Photon Impact Research Articles

Cathode spot formation possibly explained by cathode electron emission from impact of excited state atoms

A theory is given for electron emission from electric arcs with non-thermionic cathodes, for example the iron cathodes used in arc welding, proposing that the principal process for electron emission is from the impact of excited atomic states at the cathode. For plasmas in contact with a metal cathode, a sheath of positive ions is formed over the cathode surface because electrons move away from the cathode. A sheath thickness of only 0.1 µm causes secondary electrons produced at the cathode surface to gain ~12 V. The electron transit through this thin sheath is collision-less. These 12 V electrons produce excited states of atoms to a distance of approximately ~10 µm from the cathode. Diffusion and direct transfer of these excited atoms back to the cathode in the almost collision free region next to the cathode is calculated to be in times of ~10−8 s. This time is less than the radiative lifetimes of almost all atomic transitions except for some resonance transitions where there would be an enhancement of lifetimes due to imprisonment of radiation. If these transfer times are less than the radiative lifetimes, secondary emission coefficients of electrons from the cathode are likely to be approximately 1, the measured coefficients for metastable atoms. The energies of these excited atoms are ~12 V or more, much larger than the usual work function of ~4.5 V required to eject electrons. If such secondary electrons, together with secondary electrons produced from the impact of positive ions and photons, replace or exceed the original number of electrons, amplification is possible in successive avalanches to cause electron densities to grow to supply the total arc current. The mechanism would then explain why observed cathode sheath voltages are independent of arc current for currents varying from 10 to 20 000 A for various low melting point electrodes and arc gases. Electron emission from cold cathodes, when temperatures are too low for thermionic emission, would also be explained. Solutions of the densities of electrons, positive ions and excited state atoms as a function of time and distance from the cathode are presented which give a physical explanation for the formation of cathode spots.

Benefit from Photon Recycling at the Maximum-Power Point of State-of-the-Art Perovskite Solar Cells

Photon recycling is required for a solar cell to achieve an open-circuit voltage ($V_{OC}$) and power conversion efficiency (PCE) approaching the Shockley-Queisser theoretical limit. In metal halide perovskite solar cells, the achievable performance gains from photon recycling remain uncertain due to high variability in perovskite material quality and the non-radiative recombination rate ($k_{1}$). In this work, we study state-of-the-art $\textrm{Cs}_{0.05}(\textrm{MA}_{0.17}\textrm{FA}_{0.83})_{0.95}\textrm{Pb}(\textrm{I}_{0.83}\textrm{Br}_{0.17})_{3}$ films and analyze the impact of varying non-radiative recombination rates on photon recycling and device performance. Importantly, we predict the impact of photon recycling at the maximum power point (MPP), demonstrating an absolute PCE increase of up to 2.0% in the radiative limit, primarily due to a 77 mV increase in $V_{MPP}$. Even with finite non-radiative recombination, benefits from photon recycling can be achieved when non-radiative lifetimes and external LED electroluminescence efficiencies measured at open-circuit, $Q_{e}^{LED}(\textrm{V}_{OC})$, exceed 2 $\mu$s and 10%, respectively. This analysis clarifies the opportunity to fully exploit photon recycling to push the real-world performance of perovskite solar cells toward theoretical limits.

Photon recycling in nanopatterned perovskite thin-films for photovoltaic applications

Perovskites are a class of recently established materials that triggered enormous interest particularly for solar cell applications. Recent studies have pointed out the extraordinary luminescence quantum yield in perovskite materials. The concept of photon recycling investigated in this work promises a route to reharvest the radiatively emitted photons and, thus, lead to an increase in the open-circuit voltage in perovskite solar cells. In this light, this work investigates the role of nanostructured perovskite absorber layers. While the change of the open-circuit voltage due to photon recycling is understood at a conceptional level, the actual impact of a nanostructured interface on the photon recycling has not yet been studied quantitatively. Here, we rely on full-wave optical simulations to quantify the impact of photon recycling on the open-circuit voltage in a nanotextured biperiodic perovskite thin-film layer and additionally with the perovskite layer integrated into a complete solar cell multilayer stack. The validity of the optical simulations is confirmed by far-field measurements of the emission characteristics from fabricated devices. We find that the considered nanostructure provides around 2% increase to a typically achievable open-circuit voltage in perovskite solar cells. We thereby show that, while the main focus for the design of nanostructures is the optimization of light harvesting, photon recycling might be of interest in future designs of solar cell devices.

Performance comparison of large-area SiPM arrays suitable for gamma ray detectors

The use of photosensors of the type Silicon Photomultpliers (SiPM) has widely been extended in recent years for multiple applications in both research and industry. However, there is a lack of comparative studies of different SiPMs under the same conditions, making it difficult to choose the most appropriate one for a specific application. SiPM arrays are suitable for gamma rays detectors, especially when covering large active areas. They are used either in clinical or pre-clinical scenarios, constituting Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) scanners, or just gamma cameras. The current work presents, for the first time, a comparative study between SensL, Hamamatsu Photonics and KETEK arrays of 12 × 12 SiPMs, with individual active areas of 3 mm × 3 mm, thus covering a total active area of about 5 cm × 5 cm. In this study, we have first evaluated their bias and temperature dependencies, resulting in a very similar behaviour with just a slightly larger dependency of the tested Hamamatsu parts. We also report the performance of detector blocks based on these three SiPM arrays, when coupled to pixelated crystal arrays and monolithic scintillators, in terms of photon impact estimation accuracy, as well as energy resolution. In the case of the monolithic crystals, also the depth of interaction resolution was determined. Both monolithic and pixelated blocks are nowadays widely used in academia or are commercially available for molecular imaging systems. The results obtained for the three SiPM arrays when using those crystals, are comparable, without observing any significant different among them.

Tracing mechanism of optically and thermally stimulated luminescence in Lu2O3:Tb,M (M = Hf, Zr, Ti) ceramic storage phosphors

A series of energy storage phosphors, Lu2O3:Tb,M (M = Hf, Zr, Ti), were investigated by means of photo- and thermoluminescence techniques to compare the effect of the co-dopant from the same group of periodic table of elements on charge carriers trapping capability and trap parameters. Most of the investigated processes were tracked individually for the Tb3+ in the two different metal sites offered by the Lu2O3 host – noncentrosymmetric C2 and centrosymmetric C3i. It is proved that both Tb3+ ions participate in hole trapping and electrons are immobilized in traps whose depths are defined by the co-dopant. Deepest traps, ∼1.79 eV, appear upon Ti addition, while Zr and Hf generate traps of very similar, ∼1.40–1.44 eV, depths. The stored energy may be released not only by means of thermal stimulation but also upon the impact of optical photons. Light from the 390–430 nm range of wavelengths was found the most efficient in the latter process. Also deep red radiation of 780 nm releases most of the trapped electrons in Zr/Hf co-doped ceramics but is much less effective in the case of Tb, Ti material. Consistent scheme of electronic levels engaged in charge carriers trapping and subsequent generation of thermo- and optically stimulated luminescence was constructed.

Theoretical study of the electron correlation and excitation effects on energy distribution in photon impact ionization

We performed a detailed theoretical study of the electron correlation and core excitation effects on the energy distribution of the ejected electrons in the process of photon impact tunnel ionization. We used the Landau-Dykhne approach to obtain analytical formulas for the transition rate and the energy distribution with included these effects. We have limited ourselves to a non-relativistic domain, in which the rate and distribution are determined by electrical component of the laser field while the influence of magnetic can be neglected. We observed helium and helium like atoms. We have shown that the tunneling ionization mechanism may be understood as the combination of mentioned processes. We considered the case of a monochromatic wave with an elliptically polarized laser field. We compared our results with experimental and shown that ellipticity plays an important role and that inclusion of additional processes significantly influences the transition rate, as well as the energy distribution of the ejected photoelectrons.

Enhanced spatial resolution of commercial soft X-ray CCD detectors by single-photon centroid reconstruction

Abstract In many back-illuminated Charge Coupled Device (CCD) detectors the electrons liberated by the absorption of one X-ray photon quickly spread and generate charge-spots larger than the pixel dimensions. Such detectors are considered here. In the soft X-ray range this phenomenon drastically limits the effective spatial resolution to approximately 25 μ m , irrespective of the pixel’s lateral size. For very low flux the charge-cloud centroid determination can be used, on each individual spot, to estimate the actual photon impact position with sub-pixel precision. The readout noise and speed, together with the charge and spatial undersampling, are the main factors limiting the accuracy of this procedure in commercial devices. We have experimentally measured a position uncertainty better than 7 μ m and 10 μ m for 13.5 μ m and 20.0 μ m pixel sizes respectively, around 1 keV photon energy using centroiding algorithms. This study was motivated by the need of high resolution detectors in resonant inelastic (soft) X-ray scattering (RIXS).

Impact of VUV photons on SiO2 and organosilicate low-k dielectrics: General behavior, practical applications, and atomic models

This paper presents an in-depth overview of the application and impact of UV/VUV light in advanced interconnect technology. UV light application in BEOL historically was mainly motivated by the need to remove organic porogen and generate porosity in organosilicate (OSG) low-k films. Porosity lowered the film's dielectric constant, k, which enables one to reduce the interconnect wiring capacitance contribution to the RC signal delay in integrated circuits. The UV-based low-k film curing (λ > 200 nm) proved superior to thermal annealing and electron beam curing. UV and VUV light also play a significant role in plasma-induced damage to pSiCOH. VUV light with λ < 190–200 nm is able to break Si-CH3 bonds and to make low-k materials hydrophilic. The following moisture adsorption degrades the low-k properties and reliability. This fact motivated research into the mechanisms of UV/VUV photon interactions in pSiCOH films and in other materials used in BEOL nanofabrication. Today, the mechanisms of UV/VUV photon interactions with pSiCOH and other films used in interconnect fabrication are fairly well understood after nearly two decades of research. This understanding has allowed engineers to both control the damaging effects of photons and utilize the UV light for material engineering and nanofabrication processes. Some UV-based technological solutions, such as low-k curing and UV-induced stress engineering, have already been widely adopted for high volume manufacturing. Nevertheless, the challenges in nanoscaling technology may promote more widespread adoption of photon-assisted processing. We hope that fundamental insights and prospected applications described in this article will help the reader to find the optimal way in this wide and rapidly developing technology area.

Luminescence lifetime encoding in time-domain flow cytometry

Time-resolved flow cytometry represents an alternative to commonly applied spectral or intensity multiplexing in bioanalytics. At present, the vast majority of the reports on this topic focuses on phase-domain techniques and specific applications. In this report, we present a flow cytometry platform with time-resolved detection based on a compact setup and straightforward time-domain measurements utilizing lifetime-encoded beads with lifetimes in the nanosecond range. We provide general assessment of time-domain flow cytometry and discuss the concept of this platform to address achievable resolution limits, data analysis, and requirements on suitable encoding dyes. Experimental data are complemented by numerical calculations on photon count numbers and impact of noise and measurement time on the obtained lifetime values.

Transcranial Near-infrared Laser Therapy in Improving Cognitive Recovery of Function Following Traumatic Brain Injury.

: Traumatic brain injury (TBI) has turned into a major health and socioeconomic problem affecting young people and military personnel. Numerous TBI patients experienced the sequela of brain injury called cognitive impairment, which re-duced functions in attention, working memory, motivation, and execution. In recent years, transcranial near-infrared laser therapy (tNiRLT) as a possible therapy has been gradually applied in treating cognitive impairment post-TBI. In the present review, the biological mechanisms of transcranial tNiRLT for TBI are synthesized mainly based on the photonic impact of chronic mild TBI. Various exciting molecular events possibly occur during the procedure, such as stimulation of ATP pro-duction, regional cerebral blood flow, acupoint, neurogenesis and synaptogenesis, as well as a reduction in anti-inflammatory effect. Some animal experiments and clinical studies of tNiRLT for TBI are outlined. Several labs have displayed that tNiRLT is effective not only in improving neurological functions but also in increasing memory and learning capacity in ro-dent animals’ model of TBI. In a 2 patients case report and a 11-case series, cognitive functions were ameliorated. Efficacy on cognitive and emotional effects was also observed in a double-blind, controlled clinical study. Several Randomized, paral-lel, double blind, sham-controlled trials are underway, aiming to evaluate the efficacy of tLED on cognitive functions and neuropsychiatric status in participants post-TBI. Therefore, tNiRLT is a promising method applied to cognitive impairment following TBI.

Fractional integral-like processing in retinal cones reduces noise and improves adaptation

In the human retina, rod and cone cells detect incoming light with a molecule called rhodopsin. After rhodopsin molecules are activated (by photon impact), these molecules activate the rest of the signalling process for a brief period of time until they are deactivated by a multistage process. First, active rhodopsin is phosphorylated multiple times. Following this, they are further inhibited by the binding of molecules called arrestins. Finally, they decay into opsins. The time required for each of these stages becomes progressively longer, and each stage further reduces the activity of rhodopsin. However, while this deactivation process itself is well researched, the roles of the above stages in signal (and image) processing are poorly understood. In this paper, we will show that the activity of rhodopsin molecules during the deactivation process can be described as the fractional integration of an incoming signal. Furthermore, we show how this affects an image; specifically, the effect of fractional integration in video and signal processing and how it reduces noise and the improves adaptability under different lighting conditions. Our experimental results provide a better understanding of vertebrate and human vision, and why the rods and cones of the retina differ from the light detectors in cameras.

Relative importance of various stochastic terms and EUV patterning

An error propagation stochastic model is described and used to study the impact of both photon and photoresist material sources of line-width roughness (LWR). Based on typical chemically amplified resist parameters, material sources of LWR are shown to be of equal importance to photon sources. Of the material sources, quencher is shown to be the most important input noise term. The results show that it is not the relative quencher noise that ultimately matters but rather the absolute quencher noise relative to the mean produced acid count. The results also show that chemical yield is critical and that benefiting from increased absorptivity also requires the chemical yield to be maintained.

Hard X-ray induced fast secondary electron cascading processes in solids

Recent studies confirmed that the materials used in the extreme UV and soft X-ray regime for precise characterization of intense free-electron laser pulses (e.g., Si3N4) do not work efficiently in the hard X-ray regime, which is due to the fact that the impact of a hard X-ray photon is followed by a series of electron cascading processes. Following theoretical indication, we show that this limitation can be circumvented and the cascading time can be significantly reduced if the X-ray photon energy is double the ionization energy. We investigate an alternative material for pulse diagnostics, SnO2, using the Linac Coherent Light Source at photon energies of 5 keV and 9 keV. We prove the validity of the concept and show that it has a large potential for practical applications. By applying the proposed criteria, the temporal accuracy of the non-invasive pulse diagnostic tools can be improved in current and emerging hard X-ray facilities.

Diamond: a gem for micro-optics

Photonics have developed greatly over the last several decades and brought about a variety of new concepts in communica tions, sensing, solar energy and many other fields. With the emerging applications of quantum technologies and all optical data processing, the impact of photonics on technology is unde niable, as evidenced through the International Year of Light and Light based Technologies in 2015.

Rigorous wave-optical treatment of photon recycling in thermodynamics of photovoltaics: Perovskite thin-film solar cells

The establishment of a rigorous theory on thermodynamics of light management in photovoltaics that accommodates various loss mechanisms as well as wave-optical effects in the absorption and reemission of light is at stake in this contribution. To this end, we propose a theoretical framework to calculate the open-circuit voltage enhancement resulting from photon recycling ($\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$) with rigorous wave-optical treatment. It can be applied to both planar thin-film and nanostructured single-junction solar cells. We derive an explicit expression for $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$, which reveals its dependence on internal quantum luminescence efficiency, parasitic reabsorption, and on photon escape probabilities of reemmited photons. While the internal quantum luminescence efficiency is an intrinsic material property, both latter quantities can be determined rigorously for an arbitrary solar cell architecture by three-dimensional electrodynamic dipole emission calculations. We demonstrate the strengths and validity of our framework by determining the impact of photon recycling on the $V_{\mathrm{oc}}$ of a conventional planar organo-metal halide perovskite thin-film solar cell and compare it to established reference cases with perfect antireflection and Lambertian light scattering. Our calculations reveal $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$ values of up to 80 mV for the considered device stack in the absence of angular restriction and up to 240 mV when the escape cone above the cell is restricted to $\theta_{\mathrm{out}}=2.5^\circ$ around the cell normal. These improvements impose severe constraints on the parasitic absorption as a parasitic reabsorption probability of only 2\% reduces the $\Delta V^{\mathrm{PR}}_{\mathrm{oc}}$ to 100 mV for the same angular restriction. Our work here can be used to provide design guidelines.

VUV Dissociative Photoionization of Quinoline in the 7–26 eV Photon Energy Range

Abstract The dissociative photoionization of quinoline was studied by photoionization mass spectrometry and ion yield measurements over a synchrotron photon excitation energy range 7–26 eV. The ionic and neutral products were identified with the aid of thermochemical calculations that, in some cases, led to deeper understanding of photodissociation pathways and the determination of upper limits of heats of formation of ionic and neutral dissociation products. A detailed comparison between the 20 eV photon excitation and 70 eV electron impact mass spectra, coupled with estimation of thermochemical appearance energies, leads to assignment of the dissociative ionization cation and neutral products for each detected ion. Reaction schemes for formation of these products are proposed in a number of cases. Ion intensities in the photon and electron impact mass spectra were used to consider extending a rule of charge retention in simple bond cleavage to more complex cases of dissociative ionization.

Ionization Efficiency in the Dayside Martian Upper Atmosphere

Abstract Combining the Mars Atmosphere and Volatile Evolution measurements of neutral atmospheric density, solar EUV/X-ray flux, and differential photoelectron intensity made during 240 nominal orbits, we calculate the ionization efficiency, defined as the ratio of the secondary (photoelectron impact) ionization rate to the primary (photon impact) ionization rate, in the dayside Martian upper atmosphere under a range of solar illumination conditions. Both the CO2 and O ionization efficiencies tend to be constant from 160 km up to 250 km, with respective median values of 0.19 ± 0.03 and 0.27 ± 0.04. These values are useful for fast calculation of the ionization rate in the dayside Martian upper atmosphere, without the need to construct photoelectron transport models. No substantial diurnal and solar cycle variations can be identified, except for a marginal trend of reduced ionization efficiency approaching the terminator. These observations are favorably interpreted by a simple scenario with ionization efficiencies, as a first approximation, determined by a comparison between relevant cross sections. Our analysis further reveals a connection between regions with strong crustal magnetic fields and regions with high ionization efficiencies, which are likely indicative of more efficient vertical transport of photoelectrons near magnetic anomalies.

Influence of 68Ga-DOTATOC on sparing of normal tissue for radiation therapy of skull base meningioma: differential impact of photon and proton radiotherapy

BackgroundTo evaluate the impact of 68Ga-DOTATOC-PET on treatment planning and sparing of normal tissue in the treatment of skull base meningioma with advanced photons and protons.MethodsFrom the institutional database consisting of 507 skull base meningiomas 10 patients were chosen randomly for the present analysis. Target volume definition was performed based on CT and MRI only, as well as with additional 68Ga-DOTATOC-PET. Treatment plans were performed for Intensity Modulated Radiotherapy (IMRT) and proton therapy using active raster scanning on both target volumes. We calculated doses to relevant organs at risk (OAR), conformity indices as well as differences in normal tissue sparing between both radiation modalities based on CT/MRI planning as well as CT/MRI/PET planning.ResultsFor photon treatment plans, PET-based treatment plans showed a reduction of brain stem Dmax and Dmedian for different levels of total dose. At the optic chiasm, use of 68Ga-DOTATOC significantly reduces Dmax; moreover, the Dmedian is reduced in most cases, too. For both right and left optic nerve, reduction of dose by addition of 68Ga-DOTATOC-PET is minimal and depends on the anatomical location of the meningioma. In protons, the impact of 68Ga-DOTATOC-PET is minimal compared to photons.ConclusionAddition of 68Ga-DOTATOC-PET information into treatment planning for skull base meningiomas has a significant impact on target volumes. In most cases, PET-planning leads to significant reductions of the treatment volumes. Subsequently, reduced doses are applied to OAR. Using protons, the benefit of additional PET is smaller since target coverage is more conformal and dose to OAR is already reduced compared to photons. Therefore, PET-imaging has the greatest margin of benefit in advanced photon techniques, and combination of PET-planning and high-precision treatment leads to comparable treatment plans as with protons.

Decoupling of ion- and photon-activation mechanisms in polymer surfaces exposed to low-temperature plasmas

The modification of polypropylene (PP) by argon ions only is studied by means of a particle beam experiment. The contribution of UV photons from the plasma ion source is blocked by means of a 90° sector field. In situ FTIR measurements shows an etching dynamics governed by surface graphitization. The combined impact of ions and UV photons yields an anti‐synergistic effect on PP etching due to cross‐linking of the hydrocarbon matrix.

Inner shell alignment of atoms in photon impact ionization

The ionization of an atom can result in alignment of the residual ion. In the present work alignment studies are made for some of the high Z elements using different approaches. The theoretical values of the alignment parameter (A2) have been calculated by using the non-relativistic dipole approximation in a point Coulomb potential and analytical perturbation theory in a screened Coulomb potential. Empirical alignment evaluations are done by interpolating experimental L x-ray fluorescence cross-sections along with radiative decay rates. The alignment results are further compared with available theoretical and experimental measurements.