27 μmol photons m−2 s−1. C. vulgaris, Nannochloropsis sp., and I. galbana exhibited the highest cell densities when cultured under 24L:0D, whereas D. salina grew better under the alternating light/dark regime. I. galbana accumulated high levels of proteins, sugars, and lipids and exhibited the highest carotenoid content under 24L:0D. Protein production was enhanced in C. vulgaris under 24L:0D. The highest total lipid content was recorded for D. salina, reaching 74.6 % of total proteins, sugars, and lipids in cells at the stationary phase when grown under 12L:12D. The light/dark regime at low PFD was sufficient to stimulate the accumulation of monounsaturated and polyunsaturated fatty acids in all four algae. Their levels, like those of saturated fatty acids, did not differ significantly under the two light regimes. D. salina was an important source of tetradecenoic acid 14:1(n-5). Nannochloropsis sp. produced a large amount of the essential eicosapentaenoic acid, which reached 20 % of total fatty acids under 12L:12D, while I. galbana exhibited the highest level of docosahexaenoic acid, which reached 21 % under both light regimes. This study demonstrated the feasibility of culturing microalgae under low PFD in order to produce large quantities of valuable metabolites, especially various lipids with neutraceutical value.

Spectral Radiation Dependent Photoprotective Mechanism in the Diatom Pseudo-nitzschia multistriata

Phytoplankton, such as diatoms, experience great variations of photon flux density (PFD) and light spectrum along the marine water column. Diatoms have developed some rapidly-regulated photoprotective mechanisms, such as the xanthophyll cycle activation (XC) and the non-photochemical chlorophyll fluorescence quenching (NPQ), to protect themselves from photooxidative damages caused by excess PFD. In this study, we investigate the role of blue fluence rate in combination with red radiation in shaping photoacclimative and protective responses in the coastal diatom Pseudo-nitzschia multistriata. This diatom was acclimated to four spectral light conditions (blue, red, blue-red, blue-red-green), each of them provided with low and high PFD. Our results reveal that the increase in the XC pool size and the amplitude of NPQ is determined by the blue fluence rate experienced by cells, while cells require sensing red radiation to allow the development of these processes. Variations in the light spectrum and in the blue versus red radiation modulate either the photoprotective capacity, such as the activation of the diadinoxanthin-diatoxanthin xanthophyll cycle, the diadinoxanthin de-epoxidation rate and the capacity of non-photochemical quenching, or the pigment composition of this diatom. We propose that spectral composition of light has a key role on the ability of diatoms to finely balance light harvesting and photoprotective capacity.

Measurements of Silicon Photomultipliers Responsivity in Continuous Wave Regime

We report on the electrical and optical characterization, in continuous wave regime, of a novel class of silicon photomultipliers fabricated in standard planar technology on a silicon p-type substrate. Responsivity measurements, performed with an incident optical power down to tenths of picowatts, at different reverse bias voltages and on a broad (340-820 nm) spectrum, will be shown and discussed. The device temperature was monitored, allowing us to give a physical interpretation of the measurements. The obtained results demonstrate that such novel silicon photomultipliers are suitable as sensitive power meters for low photon fluxes.

Investigating the feasibility of photon‐counting K‐edge imaging at high x‐ray fluxes using nonlinearity corrections

Pulse pileup occurring at high x-ray fluxes can severely degrade the energy resolution provided by a photon counting detector, which can represent a problem in spectroscopic CT when performing quantitative material discrimination tasks. As the effects of pileup can be most easily seen as a degradation of a detector's count rate linearity at high fluxes, it has been proposed previously to quantify and correct these nonlinearities. While this strategy has been applied successfully to materials without K-edges, it is currently unknown if this still prevails when using medical contrast agents. The purpose of this study is to close this gap. A Medipix2MXR Hexa detector was employed, featuring a pixel pitch of 165 [micro sign]m and a 1 mm thick CdTe sensor. A phantom containing various concentrations of iodine and gadolinium contrast agents was subject to energy selective CT acquisitions, using a pulsable x-ray source operated at 70 kVp. These acquisitions were obtained at low and high photon fluxes of 1.0 × 10(6) and 1.3 × 10(7) mm(-2) s(-1), respectively. Nonlinearity corrections were applied to the high-flux projections and for each pixel separately. The results were compared to the results at low photon fluxes. At high fluxes, a general reduction of the reconstructed attenuation coefficients was observed, which could be partially recovered using the correction strategy applied. The spectroscopic separation of iodine from the phantom material, however, degraded with increasing x-ray flux. In contrast to this, gadolinium could still be discriminated almost as well as in the low flux case. Nonlinearity corrections applied to high flux measurements can help to recover attenuation coefficients normally obtained at low fluxes for low-Z materials, which do not exhibit an absorption edge in the relevant energy range. However, as a result of a significant change of the x-ray spectrum, the spectroscopic contrast normally observed for iodine was found to vanish with increasing x-ray flux. In other words, the authors' results indicate that nonlinearity corrections may be feasible only when the K-edge of interest is sufficiently high compared to the mean photon energy, and that spectroscopic CT at high x-ray fluxes may suffer from less limitations when using high-Z materials as contrast agents. A future study should aim to confirm these findings under clinical conditions.

Leaf form and photosynthetic physiology of Dryopteris species distributed along light gradients in eastern North America

Summary Despite the ubiquity of ferns and at least tacit recognition by botanists that their physiology is unique among land plants, most studies on fern physiology have focused on only a few, locally distributed, usually distantly related species. No previous study has attempted to examine physiological adaptations in a group of widespread taxa that are closely related and whose relationships are well understood. Here we report leaf form and physiological measures for such a group, the 11 eastern North American species of Dryopteris (Dryopteridaceae), and examine differences in these parameters for evidence of adaptation to light availability. Economic theory predicts that species from sunnier habitats should have narrower, more steeply inclined leaves, lower specific leaf area, higher stomatal density, higher rates of maximum photosynthesis, respiration and stomatal conductance, and require more light to saturate photosynthesis. Species should show adaptive crossover in net carbon uptake per unit leaf mass, with a relative advantage by sun‐associated species at high photon flux densities (PFDs) and by shade‐associated species at low PFDs. Field studies allow us to begin characterizing the range of native light environments occupied by members of this group, and to examine interspecific variation in several aspects of leaf form and photosynthetic light response for evidence of adaption to light availability. We also present a novel means for incorporating phylogeny in tests of correlated evolution in a reticulate lineage. Synthesis. Observed trends in physiology and morphology generally agree with qualitative predictions but are often not statistically significant. We found no support for adaptive crossover in mass‐based carbon uptake, and thus for light availability being the most important variable driving morphological and physiological adaptation in these ferns. We propose that hydraulic factors related to water balance may have played a larger role in determining their morphological and physiological variation. Allopolyploid hybrids did not show transgression in any physiological parameter that may have allowed them to coexist regionally with their parents. The results of our phylogenetically structured analyses highlight the importance of incorporating phylogeny into comparative studies, particularly when hybrid or polyploid taxa are present.