Showing papers on "Absorption (electromagnetic radiation) published in 1987"
TL;DR: In this article, the authors calculate the electric field dependence of the linear and the third-order nonlinear intersubband optical absorption coefficients of a semiconductor quantum well in the infrared regime.
Abstract: Analytic forms of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained for general asymmetric quantum well systems using the density matrix formalism, taking into account the intrasubband relaxation. Based on this model, we calculate the electric field dependence of the linear and the third-order nonlinear intersubband optical absorption coefficients of a semiconductor quantum well. The energy of the peak optical intersubband absorption is around 100 meV (wavelength is 12.4 μm). Thus, electrooptical modulators and photodetectors in the infrared regime can be built based on the physical mechanisms discussed here. The contributors to the nonlinear absorption coefficient due to the electric field include 1) the matrix element variation and 2) the energy shifts. Numerical results are illustrated.
384 citations
TL;DR: It is shown that the depth dependence of the distribution of photon absorption events can be inferred from measured parameters of the surface emission profile, and illustrative experimental data are shown to be in accord with the theory.
Abstract: Various characteristics of photon diffusion in turbid biological media are examined. Applications include the interpretation of data acquired with laser Doppler blood-flow monitors and the design of protocols for therapeutic excitation of tissue chromophores. Incident radiation is assumed to be applied at an interface between a turbid tissue and a transparent medium, and the reemission of photons from that interface is analyzed. Making use of a discrete lattice model, we derive an expression for the joint probability Γ(n, ρ)d2ρ that a photon will be emitted in the infinitesimal area d2ρ centered at surface point ρ = (x, y), having made n collisions with the tissue. Mathematical expressions are obtained for the intensity distribution of diffuse surface emission, the probability of photon absorption in the interior as a function of depth, and the mean path length of detected photons as a function of the distance between the site of the incident radiation and the location of the detector. We show that the depth dependence of the distribution of photon absorption events can be inferred from measured parameters of the surface emission profile. Results of relevant computer simulations are presented, and illustrative experimental data are shown to be in accord with the theory.
335 citations
TL;DR: In this paper, the spectral characteristics of intact cells of phytoplankton have been the subject of various studies because of their importance in primary production, radiative transfer in seawater, and in passive remote-sensing.
Abstract: Absorption spectra of laboratory cultures of eight species of phytoplankton were studied. These spectra, normalized per unit of chlorophyll a concentration (specific absorption) show variability in magnitude and spectral form. Specific absorption at 440 nm varied over a factor of three. Spectra of phytoplankton belonging to the same taxonomic group tended to have similar forms. Over 95% of the variability in specific absorption could be explained in terms of changes in cell size, intracellular pigment concentration, and the relative importance of auxiliary pigments. Shape was not an important factor for this sample set. The spectral characteristics of absorption by intact cells of phytoplankton have been the subject of various studies because of their importance in primary production, radiative transfer in seawater, and in passive remote-sensing of phytoplankton. At sufficiently low concentrations, the total absorption coefficient a of a given substance at a given wavelength X can be expressed as a product of a specific absorption coefficient a*c and the concentration of the substance C. That is,
314 citations
TL;DR: In this article, the infrared intersubband absorption at 8.2 μm in doped GaAs/AlAs quantum well superlattices was measured, with 95% of the incident infrared energy being absorbed.
Abstract: We have measured the infrared intersubband absorption at 8.2 μm in doped GaAs/AlAs quantum well superlattices. Waveguide geometry experiments demonstrate strong absorption with 95% of the incident infrared energy being absorbed.
291 citations
TL;DR: In this paper, the absorption cross-sections of NO 2 have been measured in the wavelength range 200 - 700 nm at 298 K with a spectral resolution of 0.04 nm, and averaged over 1 nm intervals and over the atmospheric wavelength intervals used in solar photolysis calculations.
Abstract: The absorption cross-sections of NO 2 have been measured in the wavelength range 200 – 700 nm at 298 K with a spectral resolution of 0.04 nm. The data were acquired digitally, allowing post-processing such as integration in different wavelength intervals. The cross-sections are averaged over 1 nm intervals and over the atmospheric wavelength intervals used in solar photolysis calculations.
247 citations
TL;DR: In this article, a series of observations designed to probe the nature of sunspots by detecting their influence on high-degree p-mode oscillations in the surrounding photosphere is presented.
Abstract: The paper presents the initial results of a series of observations designed to probe the nature of sunspots by detecting their influence on high-degree p-mode oscillations in the surrounding photosphere. The analysis decomposes the observed oscillations into radially propagating waves described by Hankel functions in a cylindrical coordinate system centered on the sunspot. From measurements of the differences in power between waves traveling outward and inward, it is demonstrated that sunspots appear to absorb as much as 50 percent of the incoming acoustic waves. It is found that for all three sunspots observed, the amount of absorption increases linearly with horizontal wavenumber. The effect is present in p-mode oscillations with wavelengths both significantly larger and smaller than the diameter of the sunspot umbrae. Actual absorption of acoustic energy of the magnitude observed may produce measurable decreases in the power and lifetimes of high-degree p-mode oscillations during periods of high solar activity.
224 citations
TL;DR: A computer program, ABSORB, has been written for correcting the observed structure factors using the differences obtained in the last cycle of the isotropic refinement, to model an absorption surface as a Fourier series in the polar angles of incident and diffracted beams.
214 citations
TL;DR: The upconversion of infrared radiation into green and red fluorescence has been studied for Er3+ and Yb3+ ions in BaF2/ThF4 fluoride glass over a wide temperature range and several dopant concentrations.
Abstract: The upconversion of infrared radiation into green and red fluorescence has been studied for Er3+ and Yb3+ ions in BaF2/ThF4 fluoride glass over a wide temperature range and several dopant concentrations. At room temperature an upconversion efficiency of 3.3×10−5 has been obtained for the green emission from the glass with 1 mol % ErF3 and 19 mol % YbF3 pumped by 973‐nm radiation with intensity of 16.5 mW/cm2. For an absorbed intensity of 10 W/cm2 the efficiency for green upconversion emission is predicted to be 2%. Radiative transition rates for the excited states of Er3+ were calculated using Judd–Ofelt theory and intensity parameters obtained from measured integrated absorption coefficients. The calculated values of the multiphonon relaxation and the upconversion fluorescence intensity, obtained from the rate equation model of Wright, are discussed in light of the experimental results.
201 citations
TL;DR: In this article, a spectrophotometric study was made of the absorption of light by yellow substance (gilvin, gelbstofl) in 12 freshwater lakes of diverse optical and biochemical character in which concentrations of yellow substance ranged 50-fold.
Abstract: A spectrophotometric study was made of the absorption of light by yellow substance (gilvin, gelbstofl) in 12 freshwater lakes of diverse optical and biochemical character in which concentrations of yellow substance ranged 50-fold. The shapes of the spectra of yellow substance absorption, g,, suitably corrected for residual light scattering in the spectrophotometer, were well described by an exponential function of wavelength, X, at near-UV to visible wavelengths: in which R is a reference wavelength and S is the characteristic exponential slope parameter. The average value of S was 0.0 187 nm- I, somewhat higher than previous workers have found in seawaters and other freshwaters. Absorption coefficients of yellow substance at reference-visible wavelengths (e.g. 440 nm) can be predicted within about f 15% from near-UV absorption measurements with the exponential model of spectral shape.
194 citations
TL;DR: In this article, the use of transient fluorescence spectroscopy of polar aromatics in solution was investigated as a method to determine microscopic solvation dynamics, and it was shown that the compounds previously employed as polar fluorescent probes tend to fall into three photophysical classes depending upon: (i) whether the photon induced change in μ occurs simultaneously with photon absorption (ii) whether solvent motion subsequent to photon absorption is required to induce the change inμ; or (iii) whether two excited-state isomers with different μ's are present simultaneously.
Abstract: This paper reinvestigates the use of transient fluorescence spectroscopy of polar aromatics in solution as a method to determine microscopic solvation dynamics. It is shown that the compounds previously employed as polar fluorescent probes tend to fall into three photophysical classes depending upon: (i) whether the photon induced change in μ occurs simultaneously with photon absorption (ii) whether solvent motion subsequent to photon absorption is required to induce the change in μ; or (iii) whether two excited‐state isomers with different μ’s are present simultaneously. The consequence of the different classes on microscopic solvation dynamic measurements is discussed with a molecular example for each class: (i) 4‐aminophthalimide, (ii) 4‐(9‐anthryl)‐N, N‐dimethylaniline, and (iii) bianthryl, respectively. In addition, we introduce a new transient fluorescence procedure for the determination of solvation dynamics that has advantages over the traditional transient Stokes‐shift method. Finally, for the fi...
194 citations
TL;DR: In this paper, the absorption and the steady-state fluorescence emission of a class of donor-acceptor-donor (D-A-D) molecules, bis[4-(dimethy1amino)phenyllsquaraine and its derivatives (1-19), have been studied.
Abstract: The absorption and the steady-state fluorescence emission of a class of donor-acceptor-donor (D-A-D) molecules, bis[4-(dimethy1amino)phenyllsquaraine and its derivatives (1-19), have been studied. Squaraines generally exhibit intense solution absorption in the red ( t 3 X lo5 cm-' M-I). All substituents studied in this work exert a bathochromic effect on the absorption. The effect is small and is attributable to the minor involvement of the donor group in the So SI excitation. In conjunction with emission data, we are able to show that the bathochromic shift originates from the solute-solvent complex and that the complexation constant increases as the D-A-D charge-transfer character of squaraine increases. Electronic and steric factors affecting the D-A-D character and subsequently the absorption maximum are discussed in terms of the solute-solvent complex model. Multiple-emission bands are observed in the fluorescence spectra of 1-19. Three bands, CY. p, and y, are identified from their typical Stokes shifts. Controlled experiments showed that the multiple-emission bands are intrinsic emissive properties of squaraines. Results from mixed-solvent experiments, solvent-effect studies, and temperature-effect studies show that squaraines form solute-solvent complexes in organic solvents. A photophysical model to account for the multiple emission is proposed. Excitation of squaraine in solution results in two excited states, namely, the excited state of free squaraine and the excited state of the solute-solvent complex. These two excited states emit photons to give the a-emission and the &emission. Rotational relaxation(s) (around the C-C bond between the phenyl ring and the four-membered ring of squaraine) is (are) shown to be the major radiationless decay process(es) of these two excited states. As a result, a twisted relaxed excited state is generated. This relaxed excited state can undergo a rotational relaxation to the ground state or emit a photon to give the y-emission. The effect of structural changes on the multiple emission is discussed.
TL;DR: In this article, optical measurements were performed near the fundamental absorption edge for singlecrystal AlxGa1−x N epitaxial layers in the composition range of 0≤x≤0.4.
Abstract: Optical measurements are performed near the fundamental absorption edge for single‐crystal AlxGa1−x N epitaxial layers in the composition range of 0≤x≤0.4. The dependence of the energy band gap on composition is found to deviate downwards from linearity, the bowing parameter being b=1.0±0.3 eV. The origin of the large bowing is discussed in terms of the pseudopotential of Al and Ga based on the pseudopotential of the Heine–Abarenkov type. With increasing x the absorption edges broaden, which is attributed to the increase of the compositional nonuniformity.
TL;DR: In this paper, a semi-empirical calculation of the refractive index changes induced by a perpendicular electric field in quantum wells is presented. But this calculation is based on experimental electroabsorption data, together with absorption sum rules that have recently been developed.
Abstract: We have performed a semi‐empirical calculation of the refractive index changes induced by a perpendicular electric field in quantum wells. This calculation is based on experimental electroabsorption data, together with absorption sum rules that have recently been developed. We find good agreement with published experimental results. Our results are important to the development of electro‐optic and high‐speed electroabsorption quantum well devices.
TL;DR: In this article, the dielectric behavior of living tissues and a number of biological materials was examined by new equipment of the time domain reflectometry method in a wide frequency range of 10/sup 7/-10/sup 10/ Hz.
Abstract: The dielectric behavior of living tissues and a number of biological materials was examined by new equipment of the time domain reflectometry method in a wide frequency range of 10/sup 7/-10/sup 10/ Hz. The authors found two peaks of Debye absorption around 100 MHz and 20 GHz for all the materials. The low-frequency absorption is probably due to bound water while the high-frequency absorption to free water. From the observed relaxation times of bound water a hypothesis is ventured on the structure of bound water and its relaxation mechanism.
Patent•
16 Oct 1987TL;DR: An apparatus for noninvasive determination of constituent concentrations utilizing light wave absorption measurements and methods for processing signals generated by such measurements is described in this article, where the authors present a method for processing the signals generated from such measurements.
Abstract: An apparatus for noninvasive determination of constituent concentrations utilizing light wave absorption measurements and methods for processing signals generated by such measurements.
TL;DR: In this article, the authors achieved a high responsivity, R=1.9 A/W, 10 μm infrared detector using intersubband absorption in GaAs/AlxGa1−xAs quantum well superlattices.
Abstract: We have achieved a high responsivity, R=1.9 A/W, 10 μm infrared detector using intersubband absorption in GaAs/AlxGa1−xAs quantum well superlattices. The photocurrent is produced by intersubband absorption followed by efficient photoexcited tunneling. This responsivity is nearly four times higher than our previous results and has been obtained by using thicker and higher AlxGa1−xAs superlattice barriers thereby reducing the dark current and allowing the detector to be operated at higher biases.
TL;DR: A powerful new technique is reported which enables realistic calculation of the optical energy gap of absorbing thin solid films by an analysis of measured transmittance and reflectance spectra in the fundamental absorption region.
Abstract: A powerful new technique is reported which enables realistic calculation of the optical energy gap of absorbing thin solid films by an analysis of measured transmittance and reflectance spectra in the fundamental absorption region. At the same time a new analytical method allows the thickness of films to be evaluated by measurements of transmittance only.
TL;DR: In this article, a linear absorption model with an inverse absorption length of 2.3×105 cm−1 was proposed to explain the dependence of the etch depth as a function of the number of laser pulses.
Abstract: Etching of Y‐Ba‐Cu‐O superconducting thin films has been accomplished using a pulsed excimer laser (248 nm, 30 ns). Etch depth as a function of the number of laser pulses was linear over a wide range of incident laser energy densities. An etch threshold energy density of 0.11 J/cm2 was observed and etch rate per pulse scaled linearly with the logarithm of the incident energy density. The dependence is adequately explained by a linear absorption model with an inverse absorption length of 2.3×105 cm−1.
Patent•
30 Nov 1987
TL;DR: In this article, a waveguide device for producing absorption or attenuation is described, where the wave section is provided with coupling apertures via which the absorber material is in connection with the interior of the waveguide.
Abstract: A waveguide device for producing absorption or attenuation includes a waveguide section which is provided with an external absorber material. For allowing a transfer of the high-frequency power into the absorber material, the wave section is provided with coupling apertures via which the absorber material is in connection with the interior of the waveguide section.
TL;DR: In this paper, the sub-band-gap optical absorption spectra of high-quality hydrogenated amorphous silicon (a•Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not.
Abstract: The sub‐band‐gap optical absorption spectra of high‐quality hydrogenated amorphous silicon (a‐Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as‐deposited and light‐induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.
TL;DR: For increasing electric field the absorption peak corresponding to the transition of states 1\ensuremath{\rightarrow}2 is shifted higher in energy and the peak amplitude is increased and these features are different from those of the exciton absorption.
Abstract: We present new results for the electric field dependence of the intersubband optical absorption within the conduction band of a quantum well. We show that for increasing electric field the absorption peak corresponding to the transition of states 1\ensuremath{\rightarrow}2 is shifted higher in energy and the peak amplitude is increased. These features are different from those of the exciton absorption. It is also found that the transition 1\ensuremath{\rightarrow}3, forbidden when F=0, is possible when F is nonzero.
TL;DR: The far-infrared and submillimeter absorption properties of random aggregates of conducting spheres have been studied as a possible source for the long-wavelength absorption of interstellar dust grains as discussed by the authors.
Abstract: The far-infrared and submillimeter absorption properties of random aggregates of conducting spheres have been studied as a possible source for the long-wavelength absorption of interstellar dust grains. The complex shapes of random aggregates have various fractal dimensions, depending on the process that forms them. The long wavelength absorption versus frequency is affected more by the shape of a conducting grain than by its composition.
TL;DR: In this article, the optical absorption and photovoltaic properties of thin films of Cu 2 SnS 3 were investigated and a Schottky type solar cell was developed from it.
TL;DR: The absorption spectrum of OClO has been measured by using a diode array spectrometer with a resolution of 0.25 nm at 204, 296, and 378 K in the wavelength range of 240-480 nm.
Abstract: The absorption spectrum of OClO has been measured by using a diode array spectrometer with a resolution of 0.25 nm at 204, 296, and 378 K in the wavelength range of 240-480 nm. Absolute absorption cross sections were determined by measuring OClO concentration using two independent methods. The general features of the spectrum at 396 K agree with previous observations. The individual bands of OClO get sharper and the peak cross sections increase as the temperature is lowered from 378 to 204 K.
TL;DR: In this article, an absorption at 2104 cm−1 as ν2+ν3 of ground-state HC2 is discussed and the near infrared absorption band system has been assigned to the A 2 Π −X 2 Σ+ transition of HC2, extensively perturbed by interaction with high vibrational levels of the ground state.
Abstract: Fourier transform absorption spectra have been obtained between 700 and 7900 cm−1 at a resolution of 0.2 cm−1 for Ar:C2H2 samples codeposited at 12 K with a beam of argon atoms that had been excited in a microwave discharge. Detailed isotopic substitution studies have confirmed that the predominant product species is HC2, which contributes not only the absorptions previously assigned to its two stretching fundamentals but also several weaker absorptions in the 2000–3600 cm−1 spectral region and a prominent, complicated pattern of absorptions between 3600 and 7800 cm−1. The previous assignment of the 3611 cm−1 HC2 absorption as the CH‐stretching fundamental is reviewed, and the assignment of an absorption at 2104 cm−1 as ν2+ν3 of ground‐state HC2 is discussed. The near infrared absorption band system has been assigned to the A 2Π–X 2Σ+ transition of HC2, extensively perturbed by interaction with high vibrational levels of the ground state. The position of the transition origin could not be definitively e...
TL;DR: In this paper, a simple procedure for calculating the strengths of the more important absorption features and the concentrations of the corresponding defect centres as functions of the radiation dose and of the carbon and oxygen concentrations in the silicon was described.
Abstract: Room-temperature irradiation of silicon with 2 MeV electrons can create many defects that give rise to optical absorption lines. The authors describe a simple procedure for calculating the strengths of the more important absorption features and the concentrations of the corresponding defect centres as functions of the radiation dose and of the carbon and oxygen concentrations in the silicon. The following absorption features and centres are considered: the 969 meV line (two-carbon-atom centre), the 865 cm-1 line (C+O centre), the 1020 cm-1 line (C+O+self-interstitial), the 835 cm-1 line (vacancy+O'A centre'), the 1.7 mu m band (di-vacancy) and the 488 meV line (C+O+vacancy centre).
TL;DR: In this paper, a fiber-optic sensor was developed containing a fluorophore, eosin, and an absorber, phenol red, coimmobilized on the distal end of an optical fiber.
Abstract: A fiber-optic sensor has been developed containing a fluorophore, eosin, and an absorber, phenol red, coimmobilized on the distal end of an optical fiber. When an argon laser is used to excite eosin with light of lambda 488 nm, a region of the spectrum where phenol red does not absorb, eosin emits light in a spectral region that overlaps significantly with the absorption spectru of the basic form of phenol red. Consequently, nonradiative energy transfer occurs from eosin (donor) to phenol red (acceptor). The amount of energy transfer increases as the pH increases resulting in a diminished fluorescence intensity. Thus, changes in the absorption of phenol red as a function of pH are detected as changes in the fluorescent signal. In this manner a pH sensor optimized for physiological pH measurement has been prepared. The fiber exhibits a precision of at least 0.01 pH units.
TL;DR: The contribution of radiation heat transfer to the overall conductivity of foam insulations has been examined in this paper, where the absorption and scattering coefficients as well as the phase function were measured for foam and glassfiber insulation.
TL;DR: In this article, the photorefractive properties of doped CdTe have been investigated for optical processing applications, and it has been shown that the effect of doping with V and Ti impurities is negligible.
Abstract: The first study of the photorefractive properties of doped CdTe has demonstrated high sensitivity for optical processing applications. Of the binary II‐VI and III‐V semiconductors, CdTe has the highest electro‐optic coefficient r41 in the infrared, some three times larger than that of GaAs and InP. Deep levels introduced into CdTe exhibit appropriate absorption and photoconductivity at 1.06 μm by doping with V and Ti impurities. Photorefractive beam coupling experiments in CdTe:V gave small signal gains of 0.7 cm−1, and diffraction efficiencies with no applied electrical field of 0.7%. Thus, CdTe appears to be superior to previously studied III‐V semiconductors, in the near‐infrared spectrum. Optimization of doping and trap densities is expected to result in gain which exceeds the absorption loss, thereby allowing phase conjugation with infrared injection lasers.