Bulk Absorption Research Articles

Feedback-controlled running holograms in strongly absorbing photorefractive materials

We propose a mathematical model for the movement in absorbing materials of photorefractive holograms under feedback constraints. We use this model to analyze the speed of a fringe-locked running hologram in photorefractive sillenite crystals that usually exhibit a strong absorption effect. Fringe-locked experiments permit us to compute the quantum efficiency for the photogeneration of charge carriers in photorefractive crystals if the effect of bulk absorption and the effective value of the externally applied field are adequately taken into consideration. A Bi12TiO20 sample was measured with the 532-nm laser wavelength, and a quantum efficiency of Φ=0.37 was obtained. Disregarding absorption leads to large errors in Φ.

Calculation of exciton absorption in arbitrary layered semiconductor nanostructures with exact treatment of the Coulomb singularity

An accurate finite-element exciton calculation is presented. The Coulomb singularity is exactly taken into account without any further assumptions because analytical expressions are calculated for the Coulomb matrix elements even for the general three-dimensional case. Due to open boundary conditions, this is an accurate and fast model for calculating the optical properties of arbitrary semiconductor structures. The advantage will be demonstrated by an investigation of bulk, quantum-well, and multiquantum-well absorption.

Novel method to measure bulk absorption in optically transparent materials

A novel method to measure low absorption of pulsed laser radiation by materials with high optical transparency is demonstrated. The absorbed energy generates a temperature gradient and thus a refractive index profile. A probe laser beam is deflected by the refractive index profile and its deflection is a measure of pure volume absorption without interference from surface effects. Numerical calculations were carried out to estimate the sensitivity of the discussed laser induced deflection (LID) arrangement and to optimize the path of the probe beam in relation to the irradiated field within the sample. The experimental results agree sufficiently with numerical calculations by finite element method combined with raytracing procedures. Because the transparency is not measured directly a calibration is necessary. To provide this, a sample with known bulk absorption coefficient and thermal properties was used. In order to compensate probe beam fluctuations in air outside the sample, a double beam arrangement was designed. The LID method is applied to investigate fused silica with high ultraviolet (UV) transparency under KrF excimer laser irradiation (λ=248 nm). A He/Ne laser serves as probe beam source. Using sample dimensions of 20×20×10 mm3 and UV beam cross sections of 5×5 mm2 absorbed power down to 1 mW is detected. At typical irradiation conditions (50 s−1, 300 mJ/cm2), this value corresponds with an absorption coefficient of around 2.5×10−4 cm−1. The limitation of the absorption measurement depends on the input power of the pump laser.

Dielectric relaxation time measurement in absorbing photorefractive materials

We show that absorbing photorefractive materials characterized by a space-charge exponential relaxation time law exhibit an overall hologram optical erasure that is described by the so-called exponential integral function, as far as self-diffraction can be neglected. This fact is due to the bulk absorption producing an exponentially decreasing distribution of the erasure beam irradiance along the sample thickness that results in a correspondingly increasing dielectric relaxation time. The theoretical development in this paper is experimentally verified by the analysis of the holographic erasure in a nominally undoped Bi 12TiO 20 photorefractive crystal using the 514.5 nm laser wavelength where this material exhibits a relatively strong bulk absorption. Neglecting absorption in this experiment leads to a relaxation time that is about 4-fold larger than the actual value.

Photoreactions of small molecules at the surface of alkali metal halides

The photostimulated reactions of small molecules, i.e., O 2 photoadsorption and photodesorption, H 2 and CO photooxidation, and CO 2 photodecomposition on alkali halides (alkali halides) surface are discussed in this work. Most of 19 alkali halides show the activity in the reactions mentioned above. Three spectral regions of alkali halides stimulation by light are distinguished in relation to kinetics of oxygen photoadsorption and reactivity of adsorbed oxygen species. They are in correspondence with: (i) bulk exciton absorption bands; (ii) extrinsic and intrinsic bulk and surface absorption bands and (iii) color centers absorption bands. In the latter case the activity of alkali halides arises after pre-excitation in exciton bands, which leads to sensitization of alkali halides to visible light. The H 2 and CO photooxidation on alkali halides is two-step photoprocess involving some excited states of adsorbed oxygen. The latter is quenching by gaseous oxygen as it is shown for hydrogen oxidation over KBr. The photodecomposition of carbon dioxide is treated as CO 2 dissociative adsorption with CO evolution into the gas phase.

Photo-excitation from dia- to ferri-magnetism in a Rb-Co-hexacyanoferrate Prussian blue analogue

We studied the photo-excitation process, the relaxation of the photo-excited state towards the stable state, and the photo-induced magnetic properties of the Prussian blue analogue Rb0.52Co [Fe(CN)6]0.84, 2.3 H2O. Magnetic, Mossbauer and reflectivity measurements have been performed during and after illumination. The efficiency of the photo-excitation device is maximum at \(\lambda \cong 700 - 750\) nm. The process, however, is severely hindered by bulk absorption of the light; it is rapidly completed at the surface of the sample and then proceeds slowly in the bulk. Under the effect of photo-excitation the system turns from a dia- to a ferri- magnet, with a Tc value ≥21 K, indicative of the transformation of the material due to the following optical electron transfer: \(F{e^{II}}(LS) + C{o^{III}}(LS) \to F{e^{III}}(LS) + C{o^{II}}(HS)\) Thermal relaxation towards the stable electronic state is observed in the 95-110 K temperature interval, obeying a self-accelerated kinetics. At low temperature, a weak, non-exponential, relaxation is detected. These features are discussed in terms of co-operative effects in the frontal process of photo- excitation. The peculiarities of a photo-excited state created below the magnetic ordering temperature are discussed. A metastable magnetic state has been observed in low fields, denoted “Raw Photo-Induced State" (RPI), with a magnetization curve in-between the field-cooled and zero-field-cooled curves.

Carrier concentration and lattice absorption in bulk and epitaxial silicon carbide determined using infrared ellipsometry

We have measured the dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates from 700 to 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ using Fourier-transform infrared spectroscopic ellipsometry. Photon absorption by transverse optical phonons produces a strong reststrahlen band between 797 and 1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with the effects of phonon anisotropy being observed in the region of the longitudinal phonon energy (960 to 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). The shape of this band is influenced by plasma oscillations of free electrons, which we describe with a classical Drude equation. For the 6H-SiC samples, we modify the Drude equation to account for the strong effective mass anisotropy. Detailed numerical regression analysis yields the free-electron concentrations, which range from $7\ifmmode\times\else\texttimes\fi{}{10}^{17}$ to ${10}^{19}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3},$ in good agreement with electrical and secondary ion mass spectrometry measurements. Finally, we observe the Berreman effect near the longitudinal optical phonon energy in $n\ensuremath{-}/n+$ homoepitaxial 4H SiC and hydrogen implanted samples, and we are able to determine the thickness of these surface layers.

Deconvolution and measurement of bulk and surface optical absorptions in Ti:Al2O3 crystals using photopyroelectric interferometry

The extension of our earlier single-layer (monolithic) photopyroelectric (PPE) interferometric theory to include surface and bulk optical absorptions has allowed the measurement of both bulk absorption coefficient and surface absorptance in one single experiment. Based on purely thermal-wave interferometry, the thermal-wave cavity lengths of a PPE interferometer were scanned using pairs of Ti: sapphire crystals with appropriate combinations of figure of merit, surface polish, and thickness. In the conventional single-ended (noninterferometric) PPE technique, the surface reflectivity, surface absorptance, and bulk absorption coefficient are always coupled together. However, PPE destructive interferometry provides a method for extracting highly precise values of one of these optical parameters, without the requirement of equally precise knowledge of the values of the others.

Photovoltaic spectroscopic study of GaN epilayers and InGaN quantum well structures

Room-temperature photovoltaic spectroscopy was applied to study undoped GaN, n-type GaN, and InGaN quantum well structures. Clear exciton absorption was observed in the photovoltaic spectra of the undoped GaN, and polarization measurements were made to identify the exciton absorption. For the n-type GaN sample, instead of the exciton absorption we observed only bulk absorption edge, which may be due to the free carrier screening effect. For the InGaN quantum well structures, the photovoltaic spectra showed relatively complicated line shape due to the overlap of the signals from different layers. By changing the reference phase of the lock-in amplifier, we were able to suppress some of the signals and thus identify the origin of the corresponding signal.

Altered formation and bulk absorption of cerebrospinal fluid in FGF-2-induced hydrocephalus.

Upregulation of certain growth factors in the central nervous system can alter brain fluid dynamics. Hydrocephalus was produced in adult Sprague-Dawley rats by infusing recombinant basic fibroblast growth factor (FGF-2) at 1 microg/day into a lateral ventricle for 2, 3, 5, or 10-12 days. Lateral and third ventricular enlargement progressively increased from 2 to 10 days. Ventriculomegaly was also induced by a 75% reduced dose of FGF-2. At 10-12 days, there was a 29% attenuation in cerebrospinal fluid (CSF) formation rate, from 2. 5 to 1.8 microliter/min (P < 0.01). Choroid plexus, the main site of CSF secretion, had an augmented number of dark epithelial cells, which have previously been associated with decreased choroidal fluid formation. The twofold elevated resistance to CSF absorption, i.e., 0.8 to 1.7 mmHg. min(-1). microliter(-1), was attributable, at least in part, to enhanced fibrosis and collagen deposits in the arachnoid villi, a major site for CSF absorption. Normal CSF pressure (2-3 mmHg) was consistent with a patent cerebral aqueduct and reduced CSF formation rate. The FGF-2-induced ventriculomegaly is interpreted as an ex vacuuo hydrocephalus brought about by an altered neuropil and interstitium of the brain.

Attenuated total internal reflection spectroscopy with an intracavity laser absorption spectrometer

A new type of intracavity laser absorption spectrometer (ICLAS) which employs an intracavity prism in total internal reflection (TIR) configuration is presented. We report a theoretical background and a full characterisation of the spectrometer by measuring absorption spectra of NO 2 and I 2 vapour at room temperature in the 0.6 μm spectral region. The ICLAS spectra measured in the attenuated TIR configuration correspond to bulk absorption with an equivalent number of reflections of 2×10 4. They are identical to the conventional gas phase absorption spectra with no appreciable contributions from (surface) adsorbed molecules.

Heating and structural alterations in cartilage under laser radiation

The authors have developed a theoretical model to calculate the temperature field and the spatial distribution of modified cartilage following laser irradiation. The model incorporates both thermal and mass transfer in a tissue with bulk absorption of laser radiation, water evaporation from the surfaces of a slab, and temperature dependence of the diffusion coefficient. The authors propose that water undergoes a bound-to-free phase transition in cartilage heated to about 70/spl deg/C and the mobility of proteoglycan units in the cartilage matrix increases. Movement of the proteoglycan units takes place only when the temperature exceeds 70/spl deg/C and results in tissue denaturation. Using their model, the authors show: (1) the maximal temperature is reached not on the surface irradiated but rather at some distance below; (2) surface temperature reaches its asymptotic value quicker than the maximal temperature; and (3) the depth of the denatured tissue volume strongly depends on laser fluency, wavelength, exposure time, and cartilage thickness. The model allows for the prediction and control of temperature and depth of structural alterations during the course of laser reshaping and treatment of cartilage.

Absorption and scattering of high-power laser radiation in low-density porous media

We have experimentally investigated the interaction of high-power neodymium laser pulses in the intensity range 1013–1014 W/cm2 with flat low-density (0.5–10 mg/cm3) agar-agar targets under conditions of interest for problems of inertial nuclear fusion. Optical and x-ray methods with high temporal and spatial resolution were used to examine the dependence of absorption and scattering of the incident beam on the initial mean density and thickness of the irradiated samples. We show that when a porous target is irradiated, a bulk absorption layer of high-temperature plasma is produced inside the target whose dimensions are determined by the initial density of the material. The time dependence and spectral composition of the harmonics 2ω0 and 3ω0/2 observed in the plasma-scattered radiation are measured. A theoretical model is developed that describes the interaction of high-power laser pulses with a porous medium. Predictions of the model, based on the hypothesis of two stages of homogenization of the target material—a fast stage (0.1–0.3 ns) and a slow stage (1–3 ns), are in good agreement with the experimental data.

Dissipative interaction and anomalous surface absorption of bulk phonons at a two-dimensional defect in a solid

We predict an extreme sensitivity to the dissipative losses of the resonant interaction of bulk phonons with a 2D defect in a solid. We show that the total resonant reflection of the transverse phonon at the 2D defect, described earlier without an account for dissipation, occurs only in the limit of extremely weak dissipation and is changed into almost total transmission by relatively weak bulk absorption. Anomalous surface absorption of the transverse phonon, when one half of the incident acoustic energy is absorbed at the 2D defect, is predicted for the case of “intermediate” bulk dissipation.

The mechanism of copper activation of sphalerite

On the basis of recent SIMS and XAFS measurements in conjunction with already published XPS results, a mechanism for the adsorption/absorption of Cu onto sphalerite is proposed. Under conditions of high pH and high nominal surface coverage of the sphalerite by the Cu, Cu(OH) 2 colloidal particles are observed on the sphalerite surfaces using SIMS. Under other conditions, SIMS measurements have indicated that adsorption of the Cu is essentially uniform over the sphalerite surface and is not related to low coordination sites on the surface of the sphalerite. Depth profiling of sphalerite surfaces with Cu adsorbed under conditions that do not result in Cu(OH) 2 colloidal particles show that the Cu adsorbed/absorbed on the sphalerite surface is largely in the first few atomic layers. XAFS analysis of Cu activated sphalerite has indicated that the Cu occupies a distorted trigonal planar geometry, coordinated to three S atoms, in both surface and bulk sites. In addition Cu(1s), absorption edges in XAFS show that both bulk and surface adsorbed copper have an oxidation state less than +1 with the surface Cu being slightly more oxidised than the bulk absorbed Cu. On the basis of the combined XPS, SIMS, XAFS and solution studies, a model is proposed that, on surface adsorption of Cu, the surface Zn(II) atoms are replaced by Cu(II) atoms which are then reduced in situ to Cu(I). This reduction is accompanied by the oxidation of the three neighbouring S atoms to an oxidation state of approximately −1.5. On bulk absorption of Cu atoms into the sphalerite lattice a distorted trigonal planar configuration is achieved through the breakage of a formerly tetrahedral Zn–S bond. The breakage of this bond results in a 3-fold coordinated Cu plus one S 3-fold coordinated to Zn atoms. The breakage of this bond leads to a greater reduction of the Cu than on surface absorption and also oxidation of the 3-fold coordinated S atom to an approximately −0.5 oxidation state. This model does not invoke any polysulfite or S–S bonded species to explain the higher binding energy components of the S(2p) XPS spectra.

The effect of abdominal wall morphology on ultrasonic pulse distortion. Part I. Measurements.

The relative importance of the fat and muscle layers of the human abdominal wall in producing ultrasonic wavefront distortion was assessed by means of direct measurements. Specimens employed included six whole abdominal wall specimens and twelve partial specimens obtained by dividing each whole specimen into a fat and a muscle layer. In the measurement technique employed, a hemispheric transducer transmitted a 3.75-MHz ultrasonic pulse through a tissue section. The received wavefront was measured by a linear array translated in the elevation direction to synthesize a two-dimensional aperture. Insertion loss was also measured at various locations on each specimen. Differences in arrival time and energy level between the measured waveforms and computed references that account for geometric delay and spreading were calculated. After correction for the effects of geometry, the received waveforms were synthetically focused. The characteristics of the distortion produced by each specimen and the quality of the resulting focus were analyzed and compared. The measurements show that muscle produces greater arrival time distortion than fat while fat produces greater energy level distortion than muscle, but that the distortion produced by the entire abdominal wall is not equivalent to a simple combination of distortion effects produced by the layers. The results also indicate that both fat and muscle layers contribute significantly to the distortion of ultrasonic beams by the abdominal wall. However, the spatial characteristics of the distortion produced by fat and muscle layers differ substantially. Distortion produced by muscle layers, as well as focal images aberrated by muscle layers, show considerable anisotropy associated with muscle fiber orientation. Distortion produced by fat layers shows smaller-scale, granular structure associated with scattering from the septa surrounding individual fat lobules. Thick layers of fat may be expected to cause poor image quality due to both scattering and bulk absorption effects, while thick muscle layers may be expected to cause focus aberration due to large arrival time fluctuations. Correction of aberrated focuses using time-shift compensation shows more complete correction for muscle sections than for fat sections, so that correction methods based on phase screen models may be more appropriate for muscle layers than for fat layers.

Absolute deuterium coverage determination for the Mo 0.75Re 0.25(100), (110), and (111) surfaces

Adsorption of hydrogen (deuterium) on the (100), (110), and (111) surfaces of the random alloy Mo 0.75Re 0.25 has been studied with low-energy electron diffraction (LEED), temperature-programmed desorption, and nuclear reaction analysis. The relative distribution of Mo and Re in the outermost layers of these surfaces is orientation-dependent. Following exposure to D 2, only (1×1) patterns are observed with LEED for all surfaces and D coverages, indicating no adsorbate-induced reconstruction. Using the D( 3He, p) 4He reaction, the saturation coverages for the (100), (110), and (111) surfaces are determined to be 2.02±0.10, 0.99±0.05, and 2.86±0.24 ML, respectively. No bulk absorption of D is observed after exposure to either D 2 or D for all three surfaces.

Computing method for evaluating the absorption coefficient of infrared optical elements

The paper presents a new computing method for the bulk absorption coefficient of an optical component. The method is based on the general solution of the heat diffusion equation calculated for the case where the component is irradiated by a laser beam. From the practical point of view this new method simplifies the measuring process of the absorption coefficient because it requires, as input data, the value of the temperature rise in a single point at one moment. To establish the most suitable moment and position for the measuring point in order to reduce the errors, this method was numerically simulated on GaAs and ZnSe samples irradiated with a TEM 00 CO 2 laser beam. An error analysis was conducted to find the required precision for the experimental measurements to obtain accurate data. To experimentally test this method, an experimental setup, based on a laser calorimeter was developed and the absorption coefficient of four ZnSe samples was evaluated. The results were proved to be in good agreement to the results provided by the pulse and gradient method mentioned in ISO 11551, applied in the same experimental conditions as the proposed method.

Calculation of the kinetics of heating and structural changes in the cartilaginous tissue under the action of laser radiation

A theoretical model is developed for the calculation of the temperature fields and determination of the size of a zone with structural changes in the cartilaginous tissue. The model is based on a simultaneous analysis of the heat and mass transfer processes and it takes into account the bulk absorption of laser radiation by the tissue, surface evaporation of water, and temperature dependences of the diffusion coefficients. It is assumed that under the influence of a phase transition between free and bound water, caused by heating of the cartilage to 70°C, the proteoglycans of the cartilage matrix become mobile and, as a result of such mass transfer, structural changes are induced in the cartilaginous tissue causing relaxation of stresses or denaturation. It is shown that the maximum temperature is then reached not on the irradiated surface but at some distance from it, and that the size of the zones of structural changes (denaturation depth) depends strongly on the energy density of the laser radiation and its wavelength, on the duration of the irradiation, and on the cartilage thickness. This model makes it possible to calculate the temperature fields and the depth of structural changes in laser-induced relaxation of stresses and changes in the shape of the cartilaginous tissue.

Gain spectra of beam coupling in photorefractive semiconductors

The photorefractive recording dynamics of two-beam coupling in semi-insulating semiconductors by beams with slightly different frequencies are studied theoretically and experimentally. The influences of bulk absorption, Gaussian beam profiles, and experimental geometry on the temporal response are analyzed. These effects act to narrow the bandwidth. Measurement of the material photorefractive time constant is discussed.