Showing papers on "Absorption (electromagnetic radiation) published in 1998"

High-Q measurements of fused-silica microspheres in the near infrared.

Abstract: Measurements of the quality factor Q ~ 8 x 10^9 are reported for the whispering-gallery modes (WGM’s) of quartz microspheres for the wavelengths 670, 780, and 850 nm; these results correspond to finesse F ~ 2.2 x 10^6. The observed independence of Q from wavelength indicates that losses for the WGM’s are dominated by a mechanism other than bulk absorption in fused silica in the near infrared. Data obtained by atomic force microscopy combined with a simple model for surface scattering suggest that Q can be limited by residual surface inhomogeneities. Absorption by absorbed water can also explain why the material limit is not reached at longer wavelengths in the near infrared.

Effects of black carbon content, particle size, and mixing on light absorption by aerosols from biomass burning in Brazil

Abstract: Black carbon mass absorption efficiencies of smoke particles were measured for various types of biomass fires during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment using thermal evolution measurements for black carbon and optical absorption methods. The obtained results range between 5.2 and 19.3 m2 g−1 with an average value of 12.1±4.0 m2 g−1. Particle size distributions and optical properties were also measured to provide a full set of physical parameters for modeling calculations. Mie theory was used to model the optical properties of the particles assuming both internal and external mixtures coupling the modeling calculations with the experimental results obtained during the campaign. For internal mixing, a particle model with a layered structure consisting of an absorbing black carbon core, surrounded by a nonabsorbing shell, was assumed. Also, for internal mixing, a discrete dipole approximation code was used to simulate packed soot clusters commonly found in electron microscopy photographs of filters collected during the experiment. The modeled results for layered spheres and packed clusters explain black carbon mass absorption coefficients up to values of about 25 m2 g−1, but measurements show even higher values which were correlated with the chemical composition and characteristics of the structure of the particles. Unrealistic high values of black carbon absorption efficiencies were linked to high concentrations of K, which influence the volatilization of black carbon (BC) at lower temperatures than usual, possibly causing artifacts in the determination of BC by thermal technique. The modeling results are compared with nephelometer and light absorption measurements.

Method for non-invasive measurement of an analyte

Abstract: Disclosed is a method and apparatus for measuring an analyte in a tissue of a subject. The method comprises contacting the tissue with electromagnetic radiation having a first excitation wavelength, wherein the first excitation wavelength is substantially equal to an absorption wavelength of a temperature probe within the tissue. The temperature probe and the analyte are sufficiently proximate to one another that energy deposited into one by absorption of radiation is transferred to the other. The Raman spectra emitted by the tissue are collected and analyzed to determine a concentration of analyte present in the tissue. The analysis can comprise measuring the Raman spectra associated with the temperature probe. In addition, the method can include simultaneously contacting the tissue with electromagnetic radiation having the first excitation wavelength and with electromagnetic radiation having a second excitation wavelength, wherein the second excitation wavelength is substantially equal to an absorption wavelength of the analyte. The analysis comprises comparing the spectra emitted in response to the first excitation wavelength in the presence and in the absence of the second excitation wavelength. In another embodiment, the analysis comprises measuring the anti-Stokes component of the Raman spectra associated with the analyte. The method provides a non-invasive measurement of blood glucose, using hemoglobin as the temperature probe.

Dynamics of Electron Injection in Nanocrystalline Titanium Dioxide Films Sensitized with (Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2) by Infrared Transient Absorption

Abstract: We have used femtosecond pump−probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4‘-dicarboxy-2,2‘-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge-transfer absorption peaks and probing the absorption by injected electrons in the TiO2 at 1.52 μm and in the range of 4.1−7.0 μm, we have directly observed the arrival of electrons injected into the TiO2 film. Our measurements indicate an instrument-limited ∼50 fs upper limit on the electron injection time. We have compared the infrared transient absorption for noninjecting systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2 and found no indication of electron injection at probe wavelengths in the mid-IR (4.1−7.0 μm).

Polarizing energy transfer in photoluminescent materials for display applications

Abstract: Combinations of sheet polarizers and colour filters form the basis of numerous products1,2,3,4 — most notably colour liquid-crystal displays2,4 — that require polarized chromatic light. But this combination of elements does not make efficient use of light, as a substantial fraction of the incident light is converted into thermal energy3,4, limiting the brightness and energy efficiency of the resulting devices. Here we show that these limitations can be overcome by using polymer-based photoluminescent polarizers. Our polarizers operate on a two-step principle: randomly orientated ‘sensitizer’ molecules harvest the incident light by isotropic absorption and then efficiently transfer the energy to a uniaxially orientated photoluminescent polymer, from which coloured light with a high degree of linear polarization is emitted. In principle, isotropic-to-polarized conversion efficiencies approaching unity could be attainable by this approach.

Temperature dependence of the absorption coefficient of cosmic analog grains in the wavelength range 20 microns to 2 millimeters

Abstract: We have measured the absorption coefficient per unit mass of cosmic dust analog grains, crystalline fayalite and forsterite, amorphous fayalite, and two kinds of disordered carbon grains, between 20 μm and 2 mm over the temperature range 295-24 K. The results provide evidence of a significant dependence on temperature. The opacity systematically decreases with decreasing temperature; at 1 mm, it varies by a factor of between 1.9 and 5.8, depending on the material, from room temperature to 24 K. The variations are more marked for the amorphous grains. The wavelength dependence of the absorption coefficient is well fitted by a power law with exponent β that varies with temperature. For the two amorphous carbons, β(24 K) ~1.2 with increases of 24% and 50% with respect to the room-temperature values. A 50% increase is found for amorphous fayalite, characterized by β(24 K) = 2. A less pronounced change of β with temperature, 14% and 10%, is observed for crystalline forsterite, β(24 K) = 2.2, and fayalite, β(24 K) = 2.3, respectively. For amorphous fayalite grains, the millimeter opacity at 24 K is larger by a factor of ~4 than that of the crystalline counterpart. In addition, a temperature dependence of the infrared bands present in the spectrum of the two crystalline silicates is found. The features become more intense, sharpen, and shift to slightly higher frequencies with decreasing temperature. The results are discussed in terms of intrinsic far-infrared-millimeter absorption mechanisms. The linear dependence of the millimeter absorption on temperature suggests that two-phonon difference processes play a dominant role. The absorption coefficients reported in this work can be useful in obtaining a more realistic simulation of a variety of astronomical data concerning dust at low temperatures and give hints to better identify its actual properties. In particular, they are used to discuss the origin of the diffuse far-infrared-millimeter interstellar dust emission spectrum. It is proposed that composite particles formed of silicate and amorphous carbon grains can reproduce the observations. The presence of these particles in the diffuse medium is consistent with the recent interstellar extinction model by Mathis.

Anisotropy in the absorption and scattering spectra of chicken breast tissue.

Abstract: Oblique incidence reflectometry is a simple and accurate method for measuring the absorption and the reduced-scattering coefficients of turbid media. We used this technique to deduce absorption and reduced-scattering spectra from wavelength-resolved measurements of the relative diffuse reflectance profile of white light as a function of source–detector distance. In this study, we measured the absorption and the reduced-scattering coefficients of chicken breast tissue in the visible range (400–800 nm) with the oblique incidence probe oriented at 0° and 90° relative to the muscle fibers. We found that the deduced optical properties varied with the probe orientation. Measurements on homogenized chicken breast tissue yielded an absorption spectrum comparable with the average of the absorption spectra for 0° and 90° probe orientations measured on the unhomogenized tissue. The reduced-scattering spectrum for homogeneous tissue was greater than that acquired for unhomogenized tissue taken at either probe orientation. This experiment demonstrated the application of oblique-incidence, fiber-optic reflectometry to measurements on biological tissues and the effect of tissue structural anisotropy on optical properties.

Two-photon-absorbed near-infrared photopolymerization for three-dimensional microfabrication

Abstract: We present three-dimensional (3-D) microstructures fabricated from photopolymerizable resin with a resolution of 0.62 /spl mu/m. The method used to fabricate such small structures has been developed using a femto-second-pulsed near-infrared (IR) laser. Structures such as microcoils and microtubes are presented which demonstrate the effectiveness of our method. In this method, a two-photon absorption process is successfully utilized to confine the solidification of photopolymerizable resin to the focused spot of the laser, which is possible due to the quadratic dependence of the two-photon absorption rate on light intensity. The experimental system, experimental conditions, and related issues for this novel method of photopolymerization are also discussed.

Absorption of High-Energy Gamma Rays by Interactions with Extragalactic Starlight Photons at High Redshifts and the High-Energy Gamma-Ray Background

Abstract: In this paper, we extend previous work on the absorption of high-energy γ-rays in intergalactic space by calculating the absorption of 10-500 GeV γ-rays at high redshifts. This calculation requires the determination of the high-redshift evolution of the intergalactic starlight photon field, including its spectral energy distribution out to frequencies beyond the Lyman limit. To estimate this evolution, we have followed a recent analysis by Fall, Charlot, & Pei, which reproduces the redshift dependence of the starlight background emissivity obtained by the Canada-France Redshift Survey group. We give our results for the γ-ray opacity as a function of redshift out to a redshift of z = 3. We also give predicted γ-ray spectra for selected blazars and extend our calculations of the extragalactic γ-ray background from blazars to an energy of 500 GeV with absorption effects included. Our results indicate that the extragalactic γ-ray background spectrum from blazars should steepen significantly above 20 GeV, owing to extragalactic absorption. Future observations of a such a steepening would thus provide a test of the blazar origin hypothesis for the γ-ray background radiation. We also note that our absorption calculations can be used to place limits on the redshifts of γ-ray bursts; for example, our calculated opacities indicate that the 1994 February 17 burst observed by EGRET most probably originated at z ≤ ~2.

Ring effect: impact of rotational raman scattering on radiative transfer in earth’s atmosphere

Abstract: One significant limitation to the accuracy of the remote sensing of trace gas constituents in the atmosphere, using UV-visible spectroscopy and scattered sunlight, has often been a reliable knowledge of the so-called Ring effect. In this study it is demonstrated that the filling-in of Fraunhofer and gas absorption features, resulting from Rotational Raman scattering (RRS), explains to high accuracy the Ring effect. A radiative transfer model has been adapted to include RRS and carefully validated by comparison with Ring effect data by other models and from ground-based and satellite data. The analysis of the principle components of the simulated Ring spectra enabled the Fraunhofer and gas absorption filling-in to be separated. This yields a simple, and therefore computational fast, parameterization of the Ring effect suitable for trace gas retrievals. This approach was tested for the retrieval of NO 2 which is considered to be a worst case with respect to absorption feature filling-in for a trace gas retrieved from scattered light. Analysis of the errors in the vertical column of NO 2 derived using differential optical absorption spectroscopy (DOAS) technique indicate that they are dependent on the amount of NO 2 present in the atmosphere when regarding the experimental Ring spectra. This implies that calculated Ring spectra may be superior for DOAS retrievals, compared to the experimentally determined Ring spectra.

Heating by absorption in the focus of an objective lens.

Abstract: We derive an integral solution for the local heating of a linearly absorbing, uniform medium exposed to strongly focused light. Numerical results for local heating under typical multiphoton microscopy and optical trapping conditions are presented for various aperture angles. In contrast with common Gaussian beam approximations, our model employs the focal-intensity distribution as described by the point spread function of the lens. In this way, the model also accounts for axial heat transportation, which results in a lower prediction for the temperature increase. For an aperture of 1.2 (water immersion), irradiation with 100??mW of 850-nm light for 1??s increases the local temperature of water by 0.2??K.?Heating of water by linear absorption can be ruled out as a limiting factor in standard multiphoton-excitation microscopy.

High-density flat plasma production based on surface waves

Abstract: Recent development of large-diameter (>30 cm) high-density microwave plasma production at low pressures (<20 mTorr) without an external DC magnetic field is reviewed in view of application to the next generation ULSI devices and flat panel displays. Understanding the discharge physics - excitation, propagation and absorption of the surface wave in a flat plasma geometry under overdense conditions - is indispensable for controlling the plasma. Experimental evidence of discrete surface-wave modes is clearly found in optical emission and microwave field measurements. The analysis of the full-wave electromagnetic dispersion successfully identified the observed eigenmodes. Stability analysis of the wave-plasma interaction resulted in a stability criterion predicting hysteresis loops in the power-density dependence, which were found in the experiment. A possibility of collisionless absorption of surface waves, i.e. mode conversion to electron plasma waves at the resonant layer, is discussed with the recent experimental results taken into account. From the plasma technology point of view, examples of surface-wave plasma tools (some of them commercially available) are introduced and the significance of the antenna structure is emphasized. Finally, the advantages of the surface-wave plasma source in comparison with other high-density sources are summarized.

Differential absorption imaging with optical coherence tomography

Abstract: The spatial variation of the backscattering cross section is the primary source of contrast in present applications of optical coherence tomography (OCT). We introduce and analyze a technique for obtaining OCT images of the local concentration of an absorbing compound in biological tissues and other highly scattering media. A pair of light-emitting diodes, one emitting in a vibrational absorption band of the chemical compound of interest and the other emitting just outside this band, are used as sources at the input of the interferometer. The differential absorption of the probe beam is determined by Fourier transformation and ratiometric processing of the measured interference signals. The ability of the technique to distinguish lipid and water inclusions in a scattering material is demonstrated with an OCT system that uses a pair of light-emitting-diode sources with center wavelengths of 1.3 µm and 1.46 µm.

Theoretical and experimental approaches to improve the accuracy of particulate absorption coefficients derived from the quantitative filter technique

Abstract: The quantitative filter technique (QFT), whereby particles are concentrated onto glass-fiber filters and analyzed in a spectrophotometer, is used extensively to estimate the absorption coefficients of aquatic particles. A number of empirically derived correction factors (β) have been developed to account for the amplified optical path length associated with the highly scattering glass-fiber filters. Published results are inconsistent, and β remains the largest source of uncertainty in estimated absorption coefficients. In this study, path-length amplification was estimated from the average cosine of diffusely traveling photons in the filter pad using a theoretical approach. This amplification factor, combined with variability in blank filter pad optical density, explains many of the confounding observations in the literature. Absorption coefficients for phytoplankton cultures and field samples were estimated from a modified QFT using the new model for path-length amplification and tested against absorption coefficients measured with a nine-wavelength absorption and attenuation meter (ac9, WETLabs). A linear regression between the modeled and measured particulate absorption coefficients was highly significant (r 2 > 0.99, n = 99), with estimated slope and intercept not significantly different from 1 and 0, respectively (P < 0.001). The model out-performs published, empirically derived correction factors over a broad range of absorption coefficients and particulate compositions. Results indicate that the modified QFT combined with the new model for path-length amplification yields accurate estimates of spectral particulate absorption coefficients regardless of the concentration or composition of the particulate material.

Application of Green's-function technique to the calculation of multiphoton absorption coefficients of crystalline solids

Abstract: The Green's-function method is introduced to avoid the sums over intermediate states occurring in the calculation of multiphoton absorption coefficients of crystalline solids. Up to four-photon absorption coefficients of NaCl are obtained by this method. The results are in accord with those obtained by carrying out the summations directly. The advantages of the method and its further applications are also discussed in the paper.

Absorption Spectra Measurements for the Ozone Molecule in the 350-830 nm Region

Abstract: Absolute absorption cross-sections of ozone have been measured at ambient (295 K) and low temperature (218 K) in the visible region corresponding to the Chappuis bands. The temperature effect has been studied and found to be very small. The minimum of the absorption between the Hartley and the Chappuis bands is observed for the first time at 377,5 nm.

Optical Characterization of Dielectric and Semiconductor Thin Films by Use of Transmission Data

Abstract: A method to calculate the optical functions n(lambda) and k(lambda) by use of the transmission spectrum of a dielectric or semiconducting thin film measured at normal incidence is described. The spectrum should include the low-absorption region and the absorption edge to yield the relevant optical characteristics of the material. The formulas are derived from electromagnetic theory with no simplifying assumptions. Transparent films are considered as a particular case for which a simple method of calculation is proposed. In the general case of absorbing films the method takes advantage of some properties of the transmittance T(lambda) to permit the parameters in the two regions mentioned above to be calculated separately. The interference fringes and the optical path at the extrema of T(lambda) are exploited for determining with precision the refractive index and the film thickness. The absorption coefficient is computed at the absorption edge by an efficient iterative method. At the transition zone between the interference region and the absorption edge artifacts in the absorption curve are avoided. A small amount of absorption of the substrate is allowed for in the theory by means of a factor determined from an independent measurement, thus improving the quality of the results. Application of the method to a transmission spectrum of an a:Si(x)N(1-x):H film is illustrated in detail. Refractive index, dispersion parameters, film thickness, absorption coefficient, and optical gap are given with the help of tables and graphs.

Radio Emission and Particle Acceleration in SN 1993J

Abstract: The radio light curves of SN 1993J are discussed. We find that a fit to the individual spectra by a synchrotron spectrum, suppressed by external free-free absorption and synchrotron self-absorption, gives a superior fit to models based on pure free-free absorption. A standard r-2 circumstellar medium is assumed and is found to be adequate. From the flux and cutoff wavelength, the magnetic field in the synchrotron-emitting region behind the shock is determined to B ≈ 64(Rs/1015 cm)-1 G. The strength of the field argues strongly for turbulent amplification behind the shock. The ratio of the magnetic and thermal energy density behind the shock is ~0.14. Synchrotron losses dominate the cooling of the electrons, whereas inverse Compton losses due to photospheric photons are less important. For most of the time also Coulomb cooling affects the spectrum. A model where a constant fraction of the shocked, thermal electrons are injected and accelerated, and subsequently lose their energy due to synchrotron losses, reproduces the observed evolution of the flux and number of relativistic electrons well. The injected electron spectrum has dn/dγ ∝ γ-2.1, consistent with diffusive shock acceleration. The injected number density of relativistic electrons scales with the thermal electron energy density, ρV2, rather than the density, ρ. The evolution of the flux is strongly connected to the deceleration of the shock wave. The total energy density of the relativistic electrons, if extrapolated to γ ~ 1, is ~5 × 10-4 of the thermal energy density. The free-free absorption required is consistent with previous calculations of the circumstellar temperature of SN 1993J, Te ~ (2-10) × 105 K, which failed in explaining the radio light curves by pure free-free absorption. Implications for the injection of the relativistic electrons, and the relative importance of free-free absorption, Razin suppression, and the synchrotron self-absorption effect for other supernovae, are also briefly discussed. It is argued that especially the expansion velocity, both directly and through the temperature, is important for determining the relative importance of the free-free absorption and synchrotron self-absorption. Some guidelines for the modeling and interpretation of VLBI observations are also given.

Interfacial Electron Transfer between Fe(II)(CN)64- and TiO2 Nanoparticles: Direct Electron Injection and Nonexponential Recombination

Abstract: Photoinduced electron-transfer (ET) dynamics in Fe(II)(CN)64- sensitized TiO2 nanoparticles in D2O solution are studied by subpicosecond tunable laser spectroscopy in the mid-infrared and visible region. The dynamics of the injected electrons are monitored by the mid-IR absorption of electrons in the semiconductor, and the corresponding dynamics of the adsorbate are monitored by the vibrational spectra of the CN stretching mode region and electronic absorption in the visible. After 400 nm excitation, the forward electron injection time from Fe(II)(CN)64- to TiO2 occurs in 1 ns (35%). Combining with previous measurements in the nanosecond to microsecond time scale (Lu et al. J. Am. Chem. Soc. 1993, 115, 4927 and Vrachnou et al...

Influence of layered tissue architecture on estimates of tissue optical properties obtained from spatially resolved diffuse reflectometry.

Abstract: Most instruments used to measure tissue optical properties noninvasively employ data-analysis algorithms that rely on the simplifying assumption that the tissue is semi-infinite and homogeneous. The influence of a layered tissue architecture on the determination of the scattering and absorption coefficients has been investigated in this study. Reflectance as a function of distance from a point source for a two-layered tissue architecture that simulates skin overlying fat was calculated by using a Monte Carlo code. These data were analyzed by using a diffusion theory model for a homogeneous semi-infinite medium to calculate the scatter and absorption coefficients. Depending on the algorithm and the radial distance, the estimated tissue optical properties were different from those of either layer, and under some circumstances, physically impossible. In addition, the sensitivity and cross talk of the estimated optical properties to changes in input optical properties were calculated for different layered geometries. For typical optical properties of skin, the sensitivity to changes in optical properties is highly dependent on the layered architecture, the measurement distance, and the fitting algorithm. Furthermore, a change in the input absorption coefficient may result in an apparent change in the measured scatter coefficient, and a change in the input scatter coefficient may result in an apparent change in the measured absorption coefficient.

Radio absorptive material

Abstract: PROBLEM TO BE SOLVED: To obtain radio-wave absorption in a high-frequency range by wrapping a water absorption material having a large dielectric constant and dielectric loss tangent and absorbing moisture by a water-repellent coating in the high-frequency range. SOLUTION: A water absorption material 1, in which water is absorbed to the water absorption material for absorbing moisture, is wrapped by a coating 2 having water repellency. When the water absorption material having extremely large absorptivity is used at that time, the electrical characteristics of the water absorption material 1 at the time of water absorption can be equalized approximately to water. Consequently, when the thickness of the water absorption material 1 under an absorbing state is set in (d), the electric field E0 of arrival electromagnetic waves vertically projected to a radio absorptive material is attenuated up to an electric field E1 by the water absorption material 1 under the absorbing state. The electric field E1 represents that of arrival electromagnetic waves after passage through the water absorption material 1 under the absorbing state at that time. Accordingly, large radio absorption is obtained in a high-frequency range of 1 GHz or more.

Ultraviolet laser ablation of polymers: spot size, pulse duration and plume attenuation effects explained.

Abstract: A versatile model for ultraviolet (UV) laser ablation of polymers is presented, which is very successfully applied to the calculation of a variety of different properties of this process, including the influence of plume attenuation dynamics. The polymer is described as a system of chromophores with two possible electronic states. The model is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically excited state. Laser induced chemical modifications are incorporated via different absorption coefficients for the initial and for the modified polymer after absorption of UV light. Dynamic attenuation of the expanding ablation plume and heat conduction are taken into account. The results of the theoretical calculations are compared with the results of three different series of experiments performed with polyimide (PI) and polymethylmethacrylate at the excimer laser wavelength 248 nm and with PI also at 308 nm: (1) Measurement of the ablation rate as a function of fluence for four different pulse durations between 20 and 250 ns; (2) Measurements of the ablation rate as a function of fluence for five different laser irradiation spot radii between 10 and 150 μm, and (3) Time resolved measurement of the dynamic plume attenuation at the ablating laser wavelength as a function of fluence for four different pulse durations between 20 and 250 ns. The model leads to a prediction of etch rates, ablation thresholds, plume attenuation, and surface temperatures during the ablation process, which is in good agreement with the experimental results. The observed increase of the ablation rate with increasing pulse length and with decreasing laser spot size can be explained by the model as a consequence of laser induced modified absorption in combination with the dynamic shielding of the expanding plume.

Large nonlinear absorption in coated ag2s/cds nanoparticles by inverse microemulsion

Abstract: A modified inverse microemulsion technique was used to synthesize coated Ag2S/CdS nanocomposites of ∼10 nm in diameter. Their nonlinear absorption was observed with picosecond and nanosecond laser radiation of 532-nm wavelength. The nonlinear absorption in the Ag2S/CdS nanocomposites was enhanced, in comparison with the CdS nanoparticles, due to free-carrier absorption. In addition, the relaxation times of photoexcited free carriers in the Ag2S/CdS nanocomposites were also determined to be a few nanoseconds.

On absolute calibration with xenon of laser diagnostic methods based on two-photon absorption

Abstract: A novel calibration method for the determination of absolute species densities by laser spectroscopy with two-photon absorption is presented. The method is based on a comparative measurement with a noble gas which has a two-photon resonance spectrally close to the transition investigated. Application of this scheme for the calibration of LIF measurements to determine atomic oxygen, generated in a capacitively coupled rf reactor, and for calibration of a RIS measurement of atomic oxygen and carbon generated by sputtering is demonstrated. The relevant excitation cross section of the involved 7p[3/2]2 xenon state, which is used for calibration of atomic oxygen densities, is determined.