Showing papers on "Absorption (electromagnetic radiation) published in 2008"

Gold nanoparticles in biology: beyond toxicity to cellular imaging

Abstract: Gold, enigmatically represented by the target-like design of its ancient alchemical symbol, has been considered a mystical material of great value for centuries. Nanoscale particles of gold now command a great deal of attention for biomedical applications. Depending on their size, shape, degree of aggregation, and local environment, gold nanoparticles can appear red, blue, or other colors. These visible colors reflect the underlying coherent oscillations of conduction-band electrons (“plasmons”) upon irradiation with light of appropriate wavelengths. These plasmons underlie the intense absorption and elastic scattering of light, which in turn forms the basis for many biological sensing and imaging applications of gold nanoparticles. The brilliant elastic light-scattering properties of gold nanoparticles are sufficient to detect individual nanoparticles in a visible light microscope with ∼102 nm spatial resolution. Despite the great excitement about the potential uses of gold nanoparticles for medical diag...

The electronic origin of the visible-light absorption properties of C-, N- and S-doped TiO2 nanomaterials.

Abstract: The origin of the visible-light absorption of main-group element (C, N, S) doped TiO2 nanostructures is investigated via diffuse reflectance and valence band X-ray photoelectron spectroscopy. The synthesized C-, N-, and S-doped titania nanomaterials show an increased electron density of states above the valence band of TiO2, which explains the red-shifted light absorption of these potential photocatalysts and simultaneously suggests a lowered potential as photooxidants compared to Degussa P25 TiO2.

Optical constants of ice from the ultraviolet to the microwave: A revised compilation

Abstract: [1] A compilation of the spectral absorption coefficient of ice Ih is presented for temperatures near the melting point, superseding the compilation of Warren (1984). Significant changes are made to nearly all spectral regions. The blue and near-ultraviolet absorption is much weaker than the prior estimates, which were already very small. The near-infrared absorption coefficient differs by as much as a factor of 2 from the prior compilation at some wavelengths. The midinfrared absorption coefficient is rather uncertain in the weakly absorbing regions near 9 and 20 μm. New far-infrared measurements at low temperatures are extrapolated to higher temperatures, which shifts the peak positions. New microwave measurements find absorption much weaker than previously reported, by factors of 2–5. The real part of the index of refraction is computed using Kramers-Kronig analysis; it differs from the prior compilation only in the far infrared. Tables of the revised optical constants are available on a website.

Two-photon absorption standards in the 550–1600 nm excitation wavelength range

Abstract: We present absolute two-photon absorption (2PA) spectra of 15 commercial organic dyes covering an extended range of excitation wavelengths, 550–1600 nm. The 2PA is measured with an estimated accuracy ±10% using a femtosecond fluorescence excitation method. The data are corrected for the variations of the pulse duration and the beam profile with the excitation wavelength, and are applicable as reference standards for 2PA measurements.

Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells

Abstract: We demonstrate that subwavelength scatterers can couple sunlight into guided modes in thin film Si and GaAs plasmonic solar cells whose back interface is coated with a corrugated metal film. Using numerical simulations, we find that incoupling of sunlight is remarkably insensitive to incident angle, and that the spectral features of the coupling efficiency originate from several different resonant phenomena. The incoupling cross section can be spectrally tuned and enhanced through modification of the scatterer shape, semiconductor film thickness, and materials choice. We demonstrate that, for example, a single 100 nm wide groove under a 200 nm Si thin film can enhance absorption by a factor of 2.5 over a 10 μm area for the portion of the solar spectrum near the Si band gap. These findings show promise for the design of ultrathin solar cells that exhibit enhanced absorption.

Counting near-infrared single-photons with 95% efficiency

Abstract: Single-photon detectors operating at visible and near-infrared wavelengths with high detection efficiency and low noise are a requirement for many quantum-information applications. Superconducting transition-edge sensors (TESs) are capable of detecting visible and near-infrared light at the single-photon level and are capable of discriminating between one-and two-photon absorption events; however these capabilities place stringent design requirements on the TES heat capacity, thermometry, and optical detection efficiency. We describe the fabrication and evaluation of a fiber-coupled, photon-number-resolving TES detector optimized for absorption at 1550 and 1310 nm wavelengths. The measured system detection efficiency at 1556 nm is 95 %±2 %, which to our knowledge is the highest system detection efficiency reported for a near-infrared single-photon detector.Work of US government: not subject to US copyright

Coherent multiheterodyne spectroscopy using stabilized optical frequency combs.

Abstract: The broadband, coherent nature of narrow-linewidth fiber frequency combs is exploited to measure the full complex spectrum of a molecular gas through multiheterodyne spectroscopy. We measure the absorption and phase shift experienced by each of 155 000 individual frequency-comb lines, spaced by 100 MHz and spanning from 1495 to 1620 nm, after passing through hydrogen cyanide gas. The measured phase spectrum agrees with the Kramers-Kronig transformation of the absorption spectrum. This technique can provide a full complex spectrum rapidly, over wide bandwidths, and with hertz-level accuracy.

The Relation Between Open‐Circuit Voltage and the Onset of Photocurrent Generation by Charge‐Transfer Absorption in Polymer : Fullerene Bulk Heterojunction Solar Cells

Abstract: Photocurrent generation by charge-transfer (CT) absorption is detected in a range of conjugated polymer–[6,6]-phenyl C61 butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier-transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground-state charge-transfer complexes in the studied polymer–fullerene blends. The effective band gap (Eg) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all devices, under various preparation conditions, the open-circuit voltage (Voc) scales linearly with Eg. The redshift of the CT band upon thermal annealing of regioregular poly(3-hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open-circuit voltage of high-temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of Eg, proportional with the observed changes in Voc for different PPV:PCBM ratios. As Eg corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.

Measurement of the optical absorption spectra of epitaxial graphene from terahertz to visible

Abstract: We present experimental results on the optical absorption spectra of epitaxial graphene from the visible to the terahertz frequency range. In the terahertz range, the absorption is dominated by intraband processes with a frequency dependence similar to the Drude model. In the near-IR range, the absorption is due to interband processes and the measured optical conductivity is close to the theoretical value of e2/4ℏ. We extract values for the carrier densities, the number of carbon atom layers, and the intraband scattering times from the measurements.

Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals

Abstract: A facile method for the synthesis of size- and shape-controlled CuInS2 semiconductor nanocrystals was developed by thermolysis of a mixed solution of CuAc, In(Ac)3 (molar ratio of CuAc to In(Ac)3 = 1:1) and dodecanethiol in noncoordinating solvent 1-octadecene (ODE) at 240 °C. CuInS2 nanoparticles with size of 2 to ∼5 nm and nanorods with aspect ratio of 1 to ∼3 were obtained by adjusting the reaction parameters such as temperature and time. The as-prepared nanoparticles were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, selected area electron diffraction spectroscopy, inductively coupled plasma atomic emission spectroscopy, UV−vis absorption, and photoluminescence (PL) spectroscopy. The nanoparticle solutions exhibit tunable absorption and PL spectra with the absorption edge ranging from 550 to 750 nm and PL emission peaks from 600 to 750 nm, indicating a strong size-dependent quantum confinement effect. Optical measurements of the CuI...

Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics

Abstract: Plasmon-active silver nanoparticle layers were included in solution-processed bulk-heterojunction solar cells. Nanoparticle layers were fabricated using vapor-phase deposition on indium tin oxide electrodes. Owing to the increase in optical electrical field inside the photoactive layer, the inclusion of such particle films lead to increased optical absorption and consequently increased photoconversion at solar-conversion relevant wavelengths. The resulting solar energy conversion efficiency for a bulk heterojunction photovoltaic device of poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester was found to increase from 1.3%±0.2% to 2.2%±0.1% for devices employing thin plasmon-active layers. Based on six measurements, the improvement factor of 1.7 was demonstrated to be statistically significant.

Omnidirectional absorption in nanostructured metal surfaces

Abstract: Light absorbers are not 100% efficient, and it is a challenge to absorb light completely for any direction of incidence. Using nanostructured metal surfaces, de Abajo and colleagues show that such omnidirectional absorption is now possible, potentially leading to more efficient solar cells.

Remotely triggered liposome release by near infrared light absorption via hollow gold nanoshells

Abstract: An elusive goal for systemic drug delivery is to provide both spatial and temporal control of drug release. Liposomes have been evaluated as drug delivery vehicles for decades, but their clinical significance has been limited by slow release or poor availability of the encapsulated drug. Here we show that near-complete liposome release can be initiated within seconds by irradiating hollow gold nanoshells (HGNs) with a near-infrared (NIR) pulsed laser. Our findings reveal that different coupling methods such as having the HGNs tethered to, encapsulated within, or suspended freely outside the liposomes, all triggered liposome release but with different levels of efficiency. For the underlying content release mechanism, our experiments suggest that the microbubble formation and collapse due to the rapid temperature increase of the HGN is responsible for liposome disruption, as evidenced by the formation of solid gold particles after the NIR irradiation and the coincidence of a laser power threshold for both triggered release and pressure fluctuations in the solution associated with cavitation. These effects are similar to those induced by ultrasound and our approach is conceptually analogous to the use of optically triggered nano-"sonicators" deep inside the body for drug delivery. We expect HGNs can be coupled with any nanocarriers to promote spatially and temporally controlled drug release. In addition, the capability of external HGNs to permeabilize lipid membranes can facilitate the cellular uptake of macromolecules including proteins and DNA and allow for promising applications in gene therapy.

Sodium Absorption from the Exoplanetary Atmosphere of HD 189733b Detected in the Optical Transmission Spectrum

Abstract: We present the first ground-based detection of sodium absorption in the transmission spectrum of an extrasolar planet. Absorption due to the atmosphere of the extrasolar planet HD 189733b is detected in both lines of the Na I doublet. High spectral resolution observations were taken of 11 transits with the High Resolution Spectrograph (HRS) on the 9.2 m Hobby-Eberly Telescope (HET). The Na I absorption in the transmission spectrum due to HD 189733b is (− 67.2 ± 20.7) × 10−5 deeper in the "narrow" spectral band that encompasses both lines relative to adjacent bands. The 1 σ error includes both random and systematic errors, and the detection is >3 σ. This amount of relative absorption in Na I for HD 189733b is ~3 times larger than that detected for HD 209458b by Charbonneau et al. (2002) and indicates that these two hot Jupiters may have significantly different atmospheric properties.

A Low Band Gap, Solution Processable Oligothiophene with a Diketopyrrolopyrrole Core for Use in Organic Solar Cells

Abstract: We synthesized a low band gap, solution processable oligothiophene derivative functionalized with a diketopyrrolopyrrole core, which is a highly absorbing chromophoric group. This design leads to optical absorption that extends to 720 nm in solution and to 820 nm in the film. Cyclic voltammetry shows quasireversible oxidation and reduction processes. Bulk heterojunction solar cells using [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron acceptor were fabricated. The power conversion efficiencies (PCEs) depend on the donor−acceptor ratio. A 70:30 donor/acceptor by weight mixture provided a PCE of 2.3% under simulated AM 1.5 solar irradiation of 100 mW/cm 2. To date, this is the highest efficiency reported for small molecule-based solution processed bulk heterojunction solar cells.

Application of quantum cascade lasers to trace gas analysis

Abstract: Quantum cascade (QC) lasers are virtually ideal mid-infrared sources for trace gas monitoring. They can be fabricated to operate at any of a very wide range of wavelengths from ∼ 3 μm to ∼ 24 μm. Seizing the opportunity presented by mid-infrared QC lasers, several groups world-wide are actively applying them to trace gas sensing. Real world applications include environmental monitoring, industrial process control and biomedical diagnostics. In our laboratory we have explored the use of several methods for carrying out absorption spectroscopy with these sources, which include multipass absorption spectroscopy, cavity ring down spectroscopy (CRDS), integrated cavity output spectroscopy (ICOS), and quartz-enhanced photoacoustic spectroscopy (QEPAS).

Low-bandgap poly(thiophene-phenylene-thiophene) derivatives with broaden absorption spectra for use in high-performance bulk-heterojunction polymer solar cells.

Abstract: Two low-bandgap (LGB) conjugated polymers (P1 and P2) based on thiophene-phenylene-thiophene (TPT) with adequate energy levels have been designed and synthesized for application in bulk-heterojunction polymer solar cells (PSCs). The absorption spectral, electrochemical, field effect hole mobility and photovoltaic properties of LGB TPT derivatives are investigated and compared with poly(3-hexylthiophene) (P3HT). Photophysical studies reveal bandgaps of 1.76 eV for P1 and 1.70 eV for P2, which could effectively harvest broader solar spectrum. In addition, the thin film absorption coefficients of P1 and P2 are 1.6 × 105 cm−1 (λ ≈ 520 nm) and 1.4 × 105 cm−1 (λ ≈ 590 nm), respectively. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open circuit voltage while blending them with fullerene derivatives as electron acceptors. Furthermore, both materials show sufficient hole mobility (3.4 × 10−3 cm2/Vs for P2) allowing efficient charge extraction and a good fill-factor for PSC applica...

Photonic crystal enhanced light-trapping in thin film solar cells

Abstract: We utilize photonic crystals to simulate enhanced light-trapping in a-Si:H thin film solar cells A one dimensional photonic crystal or distributed Bragg reflector with alternating dielectric layers acts as low loss backreflector A two dimensional photonic crystal between the absorber layer and the Bragg reflector diffracts light at oblique angles within the absorber The photonic crystal geometry is optimized to obtain maximum absorption The photonic crystal provides lossless diffraction of photons, increasing the photon path length within the absorber layer The simulation predicts significantly enhanced photon harvesting between 600 and 775nm below the band edge, and an absorption increase by more than a factor of 10 near the band edge The optical path length ratio can exceed the classical limit predicted for randomly roughened scattering surfaces at most wavelengths near the band edge The optical modeling is performed with a rigorous scattering matrix approach where Maxwell’s equations are solved

Electromagnetic wave absorption properties of porous carbon/Co nanocomposites

Abstract: Porous carbon/Co nanocomposites were fabricated by a sol-gel method. The electromagnetic parameters were measured in the 2–18GHz range. Compared with porous carbon composite, porous carbon/Co nanocomposite has larger dielectric loss due to the enhanced interfacial polarization relaxation loss and Ohmic loss. The maximum reflection loss of the porous C(Co) nanocomposite can reach 40dB at 4.2GHz with 5mm in thickness and the primary microwave absorptive mechanism is ascribed to the dielectric loss. The effect of porous structure on microwave absorption property of the carbon/Co nanocomposite was also discussed.

Catalysis of recombination and its limitation on open circuit voltage for dye sensitized photovoltaic cells using phthalocyanine dyes.

Abstract: In order to increase the energy efficiency of dye-sensitized solar cells beyond 10%, an improved dye needs to be developed with greater light absorption in the red and near-infrared. Many dyes have been tested for this purpose; however, no dye with significant absorption beyond 750 nm has functioned properly. We have examined a series of ruthenium phthalocyanines, a dye class with large and tunable absorption in the red. For these dyes we observe a large reduction in the output voltage of the cells relative to the benchmark dye (N719). By examination of photovoltage transients and charge density measurements, we demonstrate that this reduction in voltage is caused by a 100-fold increase in the rate constant for recombination (iodine reduction) at the TiO2/electrolyte interface. N719, however, does not seem to catalyze this reaction. By examination of the literature, we propose that catalysis of the recombination reaction may be occurring for many other classes of potentially useful dyes including porphyri...

Nanoparticle plasmon-assisted two-photon polymerization induced by incoherent excitation source.

Abstract: We demonstrate the possibility to achieve optical triggering of photochemical reactions via two-photon absorption using incoherent light sources. This is accomplished by the use of arrays of gold nanoparticles, specially tailored with high precision to obtain high near-field intensity enhancement.

Strong spectral variation of biomass smoke light absorption and single scattering albedo observed with a novel dual-wavelength photoacoustic instrument

Abstract: [1] A dual-wavelength photoacoustic instrument operating at 405 and 870 nm was used during the 2006 Fire Lab at Missoula Experiment to measure light scattering and absorption by smoke from the combustion of a variety of biomass fuels. Simultaneous measurements of aerosol light scattering by reciprocal nephelometry within the instrument's acoustic resonator accompany photoacoustic aerosol light absorption measurements. Single scattering albedo values at 405 nm ranging from 0.37 to 0.95 were measured for different fuel types, and the spectral dependence of absorption was quantified using the Angstrom exponent of absorption. An absorption Angstrom exponent near unity is commonly observed for motor vehicle emission-generated black carbon aerosol. For biomass smoke, Angstrom exponents as high as 3.5 were found in association with smoke having single scattering albedo near unity. The measurements strongly suggest that light-absorbing organic material is present in wood smoke. A second single-wavelength photoacoustic instrument with reciprocal nephelometry was used to quantify aerosol scattering and absorption at 532 nm. Absorption Angstrom exponents calculated using 532 and 870 nm data were as large as 2.5 for smoke with single scattering albedos near unity. The spectral variation in optical properties provides insight into the differentiation of aerosols from mobile or industrial sources versus those from biomass burning. Optical properties of biomass smokes could be classified by general fuel type such as flowering shrubs versus pine needle litter.

Performance analysis of near-field thermophotovoltaic devices considering absorption distribution

Abstract: This paper elucidates the energy transfer and conversion processes in near-field thermophotovoltaic (TPV) systems, considering local radiation absorption and photocurrent generation in the TPV cell. Radiation heat transfer in a multilayered structure is modeled using the fluctuation–dissipation theorem, and the electric current generation is evaluated based on the photogeneration and recombination of electron–hole pairs in different regions of the TPV cell. The effects of near-field radiation on the photon penetration depth, photocurrent generation, and quantum efficiency are examined in the spectral region of interest. The detailed analysis performed in the present work demonstrates that, while the near-field operation can enhance the power throughput, the conversion efficiency is not much improved and may even be reduced. Subsequently, a modified design of near-field TPV systems is proposed to improve the efficiency.

X-ray diffraction contrast tomography: a novel technique for three-dimensional grain mapping of polycrystals. I. Direct beam case

Abstract: The principles of a novel technique for nondestructive and simultaneous mapping of the three-dimensional grain and the absorption microstructure of a material are explained. The technique is termed X-ray diffraction contrast tomography, underlining its similarity to conventional X-ray absorption contrast tomography with which it shares a common experimental setup. The grains are imaged using the occasionally occurring diffraction contribution to the X-ray attenuation coefficient each time a grain fulfils the diffraction condition. The three-dimensional grain shapes are reconstructed from a limited number of projections using an algebraic reconstruction technique. An algorithm based on scanning orientation space and aiming at determining the corresponding crystallographic grain orientations is proposed. The potential and limitations of a first approach, based on the acquisition of the direct beam projection images only, are discussed in this first part of the paper. An extension is presented in the second part of the paper [Johnson, King, Honnicke, Marrow & Ludwig (2008). J. Appl. Cryst. 41, 310–318], addressing the case of combined direct and diffracted beam acquisition.

Optical properties of graphene nanoribbons: The role of many-body effects

Abstract: We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons GNRs. We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination.

Metal and dielectric nanoparticle scattering for improved optical absorption in photovoltaic devices

Abstract: The influence of electromagnetic scattering by Au and silica nanoparticles placed atop silicon photovoltaic devices on absorption and photocurrent generation has been investigated. The nanoparticles produce substantial increases in power transmission into the semiconductor and consequently photocurrent response from ∼500to>1000nm. Increases in power conversion efficiency under simulated solar irradiation of up to 8.8% are observed experimentally, and numerical simulations provide quantitatively accurate predictions of these observed enhancements. Additional simulations indicate that these concepts can be applied to a broad range of photovoltaic device structures, including those based on low-index materials for which conventional antireflection coatings are problematic.

Light extraction and optical loss mechanisms in organic light-emitting diodes: Influence of the emitter quantum efficiency

Abstract: The internal quantum efficiency of organic light-emitting diodes (OLEDs) can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used; however, the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are, for example, waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. Additionally, absorption in the organic layers and the electrodes can play a role. In this work we use numerical simulations to identify and quantify different loss mechanisms. Changing simulation parameters, for example, the distance of the emitter material to the cathode or thicknesses of the various layers, enables us to study their influence on the fraction of light leaving the OLED. An important parameter in these simulations and for the actual device is the radiative quantum efficiency q, which is defined as the efficiency of radiative exciton decay in an unbounded space filled by th...