Low Photon Flux Research Articles

High-energy isolated attosecond pulses generated by above-saturation few-cycle fields

The applications of isolated attosecond pulses reported to date, which have demonstrated the great potential of attosecond technology in the investigation of ultrafast electronic processes, have been limited by the low photon flux of the available attosecond sources. We report on the generation of isolated sub-160-as pulses (at a photon energy of ∼30 eV) with a pulse energy, on target, of a few nanojoules. The efficient generation of isolated attosecond pulses in noble gases is produced by 5-fs driving pulses with controlled electric field and peak intensity beyond the gas saturation intensity. The availability of attosecond sources with high peak intensities has potential in opening new avenues for attosecond-pump/attosecond-probe studies of electronic processes in atomic and molecular physics, with interesting prospects in the field of coherent control of electronic motion in complex systems in the attosecond temporal regime. Researchers report the generation of isolated sub-160-attosecond pulses that have photon energies of 30 eV, resulting in an on-target pulse energy of a few nanojoules. The availability of attosecond sources with high peak intensities may open new avenues for attosecond pump/probe studies of electronic processes in atomic and molecular physics.

EFFECTS OF ARTIFICIAL LIGHT INTENSITY AND AMBIENT CO2 LEVEL ON PHOTOSYNTHESIS OF ARACEAE SPECIES COMMONLY USED FOR INTERIOR LANDSCAPING

ISHS II International Conference on Landscape and Urban Horticulture EFFECTS OF ARTIFICIAL LIGHT INTENSITY AND AMBIENT CO2 LEVEL ON PHOTOSYNTHESIS OF ARACEAE SPECIES COMMONLY USED FOR INTERIOR LANDSCAPING

Photosynthesis of Lolium perenne L. at low temperatures under low irradiances

To develop mechanistic models for winter survival of grasses under climate change, more knowledge is needed of photosynthetic activity at low irradiance and temperature. Photosynthetic activity of small stands of Lolium perenne was continuously studied for up to two weeks under low photosynthetic photon flux density (PPFD) in the air temperature range from −3 °C to +9 °C. The photosynthetic rate of plants growing at 2, 6 and 9 °C was similar at 120 μmol m −2 s −1 PPFD but the rate increased with increasing PPFD at all temperatures, particularly 9 °C. Light saturation of photosynthesis was reached at approx. 300 μmol m −2 s −1 at 2 and 6 °C, and at approx. 600 μmol m −2 s −1 at 9 °C. At 300 μmol m −2 s −1, the CO 2 exchange rate (CER) at 2 °C was approx. 60% of that at 9 °C. When the temperature increased gradually from 0 °C at start to +5 °C at the end of the 8-h photoperiod, CER decreased by about 20% compared with that at constant 5 °C. Changing the temperature from constant +5 °C to a diurnal variation between −2 and +2 °C gradually decreased daytime CER to approx. 10% after five days, partly due to leaf area losses (∼50% loss) when the poorly acclimatised plants were exposed to frost. At start of the photoperiod at −2 °C CER was negative, but became slightly positive when temperature exceeded −1 °C after two hours. Total daily photosynthesis was negative due to night-time respiration in this treatment. Soil heating to avoid freezing when the diurnal air temperature fluctuated between −3 and +3 °C had no effect on CER at this low PPFD level (150 μmol m −2 s −1). In contrast to the −2/+2 °C treatment, total daily photosynthesis was slightly positive in the −3/+3 °C treatment, where the plants were better acclimatised to frost. Increasing the CO 2 concentration from 350 to 600 μmol mol −1 had no effect on CER at 2 and 6 °C, but increased it by 20% at 9 °C. The data indicate that the minimum temperature for photosynthesis in the cultivar studied is about −4 °C. The results can be applied in different photosynthesis models.

A solid‐state amorphous selenium avalanche technology for low photon flux imaging applications

The feasibility of a practical solid-state technology for low photon flux imaging applications was investigated. The technology is based on an amorphous selenium photoreceptor with a voltage-controlled avalanche multiplication gain. If this photoreceptor can provide sufficient internal gain, it will be useful for an extensive range of diagnostic imaging systems. The avalanche photoreceptor under investigation is referred to as HARP-DRL. This is a novel concept in which a high-gain avalanche rushing photoconductor (HARP) is integrated with a distributed resistance layer (DRL) and sandwiched between two electrodes. The avalanche gain and leakage current characteristics of this photoreceptor were measured. HARP-DRL has been found to sustain very high electric field strengths without electrical breakdown. It has shown avalanche multiplication gains as high as 10(4) and a very low leakage current (< or = 20 pA/mm2). This is the first experimental demonstration of a solid-state amorphous photoreceptor which provides sufficient internal avalanche gain for photon counting and photon starved imaging applications.

Linear‐mode operation of the quantum‐dot avalanche photodiode (QDAP)

AbstractWe report new results on the design, fabrication and characterization of a novel midinfrared sensor called quantum dot avalanche photodiode (QDAP). The QDAP consists of a quantum dots‐in‐a‐well (DWELL) detector coupled with an avalanche photodiode (APD) through a tunnel barrier. In the QDAP, the photons are absorbed in the DWELL active region while the APD section provides gain.Photocurrent and noise measurements at 77 K were taken to characterize the response of the device. The nonlinear increase in the photocurrent as the APD voltage increases support theoretical predictions about the QDAP capability to work in applications with low photon flux levels. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Germination Response of the Epiphytic Cactus Rhipsalis baccifera (J. S. Miller) Stearn to Different Light Conditions and Water Availability

In the forest canopy, seeds of epiphytic plants encounter heterogeneous environments created by a combination of factors such as solar radiation, humidity, and host characteristics. Germination requirements may explain the species distribution in the canopy; however, more knowledge is essential. Germination of Rhipsalis baccifera, a widespread tropical epiphytic cactus and representative of the humid montane forest in Mexico, was 80% or higher with far red, red, and white light and close to 0 in darkness. Germination was light saturated at very low photon flux density of only 13.5 μmol m−2 s−1. Germination decreased gradually at low water potentials and with increased storage time. After storage for 1 yr, no seeds germinated. Seeds have the ability to germinate in both the internal and external positions of the tree crowns. The germinative requirements of R. baccifera indicate that it could colonize a wide range of light conditions in the canopy; however, substrate humidity and seed age could limit germination.

Experimental realization of sub-shot-noise quantum imaging

Properties of quantum states have disclosed new technologies, ranging from quantum information to quantum metrology. Among them a recent research field is quantum imaging, addressed to overcome limits of classical imaging by exploiting spatial properties of quantum states of light . In particular quantum correlations between twin beams represent a fundamental resource for these studies. One of the most interesting proposed scheme exploits spatial quantum correlations between parametric down conversion light beams for realizing sub-shot-noise imaging of the weak absorbing objects, leading ideally to a noise-free imaging. Here we present the first experimental realisation of this scheme, showing its capability to reach a larger signal to noise ratio (SNR) with respect to classical imaging methods. This work represents the starting point of this quantum technology that can have relevant applications, especially whenever there is a need of a low photon flux illumination (e.g. as with certain biological samples).

Photon Counting OTDR: Advantages and Limitations

In this paper, we provide a detailed insight into photon-counting optical time-domain reflectometer (¿-OTDR) operation, ranging from Geiger-mode operation of avalanche photodiodes (APD), analysis of different APD bias schemes, to the discussion of OTDR perspectives. Our results demonstrate that an InGaAs/InP APD-based ¿-OTDR has the potential of outperforming the dynamic range of a conventional state-of-the-art OTDR by 10 dB, as well as the two-point resolution by a factor of 20. Considering the trace acquisition speed of ¿-OTDRs, we find that a combination of rapid gating for high photon flux and free running mode for low photon flux is the most efficient solution. Concerning dead zones, our results are less promising. Without additional measures, e.g., an optical shutter, the photon counting approach is not competitive.

Quantum transport in quantum well infrared photodetectors in the tunneling regime

Quantum well infrared photodetectors (QWIP) are good candidates for low photon flux detection in the 12–20 μm range. For particularly low incident power applications, it can be interesting to reduce the operating temperature to reach the ultimate performance of the QWIP (low dark current, low noise, high detectivity). Nevertheless, once the QWIP operates in the tunneling regime, the dark current is no longer improved by reducing the temperature. Thus, further improvement of the performance needs a microscopic understanding of the physical phenomena involved in QWIP operation in the tunneling regime. In this paper we focus on the dark current of QWIP operated at very low temperature (4–20 K). Experimental results obtained on a 14.5 μm peaking device revealed a plateau regime in the IV curves. We first modeled the dark current using the WKB approximation, but it failed to reproduce the shape and order of magnitude of the phenomenon. As an improvement, we developed a scattering formalism. Our model includes all the most common interactions observed in GaAs: optical phonon, acoustical phonon, alloy disorder, interface roughness, interaction with ionized impurities and between carriers. We demonstrate that, as far as the tunneling regime is concerned, the dominant interaction is the one between electron and ionized impurities, which allows us to conclude on the influence of the doping profile on the dark current.

The spectral photon flux of the radiometric calibration spectral source for the NIRSpec instrument of the James Webb Space Telescope

The radiometric calibration spectral source (RCSS) is a very low photon flux radiation source based on an integrating sphere, operated under vacuum and cryogenic conditions, for the testing and calibration of the near infrared multiobject dispersive spectrograph (NIRSpec). NIRSpec itself is a part of the scientific instrumentation of the James Webb Space Telescope. For the traceable calibration of the photon flux emitted by the RCSS in the wavelength range from 0.7 µm to 5 µm, dedicated calibration schemes were developed at PTB. To achieve this goal, the Spectral Radiance Comparator Facility and the Reduced Background Calibration Facility of the PTB have been significantly improved with respect to their detector instrumentation.

Application of Photoconductor Physical Transient Model to Fourier Transform Spectrometer Data of AKARI/Far-Infrared Surveyor (FIS)

ABSTRACTAKARI carries on board the Fourier transform spectrometer (FTS) with the gallium-doped germanium (Ge:Ga) photoconductor arrays of the far-infrared surveyor (FIS). Ge:Ga photoconductors are the most sensitive detectors for far-infrared satellite instruments. However, the performance of these detectors is known to be severely influenced by undesirable slow transient behaviors under a low photon flux condition in space. In fact, we have found that the in-orbit AKARI/FTS data are heavily distorted by the transient response, which severely affects their spectroscopic accuracies. Therefore we have performed transient correction of in-orbit FTS data by using our physical transient model with numerical calculation, by which we have found that the distortion is improved significantly. We have demonstrated the applicability of the physical transient model to FTS data by using the observational data of the Galactic center region.

Kinetics rate model of the photocatalytic oxidation of trichloroethylene in air over TiO 2 thin films

The photocatalytic oxidation of trichloroethylene (TCE) over a TiO 2 thin film was investigated in a flow-through photocatalytic reactor. The effects of TCE concentration and water vapor concentration on the oxidation rates were investigated. Rate models based on variations of the Langmuir–Hinshelwood kinetic model were found to represent the results unsatisfactorily. Therefore, a general rate equation for the oxidation of TCE was derived from an elementary reaction mechanism of TCE photocatalytic oxidation over TiO 2. The model, based on chlorine atom attack of the TCE molecule, yields a relatively complex equation including the explicit dependence on water vapor concentration, TCE concentration, photon flux and quantum yield. The rate equation yields half-order dependence on the photon flux at high photon fluxes and first-order dependence at low photon fluxes and reduces to first-order dependence on TCE and inverse dependence on water vapor under specific conditions. The kinetic parameters of the photocatalytic oxidation of TCE on TiO 2 thin film were estimated by fitting the model to the experimental results. The approach demonstrated in this paper represents a more rational method of kinetic analysis than the mechanical adoption of a Langmuir–Hinshelwood type rate equation often reported in the literature.

Ancient Photosynthetic Eukaryote Biofilms in an Atacama Desert Coastal Cave

Caves offer a stable and protected environment from harsh and changing outside prevailing conditions. Hence, they represent an interesting habitat for studying life in extreme environments. Here, we report the presence of a member of the ancient eukaryote red algae Cyanidium group in a coastal cave of the hyperarid Atacama Desert. This microorganism was found to form a seemingly monospecific biofilm growing under extremely low photon flux levels. Our work suggests that this species, Cyanidium sp. Atacama, is a new member of a recently proposed novel monophyletic lineage of mesophilic "cave" Cyanidium sp., distinct from the remaining three other lineages which are all thermo-acidophilic. The cave described in this work may represent an evolutionary island for life in the midst of the Atacama Desert.

Possible involvement of distinct photoreceptors in the photoperiodic induction of diapause in the flesh fly Sarcophaga similis

Physiological characteristics of the photoreceptors involved in the photoperiodic induction of diapause were investigated in the flesh fly Sarcophaga similis. Both the early and late phases of scotophase were sensitive to light and a light pulse during each of these phases prevented diapause. Certain physiological differences between the phases were, nevertheless, detected. Compared with early scotophase, late scotophase required a light pulse with a long period and a large number of night interruption photoperiodic cycles in order to effectively prevent diapause. The diapause-averting effects of a light pulse during early scotophase were canceled by an additional long dark period, but those during late scotophase were not. Thus, the diapause-averting effects produced during early scotophase are different to those produced during late scotophase. The early scotophase was sensitive to light at wavelengths of 470 nm or shorter, but not to light of 583 nm or longer. In contrast, the late scotophase was sensitive to light of a broad range of wavelengths, ranging from 395 to 660 nm. Furthermore, the early scotophase was considerably more sensitive to monochromatic light with low photon flux density than the late scotophase. These results suggest that different types of photoreceptor are involved in the photoperiodic response.

Single-photon-counting detector for increased sensitivity in two-photon laser scanning microscopy

We present the use and characterization of a photon-counting detector for increased sensitivity at low signal levels in fluorescence laser scanning microscopy (LSM). Conventional LSM photomultiplier tube detectors utilize analog current integration and thus suffer from excessive noise at low signal levels, introduced during current measurement. In this Letter we describe the implementation of a fast single-photon-counting (SPC) detector on a conventional two-photon laser scanning microscope and detail its use in imaging low fluorescence intensities. We show that for a low photon flux, the SPC detector is shot-noise limited and thus provides increased detection sensitivity compared with analog current integration.

Estimation of the degree of polarization from a single speckle intensity image with photon noise

It is demonstrated that, if not appropriately taken into account, photon noise leads to a biased estimation of the degree of polarization from a single speckle intensity image. A new method for estimating the degree of polarization from a single speckle intensity image acquired at low photon flux level is thus proposed. Its precision and the limit critical photon level that does not deteriorate the performance up to a given precision are determined.

An original laboratory X-ray diffraction method for in situ investigations on the water dynamics in a fuel cell proton exchange membrane

A key issue of the polymer membrane-based fuel cells is the water management, since the polymeric electrolyte acts as a good protonic conductor only if sufficiently hydrated. Although models and theoretical studies are widely reported in the literature, experimental investigations are much less available, due to inherent difficulties to sample the membrane in situ. Among the techniques utilized for this purpose, the synchrotron radiation X-ray diffraction has been demonstrated to be a suitable tool to observe the real-time water dynamics in the polymeric membranes. To export this method in laboratories would represent a big advantage because of the limited access to synchrotron facilities. Clearly, the main problem in performing such studies by using conventional X-ray tubes consists of their much lower photon flux, particularly relevant when non-crystalline samples, as polymers, are to be investigated and when extreme collimations of the X-ray beams are required, as in the case of thin membranes. In the present work, we propose a X-ray method, based on the energy dispersive X-ray diffraction, capable to overcome these drawbacks.

WE-C-342-01: X-Ray Image Acquisition- Two Long Standing Problems and a New Promise

In the last two decades, there have been rapid advances in projection x‐ray imaging in the areas of x‐ray detector and infrastructures for image management, processing and display. Following the development and commercialization of flat panel detectors, there have also been rapid advances in using the large area detectors to implement reconstructive 3‐D imaging techniques, including the cone beam CT and digital tomosynthesisimaging techniques. In contrast to these advances, the issues of x‐ray scatter and heavy patient attenuation remain the two biggest challenges in our effort to improve the image quality while keeping the patient dose in check. The presence of the x‐ray scatter component in the image signals biases the transmitted x‐ray intensity and results in erroneous x‐ray attenuation measurements which degrades the image quality and prevents accurate quantitative analysis in both projection and reconstructed images. Heavy patient attenuation could result in excessively low photon flux in certain anatomical regions, such as abdomen or retrocardium. This can combine with the lowered detective quantum efficiencies (DQEs) to lead to excessively low and unusable image signal‐to‐noise ratios (SNRs) Along with these two long standing issues is the long awaiting wish to develop a “weightless” x‐ray source that can be digitally controlled to shift without having to move a bulky and heavy housing. The development of a CT or digital tomosynthesisimaging system with no moving parts has become the holy grail of x‐ray imaging research. In this paper, efforts to address the scatter and exposure issues and to develop a “weightless” x‐ray source in the past two decades are reviewed with an educated guess on where we might head to in the future. Educational Objectives: 1. To review the issues of and solutions to the scatter problem in x‐ray image acquisition. 2. To review the issues of and solutions to the problem of excessive patient attenuation. 3. To review the efforts and potential use of digitally addressable “weightless” x‐ray source

Effects of inelastic capture, tunneling escape, and quantum confinement on surface acoustic wave-dragged photocurrents in quantum wells

We developed a dual-charged-fluid model for studying the steady-state transport of surface acoustic wave (SAW)-dragged photocurrents of one-dimensional (1D) confined-state carriers. This model includes the effects of quantum confinement and the escape via tunneling of SAW-dragged 1D carriers, as well as the effects of the inelastic capture of two-dimensional continuous-state carriers and the self-consistent space-charge field. Our numerical results revealed a high photocurrent gain due to the suppressed recombination of 1D carriers in a crossover region of the sample between an absorption strip and a surface gate. Based on this model, responsivities for the SAW-dragged photocurrents in a quantum well are calculated as functions of the gate voltage, photon flux, SAW power and frequency, and temperature, respectively. A responsivity as high as 103 A/W was found for high gate voltages and SAW powers, as well as for low photon fluxes and SAW frequencies.

High Optical Response in Forward Biased (In,Ga)N–GaN Multiquantum-Well Diodes Under Barrier Illumination

The authors report on the current-voltage (I-V) characteristic under forward biases obtained in low leakage, small size p-(In,Ga)N-GaN-n multiquantum well diodes. Under barrier illumination, the devices present a high optical response with capabilities to detect optical powers in the pW range without further amplification. This response is attributed to the screening of the internal electric fields. Recombination times of a few seconds are found to be associated to this mechanism. Moreover, a step-like feature is found in the I-V characteristic before the diode turn-on voltage. Our model proposes tunneling current through the multiquantum-well structure as responsible of this feature. Fast modulation of the tunneling effect under barrier illumination is used to evaluate the detection of low photon fluxes.