Showing papers on "Absorption (electromagnetic radiation) published in 2010"
TL;DR: The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.
Abstract: The use of silicon nanostructures in solar cells offers a number of benefits, such as the fact they can be used on flexible substrates. A silicon wire-array structure, containing reflecting nanoparticles for enhanced absorption, is now shown to achieve 96% peak absorption efficiency, capturing 85% of light with only 1% of the silicon used in comparable commercial cells. Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.
1,346 citations
TL;DR: Progress in light sensing using nanostructured materials is reviewed, focusing on solution-processed materials such as colloidal quantum dots and metal nanoparticles.
Abstract: The detection of photons underpins imaging, spectroscopy, fibre-optic communications and time-gated distance measurements. Nanostructured materials are attractive for detection applications because they can be integrated with conventional silicon electronics and flexible, large-area substrates, and can be processed from the solution phase using established techniques such as spin casting, spray coating and layer-by-layer deposition. In addition, their performance has improved rapidly in recent years. Here we review progress in light sensing using nanostructured materials, focusing on solution-processed materials such as colloidal quantum dots and metal nanoparticles. These devices exhibit phenomena such as absorption of ultraviolet light, plasmonic enhancement of absorption, size-based spectral tuning, multiexciton generation, and charge carrier storage in surface and interface traps.
1,253 citations
TL;DR: In this article, an ultrathin, wide-angle, subwavelength high performance metamaterial absorber for optical frequencies is presented. But the experimental results show that an absorption peak of 88% is achieved at the wavelength of ∼1.58μm, though theoretical results give near perfect absorption.
Abstract: High absorption efficiency is particularly desirable at present for various microtechnological applications including microbolometers, photodectors, coherent thermal emitters, and solar cells. Here we report the design, characterization, and experimental demonstration of an ultrathin, wide-angle, subwavelength high performance metamaterial absorber for optical frequencies. Experimental results show that an absorption peak of 88% is achieved at the wavelength of ∼1.58 μm, though theoretical results give near perfect absorption.
1,147 citations
TL;DR: This work demonstrates, for the first time, a spatially dependent metamaterial perfect absorber operating in the infrared regime, and achieves an experimental absorption of 97% at a wavelength of 6.0 μm.
Abstract: We demonstrate, for the first time, a spatially dependent metamaterial perfect absorber operating in the infrared regime We achieve an experimental absorption of 97% at a wavelength of 60 microns, and our results agree well with numerical full-wave simulations By using two different metamaterial sublattices we experimentally demonstrate a spatial and frequency varying absorption which may have many relevant applications including hyperspectral sub-sampling imaging
1,045 citations
TL;DR: In this paper, the experimental results on solar collectors based on nanofluids made from a variety of nanoparticles (carbon nanotubes, graphite, and silver) were reported.
Abstract: Solar energy is one of the best sources of renewable energy with minimal environmental impact. Direct absorption solar collectors have been proposed for a variety of applications such as water heating; however the efficiency of these collectors is limited by the absorption properties of the working fluid, which is very poor for typical fluids used in solar collectors. It has been shown that mixing nanoparticles in a liquid (nanofluid) has a dramatic effect on the liquid thermophysical properties such as thermal conductivity. Nanoparticles also offer the potential of improving the radiative properties of liquids, leading to an increase in the efficiency of direct absorption solar collectors. Here we report on the experimental results on solar collectors based on nanofluids made from a variety of nanoparticles (carbon nanotubes, graphite, and silver). We demonstrate efficiency improvements of up to 5% in solar thermal collectors by utilizing nanofluids as the absorption mechanism. In addition the experiment...
759 citations
TL;DR: The aim of reproducing some of the fundamental steps occurring in natural photosynthesis, one of the most important being the photoinduced charge separation (CS), are reproduced.
Abstract: Photosynthesis is used by nature to convert light energy into chemical energy in some living systems. In such a process, a cascade of very efficient, short-range energy and electron transfer events between well-arranged, light-harvesting organic donor and acceptor pigments takes place within the photosynthetic reaction center, leading to the overall generation of chemical energy from sunlight with near quantum efficiency.1-8 During the past decade, a significant effort has been made by the scientific community toward the preparation of synthetic model compounds of natural photosynthetic systems able to convert light into other energy sources,9 probably fostered by the increasing concerns related to the utilization of fossils fuels for the production of electricity in terms of both availability and environmental issues. However, considering the structural complexity presented by the natural photosynthetic systems, much of the scientific effort has been devoted toward the preparation and study of structurally simpler systems, with the aim of reproducing some of the fundamental steps occurring in natural photosynthesis, one of the most important being the photoinduced charge separation (CS).10-12 Among the chromophores that have been used as molecular components in artificial photosynthetic systems, porphyrinoids, the ubiquitous molecular building blocks employed by nature in natural photosynthesis, have been the preferred and obvious choice, due to their intense optical absorption and rich redox chemistry.13-20 Within the large family of porphyrinoid systems, phthalocyanines (Pcs) enjoy a privileged position (Figure 1a). These chromophores, which have a two-dimensional 18-πelectron aromatic system isoelectronic with that of porphyrins (Pors), possess in fact unique physicochemical properties which render these macrocycles valuable building blocks in materials science.21-32 Pcs are thermally and chemically stable compounds which present an intense absorption in the red/near-infrared (IR) region of the solar spectrum with extinction coefficients (as high as 200 000 M-1 cm-1) and fluorescence quantum yields * To whom correspondence should be addressed. E-mail: tomas.torres@uam.es (T.T.); dirk.guldi@chemie.uni-erlangen.de (D.M.G.). † Universidad Autonoma de Madrid. ‡ Friedrich-Alexander-Universitat Erlangen-Nurnberg. § IMDEA-Nanociencia. Chem. Rev. 2010, 110, 6768–6816 6768
731 citations
12 Oct 2010
TL;DR: A statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach is developed, revealing that the conventional limit can be substantially surpassed when optical modes exhibit deep-subwavelength-scale field confinement, opening new avenues for highly efficient next-generation solar cells.
Abstract: We use a rigorous electromagnetic approach to develop a light-trapping theory, which reveals that the standard limit developed by Yablonovitch can be substantially surpassed in nanophotonic regimes, opening new avenues for highly efficient solar cells
703 citations
21 Jun 2010
TL;DR: When femtosecond laser pulses are focused tightly into a transparent material, the intensity in the focal volume can become high enough to cause nonlinear absorption of laser energy, which can lead to permanent structural or chemical changes as discussed by the authors.
Abstract: When femtosecond laser pulses are focused tightly into a transparent material, the intensity in the focal volume can become high enough to cause nonlinear absorption of laser energy. The absorption, in turn, can lead to permanent structural or chemical changes. Such changes can be used for micromachining bulk transparent materials. Applications include data storage and the writing of waveguides and waveguide splitters in bulk glass, fabrication of micromechanical devices in polymers, and subcellular photodisruption inside single cells. Article not available.
698 citations
TL;DR: It is shown that for typical OPVs, precise knowledge of the real part of the complex index of refraction is not required for the active layer absorption, and it is critically important to take this parasitic absorption into account when calculating internal quantum effi ciency.
Abstract: In solar cells, internal quantum effi ciency (IQE) is the ratio of the number of charge carriers extracted from the cell to the number of photons absorbed in the active layer. Because IQE measurements normalize the current generation effi ciency by the light absorption effi ciency, they separate electronic properties from optical properties and provide useful information about the electrical properties of cells that external quantum effi ciency measurements alone cannot. The magnitude of the IQE is inversely related to the amount of recombination that is occurring in the cell, while the spectral shape of the curve can provide information about the effi ciency of harvesting excitons in the cell or spatial dependence of charge recombination. [ 1 , 2 ] Effects like multiple exciton generation [ 3‐5 ] and singlet exciton fi ssion [ 6 ] as well as bias-dependent photoconductivity [ 7 ] can lead to interesting spectral shapes and be detected by measuring IQEs greater than 100%. Despite its usefulness as a characterization tool, IQE is rarely reported. When IQE is reported, absorption is frequently not measured in actual devices; this can lead to errors since refl ective electrodes induce strong interference effects that substantially affect absorption. When absorption is measured in actual devices, parasitic absorptions are almost never taken into account. We hope that by demonstrating a straightforward method of measuring IQE, it will become a standard measurement and the community may benefi t from a better understanding of how the best performing cells work. Organic photovoltaics (OPVs) and other ultra-thin solar cells [ 8‐11 ] are made as a stack of materials including an active semiconducting layer, electrodes, and in some cases modifi er layers such as charge blocking layers and optical spacers. [ 12‐15 ] The active layer is responsible for all charge generation in the cell. Typically 5‐10% of the incident light is absorbed in the electrodes. In many solar cells, the IQE should not vary with wavelength. Since parasitic absorption does vary with wavelength, one must account for it to observe the correct spectral shape. [ 1 ] Consequently in the general case, it is critically important to take this parasitic absorption into account when calculating internal quantum effi ciency. Determining the active layer’s contribution to the total absorption can be a challenge, as it generally requires optical modeling to relate the experimentally measurable total absorption to the absorption in each layer. The absorption of each layer cannot independently be measured because, due to interference effects, the optical density of the stack is not simply the sum of the optical densities of each layer. The most accurate commonly used model uses a transfer matrix formalism to calculate the interference of coherent refl ected and transmitted waves at each interface in the stack. [ 16 , 17 ] This calculation requires knowledge of the wavelengthdependent complex index of refraction of each material. The imaginary part, k , is related to the extinction coeffi cient and is responsible for absorption in a medium. The real part, n , determines the wavelength of light of a given energy in a material and is important for calculating where areas of constructive and destructive interference occur. Typically the optical constants are measured using variable angle spectroscopic ellipsometry (VASE). [ 18‐22 ] The data produced by this technique when measuring anisotropic organic materials are diffi cult to interpret and require complicated modeling not available to many research groups. In blended donor-acceptor fi lms, the optical properties depend strongly on morphology and therefore on processing conditions. Thus fi lms of different thicknesses, cast from different solvents, or dried for different amounts of time have different optical constants. [ 23 , 24 ] In such composite materials, morphology is also a function of depth due to vertical phase segregation. [ 24 , 25 ] In these cases the optical constants are spatially dependent and the data gathered by these methods are approximations themselves. It is not always feasible to use VASE to measure n and k for each fi lm, so a simpler method of determining active layer absorption is desirable. In this article we show that for typical OPVs, precise knowledge of the real part of the complex index of refraction of the active layer is not required for making the measurements of the active layer absorption necessary for calculating IQE. We have investigated several methods to calculate the active layer absorption using published values of the optical constants. [ 18‐22 ] We propose a method that minimizes error by using an optical model to calculate the parasitic absorption (the absorption by the layers that do not contribute to photocurrent) and subtracting this from the experimentally measured total absorption.
665 citations
TL;DR: The design, fabrication, and measurement of ultrathin film a-Si:H solar cells with nanostructured plasmonic back contacts are reported, which demonstrate enhanced short circuit current densities compared to cells having flat or randomly textured back contacts.
Abstract: We report on the design, fabrication, and measurement of ultrathin film a-Si:H solar cells with nanostructured plasmonic back contacts, which demonstrate enhanced short circuit current densities compared to cells having flat or randomly textured back contacts. The primary photocurrent enhancement occurs in the spectral range from 550 nm to 800 nm. We use angle-resolved photocurrent spectroscopy to confirm that the enhanced absorption is due to coupling to guided modes supported by the cell. Full-field electromagnetic simulation of the absorption in the active a-Si:H layer agrees well with the experimental results. Furthermore, the nanopatterns were fabricated via an inexpensive, scalable, and precise nanopatterning method. These results should guide design of optimized, non-random nanostructured back reflectors for thin film solar cells.
653 citations
TL;DR: The nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation is revealed and successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.
Abstract: An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.
TL;DR: In this article, the authors performed a blind search for narrow absorption features at energies greater than 6.4 kV and detected 36 narrow absorption lines on a total of 101 XMM-Newton EPIC pn observations.
Abstract: Context. Blue-shifted Fe K absorption lines have been detected in recent years between 7 and 10 keV in the X-ray spectra of several radio-quiet AGNs. The derived blue-shifted velocities of the lines can often reach mildly relativistic values, up to 0.2–0.4c. These findings are important because they suggest the presence of a previously unknown massive and highly ionized absorbing material outflowing from their nuclei, possibly connected with accretion disk winds/outflows.Aims. The scope of the present work is to statistically quantify the parameters and incidence of the blue-shifted Fe K absorption lines through a uniform analysis on a large sample of radio-quiet AGNs. This allows us to assess their global detection significance and to overcome any possible publication bias.Methods. We performed a blind search for narrow absorption features at energies greater than 6.4 keV in a sample of 42 radio-quiet AGNs observed with XMM-Newton . A simple uniform model composed by an absorbed power-law plus Gaussian emission and absorption lines provided a good fit for all the data sets. We derived the absorption lines parameters and calculated their detailed detection significance making use of the classical F-test and extensive Monte Carlo simulations.Results. We detect 36 narrow absorption lines on a total of 101 XMM-Newton EPIC pn observations. The number of absorption lines at rest-frame energies higher than 7 keV is 22. Their global probability to be generated by random fluctuations is very low, less than 3 × 10-8 , and their detection have been independently confirmed by a spectral analysis of the MOS data, with associated random probability 10-7 . We identify the lines as Fe XXV and Fe XXVI K-shell resonant absorption. They are systematically blue-shifted, with a velocity distribution ranging from zero up to ~0.3c, with a peak and mean value at ~0.1c. We detect variability of the lines on both EW s and blue-shifted velocities among different XMM-Newton observations even on time-scales as short as a few days, possibly suggesting somewhat compact absorbers. Moreover, we find no significant correlation between the cosmological red-shifts of the sources and the lines blue-shifted velocities, ruling out any systematic contamination by local absorption. If we define ultra-fast outflows (UFOs) those highly ionized absorbers with outflow velocities higher than 104 km s-1 , then the majority of the lines are consistent with being associated to UFOs and the fraction of objects with detected UFOs in the whole sample is at least ~35%. This fraction is similar for type 1 and type 2 sources. The global covering fraction of the absorbers is consequently estimated to be in the range C ∼ 0.4-0.6, thereby implying large opening angles.Conclusions. From our systematic X-ray spectral analysis on a large sample of radio-quiet AGNs we have been able to clearly assess the global veracity of the blue-shifted Fe K absorption lines at E > 7 keV and to overcome their publication bias. These lines indicate that UFOs are a rather common phenomenon observable in the central regions of these sources and they are probably the direct signature of AGN accretion disk winds/ejecta. The detailed photo-ionization modeling of these absorbers is presented in a companion paper.
TL;DR: In this article, the authors examined absorption spectra of primary organic carbon (OC) emitted from solid fuel pyrolysis and found that more than 92% was extractable by methanol or acetone compared with 73% for water and 52% for hexane.
Abstract: . Carbonaceous aerosols affect the radiative balance of the Earth by absorbing and scattering light. While black carbon (BC) is highly absorbing, some organic carbon (OC) also has significant absorption, especially at near-ultraviolet and blue wavelengths. To the extent that OC absorbs visible light, it may be a non-negligible contributor to positive direct aerosol radiative forcing. Quantification of that absorption is necessary so that radiative-transfer models can evaluate the net radiative effect of OC. In this work, we examine absorption by primary OC emitted from solid fuel pyrolysis. We provide absorption spectra of this material, which can be related to the imaginary refractive index. This material has polar character but is not fully water-soluble: more than 92% was extractable by methanol or acetone, compared with 73% for water and 52% for hexane. Water-soluble OC contributes to light absorption at both ultraviolet and visible wavelengths. However, a larger portion of the absorption comes from OC that is extractable only by methanol. Absorption spectra of water-soluble OC are similar to literature reports. We compare spectra for material generated with different wood type, wood size and pyrolysis temperature. Higher wood temperature is the main factor creating OC with higher absorption; changing wood temperature from a devolatilizing state of 210 °C to a near-flaming state of 360 °C causes about a factor of four increase in mass-normalized absorption at visible wavelengths. A clear-sky radiative transfer model suggests that, despite the absorption, both high-temperature and low-temperature OC result in negative top-of-atmosphere radiative forcing over a surface with an albedo of 0.19 and positive radiative forcing over bright surfaces. Unless absorption by real ambient aerosol is higher than that measured here, it probably affects global average clear-sky forcing very little, but could be important in energy balances over bright surfaces.
TL;DR: In this article, the optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasiparticle spectrum to an optical absorption of such a system.
Abstract: We demonstrate that optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasiparticle spectrum to an optical absorption of such a system. These results are applied to graphene deposited on a surface of oxidized silicon for which we measure ellipsometric spectra, extract optical constants of a graphene layer and reconstruct the electronic dispersion relation near the K point using optical transmission spectra. We also present spectroscopic ellipsometry analysis of graphene placed on amorphous quartz substrates and report a pronounced peak in ultraviolet absorption at 4.6 eV because of a van Hove singularity in graphene's density of states. The peak is asymmetric and downshifted by 0.5 eV probably due to excitonic effects.
TL;DR: It is proposed that by patterning the silicon layer in a thin film PV cell into an array of NWs, one can boost the absorption for solar radiation by 25% while utilizing less than half of the semiconductor material (250% increase in the light absorption per unit volume of material).
Abstract: Photovoltaic (PV) cells can serve as a virtually unlimited clean source of energy by converting sunlight into electrical power. Their importance is reflected in the tireless efforts that have been devoted to improving the electrical and structural properties of PV materials. More recently, photon management (PM) has emerged as a powerful additional means to boost energy conversion efficiencies. Here, we demonstrate an entirely new PM strategy that capitalizes on strong broad band optical antenna effects in one-dimensional semiconductor nanostructures to dramatically enhance absorption of sunlight. We show that the absorption of sunlight in Si nanowires (Si NWs) can be significantly enhanced over the bulk. The NW’s optical properties also naturally give rise to an improved angular response. We propose that by patterning the silicon layer in a thin film PV cell into an array of NWs, one can boost the absorption for solar radiation by 25% while utilizing less than half of the semiconductor material (250% inc...
TL;DR: In this paper, the authors demonstrate a mid-infrared silicon optical parametric amplifier that exhibits broadband gain as large as 25.4 dB and a net off-chip amplification of 13 dB using only an ultra-compact 4mm silicon chip.
Abstract: By taking advantage of the absorption reduction at wavelengths approaching the two-photon absorption bandedge of 2,200 nm, scientists demonstrate a mid-infrared silicon optical parametric amplifier that exhibits broadband gain as large as 25.4 dB and a net off-chip amplification of 13 dB using only an ultra-compact 4-mm silicon chip.
TL;DR: In this article, a microwave characterization of several carbon-based composite materials is presented for future aircraft/aerospace systems, which consist in epoxy resin reinforced with five different carbon species: micro-sized granular graphite, fullerenes, carbon nanofibers, single and multi-walled carbon nanotubes.
TL;DR: Dye-sensitized solar cells employing mesoporous TiO(2) beads have demonstrated longer electron diffusion lengths and extended electron lifetimes over Degussa P25 titania electrodes due to the well interconnected, densely packed nanocrystalline TiO (2) particles inside the beads.
Abstract: Dye-sensitized solar cells employing mesoporous TiO2 beads have demonstrated longer electron diffusion lengths and extended electron lifetimes over Degussa P25 titania electrodes due to the well interconnected, densely packed nanocrystalline TiO2 particles inside the beads. Careful selection of the dye to match the dye photon absorption characteristics with the light scattering properties of the beads have improved the light harvesting and conversion efficiency of the bead electrode in the dye-sensitized solar cell. This has resulted in a solar to electric power conversion efficiency (PCE) of greater than 10% (10.6% for Ru(II)-based dye C101 and 10.7% using C106) for the first time using a single screen-printed titania layer cell construction (that is, without an additional scattering layer).
TL;DR: In this article, the authors report the direct observation of laboratory production of spherical, carbonaceous particles from smoldering combustion of two commonly occurring dry mid-latitude fuels, called "tar balls".
Abstract: . We report the direct observation of laboratory production of spherical, carbonaceous particles – "tar balls" – from smoldering combustion of two commonly occurring dry mid-latitude fuels. Real-time measurements of spectrally varying absorption Angstrom coefficients (AAC) indicate that a class of light absorbing organic carbon (OC) with wavelength dependent imaginary part of its refractive index – optically defined as "brown carbon" – is an important component of tar balls. The spectrum of the imaginary parts of their complex refractive indices can be described with a Lorentzian-like model with an effective resonance wavelength in the ultraviolet (UV) spectral region. Sensitivity calculations for aerosols containing traditional OC (no absorption at visible and UV wavelengths) and brown carbon suggest that accounting for near-UV absorption by brown carbon leads to an increase in aerosol radiative forcing efficiency and increased light absorption. Since particles from smoldering combustion account for nearly three-fourths of the total carbonaceous aerosol mass emitted globally, inclusion of the optical properties of tar balls into radiative forcing models has significance for the Earth's radiation budget, optical remote sensing, and understanding of anomalous UV absorption in the troposphere.
TL;DR: In this article, the enhancement of light absorption by atmospheric black carbon (BC) when it is coated in mildly absorbing material (CBrown) is reduced relative to the enhancement induced by non-absorbing coatings (CClear).
Abstract: . The presence of clear coatings on atmospheric black carbon (BC) particles is known to enhance the magnitude of light absorption by the BC cores. Based on calculations using core/shell Mie theory, we demonstrate that the enhancement of light absorption (EAbs) by atmospheric black carbon (BC) when it is coated in mildly absorbing material (CBrown) is reduced relative to the enhancement induced by non-absorbing coatings (CClear). This reduction, sensitive to both the CBrown coating thickness and imaginary refractive index (RI), can be up to 50% for 400 nm radiation and 25% averaged across the visible radiation spectrum for reasonable core/shell diameters. The enhanced direct radiative forcing possible due to the enhancement effect of CClear is therefore reduced if the coating is absorbing. Additionally, the need to explicitly treat BC as an internal, as opposed to external, mixture with CBrown is shown to be important to the calculated single scatter albedo only when models treat BC as large spherical cores (>50 nm). For smaller BC cores (or fractal agglomerates) consideration of the BC and CBrown as an external mixture leads to relatively small errors in the particle single scatter albedo of 1 indicates absorption by a non-BC aerosol. Here, it is shown that BC cores coated in CClear can reasonably have an AAE of up to 1.6, a result that complicates the attribution of observed light absorption to CBrown within ambient particles. However, an AAE 1.6. Comparison of these model results to various ambient AAE measurements demonstrates that large-scale attribution of CBrown is a challenging task using current in-situ measurement methods. We suggest that coincident measurements of particle core and shell sizes along with the AAE may be necessary to distinguish absorbing and non-absorbing OC.
TL;DR: To obtain the same ultimate efficiency as a standard 300 microm crystalline silicon wafer, the calculations show that nanohole arrays have an efficiency superior to nanorod arrays for practical thicknesses.
Abstract: We investigate silicon nanohole arrays as light absorbing structures for solar photovoltaics via simulation. To obtain the same ultimate efficiency as a standard 300 μm crystalline silicon wafer, we find that nanohole arrays require twelve times less silicon by mass. Moreover, our calculations show that nanohole arrays have an efficiency superior to nanorod arrays for practical thicknesses. With well-established fabrication techniques, nanohole arrays have great potential for efficient solar photovoltaics.
TL;DR: This result provides a molecular mechanism explaining the experimentally determined sensitivity of absorption changes in the THz domain in terms of distinct, solute-induced dynamical properties in solvation shells of (bio)molecules—even in the absence of well-defined resonances.
Abstract: Solvation of molecules in water is at the heart of a myriad of molecular phenomena and of crucial importance to understanding such diverse issues as chemical reactivity or biomolecular function. Complementing well-established approaches, it has been shown that laser spectroscopy in the THz frequency domain offers new insights into hydration from small solutes to proteins. Upon introducing spatially-resolved analyses of the absorption cross section by simulations, the sensitivity of THz spectroscopy is traced back to characteristic distance-dependent modulations of absorption intensities for bulk water. The prominent peak at ≈200 cm-1 is dominated by first-shell dynamics, whereas a concerted motion involving the second solvation shell contributes most significantly to the absorption at about 80 cm-1 ≈2.4 THz. The latter can be understood in terms of an umbrella-like motion of two hydrogen-bonded tetrahedra along the connecting hydrogen bond axis. Thus, a modification of the hydrogen bond network, e.g., due to the presence of a solute, is expected to affect vibrational motion and THz absorption intensity at least on a length scale that corresponds to two layers of solvating water molecules. This result provides a molecular mechanism explaining the experimentally determined sensitivity of absorption changes in the THz domain in terms of distinct, solute-induced dynamical properties in solvation shells of (bio)molecules—even in the absence of well-defined resonances.
TL;DR: It is shown that a photothermal detection scheme can resolve absorption events by individual molecular dyes that exhibit poor fluorescence efficiency, and provides contrast for the absorbing objects only, irrespective of scattering by defects or roughness.
Abstract: So far, single-molecule imaging has predominantly relied on fluorescence detection. We imaged single nonfluorescent azo dye molecules in room-temperature glycerol by the refractive effect of the heat that they release in their environment upon intense illumination. This photothermal technique provides contrast for the absorbing objects only, irrespective of scattering by defects or roughness, with a signal-to-noise ratio of ~10 for a single molecule in an integration time of 300 milliseconds. In the absence of oxygen, virtually no bleaching event was observed, even after more than 10 minutes of illumination. In a solution saturated with oxygen, the average bleaching time was of the order of 1 minute. No blinking was observed in the absorption signal. On the basis of bleaching steps, we obtained an average absorption cross section of 4 angstroms2 for a single chromophore.
TL;DR: A double coating process is reported that greatly improves the uniformity of the carbon coating on both the primary and secondary LiFePO particles, resulting in electrodes having high volumetric energy density and rate capability.
Abstract: , and prom-ising electrochemical performance. This single-carbon-coating material was composed of micrometer-scale secondary parti-cles containing nanoscale carbon-coated primary particles; this morphology provided interconnected open pores that favor elec-trolyte absorption and signifi cantly reduce the diffusion path of the lithium ions. This feature, combined with the high tap density, resulted in electrodes having high volumetric energy density and rate capability. Here we report a double coating process that greatly improves the uniformity of the carbon coating on both the primary and secondary LiFePO
TL;DR: A mixture of SiC and SiO(2) nanoparticles yields high performance cooling at low cost within a practical cooling rig and optical properties are presented and subambient cooling performance analyzed for doped polyethylene on aluminum.
Abstract: Nanoparticles that have narrow absorption bands that lie entirely within the atmosphere’s transparent window from 7.9 to 13 μm can be used to radiatively cool to temperatures that are well below ambient. Heating from incoming atmospheric radiation in the remainder of the Planck radiation spectrum, where the atmosphere is nearly “black”, is reduced if the particles are dopants in infrared transmitting polymers, or in transmitting coatings on low emittance substrates. Crystalline SiC nanoparticles stand out with a surface phonon resonance from 10.5 to 13 μm clear of the atmospheric ozone band. Resonant SiO2 nanoparticles are complementary, absorbing from 8 to 10 μm, which includes atmospheric ozone emissions. Their spectral location has made SiC nanoparticles in space dust a feature in ground-based IR astronomy. Optical properties are presented and subambient cooling performance analyzed for doped polyethylene on aluminum. A mixture of SiC and SiO2 nanoparticles yields high performance cooling at low cost w...
TL;DR: A novel dual-diameter nanopillar structure is presented, with a small diameter tip for minimal reflectance and a large diameter base for maximal effective absorption coefficient, which enables a viable and convenient route toward shape-controlled nanopillsar-based high-performance photonic devices.
Abstract: Optical properties of highly ordered Ge nanopillar arrays are tuned through shape and geometry control to achieve the optimal absorption efficiency. Increasing the Ge materials filling ratio is shown to increase the reflectance while simultaneously decreasing the transmittance, with the absorbance showing a strong diameter dependency. To enhance the broad band optical absorption efficiency, a novel dual-diameter nanopillar structure is presented, with a small diameter tip for minimal reflectance and a large diameter base for maximal effective absorption coefficient. The enabled single-crystalline absorber material with a thickness of only 2 μm exhibits an impressive absorbance of ∼99% over wavelengths, λ = 300−900 nm. These results enable a viable and convenient route toward shape-controlled nanopillar-based high-performance photonic devices.
TL;DR: The complementary optical properties of metal and semiconductor nanostructures make them attractive components for many applications that require controlled flow of electromagnetic energy on the nanometer length scale as mentioned in this paper.
Abstract: The complementary optical properties of metal and semiconductor nanostructures make them attractive components for many applications that require controlled flow of electromagnetic energy on the nanometer length scale. When combined into heterostructures, the nanometer-scale vicinity of the two material systems leads to interactions between quantum-confined electronic states in semiconductor nanostructures and dielectric-confined electromagnetic modes in the metal counterparts. Such exciton−plasmon interactions allow design of absorption and emission properties, control of nanoscale energy-transfer processes, creation of new excitations in the strong coupling regime, and increase of optical nonlinearities. With the advancement of novel fabrication techniques, the functionalities of metal−semiconductor nanostructures will be further increased for better control of optical properties and energy flows on nanometer length and femtosecond time scales.
TL;DR: In this article, a method to compensate for the effect of acoustic absorption on the measured time domain signals is described, where the reconstruction is regularized by filtering the absorption and dispersion terms in the spatial frequency domain using a Tukey window.
Abstract: The reconstruction of photoacoustic images typically neglects the effect of acoustic absorption on the measured time domain signals. Here, a method to compensate for acoustic absorption in photoacoustic tomography is described. The approach is based on time-reversal image reconstruction and an absorbing equation of state which separately accounts for acoustic absorption and dispersion following a frequency power law. Absorption compensation in the inverse problem is achieved by reversing the absorption proportionality coefficient in sign but leaving the equivalent dispersion parameter unchanged. The reconstruction is regularized by filtering the absorption and dispersion terms in the spatial frequency domain using a Tukey window. This maintains the correct frequency dependence of these parameters within the filter pass band. The method is valid in one, two and three dimensions, and for arbitrary power law absorption parameters. The approach is verified through several numerical experiments. The reconstruction of a carbon fibre phantom and the vasculature in the abdomen of a mouse are also presented. When absorption compensation is included, a general improvement in the image magnitude and resolution is seen, particularly for deeper features.
TL;DR: In this paper, the authors summarize the most recent developments in conjugated polymers for high-efficiency organic photovoltaics (OPV) devices, focusing on correlations of polymer chemical structures with properties.
TL;DR: In this article, a dual-band metamaterial absorber with two perfect absorption points near 11.15GHz and 16.01GHz was designed and measured in the microwave region.
Abstract: In this paper, we present the design, simulation, and measurement of a dual-band metamaterial absorber in the microwave region. Simulated and experimental results show that the absorber has two perfect absorption points near 11.15GHz and 16.01GHz. Absorptions under difierent polarizations of incident EM waves are measured with magnitude of over 97% at low-frequency peak and 99% at high-frequency peak respectively. Current distribution at the dual absorptive peaks is also given to study the physical mechanism of power loss. Moreover, it is verifled by experiment that the absorptions of this kind of metamaterial absorber remain over 90% at the low-frequency peak and 92% at the high-frequency peak with wide incident angles ranging from 0 - to 60 - for both transverse electric wave and transverse magnetic wave.