Bulk Absorption Research Articles

Analysis of the sensing properties of silica microtube resonator sensors

We present a theoretical study of the sensing properties of silica microtube resonator sensors. By solving the Maxwell's equations to obtain the electric field distribution in a cylindrical coordinate system and by using the perturbation method, we have analyzed in detail the sensing properties of silica microtube resonator sensors, which include the bulk refractive index, surface, and absorption sensing sensitivities with different radial order resonant modes. We found that a type of resonant mode, different from the evanescent modes commonly employed in previous investigations, is very promising for sensing changes of the refractive index. Furthermore, the resonant mode with high electric field at the inner boundary of the microtube is ideal for surface sensing applications. The high Q factor resonant mode is useful for the absorption sensing application. These sensing properties analyzed by using the perturbation method match very well with the results from the Mie scattering method. Finally, the limitation of the perturbation method is discussed.

Bulk flow in the cerebrospinal fluid system of the dog.

Ventriculo-cisternal, intracranial subarachnoid-to-subarachnoid space and spinal subarachnoid space perfusions were carried out on dogs. The perfusate contained inulin as a tracer and the design of these experiments was based upon the concept that inulin leaves cerebrospinal fluid (CSF) compartments only by means of bulk absorption, and that actual diffusion and active transport of inulin is negligibly small. Inulin was removed from different CSF spaces by bulk absorption at rates which varied linearly with hydrostatic pressure. The rate of formation of CSF is equal to inulin clearance plus the difference between outflow and inflow rates. The total CSF formation in dogs weighing 12 to 17 kg was measured as 0.065 ml/min, and 58.5 per cent of this amount was found to come from extraventricular CSF space. The rates are independent of hydrostatic pressure in the range studied.

Changes in the Absorption and Scattering Properties in the Near-Infrared Region during the Growth of Bacillus Subtilis in Liquid Culture

Multiple scattering of light by cells poses a significant challenge in the development of near-infrared-based methodologies to reliably extract chemical and physical information contained in the spectra collected during the bacterial growth cycle. The extent of information that can be obtained from NIR spectra could, in principle, be vastly improved if the scattering and absorption effects can be effectively separated. This study focuses on the methodology for extracting the bulk optical properties over the course of the bacterial growth cycle and investigates the nature and extent of changes in the optical properties with time. By inverting the radiative transfer equation (RTE) using three measurements, total diffuse reflectance, total diffuse transmittance, and collimated transmittance, the bulk absorption coefficient (microa), the bulk scattering coefficient (micros), and the anisotropy factor (g) are extracted and their changes during the course of the growth cycle are investigated. In this study, a simple bacterial growth system consisting of Bacillus subtilis growing in an aqueous solution (minimum medium) was investigated. The changes in the optical properties of this system during bacterial growth, stationary, and decline phases were investigated by inverting the measurements using the adding-doubling method to solve the RTE in the wavelength region of 950-1850 nm. This study shows that during growth in liquid culture, the absorption and scattering property changes can be consistently extracted from measurements under multiple light scattering conditions. The estimation of the anisotropy factor was not reliable beyond 1200 nm at low bacterial cell counts, but reliability increased with increasing biomass concentration. At all stages in the growth cycle, the anisotropy factor could not be reliably extracted in the first overtone region. However, this does not appear to adversely affect the estimation of the absorption and scattering coefficients.

Effect of the chest wall on breast lesion reconstruction

The chest wall underneath the breast tissue affects near-infrared (NIR) diffusive waves measured with reflection geometry. With the assistance of a co-registered ultrasound, the depth and the tilting angle of the chest wall can be determined and are used to model the breast as a two-layer medium. Finite element method (FEM) is suitable for modeling complex boundary conditions and is adapted to model the breast tissue and chest wall. Four parameters of bulk absorption and reduced scattering coefficients of these two layers are estimated and used for imaging reconstruction. Using a two-layer model, we have systematically investigated the effect of the chest wall on breast lesion reconstruction. Results have shown that chest-wall depth, titling angle, and difference between optical properties of two layers of lesion and reference sites affect the lesion reconstruction differently. Our analysis will be valuable and informative to researchers who are using reflectance geometry for breast imaging. The analysis can also provide guidelines for imaging operators to minimize image artifacts and to produce the best reconstruction results.

Spin-transition in [FeII(L5)2][ClO4]2 [L5 = 2-[3-(2′-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)]: a further example of coexistence of features typical for disorder and cooperativity

An iron(II) complex [Fe(L5)2][ClO4]2 of a new nonplanar tridentate ligand [L5 = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)] has been synthesized and its spin-transition properties have been investigated. X-ray structural analysis of the solvated compound [Fe(L5)2][ClO4]2.CH3CN at 100 K and at 298 K revealed a mononuclear Fe(II)N6 pseudooctahedron with the tridentate ligand in the meridional coordination mode. Magnetic susceptibility measurements revealed LS (1A1) <--> HS (5T2) transition starting above room temperature and remained incomplete even up to 350 K, however providing evidence for the hysteretic behaviour of the transition. Photo-magnetic measurements using a 532 nm laser revealed incomplete light-induced excited spin-state trapping (LIESST). Relaxation curves in the dark exhibited a stretched exponential shape, typical for broad distributions of relaxation times. The complete photo-magnetic data were modelled using a non-cooperative master equation including a tunnelling process with distributed parameters and accounting for the bulk absorption of light. We also measured the diffuse reflectance properties, with detailed investigation of the absorption spectra and discussed the obtained features with respect to structural properties, including the suggested presence of disorder.

Measurement of optical absorption in polycrystalline CVD diamond plates by the phase photothermal method at a wavelength of 10.6 μm

A highly-efficient phase photothermal method is developed for quantitative measurements of the small optical absorption coefficient in thin plates made of highly transparent materials in which bulk losses significantly exceed surface losses. The bulk absorption coefficient at 10.6 μm is estimated in polycrystalline diamond plates grown from the vapour phase (a CVD diamond). The results are compared with those for natural and synthetic diamond single crystals and with the concentrations of nitrogen and hydrogen impurities. The absorption coefficient of the best samples of the CVD diamond did not exceed 0.06 cm-1, which, taking into account the high thermal conductivity of the CVD diamond (1800–2200 W mK-1 at room temperature), makes this material attractive for fabricating output windows of high-power CO2 lasers, especially for manufacturing large-size optics.

IR Spectroscopic Testing of Surfaces in Water Ice and in Icy Mixtures with Prussic Acid or Ammonia

An experimental technique is developed for spectral studies of ice films deposited at 77 K onto the ZnSe inner windows of a cell, so that the spectra could be registered in the presence of gaseous adsorbate. Pure H2O or D2O ice as well as HCN/D2O and ND3/D2O mixed icy films with different dopant/water ratios (1:10, 1:5, and 1:1) were investigated. The surface area of ice deposited at 77 K H2O estimated from adsorption measurements was about 160 m2/g. Bands of dangling hydroxyl groups disappear on raising the temperature up to 130-160 K when the changes of bulk absorption provide evidence for a phase transition from amorphous to polycrystalline ice. Surface properties of icy films were characterized by low-temperature adsorption of CO and CHF3. Insertion of small doses of HCN or ammonia does not change the acid/base strength of dangling hydroxyl groups or coordinately unsaturated surface oxygen atoms, but it changes the proportion between the concentrations of these sites as compared with pure water ice. For high dopant concentrations, the dangling hydroxyls were not observed, and the dominant adsorption sites for CO are likely to be the unsaturated oxygen atoms, while serious structural changes occur in the bulk of ices.

Light absorption and fluorescence, and photoacclimation features in the marine macroalga &lt;i&gt;Porphyra leucosticta&lt;/i&gt; (Rhodophyta)

Absorption and fluorescence spectra, photosynthetic rates, pigment concentrations, and cell size were investigated during photoacclimation of Porphyra leucosticta, a red marine macroalga that thrives during winter in the eastern Mediterranean. Optical properties were measured on intact thalli using image analysis or from crude extracts after disrupting algal tissue. Absorption spectra from crude extracts showed major peaks at 485, 620, and 664 nm for chlorophyll a (chl a) and 500, 564 and 615 nm for phycoerythrin (PE) and phycocyanin (PC). Absorption peaks from intact thalli were less conspicuous than those from crude extracts, possibly due to PE contributing to the bulk absorption of light by chl a in intact cells. Fluorescence emission of intact thalli was about threefold higher than that of crude extracts at 420 and 560 nm. Image analysis revealed differences in maximal fluorescence emissions as related to cell size, cell age, and thallus functional areas, with female cells having nearly a twofold high...

Albedo Measurements and Optical Sizing of Single Aerosol Particles

Aerosols play an important role in global climate change by their interactions with incoming solar radiation and outgoing longwave radiation from the planetary surface. The climate effects of aerosols depend on their scattering and absorption properties. This article describes the development of an instrument (ASTER: Aerosol Scattering To Extinction Ratio) that simultaneously measures the scattering and extinction of single aerosol particles. ASTER uses a high-Q cavity to amplify the extinction signal and innovative optics to collect the scattered light. It can distinguish many partially absorbing particles from a few black ones even if the bulk absorption is the same. Optical sizing and single-scattering albedo measurements were made for laboratory-generated particles with diameters from about 300 nanometers to above one micrometer. Using this prototype instrument, changes in albedo for single particles of 20% or greater were detected by measurement of the scattering and extinction. Optical sizing of the...

Electronic energy transfer on CaO surfaces

We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range from 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following electronic energy transfer from higher coordinated surface sites.

Penetration mechanisms of surface-adsorbed hydrogen atoms into bulk metals: Experiment and model

The conditions enabling the transition of surface-adsorbed hydrogen atoms into bulk metals are explored by comparing the response of chemisorbed H on Pd(100) and Ti(0001) single crystals to thermal activation in vacuum. Thermal desorption spectroscopy and $^{1}\text{H}(^{15}\text{N},\ensuremath{\alpha}\ensuremath{\gamma})^{12}\text{C}$ nuclear reaction analysis reveal that heating causes ${\text{H}}_{2}$ desorption from Pd(100), whereas H atoms on Ti(0001) reversibly exchange between the surface and the Ti bulk with negligible desorption loss of ${\text{H}}_{2}$. A general model is proposed in which the competition between desorption and bulk absorption of surface hydrogen is, on one hand, kinetically determined by the activation energy for associative ${\text{H}}_{2}$ desorption relative to the effective energy barrier for H absorption. On the other hand, the thermodynamic possibility to dissolve the surface H atoms into the metal must be considered to consistently explain the opposite behavior of H on Pd(100) and Ti(0001). The first experimental estimation of the energy of surface adsorption, ${\ensuremath{\epsilon}}_{s}=\ensuremath{-}0.92\text{ }\text{eV}$, for H/Ti(0001) is presented, in good agreement with theoretical calculations.

Chemical Imaging of the Surface of Self-Assembled Polystyrene-b-Poly(methyl methacrylate) Diblock Copolymer Films Using Apertureless Near-Field IR Microscopy

The nanoscale chemical composition variations of the surfaces of thin films of polystyrene- b-poly(methyl methacrylate) (PS- b-PMMA) diblock copolymers are investigated using apertureless near-field IR microscopy. The scattering of the incident infrared beam from a modulated atomic force microscopy (AFM) tip is probed using homodyne detection and demodulation at the tip oscillation frequency. An increase in the IR attenuation is observed in the PMMA-rich domains with a wavenumber dependence that is consistent with the bulk absorption spectrum. The results indicate that even though a small topography-induced artifact can be observed in the near-field images, the chemical signature of the sample is detected clearly.

Porous polymer fibers for low-loss Terahertz guiding

We propose two designs of effectively single mode porous polymer fibers for low-loss guiding of terahertz radiation. First, we present a fiber of several wavelengths in diameter containing an array of sub-wavelength holes separated by sub-wavelength material veins. Second, we detail a large diameter hollow core photonic bandgap Bragg fiber made of solid film layers suspended in air by a network of circular bridges. Numerical simulations of radiation, absorption and bending losses are presented; strategies for the experimental realization of both fibers are suggested. Emphasis is put on the optimization of the fiber geometries to increase the fraction of power guided in the air inside of the fiber, thereby alleviating the effects of material absorption and interaction with the environment. Total fiber loss of less than 10 dB/m, bending radii as tight as 3 cm, and fiber bandwidth of approximately 1 THz is predicted for the porous fibers with sub-wavelength holes. Performance of this fiber type is also compared to that of the equivalent sub-wavelength rod-in-the-air fiber with a conclusion that suggested porous fibers outperform considerably the rod-in-the-air fiber designs. For the porous Bragg fibers total loss of less than 5 dB/m, bending radii as tight as 12 cm, and fiber bandwidth of approximately 0.1 THz are predicted. oupling to the surface states of a multilayer reflector facilitated by the material bridges is determined as primary mechanism responsible for the reduction of the bandwidth of a porous Bragg fiber. In all the simulations, polymer fiber material is assumed to be Teflon with bulk absorption loss of 130 dB/m.

Effect of Density and Surface Roughness on Optical Properties of Silicon Carbide Optical Components

The effect of density and surface roughness on the optical properties of silicon carbide optical components is investigated. The density is the major factor of the total reflectance while the surface roughness is the major factor of the diffuse reflectance. The specular reflectance of silicon carbide optical components can be improved by increasing the density and decreasing the surface roughness, in the form of reducing bulk absorption and surface-related scattering, respectively. The contribution of the surface roughness to the specular reflectance is much greater than that of the density. When the rms surface roughness decreases to 2.228 nm, the specular reflectance decreases to less than 0.7% accordingly.

Reaction of molecular and atomic deuterium with methanol on Pd(111)

Multiplexed thermal desorption spectroscopy was used to study the influence of preadsorbed and postadsorbed deuterium on a methanol monolayer on Pd(111). In particular, the effect of atomic deuterium exposure on the reaction products was examined in detail. Preadsorbed deuterium hinders the formation of chemisorbed methanol and favors physisorption of methanol. This leads to a strong reduction of methanol dehydrogenation during sample heating. Postadsorption of molecular deuterium also changes partially chemisorbed to physisorbed methanol. No deuteration of methanol is found under these conditions. With atomic deuterium exposure, however, significant amounts of deuterated methanol, from methanol-D1 through methanol-D4, can be produced. In addition, D exposure also causes an increased dehydrogenation probability during sample heating. The probabilities for methanol deuteration, for methanol dehydrogenation, and for D-atom bulk absorption have been determined quantitatively.

Low loss porous terahertz fibers containing multiple subwavelength holes

We propose a porous polymer terahertz fiber with a core composed of a hexagonal array of subwavelength air holes. Numerical simulations show that the larger part of guided power propagates inside the air holes within the fiber core, resulting in suppression of the bulk absorption losses of the core material by a factor of ∼10–20. Confinement of terahertz power in the subwavelength holes greatly reduces effective refractive index of the guided mode but not as much as to considerably increase modal radiation losses due to bending. As a result, tight bends of several centimeter bending radii can be tolerated.

Absolute transmitted light plethysmography for assessment of dental pulp vitality through quantification of pulp chamber hematocrit by a three-layer model

After confirming that the gingival circulation had little effect on transmitted light plethysmography measurement in the upper central incisor in both in vivo experiments and numerical Monte Carlo simulation studies, a three-layer model comprising of a pulp chamber sandwiched between two dentin layers has been introduced to quantify the pulp chamber hematocrit (Hctp) from the measured optical density. Two-flux theory was utilized to derive a mathematical equation for transmitted intensity in terms of tooth dimensions, Hctp, and light-source wavelength. Each layer was assumed homogeneous so as to represent its optical properties by the bulk absorption and scattering constants. The mean error between the Hctp estimate based on the three-layer-model equation and the Hctp actual in the extracted model tooth was -0.00115 with standard deviation (SD) of 0.00733 at 522 nm wavelength, while for 810 nm +0.09157 and 0.02493. The Hctp estimate of the upper central incisor in 10 young volunteers at 522 nm using the three-layer model ranged from 0.002 to 0.061 with the mean of 0.032. The Hctp change reflects blood volume shift in the pulp microcirculation to possibly indicate dental pulp vitality.

Optical properties of low temperature prepared granular titanium dioxide on a silver substrate

Nanolayered systems composed of granular titanium dioxide layers coated on silver surfaces have been fabricated through a combined sputtering/sol–gel method. It has been demonstrated that, at a low temperature and in the absence of an additional heat treatment, nanocrystallites of titanium dioxide can be obtained without aggregation effects. The near-normal reflection spectrum showed a well-defined minimum at 314 nm corresponding to the silver bulk absorption properties and another structure beyond 400 nm that is due to surface plasma absorption. An effective medium theory is used to model the optical properties of the granular dielectric coated on a silver substrate, showing a good qualitative agreement with experiment.

Growth and optical properties of bulk KTP crystals by hydrothermal method

The high-quality bulk potassium titanyl phosphate (KTiOPO4 or KTP) crystals have been grown successfully by the hydrothermal method using an improved mixed solution of K2HPO4 (2.0 mol/l), KH2PO4 (0.1 mol/l) and H2O2 (1 wt%) as mineralizer, and spontaneous flux KTP crystals as nutrients. The growing rate of KTP crystals along (0 1 1) was 0.15 mm/day. The size of the biggest grown crystal was up to 26×83×25 mm3, 132 g in weight. The transmittance was over 80% in the range of 450–2500 nm. The optical homogeneity was up to 4.15×10−6 cm−1. The precise principal refractive indices were measured by the auto-collimation method and compared with those of KTP grown by the flux method. The Sellmeir equations were fitted with the experimental data. The test of bulk absorption at strong CW green radiation 532 nm was performed with the photo-thermal common-path interferometer. The results showed that the hydrothermal KTP crystals have great ability to anti-gray track.

Bending Effects in Finite-Cladding Single-Mode Slab Waveguides

A new approach to the physical description and quantification of bend loss in arbitrarily bent single-mode fibers is based on the coupling of the fundamental mode to cladding modes solely due to curvature change along the fiber. Using a slab model, it is shown that there is an optimum bend design criterion based on the rate of change of curvature that can reduce bend loss to almost arbitrarily low levels regardless of curvature, provided only that the fiber coating index value is sufficiently smaller than that of the cladding index. With a finite coating, the propagation of the field around the bend is influenced by both the refractive index of the coating and its bulk absorption.