Measurement Of Optical Absorption Coefficient Research Articles

Moving Beyond Simulation: Data-Driven Quantitative Photoacoustic Imaging Using Tissue-Mimicking Phantoms.

Accurate measurement of optical absorption coefficients from photoacoustic imaging (PAI) data would enable direct mapping of molecular concentrations, providing vital clinical insight. The ill-posed nature of the problem of absorption coefficient recovery has prohibited PAI from achieving this goal in living systems due to the domain gap between simulation and experiment. To bridge this gap, we introduce a collection of experimentally well-characterised imaging phantoms and their digital twins. This first-of-a-kind phantom data set enables supervised training of a U-Net on experimental data for pixel-wise estimation of absorption coefficients. We show that training on simulated data results in artefacts and biases in the estimates, reinforcing the existence of a domain gap between simulation and experiment. Training on experimentally acquired data, however, yielded more accurate and robust estimates of optical absorption coefficients. We compare the results to fluence correction with a Monte Carlo model from reference optical properties of the materials, which yields a quantification error of approximately 20%. Application of the trained U-Nets to a blood flow phantom demonstrated spectral biases when training on simulated data, while application to a mouse model highlighted the ability of both learning-based approaches to recover the depth-dependent loss of signal intensity. We demonstrate that training on experimental phantoms can restore the correlation of signal amplitudes measured in depth. While the absolute quantification error remains high and further improvements are needed, our results highlight the promise of deep learning to advance quantitative PAI.

Intercomparison of airborne and surface-based measurements during the CLARIFY, ORACLES and LASIC field experiments

Abstract. Data are presented from intercomparisons between two research aircraft, the FAAM BAe-146 and the NASA Lockheed P3, and between the BAe-146 and the surface-based DOE (Department of Energy) ARM (Atmospheric Radiation Measurement) Mobile Facility at Ascension Island (8∘ S, 14.5∘ W; a remote island in the mid-Atlantic). These took place from 17 August to 5 September 2017, during the African biomass burning (BB) season. The primary motivation was to give confidence in the use of data from multiple platforms with which to evaluate numerical climate models. The three platforms were involved in the CLouds–Aerosol–Radiation Interaction and Forcing for Year 2017 (CLARIFY-2017), ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), and Layered Atlantic Smoke and Interactions with Clouds (LASIC) field experiments. Comparisons from flight segments on 6 d where the BAe-146 flew alongside the ARM facility on Ascension Island are presented, along with comparisons from the wing-tip-to-wing-tip flight of the P3 and BAe-146 on 18 August 2017. The intercomparison flight sampled a relatively clean atmosphere overlying a moderately polluted boundary layer, while the six fly-bys of the ARM site sampled both clean and polluted conditions 2–4 km upwind. We compare and validate characterisations of aerosol physical, chemical and optical properties as well as atmospheric radiation and cloud microphysics between platforms. We assess the performance of measurement instrumentation in the field, under conditions where sampling conditions are not as tightly controlled as in laboratory measurements where calibrations are performed. Solar radiation measurements compared well enough to permit radiative closure studies. Optical absorption coefficient measurements from all three platforms were within uncertainty limits, although absolute magnitudes were too low (<10 Mm−1) to fully support a comparison of the absorption Ångström exponents. Aerosol optical absorption measurements from airborne platforms were more comparable than aircraft-to-ground observations. Scattering coefficient observations compared adequately between airborne platforms, but agreement with ground-based measurements was worse, potentially caused by small differences in sampling conditions or actual aerosol population differences over land. Chemical composition measurements followed a similar pattern, with better comparisons between the airborne platforms. Thermodynamics, aerosol and cloud microphysical properties generally agreed given uncertainties.

Measurement of the absorption coefficient of a periodically poled LiNbO3 crystal at a radiation wavelength of 3 μm

Problem formulating. In the case of nonlinear-optical conversion of high-power laser radiation in nonlinear-optical crystals the optical absorption leads to a nonuniform heating of the crystals, which results in the violation of phase matching conditions, the formation of defects, and optical damage. Goal. Measurements of optical absorption coefficient of the periodically poled lithium niobate (PPLN) crystal at 3 μm radiation wavelength using the piezoelectric resonance laser calorimetry (PRLC). Result. The optical absorption coefficient of PPLN crystal at 3.045 μm radiation wavelength was measured using the piezoelectric resonance laser calorimetry. The value (1.15±0.15) 10⋅ -2cm-1 of absorption coefficient within the measurement error limits was independent of the initial temperature of PPLN and the incident radiation power in the ranges 293–343 K and 0.2–10 W respectively. Practical meaning. The interaction of mid-IR laser radiation with PPLN crystal was studied using the piezoelectric resonance laser calorimetry.

Nd3+ doped CAS glasses: A thermo-optical and spectroscopic investigation

Previous works have showed that calcium aluminosilicate (CAS) glasses, when prepared under vacuum conditions, are good candidates for solid state laser medium hosts and optical devices due to their appropriated thermal, optical and mechanical properties. These promising results led us to investigate the thermo-optical properties and emission spectra as a function of temperature in Nd3+ doped CAS glasses. Temperature changes in optical systems can cause structural modifications to the host, as well as other effects, such as emission quenching, or self-focalization. In this work, two series of CAS glasses, doped up to 5wt.% Nd2O3, were prepared and characterized. Measurements of thermal coefficient of optical path length (dS/dT) and emission were performed on both series of Nd3+ doped CAS. In addition, measurements of optical absorption coefficient and emission lifetime were carried out. The results are discussed in terms of temperature dependence of these properties and Nd2O3 content. Comparisons with other glasses, such as LSCAS (low-silica calcium aluminosilicate) are also presented.

Simultaneous Determination of Absorption Coefficients for Skin and Muscle Tissues Using Spatially Resolved Measurements

We present a method for simultaneous measurement of optical absorption coefficients for skin (μas) and muscle (μam) tissues using spatially resolved near-infrared spectroscopy (SRS). A novel calculation algorithm was developed to determine the absorption coefficients of superficial and deep layers within a three-layered structure using Monte Carlo simulation. A method for measuring the skin and muscle absorption coefficients was proposed based on this algorithm. In vitro experiments with tissue-like phantom and in vivo tests were performed using the SRS system with four separate detectors. The results show that the absorption coefficients for both skin and muscle tissues were obtained accurately.

Evaluation of the crystalline quality of β-Ga2O3 films by optical absorption measurements

Abstract Si doped β-Ga2O3 films were grown on Si substrate by RF magnetron sputtering. The Si concentration varied from 0% to 50%. After the deposition of the amorphous Ga2O3 on the substrate, thermal annealing at 600 °C was performed in nitrogen ambient. Polycrystalline β-Ga2O3 films were grown on Si or quartz substrates; however, other mixed phases of Si, Ga and O were not observed. From the measurement of optical absorption coefficient, it is concluded that the β-Ga2O3 energy gap increases with increasing Si concentration in the deposited film.

A time-resolved single-pass technique for measuring optical absorption coefficients of window materials under 100 GPa shock pressures

An experimental method was developed to perform time-resolved, single-pass optical absorption measurements and to determine absorption coefficients of window materials under strong shock compression up to approximately 200 GPa. Experimental details are described of (i) a configuration to generate an in situ dynamic, bright, optical source and (ii) a sample assembly with a lithium fluoride plate to essentially eliminate heat transfer from the hot radiator into the specimen and to maintain a constant optical source within the duration of the experiment. Examples of measurements of optical absorption coefficients of several initially transparent single crystal materials at high shock pressures are presented.

Combined photothermal and photoacoustic characterization of silicon–epoxy composites and the existence of a particle thermal percolation threshold

Photoacoustic (PA) and photothermal radiometric (PTR) detection were used to characterize thermal properties of silicon–epoxy composite materials in the volume range 0%< x<32 vol% ( 50 μm ). PA detection was used to study the variation of the thermal diffusivity as a function of Si volume fraction, and PTR was used to determine the influence of the electronic carrier contribution to the thermal transport with the optical properties taken into consideration. The combined PA and PTR measurements show that there exists no linear relation between thermal diffusivity and silicon volume fraction. Thermal diffusivity and optical absorption coefficient measurements can be obtained by means of combined PA and PTR measurements. Both parameters exhibit anomalous behavior in the 16% Si volume fraction range, corroborating the existence of a particle percolation threshold for three-dimensional random close packed (rcp) solids.

Thermo-mechanical and optical properties of calcium aluminosilicate glasses doped with Er 3+ and Yb 3+

In this work a series of Er 2O 3 and Yb 2O 3 doped and Er 2O 3–Yb 2O 3 co-doped low silica calcium aluminosilicate glasses have been melted at 1470°C under vacuum conditions. Measurements of optical absorption coefficient, mass density, refractive index, Vickers micro-hardness, glass transformation temperature ( T g) and crystallization temperature ( T x) have been carried out. The results showed that these glasses dissolved ∼1.5 mol% Er 2O 3 and ∼1.1 mol% Yb 2O 3 in their structure without devitrification and also that only small changes (∼10%) have been measured in their thermal, mechanical and optical properties.

Absorption and photoluminescence spectra of the diluted magnetic semiconductor Ga1−xFexSb

Bulk growth of the III–V diluted magnetic semiconductor, Ga1−xFexSb is reported for different iron concentrations. Room-temperature optical absorption coefficient measurements were performed. The dependence of the band gap Eg on the Fe concentration was studied. The band gap decreased with increasing x, and the variation was observed to follow a nonlinear trend. Photoluminescence (PL) measurements at 18 K were also carried out. The PL spectra showed a clear peak shift due to incorporation of Fe in the lattice, thus agreeing with the optical absorption results.

The influence of sample thickness on the determination of large optical absorption coefficients in liquids

The effect of the sample thickness on the measurement of optical absorption coefficients in opaque liquids has been examined in this work both experimentally and theoretically, using the method of photoacoustic spectroscopy and taking into account the decrease of the pulse amplitude ratio with increasing propagation distance. Measurements were performed in water within a narrow wavelength range for various sample thicknesses . The results are in good agreement with data obtained by other methods for samples of up to approximately 4 mm thickness. Furthermore, the conditions for the application of this scheme are outlined.

Optical Absorption in Fluorocarbon Liquids for the High Energy Stimulated Brillouin Scattering Phase Conjugation and Compression.

An experimental setup for measurement of optical absorption coefficients at 1.054 μm in high transparent liquids by means of the thermal lens effect was built, calibrated, and tested. The critical energy for defocusing of laser beam Ecr≅3 J and the absorption coefficient α of order 10-6 cm-1 were precisely measured by this device in the fluorocarbon liquids FLUORINERT FC. Obtained data were used for an optimization of the high energy Stimulated Brillouin Scattering (SBS) phase conjugated mirror and SBS compressor based on FLUORINERT FC medium.

Microstructure and optical absorption properties of Si nanocrystals fabricated with low-pressure chemical-vapor deposition

We report a simple technique for fabricating a layer of isolated Si quantum dots on SiO2 glass substrates. This technique uses conventional low-pressure chemical-vapor deposition for an extremely short deposition time in the early stage of poly-Si film growth. The layer after a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. The measurements of optical absorption coefficient α show that the absorption edge for Si nanocrystals shifts to higher energies compared to that of bulk Si, indicating a widening of the energy gap caused by quantum size effects. The linear relationship (αhν)1/2 against hν suggests that the Si nanocrystal, whose diameter is as small as 10 nm, basically maintains the properties of an indirect band-gap semiconductor. Special attention must be paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots.

Dopant incorporation efficiencies of SiC crystals grown on {1100}-face

AbstractThe dopant incorporation efficiencies of SiC bulk single crystals were investigated by the measurement of optical absorption coefficients and chemical analysis by Glow Discharge Mass Spectroscopy. The SiC crystals were grown on the seed with the {1100} face by a sublimation method. The grown ingot consisted of approximately three parts, which were the regions grown with the {1100} face (I), the {110n} face (II) and the {110n} face (III) as a growing front. The nitrogen concentrations in these parts were in the order of II&gt;I&gt;III. The wafers exhibited the color distributions in the c-plane due to the difference of the growing facet. The nitrogen incorporation efficiencies were discussed in terms of atomic structure on the surface. The aluminum concentration in the crystal grown on a {1100}-faced seed was in the medium between those in crystals grown on Si-face and C-face. On the other hand, the boron concentration was similar to that of Si-face grown crystal, and higher than that of C-face grown crystal.

Laser lithotripsy: the non-destructive measurement of calculus-surface optical absorption coefficients

Laser thermal heating in laser lithotripsy was investigated and a model was shown to be in good agreement with experimental observation. A novel non-destructive method for absorption coefficient measurement was proposed and has been used successfully for calculi-surface optical absorption coefficient measurements.

Optical absorption measurements in the soft-x-ray range

We report measurements of optical-absorption coefficients obtained by techniques not requiring free-standing films. The results of these methods are compared with data obtained by inelastic electron scattering. We find that, as expected, absorption data obtained with smoother substrates yield results with more detail in the regions with rich structure. The spectra resulting from samples evaporated onto a Si diode vacuum-UV detector with a KCI buffer layer yield results that are quite similar to the results for inelastic electron scattering.

Optical absorption coefficient of semiconductors in the extrinsic region obtained by photoconductivity measurements: application to SI GaAs

A simple method for measurements of optical absorption coefficient in the extrinsic region of semiconductors is suggested and demonstrated for photoionisation as well as intracentre transitions using standard semi-insulating GaAs and GaAs:V wafers. Also discussed is its applicability to inhomogeneous compound semiconductors.

Optical-absorption coefficient measurements in solids and liquids using correlation photoacoustic spectroscopy

The spectroscopic information related to the optical-absorption coefficients of solids and liquids, which is contained in the signal-magnitude and time-delay channels of cross-correlation photoacoustic spectroscopy (CPAS), has been investigated. Powders of holmium oxide and aqueous solutions of black India ink of variable concentrations were used as solid and liquid samples, respectively. The experimental results were found to be in general agreement with a one-dimensional theoretical model of the CPAS-signal generation.

Measurement of Optical-Absorption Coefficient in the Deletion Region of GaAs Schottky-Barrier Photodiode

Measurement of Optical-Absorption Coefficient in the Deletion Region of GaAs Schottky-Barrier Photodiode

Errors in the measurement of optical absorption coefficients

Many workers adjust the sensitivity of their spectrometers so that the signal transmitted by a crystal under investigation is set to a baseline of 100% in a region where the crystal is known to be non-absorbing, then calculate the absorption coefficient alpha using the approximate relation alpha =t-1 ln(Ib/It) where Ib and It are the baseline and transmitted light intensities, respectively, and t is the crystal thickness. In this note the author examines the validity of this technique, and shows that significant errors (>5%) are only incurred for materials with high relative permittivity (>2.3) in regions of low absorption.