Short Wavelength Range Research Articles

Ellipsometric study on optical properties of hydrogen plasma-treated aluminum-doped ZnO thin film

Aluminum-doped zinc oxide (AZO) thin films were prepared by radio frequency (RF) sputtering at room temperature, and then post-treated by hydrogen (H2) plasma at different durations. After H2 plasma treatment under the condition of 10 W, 200 °C and 3.0 Hours, the resistivity showed a dramatically decrease from 1.6 Ω cm to 3.4 × 10−3 Ω cm, while the transmittance at the wavelength of 550 nm was improved from 90.5% to 96.0%. The optical constants of H2 plasma-treated AZO thin films were detailed characterized by a varied angle spectroscopic ellipsometer. The results show that the refractive index n decreases in the entire measured wavelength range of 350–1100 nm, while the extinction coefficient k decreases in the short wavelength range and changes negligibly at the long wavelength range. These results can provide guidelines for the design and optimization of AZO thin film-based optoelectronic applications.

Effect of photogenerated carrier distribution on performance enhancement of photomultiplication organic photodetectors

The trap-assisted photomultiplication (PM) organic photodetectors (OPDs) are very attractive for achieving high sensitivity photodetection, with external quantum efficiency (EQE) in excess of 100%. A classic structure of PM-type OPDs has a poly-3-hexylthiophene (P3HT): Phenyl-C70-butyric acid methyl ester (PC70BM) blend active layer, made with the weight ratio of P3HT to PC70BM of 100:1. The presence of a low PC70BM content in the P3HT:PC70BM blend layer forms isolated PC70BM domains serving as the defect sites to trap the photogenerated electrons. The trapped electrons induce a strong interfacial band bending at the organic/metal contact (Al) interface assisting in an enhanced hole injection via tunneling effect. In this work, we report a comprehensive study to understand the effect of the distribution of the photogenerated charge carriers on performance of the PM OPDs, made with a P3HT:PC70BM active layer with different layer thicknesses, for optimizing the PM effect. The combination effect of both the exciton generation and carrier loss properties affects the distribution of the photogenerated charge carriers and hence the PM processes in the OPDs. The optical properties of the PM OPDs were simulated. It reveals that a stronger exciton generation occurs near the organic/Al interface in PM OPD with a thinner active layer. The photoluminescence and surface morphology measurements suggested that a 230 nm thick active layer with a smooth surface had the lowest nonradiative loss. An optimal EQE of 105569% and a photoresponsivity of 344 A/W were obtained for a PM OPD with a 205 nm thick active layer, which are about 330% higher than that of the OPD with a 325 nm thick active layer. Due to the tradeoff between EQE and dark current, the OPD with a thin active layer (205 nm) yields a maximum detectivity of 1.87 × 1013 Jones at the short wavelength range while the OPD with a thick active layer (325 nm) has the highest detectivity of 2.32 × 1013 Jones at the long wavelength range. Our work contributes to the development of high performance low-cost PM OPDs for detecting weak light signals.

The potential of many-line inversions of photospheric spectropolarimetric data in the visible and near UV

Our knowledge of the lower solar atmosphere is mainly obtained from spectropolarimetric observations, which are often carried out in the red or infrared spectral range and almost always cover only a single or a few spectral lines. Here we compare the quality of Stokes inversions of only a few spectral lines with many-line inversions. In connection with this, we have also investigated the feasibility of spectropolarimetry in the short-wavelength range, 3000 Å−4300 Å, where the line density but also the photon noise are considerably higher than in the red, so that many-line inversions could be particularly attractive in that wavelength range. This is also timely because this wavelength range will be the focus of a new spectropolarimeter in the third science flight of the balloon-borne solar observatory SUNRISE. For an ensemble of state-of-the-art magneto-hydrodynamical atmospheres we synthesize exemplarily spectral regions around 3140 Å (containing 371 identified spectral lines), around 4080 Å (328 lines), and around 6302 Å (110 lines). The spectral coverage is chosen such that at a spectral resolving power of 150 000 the spectra can be recorded by a 2K × 2K detector. The synthetic Stokes profiles are degraded with a typical photon noise and afterward inverted. The atmospheric parameters of the inversion of noisy profiles are compared with the inversion of noise-free spectra. We find that significantly more information can be obtained from many-line inversions than from a traditionally used inversion of only a few spectral lines. We further find that information on the upper photosphere can be significantly more reliably obtained at short wavelengths. In the mid and lower photosphere, the many-line approach at 4080 Å provides equally good results as the many-line approach at 6302 Å for the magnetic field strength and the line-of-sight (LOS) velocity, while the temperature determination is even more precise by a factor of three. We conclude from our results that many-line spectropolarimetry should be the preferred option in the future, and in particular at short wavelengths it offers a high potential in solar physics.

Absorption coefficient of water vapor across atmospheric troposphere layer

Absorption coefficient of water vapor proposed to be responsible for an increase in temperature in the troposphere layer with altitude less than 10 km is systematically presented in this work. Since global warming plays an important role in affecting the human life, a confirmative and detailed study of global warming is essentially need. Solar irradiation within short wavelength range can be extinguished from absorption and scattering by the atmosphere, and absorbed and reflected by the Earth's surface. Radiative within high wavelength range from the Earth's surface can be absorbed by atmospheric water vapor, carbon dioxide and other gases. The difference in solar irradiation and energy escaped to the space from the atmosphere results in the atmosphere acting as the glass of a greenhouse and increase atmospheric temperature. Extending the previous work [1] for predicting absorption coefficient of carbon dioxide through the troposphere, this work further determines absorption coefficients of water vapor in different wavelength bands centered at 71, 6.3, 2.7, 1.87 and 1.38 μm across the temperature, pressure and concentration-dependent troposphere layer. Solving one-dimensional unsteady heat conduction-radiation equation with the COMSOL computer code, the predicted temperature together with water vapor density for different optical path lengths can be used to interpret in details absorption coefficient or the ratio between band intensity and effective band width by using the exponential wide band model. The results show that absorption coefficients are strongly affected by water vapor concentration. For example, absorption coefficients in the band centered at 71 μm increases from 0.3 to 1.2 m−1 at the tropopause and 0.6 to 3.1 m−1 at the Earth's surface as mole fraction of water vapor increases from 0.005 to 0.02. The predicted absorption coefficients agree with experimental and theoretical results in the literature. A more detailed and realistic temperature profile through the troposphere with optical path length of 104 m is presented.

Visible light optical coherence tomography in biomedical imaging

Optical coherence tomography (OCT) is a widely used optical imaging modality for three-dimensional structural and functional imaging. The prevalent OCT systems use an invisible light laser source beyond 800 nm and up to 1 500 nm to allow deep image penetration in biological tissues. Recently, visible light OCT (vis-OCT) using a short wavelength range between 400 nm to 700 nm has gained significant progress and attracted interest in its unique capability of high resolution imaging and spatially-resolved spectroscopy. In this article, we will briefly review the recent advance of vis-OCT imaging and its potential biomedical applications.

On Detecting the Fourth Gyrofrequency Harmonic in Microwave Emission Spectra above Sunspots

Spectral polarization observations of radio sources above sunspots are regularly carried out with the RATAN-600 radio telescope (RATAN is a Russian acronym for the Radio Astronomical Telescope of the Academy of Sciences). The in-depth studies of the spectra reveal new effects. In this paper, we analyze the manifestation of radio emission of the fourth gyrofrequency harmonic in microwave spectra obtained with 1-percent frequency resolution in a range of 3–18 GHz. Registration of the extraordinary mode in the short-wavelength range of the spectrum is compared to the model calculations of the second-to-fifth gyrofrequency harmonics against a background of the thermal bremsstrahlung emission of flocculi, surrounding the spot structure of an active region. The brightening of the extraordinary mode in the short-wavelength spectral range and the kinks in the intensity spectra of emission are analyzed. The interpretation of the RATAN-600 observational data with probable diagnostics of the emission of the fourth gyrofrequency harmonic is considered as examples.

Dithienothiapyran: An Excellent Donor Block for Building High-Performance Copolymers in Nonfullerene Polymer Solar Cells.

In this work, a new but excellent donor block dithienothiapyran (DTTP) was developed for constructing highly efficient wide band gap copolymer donors. Compared to dithienopyran (DTP), DTTP features weaken electron-donating ability and more planar-conjugated backbones. Polymer-fluorinated benzotriazole (FBTA) based on DTTP exhibits lower highest occupied molecular orbital level (-5.30 vs -5.21 eV), higher molar extinction coefficient (1.54 × 105 vs 8.65 × 104 M-1 cm-1), and better crystallinity than -FBTA based on DTP, thus producing a higher device performance of 10.51% in binary blend nonfullerene polymer solar cells (NF-PSCs) blended with IT-M. To improve the absorption strength of PDTTP-FBTA: devices in the shorter wavelength range and further optimize the blend morphology, a small molecule of , which has strong absorption at short wavelength (300-600 nm), was incorporated. Finally, the performance of the ternary blends was successfully enhanced to 11.57% and a very high fill factor of 76.5%. Our work provided a new but excellent donor block for building high-performance conjugated copolymers to achieve highly efficient NF-PSCs.

Controllable phase transformation of titanium dioxide for the high performance polymer solar cells

As an important electron transport layer, titanium dioxide is intensively studied in the polymer solar cells. Titanium dioxide exists as two main polymorphs which are anatase and rutile. However, the impacts of the presence of either or both of these phases on the performance of polymer solar cells remain unclear. In this work, high quality TiO2 films with controllable polymorphs are deposited via sputtering under room temperature. Pure phase of anatase and rutile are obtained. The distinguishing characteristics of the films were analyzed and PSCs with the structure of ITO/TiO2/PTB7-Th:PC71BM/MoOx/Ag were made. We found a better electron extraction efficiency of anatase which may originated from the band alignment of TiO2 in between ITO and active layer and smaller electron effective mass of anatase. Moreover, the anatase possesses a closer refractive index with ITO and high transparency in the short wavelength range which make it efficiently utilize the induced light. The efficiency of the PSCs has achieved 4.01% for rutile device and 8.26% for anatase device.

Single-channel UV/vis dual-band detection with ZnCdS:Mn/ZnS core/shell quantum dots

In the ultraviolet detection system, the Si-based photodetector could be sensitised with different kinds of fluorescent material to enhance its response in the short-wavelength range. Thick-shell ZnCdS:Mn/ZnS core/shell quantum dots (QDs) exhibit unique advantages in UV signal sensitisation due to their long PL lifetime, as well as stable emission matched with CCD’s response. Herein, a single-channel UV panoramic detection system based on these Mn-doped QDs has been proposed. The QDs@PMMA film was attached on a Si-based CCD camera versus a tapered fibre, and an optical chopper was mounted before the QDs@PMMA film. The long lifetime fluorescence originating from UV signal could be still collected by the CCD camera when the chopper is in the ‘off’ state, hence the UV/vis signal ratio is significantly enhanced.

The normal spectral emittance of the real surface from worked aero-engine nozzle

The radiative properties of the surfaces of aero-engine components have significant impact on inner heat transfer. It is necessary to research the radiative properties of surfaces which have experienced real working conditions. In this paper, samples were cut from an aero-engine exhaust nozzle which was in operation in the engine and have complex surface properties. Some samples were further processed by grinding or scrubbing to produce a substrate alloy sample and a sample without contaminants. The normal spectral emittances of each sample have been measured for wavelengths ranging in 2.5–20 μm at different temperatures (400–1200 K) using Fourier Transform Infrared (FT-IR) spectrometer. The emittance of the substrate alloy increases as temperature increases and wavelength decreases. By comparing different measurement results of samples, it can be inferred that there are three main factors affecting the radiative properties, including oxidation, surface roughness and contaminants. Oxidation can elevate the normal spectral emittance of sample surfaces and give more weight to short-wavelength range. Roughness would mainly affect radiative properties in long-wavelength range. It seems that contaminants change the radiative properties in the whole wavelength range. In reality, these factors usually exist simultaneously and are related to each other. Though the radiative properties of realistic alloy surfaces are relatively complex, it can be analyzed by different factors.

Absorption coefficient of carbon dioxide across atmospheric troposphere layer

Absorption coefficient affected by carbon dioxide concentration and optical path length responsible for temperature or global warming across the troposphere layer, which is less than the altitude of 10 km in the atmosphere, is systematically presented in this work. Solar irradiation within a short wavelength range can be absorbed, scattered and transmitted by the atmosphere, and absorbed and reflected by the Earth's surface. Radiative emission in high wavelength ranges from the Earth's surface at low temperature can be absorbed by atmospheric water vapor, carbon dioxide and other gases. Unbalance of radiation thus results in the atmosphere to act as the glass of a greenhouse and increase atmospheric temperature. Even though global warming strongly affects the life of the human being, the cause of global warming is still controversial. This work thus proposes a fundamental and systematical unsteady one-dimensional heat conduction-radiation model together with exponential wide band model to predict absorption coefficients affected by concentration, temperature, optical path lengths and radiation correlated parameters in different bands centered at 15, 4.3, 2.7, and 2 μm of carbon dioxide across the troposphere layer. It shows that absorption coefficient required for calculating heat transfer is strongly affected by carbon dioxide concentration and optical path length across the troposphere. Relevant values of the latter should be greater than 5,000 m. Absorption coefficients in the band centered at 4.3 μm subject to a chosen optical path length of 104 m increase from 0.04 m−1 and 0.165 m−1at the tropopause to 0.11 m−1 and 0.44 m−1 at the Earth's surface for carbon dioxide concentrations of 100 and 400 ppm, respectively. A more relevant and detailed temperature profile across the troposphere is presented.

Optical cleanliness measurement methods for aluminium sheet surfaces

The cleaning of rolled aluminium surfaces is of critical importance for most applications and is of particular importance in automotive applications. The cleanliness of the sheet surface is mainly determined by the residual amount of the near‐surface deformed layer on the alloy surface. This layer has nano‐sized grains with their grain boundaries decorated and pinned by oxide particles and lubricant residues. The deformed layer reduces the reflectivity of the sheet or strip surface to visible light, particularly in the short wavelength range, resulting in a brownish appearance. Based on the optical characteristics of the deformed layer, the use of optical microscopy, spectrophotometry, and colourimetry has been evaluated to provide a quantitative measurement of the level of cleanliness. These latter evaluations have been cross‐checked by using scanning electron microscopy and transmission electron microscopy on ultramicrotomed cross sections of aluminium sheet samples subjected to different level of cleaning to determine the amount of residual near‐surface deformed layer.

Spectral Properties of Magnetohydrodynamic Turbulence Revealed by Polarization Synchrotron Emission with Faraday Rotation

Abstract We investigate how to recover the spectral properties of underlying magnetohydrodynamic (MHD) turbulence using fluctuation statistics of synchrotron polarization radiation, based on the synthetic observations. Taking spatially coincident, separated, and compounded synchrotron emission and Faraday rotation regions into account, we extract the power spectrum of synchrotron polarization intensities integrated along the line of sight. Our results demonstrate that in the short wavelength range, the power spectra reflect fluctuation statistics of the perpendicular component of turbulent magnetic fields, and the spectra at long wavelengths reveal the fluctuation of the Faraday rotation density, which is a product of the parallel component of magnetic field and thermal electron density. We find that our numerical results (in the case of spatially coincident regions) are in agreement with the analytical prediction in Lazarian & Pogosyan, and this theoretical prediction is applicable to more complicated settings, i.e., the spatially separated and compounded regions. We simulate telescopic observations that incorporate the effects of telescope angular resolution and noise, and find that statistics of underlying MHD turbulence can be recovered successfully. We expect that the technique can be applied to a variety of astrophysical environments, with existing synchrotron data cubes and a large number of forthcoming data sets from such as the LOw Frequency Array for Radio astronomy, the Square Kilometer Array, and the Five-hundred-meter Aperture Spherical radio Telescope.

Photosensitive organic-inorganic hybrid structures based on porous silicon

In this study, the photovoltaic organic-inorganic structures were created by deposition of poly(3,4-ethylenedioxythiophene) film doped by poly(styrenesulfonate) and reduced graphene oxide on the porous silicon/silicon substrate. Formation of the hybrid structure was confirmed by means of atomic-force microscopy and Fourier transform infrared spectroscopy. The current-voltage characteristics of the obtained structures were studied. It was found the increase of electrical conductivity and photo-induced signal in organic-inorganic structures. Temporal parameters and spectral characteristics of photoresponse in the 400–1100 nm wavelength range were investigated. The widening of spectral photosensitivity in a short-wavelength range due to light absorption in various layers of the multijunction structure in comparison with single crystal silicon was revealed.

Synthesis and photovoltaic properties of a small molecule acceptor with thienylenevinylene thiophene as π bridge

A new n-type organic semiconductor (n-OS) acceptor with (E)-5-(2-(5-(2-ethylhexyl)thiophen-2-yl)vinyl)-thiophene-2-yl (TVT) functional group as π-bridge to link indacenodithiophene (IDT) core and 3-(dicyanomethylidene)indan-1-one (IC) terminals, named ITVT, was synthesized and applied in polymer solar cells (PSCs). It is the first time employing TVT unit in n-OS small molecule acceptor. Benefited from the conjugated TVT side groups, the small molecule ITVT shows a strong absorption at the short wavelength range (300–500 nm). In addition, ITVT exhibits a low optical band gap (Egopt) of 1.48 eV and a medium electron mobility of 1.85 × 10−4 cm2 V-1 s-1. A wide band gap polymer PBZ was chosen as donor to fabricate an inverted BHJ PSCs because of the complementary absorption and matched molecular energy levels. The as-cast PSCs yield a power conversion efficiency (PCE) of 5.84% with an open-circuit voltage (Voc) of 0.96 V and a short-circuit current density (Jsc) of 14.22 mA cm−2 under the illumination of AM 1.5 G at 100 mW cm−2.

Fabrication of GaAs solar cells grown with InGaP layers by hydride vapor-phase epitaxy

The elevated manufacturing cost of highly efficient III–V multijunction devices limits them to space and high-concentration terrestrial applications. Hydride vapor-phase epitaxy (HVPE), in contrast to metal–organic vapor-phase epitaxy, can reduce the manufacturing costs due to the use of cheaper group III metal sources, capability to grow crystals under a low arsenic overpressure, and its low installation cost. In this study, we characterized the abruptness of the heterointerface between the InGaP and GaAs layers in GaAs solar cells grown by HVPE. InGaP passivation layers, such as the back-surface field (BSF) and window layers, were introduced in order to enhance the performance of the GaAs solar cells. Owing to the reduction of the surface recombination, the devices fabricated with the incorporated window layer showed a significant improvement in the short-wavelength range of the external quantum efficiency response, compared with that obtained for the unpassivated cells. This provided a cell efficiency improvement from 9.25 to 20.75%. However, the introduction of the BSF layer degrades the cell efficiency to 17.92%, owing to the formation of an anomalous interlayer at the GaAs-on-InGaP heterointerface.

Upconversion Luminescence of Graphene Oxide through Hybrid Waveguide

Phonon-assisted upconversion is a promising way to generate short-wavelength emissions under excitation of long wavelength based on unique anti-Stokes luminescence properties. Graphene oxide nanosheets (GONs) exhibit excellent optical properties owing to quantum confinement and edge effects, which have driven research into fundamental principles and potential applications. Here, we experimentally demonstrate upconversion emission by exciting an easily fabricated GON hybrid waveguide (GHW) with enhanced photothermal effects. The results reveal different origins of short-wavelength range and long-wavelength range in the upconversion spectra, whereas the emissive surface defects of GONs and GHW structure play significant roles in the behavior of photoluminescence. Introducing other upconversion materials to promote emission efficiency, the hybrid waveguide system might readily provide the possibility for the construction of upconversion fiber lasers and remote control of the upconversion luminescence.

Titanium nitride selective absorber enhanced solar thermoelectric generator (SA-STEG)

Solar thermoelectric generator (STEG) system has been intensively studied due to its reliability and environment-friendly property. In a STEG system, a solar absorber is an important component because it not only absorbs the solar energy, but also emits photons to environment as radiation heat loss. In this study, a selective absorber enhanced STEG system (SA-STEG) is presented. A Titanium nitride (TiN) based 2D photonic crystal (PhC) selective absorber is fabricated, which possesses a high absorptivity at short wavelength range to fully absorb the solar energy, but a suppressed absorptivity at long wavelength range to avoid the radiation loss. The selective absorber also keeps a good wavelength selectivity even after a thermal annealing over 1000 K. The selective absorptivity data were imported into the SA-STEG model and an improvement was obtained: for a Lead telluride STEG system, a 28 K temperature increase was achieved, and for a Silicon germanium alloy STEG system a 32 K temperature increase was achieved. The increased temperature yields a larger open-circuit voltage output. These results were attained based on a 40 × 40 mm2 STEG system and this value is expected to be further increased if a larger thermoelectric generator is employed.

Species Entropies in the Kinetic Range of Collisionless Plasma Turbulence: Particle-in-cell Simulations

Abstract Three-dimensional particle-in-cell simulations of the forward cascade of decaying turbulence in the relatively short-wavelength kinetic range have been carried out as initial-value problems on collisionless, homogeneous, magnetized electron-ion plasma models. The simulations have addressed both whistler turbulence at β i = β e = 0.25 and kinetic Alfvén turbulence at β i = β e = 0.50, computing the species energy dissipation rates as well as the increase of the Boltzmann entropies for both ions and electrons as functions of the initial dimensionless fluctuating magnetic field energy density ε o in the range 0 ≤ ε o ≤ 0.50. This study shows that electron and ion entropies display similar rates of increase and that all four entropy rates increase approximately as ε o , consistent with the assumption that the quasilinear premise is valid for the initial conditions assumed for these simulations. The simulations further predict that the time rates of ion entropy increase should be substantially greater for kinetic Alfvén turbulence than for whistler turbulence.

Dual donor bulk-heterojunction to realize a quick and more sensitive organic visible photodector

We present herein, an interpenetrating organic dual donor bulk-heterojunction platform for promising visible light sensing utility. The photoactive layer of the sensor is comprised of ternary organic blend of 7,16-bis(3,3-dimethyl-3H-indol-2-yl)-5,14-dihydrodibenzo [b,i][1,4,8,11] tetraazacy clotetradecine (LH2), poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM). Dual donor (LH2 and P3HT) configuration has been utilized for spectral enhancement in visible range. The role of LH2 doping in the P3HT:PC61BM binary blend in the present sensor has been justified by the improved light absorption in the short wavelength range. The photoluminescence (PL) spectra of the LH2: P3HT binary blend has been studied to optimize the volumetric ratio of the donor moieties for improved charge transfer efficiency in the ternary blend. The photo detector’s electrical parameters have been recorded under varied illumination intensities. The current density-voltage (J–V) characteristics of the fabricated sensors exhibit a strong relationship with incident optical power densities. The sensor has revealed high photoconductive sensitivity ~ 5.33 × 10− 11 (S m/W) and significantly higher responsivity of ∼ 3.23 mA/W. Further the sensor possesses rapid change of states as function of stepped light input (response and recovery time of ∼ 340 and ~ 550 ms) and the corresponding photocurrent and dark current show a stable plateau value. Beside reliability and noteworthy sensing parameters, the eco-friendly processing and low power operation provide a practical advantage to the fabricated organic photodetector (OPD).