Changes In Absorption Coefficient Research Articles

A technique for estimating one-dimensional diffusion coefficients in low-permeability sedimentary rock using X-ray radiography: Comparison with through-diffusion measurements

The measurement of diffusive properties of low-permeability rocks is of interest to the nuclear power industry, which is considering the option of deep geologic repositories for management of radioactive waste. We present a simple, non-destructive, constant source in-diffusion method for estimating one-dimensional pore diffusion coefficients ( D p) in geologic materials based on X-ray radiography. Changes in X-ray absorption coefficient (Δ μ) are used to quantify changes in relative concentration ( C/ C 0) of an X-ray attenuating iodide tracer as the tracer solution diffuses through the rock pores. Estimated values of D p are then obtained by fitting an analytical solution to the measured concentration profiles over time. Measurements on samples before and after saturation with iodide can also be used to determine iodide-accessible porosity (ϕ I). To evaluate the radiography method, results were compared with traditional steady-state through-diffusion measurements on two rock types: shale and limestone. Values of D p of (4.8 ± 2.5) × 10 − 11 m 2·s − 1 (mean ± standard deviation) were measured for samples of Queenston Formation shale and (2.6 ± 1.0) × 10 − 11 m 2·s − 1 for samples of Cobourg Formation limestone using the radiography method. The range of results for each rock type agree well with D p values of (4.6 ± 2.0) × 10 − 11 m 2·s − 1 for shale and (3.5 ± 1.8) × 10 − 11 m 2·s − 1 for limestone, calculated from through-diffusion experiments on adjacent rock samples. Low porosity (0.01 to 0.03) and heterogeneous distribution of porosity in the Cobourg Formation may be responsible for the slightly poorer agreement between radiography and through-diffusion results for limestones. Mean values of ϕ I for shales (0.060) and limestones (0.028) were close to mean porosity measurements made on bulk samples by the independent water loss technique (0.062 and 0.020 for shales and limestones, respectively). Radiography measurements offer the advantage of time-saving for diffusion experiments because the experiment does not require steady-state conditions and also allows for visualization of the small-scale heterogeneities in diffusive properties within rocks at the mm to cm scale.

Sensitivities of the InGaAs/InGaAsP/InGaAsP Intrastep Quantum Well to Thickness and Composition Variations for Realizing Efficient Electroabsorption Modulators

We examine the sensitivities of the intrastep-quantum-well (IQW) structure with an InGaAs well to the thickness and composition variations. With the layer variations into account, we evaluate the changes in absorption coefficient as a function of electric field and the subsequent transfer functions of the electroabsorption modulator. It is concluded that the examined IQW structure is rather sensitive to the thickness variation but is robust to the composition variation. This work suggests that the thickness control is very important in actually growing the IQW structure to obtain the desired performance with the IQW.

Excitonic effects on the nonlinear optical properties of small quantum dots

The excitonic effects on the nonlinear optical properties of small quantum dots with a semiparabolic confining potential are studied under the density matrix formalism. First, within the framework of the strong confinement approximation, we present the excitonic states and then calculate the excitonic effects on the nonlinear optical properties, such as second harmonic generation, third harmonic generation, nonlinear absorption coefficient and refractive index changes. We find the explicit analytical expressions between the corresponding nonlinear optical properties with and without considering the excitonic effects. It is seen that these analytical expressions are related only to ratios of the effective masses of electron and hole. These explicit expressions indicate that the excitonic effects on the corresponding nonlinear optical properties become more important with increasing orders of the optical susceptibilities. In addition, we suggest a scaling rule for the nth-order susceptibility as γ(n+1)/2. The effect of the confining potential frequency on the corresponding nonlinear optical properties is also studied. Our results show a remarkable dependence of nonlinear optical properties on both the excitonic effects and the confining potential frequency.

Linear and nonlinear optical absorption coefficients and refractive index changes in spherical quantum dots: Effects of impurities, electric field, size, and optical intensity

In the present work, the case of a spherical quantum dot with parabolic confinement subjected to an external electric field with the presence of an impurity, the linear and third-order nonlinear optical absorption coefficients as well as refractive index changes have been calculated. The numerical method we are using for the calculation of the energy levels and the corresponding wave functions is the potential morphing method in the effective mass approximation. As our results indicate an increase of the electric field and/or the position of the impurity and/or the quantum dot radius redshifts the peak positions of the total absorption coefficient and total refractive index changes. Additionally, an increase of the position of the impurity and/or the quantum dot radius decreases the total absorption coefficient and increases the total refractive index changes. An increase also of the electric field decreases the total absorption coefficient but does not significantly affect the peak values of the total refractive index changes. Finally, an increase of the optical intensity considerably changes the total absorption coefficient as well as the total refractive index changes.

Temperature dependence of ytterbium-doped fiber amplifiers

We characterized the temperature response of a ytterbium (Yb) system by measuring its temperature-dependent emission and absorption spectra and deriving parameters characterizing transitions among Stark levels by Lorentzian curve fitting. We found that even weak transitions in the Stark levels are distinct enough for this procedure when using a linear combination of measured spectra at 0°C and 100°C. A fiber amplifier model was established to determine temperature-dependent performance of Yb-doped fiber amplifiers. We concluded that the temperature dependence of the Yb-doped fiber amplifier is mainly determined by the saturation level Pout/Ppump, when Pout/Ppump>0.4; the higher the saturation levels, the less temperature dependence. When operating a Yb-doped fiber amplifier at low saturation, the temperature dependence is mainly determined by changes in absorption and emission coefficients at the signal wavelength and is the worst when operating around the short wavelength of ~1020 nm.

Violet to deep-ultraviolet InGaN∕GaN and GaN∕AlGaN quantum structures for UV electroabsorption modulators

In this paper, we present four GaN based polar quantum structures grown on c-plane embedded in p-i-n diode architecture as a part of high-speed electroabsorption modulators for use in optical communication (free-space non-line-of-sight optical links) in the ultraviolet (UV): the first modulator incorporates ∼4–6nm thick GaN∕AlGaN quantum structures for operation in the deep-UV spectral region and the other three incorporate ∼2–3nm thick InGaN∕GaN quantum structures tuned for operation in violet to near-UV spectral region. Here, we report on the design, epitaxial growth, fabrication, and characterization of these quantum electroabsorption modulators. In reverse bias, these devices exhibit a strong electroabsorption (optical absorption coefficient change in the range of 5500–13000cm−1 with electric field swings of 40–75V∕μm) at their specific operating wavelengths. In this work, we show that these quantum electroabsorption structures hold great promise for future applications in ultraviolet optoelectronics technology such as external modulation and data coding in secure non-line-of-sight communication systems.

Measurement of laser-induced refractive index change of inverted ferroelectric domain LiNbO_3

Optical nonlinearities of periodically poled LiNbO(3) crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1 x 10(-6) cm/W and 2.6 x 10(-4) at 0.5 MW/cm(2) light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO(3). The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.

A multi-section technique for the electroabsorption measurements in waveguide semiconductor electroabsorption modulators

A technique for measuring the optical absorption spectra of waveguides, semiconductors and electroabsorption modulators is described. The technique uses a multi-sectioned optical waveguide that has electrically isolated longitudinal sections and has a p–i–n cross-section structure. Light generated in forward biased sections is used to probe the optical absorption in sections that can be reverse biased. The technique is applied to a p–i–n electroabsorption device that was designed to operate over a broadband of optical communication wavelengths. It has an active region consisting of multiple-width InGaAs–InAlGaAs quantum wells. For this device, a band edge shift of 40 meV is obtained at an applied field of 105 kV cm−1, and figures of merit Δα, Δα/F and Δα/α0 were also determined as a function of wavelength and applied field, where Δα is the absorption coefficient change due to electroabsorption, α0 static absorption coefficient of the wells at zero bias field and F the applied reverse bias field.

Acoustostimulated change in the optical absorption coefficient of gallium arsenide in the 0.81–1.77 μm wavelength range

The effect of ultrasonic waves on the spectral absorption coefficient of gallium arsenide (GaAs) in the 0.81–1.77 μm wavelength range. The treatment of GaAs single crystals with ultrasonic waves leads to a change in their coefficients of absorption of electromagnetic radiation. This phenomenon is related to the fact that an acoustic wave, being an energy carrier, modified a defect system of the crystal and produces a redistribution of impurities in the crystal lattice. The interaction of photons with acoustogenerated defects changes the optical absorption coefficient near the edge of the fundamental absorption band of GaAs.

Intraoperative monitoring of depth-dependent hemoglobin concentration changes during carotid endarterectomy by time-resolved spectroscopy

By measuring the adult human head during carotid endarterectomy, we investigate the depth sensitivity of two methods for deriving the absorption coefficient changes (Dmu(a)) from time-resolved reflectance data to absorption changes in inhomogeneous media: (1) the curve-fitting method based on the diffusion equation (DE-fit method) and (2) the time-independent calculation based on the modified Lambert-Beer law (MLB method). Remarkable differences in the determined values of Dmu(a) caused by clamping the external carotid artery and subsequently clamping the common carotid artery were observed between the methods. The DE-fit method was more sensitive to mu(a) changes in cerebral tissues, whereas the MLB method was rather sensitive to mu(a) changes in the extracerebral tissues. Our results indicated that the DE-fit was useful for monitoring the cerebral blood circulation and oxygenation during neurosurgical operations. In addition, the combined evaluation of mu(a) changes with the DE-fit and MLB methods will provide us with more available information about the hemodynamic changes in the depth direction.

Quantum yield of conversion of the photoinitiator camphorquinone

The primary absorber in dental resins is the photoinitiator, which starts the photo polymerization process. We studied the quantum yield of conversion of camphorquinone (CQ), a blue light photoinitiator, in dental resin composites using a LED lamp (3M FreeLight) and a Quartz Tungsten Halogen (QTH) lamp (VIP) as the light curing units at five different irradiances. The molar extinction coefficient, ɛ 469, of CQ was 46 ± 2 cm −1/(mol/L) at 469 nm. The reciprocity of irradiance and exposure time holds for changes of CQ absorption coefficient, that is, irradiance × exposure time (=radiant exposure) = constant. Both LED and QTH lamps yielded the same curing threshold (the radiant exposure when CQ absorption drops to 1/e) and the same quantum yield conversion under different irradiances. In our dental resin formulation (0.7 wt.% CQ with reducing agents 0.35 wt.% dimethylaminoethyl methacrylate (DMAEMA) and 0.05 wt.% butylated hydroxytoluene (BHT)) the quantum yield was measured as 0.07 ± 0.01 CQ conversion per absorbed photon.

Millisecond kinetics of photoinduced changes in the optical parameters ofa−As2S3films

We report the kinetics of photoinduced changes in absorption coefficient, optical band gap, and refractive index of $\mathrm{a}\text{\ensuremath{-}}{\mathrm{As}}_{2}{\mathrm{S}}_{3}$ films by measuring the time evolution of complete transmission spectrum between 400 and $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at every $10\phantom{\rule{0.3em}{0ex}}\mathrm{milliseconds}$. The wavelength dependence of the time constants invalidates previously assumed parallel shift of the absorption edge, and provides a better description of photodarkening and other photoinduced effects. The time evolution of the absorption coefficient and optical band gap is significantly faster than that of the refractive index. The time constants of the reversible, transient part of the changes are also wavelength dependent. We use the photon-assisted site switching model and defects around $S$ atoms to explain the observed changes.

Changes in scattering and absorption properties of esthetic filling materials after aging

Assuming that color changes after aging are related to changes in translucency of materials, the purpose of the present study was to determine the correlation between the changes in color and the changes in scattering and absorption properties after accelerated aging with representative dental esthetic restorative materials: glass ionomer, resin-modified glass ionomer, compomer, and resin composite. Color was measured according to the CIELAB color scale in the transmittance and reflectance modes and used to calculate changes in color (deltaE*(ab)), color coordinates (deltaL*, delta a*, and delta b*), translucency parameter (deltaTP), scattering coefficient (deltaS), absorption coefficient (deltaK), and light reflectivity (deltaRI) after accelerated aging. Simple correlations between each pair of the changes in optical values were calculated, and multiple regression analysis was used to determine the parameters influencing the changes in color and color coordinates (p = 0.05). In the resin composite and compomer, deltaS, deltaK, and deltaRI values were approximately zero, whereas deltaS was as high as 8.9 in the glass ionomer. For most comparisons, correlation coefficient (r) was between 0.700 and 0.997. DeltaL* was found to have a major influence on color changes, and deltaS, deltaTP, and deltaRI influenced deltaL*. Therefore, changes in scattering and absorption properties, after aging, were closely correlated with changes in color and color coordinates, especially in glass ionomer-based filling materials.

Ocean Color Reveals Phase Shift Between Marine Plants and Yellow Substance

Daily high-resolution Sea-viewing Wide Field-of-view Sensor (SeaWiFS) images of the central North Atlantic Ocean (1998-2003) show that temporal changes in the absorption coefficient of colored dissolved organic matter (CDOM) or "yellow substance" follow changes in phytoplankton pigment absorption coefficient in time. CDOM peaks (between January and March) and troughs (late summer and fall) followed pigment peaks and troughs by approximately two and four weeks, respectively. This phase shift is additional strong evidence that CDOM in the marine environment is derived from phytoplankton degradation. The common assumption of linear covariation between chlorophyll and CDOM is a simplification even in this ocean gyre. Due to the temporal changes in CDOM, chlorophyll concentration estimated based on traditional remote sensing band-ratio algorithms may be overestimated by about 10% during the spring bloom and underestimated by a similar 10% during the fall. These observations are only possible through use of synoptic, precise, accurate, and frequent measurements afforded by space-based sensors because in situ technologies cannot provide the required sensitivity or synoptic coverage to observe these natural phenomena.

Organic thin films based on a dicyanovinyl-quaterthiophene: Influence of electrode configuration on third-order nonlinear optical properties measured by electroabsorption spectroscopy

Electroabsorption (EA) studies at room temperature on organic thin films based on a dicyanovinyl-quaterthiophene 4T-V(CN) 2 are reported. An electric field modulation is applied to the samples for two different electrode geometries, i.e. sandwich and coplanar versus the organic layer. Changes in optical absorption coefficient of 4T-V(CN) 2 based thin films are measured and analyzed to determine the character of the optical transition in the visible range (400–800 nm). Depending on the experimental electrode configuration, magnitude of electroabsorption responses are different, possibly due to different distribution of the externally applied electric field. The results indicate a higher resolution of EA response for the sandwich electrode configuration and confirm the charge transfer exciton character of 4T-V(CN) 2 in contrast to the unsubstituted quaterthiophene 4T. Finally, a third-order nonlinear susceptibility χ (3) (− ω; ω, 0, 0) of 16 × 10 −12 e.s.u. is obtained.

Optical percolation and nonlinearity of sputtered Ag island films

The fractal dimension Df and the critical parameter nu of the sputtered percolation Ag island films are measured to be about 1.57 and 1.15, respectively. A modified Z-scan method including both transmittance Z-scan and reflection Z-scan is proposed to measure the pure nonlinear absorption of the semi-continuous Ag films near the percolation threshold. As the microstructure of the sputtered Ag films varies from the discontinuous to the continuous, the linear absorbance decreases from 0.97 to 0.58 while the nonlinear absorption coefficient changes from -1.4x10-5m/W to -4.6x10-6m/W at the wavelength of 800nm. The largest nonlinear absorption coefficient beta of the sputtered Ag films is obtained in the discontinuous film near the percolation threshold, which is mainly contributed by the large enhancement of local field of dis-continuous Ag particles.

Quantitative determination of gold active sites by chemisorption and by infrared measurements of adsorbed CO

Quantitative measurements of CO chemisorption in the range 140–180 K, supported by FTIR data on adsorbed CO, were performed on Au/TiO 2, Au/Fe 2O 3, and Au/CeO 2 catalysts. On the first two samples, which had similar particle size distributions, an average Au/CO chemisorption stoichiometry of about 3, referred to step-edge Au atoms, was found. On Au/CeO 2, where very small clusters and quite large particles are present, the CO-chemisorbed volume was much higher than expected, due to the prevailing contribution of very small Au clusters. On the same sample, a change in the IR absorption coefficient was observed and was reasonably explained.

The Effect of a Magnetic Field on the Absorption Coefficient of Ultrasonic Wave in Biocompatible Ferrofluid

In this paper acoustic and magnetic properties of water-based biocompatible fluid MF BSA (bovine serum albumin) are presented. The changes in the ultrasonic wave absorption coefficient show that MF had a good structural stability up to 100 kA/m. The measurements of magnetic susceptibility enabled to estimate the mean magnetic moment and mean size of magnetic particles. The obtained values are promising for future biomedical applications of such magnetic fluids.

Quantum-confined Stark effect of GaInNAs(Sb) quantum wells at 1300–1600nm

We report the measurement of electroabsorption spectra from GaInNAs and GaInNAsSb quantum wells grown on GaAs showing quantum-confined Stark effect behavior suitable for optical modulation at 1300 and 1550nm wavelength, respectively. The high quality of our material is evidenced by sharp exciton resonances with a full width at half maximum <25meV at 295K, and peak absorption coefficient of 18 000cm−1 for GaInNAs and 34 800cm−1 for GaInNAsSb. Changes in absorption coefficient 10 000cm−1 with an applied electric field were measured. Device performance from these materials is expected to be comparable to or better than the competing material grown on InP.

A Monte Carlo simulation of photon propagation in fluorescent poly(ethylene glycol) hydrogel microsensors

A Monte Carlo simulation of photon propagation through fluorophore-containing poly(ethylene glycol) hydrogel networks is described. Poly(ethylene glycol) (PEG) hydrogel microspheres are being studied for fluorescent biosensing of small molecular weight analytes such as glucose and paraoxon. A computational approach to assess the fluorescent output from these sensors was developed, in particular investigating the impact of sphere characteristics such as size, scattering, and fluorophore concentrations on sensor response. Monte Carlo simulations of both individual spheres and packed sphere beds were developed to model photon propagation through homogeneous, water-swollen PEG hydrogel microspheres with encapsulated fluorescein isothiocyanate (FITC) and tetramethylrhodamine isothiocyanate (TRITC) dyes. Experimental studies using an integrating sphere were conducted to determine the hydrogel optical properties used in the simulation and spectrofluorimetry studies performed to verify assumptions made in the model regarding quantum yield. Changes in fluorescence quantum yield, hydrogel scattering and absorption coefficients ( μ s and μ a), hydrogel scattering anisotropy ( g), and depth of the sphere bed were used to predict optimum sphere size, gel chemistry, and collection geometry.