Showing papers on "Absorption (electromagnetic radiation) published in 2000"

Laser-Induced Shape Changes of Colloidal Gold Nanorods Using Femtosecond and Nanosecond Laser Pulses

Abstract: Gold nanorods have been found to change their shape after excitation with intense pulsed laser irradiation. The final irradiation products strongly depend on the energy of the laser pulse as well as on its width. We performed a series of measurements in which the excitation power was varied over the range of the output power of an amplified femtosecond laser system producing pulses of 100 fs duration and a nanosecond optical parametric oscillator (OPO) laser system having a pulse width of 7 ns. The shape transformations of the gold nanorods are followed by two techniques: (1) visible absorption spectroscopy by monitoring the changes in the plasmon absorption bands characteristic for gold nanoparticles; (2) transmission electron microscopy (TEM) in order to analyze the final shape and size distribution. While at high laser fluences (∼1 J cm-2) the gold nanoparticles fragment, a melting of the nanorods into spherical nanoparticles (nanodots) is observed when the laser energy is lowered. Upon decreasing the...

Two-Dimensional Electronic Excitations in Self-Assembled Conjugated Polymer Nanocrystals

Abstract: Several spectroscopic methods were applied to study the characteristic properties of the electronic excitations in thin films of regioregular and regiorandom polythiophene polymers. In the regioregular polymers, which form two-dimensional lamellar structures, increased interchain coupling strongly influences the traditional one-dimensional electronic properties of the polymer chains. The photogenerated charge excitations (polarons) show two-dimensional delocalization that results in a relatively small polaronic energy, multiple absorption bands in the gap where the lowest energy band becomes dominant, and associated infrared active vibrations with reverse absorption bands caused by electron-vibration interferences. The relatively weak absorption bands of the delocalized polaron in the visible and near-infrared spectral ranges may help to achieve laser action in nanocrystalline polymer devices using current injection.

Temperature dependence of the absorption cross sections of formaldehyde between 223 and 323 K in the wavelength range 225–375 nm

Abstract: UV absorption cross sections of formaldehyde (HCHO) have been measured with a spectral resolution of 0.025 nm in the wavelength range 225–375 nm at 298 K using a diode array detector. At selected temperatures ranging from 223 to 323 K, measurements have been conducted to obtain temperature gradients in the wavelength range 250–356 nm. Error limits for the reported absorption cross sections are ±5% but at least ±3×10−22 cm2 molecule−1. For the temperature gradients, uncertainties are <8%. Spectra and temperature gradients are compared with earlier measurements.

X-ray sensitivity of photoconductors: application to stabilized a-Se

Abstract: The x-ray sensitivity of a high-resistivity photoconductor sandwiched between two parallel plate electrodes and operating under a constant field is analysed by considering charge carrier generation that follows the x-ray photon absorption profile and taking into account both electron and hole trapping phenomena but neglecting recombination, bulk space charge and diffusion effects. The amount of collected charge in the external circuit due to distributed generation of electrons and holes through the detector is calculated by integrating the Hecht collection efficiency with Ramo's theorem across the sample thickness. The results of the model allow the x-ray sensitivity to be calculated as a function of the applied field, detector thickness and electron and hole ranges (µτ), given the field and energy dependence of the electron and hole pair creation energy, W±, and the energy spectrum of incident x-ray radiation. The sensitivity model was applied to stabilized a-Se that is currently used as a successful x-ray photoconductor in the recently developed flat panel x-ray image detectors. Recent free electron-hole pair creation energy versus electric field data at room temperature and appropriate electron and hole drift mobilities were used to calculate the sensitivity for monoenergetic x-rays at 20 and at 60 keV. For the 20 keV radiation, it was shown that a typical detector thickness of 200 µm (4 × attenuation depth at 20 keV) with currently attainable electron and hole trapping parameters in a-Se was operating optimally, the sensitivity of which can only be increased by further increasing the applied field. With the receiving electrode positively biased, the sensitivity was much more dependent on the hole lifetime than electron lifetime. The absence of hole transport results in a reduction in sensitivity by a factor of about 4.4, whereas the absence of electron transport results in a sensitivity degradation of only 22%. The ratio of hole trapping limited sensitivity to electron trapping limited sensitivity is about 0.3. For a detector of thickness 200 µm operating at 10 V µm-1, the maximum sensitivity is about 220 pC cm-2 mR-1, and this sensitivity degrades by more than 10% when either the electron lifetime falls below ~20 µs or the hole lifetime falls below ~5 µs. When the hole lifetime is very short so that the sensitivity is substantially reduced, the sensitivity versus thickness dependence at a given field exhibits a maximum (an optimal thickness) that is less than that for full absorption. In the case of 60 keV x-ray photons, it is more useful to examine the sensitivity as a function of detector thickness given the practical bias voltage limit. The sensitivity versus thickness behaviour for a given bias voltage exhibits a maximum, that is an optimal thickness, that is less than that for nearly full absorption. Electron lifetimes longer than ~200 µs and hole lifetimes longer than ~10 µs do not significantly affect the sensitivity.

Hydrodynamic simulation of subpicosecond laser interaction with solid-density matter

Abstract: The interaction of ultrashort subpicosecond laser pulses with initially cold and solid matter is investigated in a wide intensity range (10(11) to 10(17) W/cm(2)) by means of the hydrodynamic code MULTI-FS, which is an extension of the long pulse version of MULTI [R. Ramis, R. Schmalz, and J. Meyer-ter-Vehn, Comput. Phys. Commun. 49, 475 (1988)]. Essential modifications for the treatment of ultrashort pulses are the solution of Maxwell's equations in a steep gradient plasma, consideration of the nonequilibrium between electrons and ions, and a model for the electrical and thermal conductivity covering the wide range from the solid state to the high temperature plasma. The simulations are compared with several absorption measurements performed with aluminum targets at normal and oblique incidence. Good agreement is obtained by an appropriate choice of the electron-ion energy exchange time (characterized by 10 to 20 ps in cold solid Al). In addition we discuss the intensity scaling of the temperature, of the pressure, and of the density, where the laser energy is deposited in the expanding plasma, as well as the propagation of the heat wave and the shock wave into the solid. For laser pulse durations >/=150 fs considered in this paper the amount of isochorically heated matter at solid density is determined by the depth of the electron heat wave in the whole intensity range.

Effect of Axial Substitution on the Optical Limiting Properties of Indium Phthalocyanines

Abstract: Nonlinear absorption, refraction, and optical limiting by a series of monoaxially chloro- and aryl-substituted indium(III) phthalocyanines are described. The absorption cross sections and temporal evolution of the low-lying excited states are also reported. A large nonlinear absorption that increased with wavelength between 500 and 590 nm was observed in each material. The nanosecond nonlinear absorption and the optical limiting are shown to be dominated by a strong excited state absorption from an orientationally averaged triplet state. We derive and experimentally confirm the relation between the molecular absorption cross sections and the fluence-dependent nonlinear absorption coefficients. The effective nonlinear refraction on the nanosecond time scale was reduced because the electronic contribution to the nonlinear refractive index was of the opposite sign from the thermal contribution. An optical limiter using the new material, p-(trifluoromethyl)phenylindium(III) tetra-tert-butylphthalocyanine [(t-...

Intersubband absorption at λ∼1.55 μm in well- and modulation-doped GaN/AlGaN multiple quantum wells with superlattice barriers

Abstract: Intersubband optical absorption around 1.55 μm has been measured in GaN/AlGaN multiple quantum wells (MQWs) grown by molecular-beam epitaxy. First, peak absorption wavelengths as short as 1.41 μm are reported for ultranarrow, ⩾11 A wide, well-doped MQWs with high, 85%, AlN mole-fraction barriers. Second, in order to enable modulation doping as well as the use of lower AlN mole-fraction barriers, we designed and fabricated QWs embedded in barriers consisting of a short period superlattice of narrow GaN QWs and only 65% AlN mole-fraction barriers. The resulting electron Bragg confinement allows peak absorption wavelengths as short as 1.52 μm. Furthermore, the structures can now be modulation doped through doping of the narrow superlattice wells and subsequent charge transfer into the active well. We observe a reduction of the absorption linewidth, from ∼200 to ∼130 meV, for these structures.

Electronic and vibronic contributions to two-photon absorption of molecules with multi-branched structures

Abstract: The electronic and vibronic contributions to the two-photon absorption of a series molecules with multi-branched structures have been studied using ab initio response theory. The results indicate that the electronic coupling between different branches alone cannot explain the experimental finding of a strong enhancement of the two-photon absorption cross section over the single branch structure, whereas it is predicted that vibronic contributions can play an important role in this respect. It is shown that for multi-branched molecules the use of circularly polarized light can increase the two-photon absorption cross section by a factor of 1.5 over linearly polarized light excitation.

An improved optical pH sensor based on polyaniline

Abstract: An optical pH sensor based on polyaniline is presented in this paper. Polyaniline films were prepared by chemical oxidation at room temperature. By increasing reaction time for up to 12 h, the stability of the polyaniline film was significantly improved. The film showed rapid reversible color change upon pH change. Solution pH could be determined by monitoring either absorption at a fixed wavelength or the maximum absorption wavelength of the film. The optical pH sensors can be exposed in air for over 1 month without any deterioration in sensor performance.

Nature of the fundamental band gap in GaNxP1−x alloys

Abstract: The optical properties of GaNxP1−x alloys (0.007⩽x⩽0.031) grown by gas-source molecular-beam epitaxy have been studied. An absorption edge appears in GaNxP1−x at energy below the indirect ΓV–XC transition in GaP, and the absorption edge shifts to lower energy with increasing N concentration. Strong photomodulation signals associated with the absorption edges in GaNxP1−x indicate that a direct fundamental optical transition is taking place, revealing that the fundamental band gap has changed from indirect to direct. This N-induced transformation from indirect to direct band gap is explained in terms of an interaction between the highly localized nitrogen states and the extended states at the Γ conduction-band minimum.

UV‐visible absorption cross sections of nitrous acid

Abstract: Nitrous acid, HONO, is a source of OH radicals in the polluted atmosphere. Although the atmospheric chemistry of HONO is qualitatively understood, not much quantitative information exists. The magnitude of the OH production by HONO photolysis depends on the spectrum of its absorption cross sections; therefore the knowledge of σ'HONO(λ) is essential. The spectrum of the differential cross sections σ'HONO(λ) is needed to detect HONO in the atmosphere by differential optical absorption spectroscopy (DOAS). Here we present measurements of the HONO UV-visible absorption cross sections with a spectral resolution better than 0.1 nm and a high signal-to-noise ratio. The maximum value of the absorption cross sections is σHONO (354 nm) = (5.19±0.26) × 10−19 cm2 and agrees well with literature data. Nevertheless, calculations based on data from this work and on literature data reveal that an uncertainty of ∼15% remains for the HONO photolysis rates. The new σHONO(λ) has been employed in DOAS measurements in Milan, Italy.

Refractive indices and absorption coefficients of MgxZn1−xO alloys

Abstract: Indices of refraction for MgxZn1−xO epitaxial films grown by pulsed-laser deposition on sapphire substrates with x up to 0.36 were determined in the range of wavelength 457–968 nm by analysis of optical transmission spectra and prism-coupled waveguide measurements. The dispersion follows the first-order Sellmeier dispersion equation. Absorption coefficients, exciton energy gaps, and binding energies of MgxZn1−xO alloys were determined by transmission spectroscopy. The excitonic absorption features were clearly visible at room temperature despite alloy broadening. These results provide important information for the design and modeling of ZnO/MgZnO heterostructure optoelectronic devices.

Optical absorption cross sections of Si nanocrystals

Abstract: Using the photoluminescence Auger saturation phenomenon, we deduce the values of the absorption cross section of silicon nanocrystals in a wide range of energies. The very large variation of their values versus energy of the absorbed light is attributed to the enhanced optical transition oscillator strength but reduced density of electronic states towards higher confinement energies. The overall spectral behavior of the absorption cross section reflects the indirect-gap nature of silicon nanocrystals.

Mechanism of nonphotochemical quenching in green plants: energies of the lowest excited singlet states of violaxanthin and zeaxanthin.

Abstract: The xanthophyll cycle is an enzymatic, reversible process through which the carotenoids violaxanthin, antheraxanthin, and zeaxanthin are interconverted in response to the need to balance light absorption with the capacity to use the energy to drive the reactions of photosynthesis. The cycle is thought to be one of the main avenues for safely dissipating excitation energy absorbed by plants in excess of that needed for photosynthesis. One of the key factors needed to elucidate the molecular mechanism by which the potentially damaging excess energy is dissipated is the energy of the lowest excited singlet (S1) state of the xanthophyll pigments. Absorption from the ground state (S0) to S1 is forbidden by symmetry, making a determination of the S1 state energies of these molecules by absorption spectroscopy very difficult. Fluorescence spectroscopy is potentially the most direct method for obtaining the S1 state energies. However, because of problems with sample purity, low emission quantum yields, and detect...

VLBA Absorption Imaging of Ionized Gas Associated with the Accretion Disk in NGC 1275

Abstract: Nearly simultaneous VLBA observations of 3C 84, the radio source associated with NGC 1275, have been made at multiple frequencies to study the free-free absorption of the northern, or counterjet, feature found by Walker, Romney, & Benson and by Vermeulen, Readhead, & Backer. Our observations confirm that the spectra are consistent with free-free absorption and eliminate the possibility that the earlier result was an effect of variability. The northern feature is well resolved spatially, so images have been made showing the distribution of the absorption over a region of about 1.5 pc on a side, beginning about 1.5 pc from the presumed location of the central object. That distribution is dominated by a strong decrease with radial distance. The magnitude of the absorption near 2.5 pc projected distance from the central object is consistent with a 104 K gas with an emission measure of about 5 × 108 pc cm-6. The geometry is consistent with absorption by ionized gas associated with an accretion disk. The data provide firm constraints for models of the outer regions of accretion disks and, perhaps, associated winds.

Real three-dimensional microstructures fabricated by photopolymerization of resins through two-photon absorption

Abstract: Effective energy windows for two-photon absorption (TPA) photopolymerization of resins were investigated and, with a properly selected laser pulse energy, exquisite three-dimensional (3D) microstructures with submicrometer spatial resolution were achieved. The results show the inherent utility of TPA in the fabrication of real 3D patterns. In particular, we propose and utilize a resin pre-exposure technique by which freely movable components affixed to an axle are built, demonstrating a new application of TPA in laser microfabrication.

Estimation of the inherent optical properties of natural waters from the irradiance attenuation coefficient and reflectance in the presence of Raman scattering.

Abstract: By means of radiative transfer simulations we developed a model for estimating the absorption a, the scattering b, and the backscattering bb coefficients in the upper ocean from irradiance reflectance just beneath the sea surface, R(0-), and the average attenuation coefficient for downwelling irradiance, 〈Kd〉1, between the surface and the first attenuation depth. The model accounts for Raman scattering by water, and it does not require any assumption about the spectral shapes of a, b, and bb. The best estimations are obtained for a and bb in the blue and green spectral regions, where errors of a few percent to <10% are expected over a broad range of chlorophyll concentration in water. The model is useful for satellite ocean color applications because the model input, R(0-) and 〈Kd〉1, can be retrieved from remote sensing and the model output, a and bb, is the major determinant of remote-sensing reflectance.

Complex resonance absorption structure in the X-ray spectrum of IRAS13349+2438

Abstract: The luminous infrared-loud quasar IRAS 13349+2438 was observed with the XMM-Newton Observatory as part of the Performance Verification program. The spectrum obtained by the Reflection Grating Spectrometer (RGS) exhibits broad (v ~ 1400 km/s FWHM) absorption lines from highly ionized elements including hydrogen- and helium-like carbon, nitrogen, oxygen, and neon, and several iron L-shell ions (Fe XVII - XX). Also shown in the spectrum is the first astrophysical detection of a broad absorption feature around lambda = 16 - 17 Ang identified as an unresolved transition array (UTA) of 2p - 3d inner-shell absorption by iron M-shell ions in a much cooler medium; a feature that might be misidentified as an O VII edge when observed with moderate resolution spectrometers. No absorption edges are clearly detected in the spectrum. We demonstrate that the RGS spectrum of IRAS 13349+2438 exhibits absorption lines from at least two distinct regions, one of which is tentatively associated with the medium that produces the optical/UV reddening.

Nitrogen dioxide and kerosene-flame soot calibration of photoacoustic instruments for measurement of light absorption by aerosols

Abstract: A nitrogen dioxide calibration method is developed to evaluate the theoretical calibration for a photoacoustic instrument used to measure light absorption by atmospheric aerosols at a laser wavelength of 532.0 nm. This method uses high concentrations of nitrogen dioxide so that both a simple extinction and the photoacoustically obtained absorption measurement may be performed simultaneously. Since Rayleigh scattering is much less than absorption for the gas, the agreement between the extinction and absorption coefficients can be used to evaluate the theoretical calibration, so that the laser gas spectra are not needed. Photoacoustic theory is developed to account for strong absorption of the laser beam power in passage through the resonator. Findings are that the photoacoustic absorption based on heat-balance theory for the instrument compares well with absorption inferred from the extinction measurement, and that both are well within values represented by published spectra of nitrogen dioxide. Photodisso...

High-resolution multiwavelength surface plasmon resonance spectroscopy for probing conformational and electronic changes in redox proteins.

Abstract: To date, surface plasmon resonance (SPR) spectroscopy identifies molecules via specific bindings with their ligands immobilized on a surface. We demonstrate here that a high-resolution multiwavelength SPR technique can measure the electronic states of the molecules and thus allow direct identification of the molecules. Using this new capability, we have studied the electronic and conformational differences between the oxidized and reduced states of cytochrome c immobilized on a modified gold electrode. When the wavelength of the incident light is far away from the optical absorption bands of the protein, a ∼0.008° decrease in the resonance angle, due to a conformational change, occurs as the protein is switched from the oxidized to reduced states. When the wavelength is tuned to the absorption bands, the resonance angle oscillates at the wavelengths of the absorption peaks, which provides electronic signatures of the protein.

Method and apparatus for determining oxygen saturation of blood in body organs

Abstract: A spectrophotometric method and apparatus for determining the degree of oxygen saturation of the hemoglobin in the blood within a body part utilizes differences in light absorption based on differences in extinction coefficients at different wavelengths. Oxygen saturation is determined by utilizing absorption at three or more wavelengths of radiation preferably in the near red and infrared region (NIR) of the spectrum, specifically tailored to two or more components to be detected by the radiation. The first (reference) wavelength is preferably chosen to be at an isosbestic point for the two components, commonly the oxygenated and deoxygenated forms of hemoglobin. The absorption at the isosbestic point is subtracted from the absorption at the other wavelengths. Using these differences in absorption, the amount of each of the components encountered by the light may be determined without determination of pathlength, which would be required to determine the concentration. In order to determine the oxygenation state, i.e. ratio of oxygenated to deoxygentaed blood components, knowledge of the relative amounts suffices. Interference from other light-absorbing components, specifically those that respond to the oxygenation state of the body part (such as cytochrome c oxidase) is in the three wavelength method eliminated by reiterative correction.

Photoluminescence and free-electron absorption in heavily phosphorus-doped Si nanocrystals

Abstract: Heavily phosphorus-doped Si nanocrystals several nanometers in diameter are studied by photoluminescence (PL) and optical absorption spectroscopy. It is demonstrated that P doping results in the quenching of the PL. The quenching is accompanied by the appearance of the optical absorption in the infrared range. The absorption was assigned to the intravalley transitions of free electrons generated by P doping (free-electron absorption). The generation of free electrons and the resultant three-body Auger recombination of excitons is considered to be responsible for the observed PL quenching.

Low- and high-ionization absorption properties of Mg II absorption-selected galaxies at intermediate redshifts. I. General properties

Abstract: We present extensive metal-line absorption properties for 45 absorption systems that were selected by their Mg II absorption at redshifts between 0.4 and 1.4. For each system the properties of several chemical species are determined, including a wide range of ionization conditions. In the optical, the absorption systems have been observed at ~6 km s-1 resolution with HIRES/Keck, which covered Mg II, several Fe II transitions, Mg I, and in some cases (depending upon redshift) Ca II, Ti II, Mn II, and Al III. Ultraviolet, lower resolution (~230 km s-1) Faint Object Spectrograph data (1600-3275 ?) were obtained from the Hubble Space Telescope archive. These spectra covered Al II, Al III, Si II, Si III, Si IV, C II, C III, C IV, N V, O VI, and several Lyman series transitions, with coverage dependent upon the absorption system redshift. From these data, we infer that Mg II-absorbing galaxies at intermediate redshifts have multiphase gaseous structures.

Solvent-induced two-photon absorption of a push-pull molecule

Abstract: Solvent-induced two-photon absorption cross sections are calculated for a push−pull molecule in solutions using both self-consistent reaction field and internal finite field approaches. It is shown analytically and numerically that the results from the two methods can be connected through induced local reaction field factors. The two-photon cross sections of the studied push−pull polyene are found to be rather insensitive to the choice of cavity shape. The solvent dependence of the two-photon absorption displays a pattern different from that of the first hyperpolarizability.

Detection of terahertz radiation with low-temperature-grown GaAs-based photoconductive antenna using 1.55 μm probe

Abstract: THz radiation is detected by a low-temperature-grown GaAs (LT-GaAs) photoconductive antenna probed with a 1.55 μm probe laser. The detection efficiency is found to be approximately 10% of that obtained with a 780 nm probe. From the nonquadratic dependence of photoconductivity on laser intensity, two-step photoabsorption mediated by midgap states in LT-GaAs is suggested, instead of the two-photon absorption, as the primary process for the photoconductivity.

PbS quantum-dot-doped glasses for ultrashort-pulse generation

Abstract: We investigate the use of PbS quantum-dot-doped glasses as saturable absorbers for ultrashort-pulse lasers by means of absorption bleaching experiments and numerical analysis of the pulse shaping process using the Haus’ master equation. We explain the mode-locking mechanism and the limitations of these absorbers. The generation of transform-limited fs pulses is predicted by soliton mode locking initiated by the absorption saturation of higher excited states of the quantum-dot saturable absorber.