Time-resolved Diffuse Reflectance Measurements Research Articles

A comparison between plausible models in layered turbid media with geometrical variations applying a Bayesian selection criterion

One possible application of Near Infrared techniques is to analyze human brain metabolic activity. Currently used models take into account the layered structure of the human head but, usually, they do not consider the non-planar surface of some of the boundaries, i.e. gray matter, which results in a much more complex structure, thus leading to more sophisticated models and longer calculation times. The main objective of this work is to determine if it is worth to replace a planar layered structure by a non-planar one. To this end we implement a Bayesian-based quantitative methodology for choosing between two competitive models describing light propagation in layered turbid media. Experiments of time-resolved diffuse reflectance measurements are performed in layered phantoms and complemented with numerical calculations. The resulting Distributions of Time of Flight of both models are compared using Bayesian model selection analysis. The non-planar interface was introduced in the simulations by a simple surface parametrization. Results suggest that, under certain conditions, a multilayer model with planar boundaries is good enough.

Defects rich g-C3N4 with mesoporous structure for efficient photocatalytic H2 production under visible light irradiation

Graphitic carbon nitride (g-C3N4) is a promising visible-light-driven photocatalyst for solar energy conversion by splitting water into H2. However, the photocatalytic performance of g-C3N4 is not sufficient because of low surface area and fast recombination of charge carriers. Here, g-C3N4 with nanosheet structure was prepared by using mixture of melamine (M) and urea (U) as the precursors for the photocatalytic H2 production from water. The use of U increased the surface area with mesoporous structure and defects of nitrogen vacancies acting as trap states in g-C3N4. The as-obtained g-C3N4-M1U2 (1 and 2 denote the weight ratios of M and U, respectively) exhibited the photocatalytic H2 production rate of 3.1 mmol g−1 h−1 under λ ≥ 400 nm light irradiation and apparent quantum efficiency (AQE) of 74% at λ = 400 nm. Single-particle photoluminescence (PL), originated from charge recombination of photogenerated charge carriers, showed longer lifetime of charge carriers in g-C3N4-M1U2 than that in g-C3N4-M. Femtosecond time-resolved diffuse reflectance measurement was performed to observe the photogenerated electrons trapping in g-C3N4-MxUy. The results show that the electron trapping by the defects as trap states occurs faster in g-C3N4-MxUy than in g-C3N4-M. It is suggested that the use of U increases the defects as electron trap states, leading efficient photocatalytic H2 production.

TiO2 mesocrystals composited with gold nanorods for highly efficient visible-NIR-photocatalytic hydrogen production

TiO2 mesocrystals (TMC) have efficient charge transport properties and long-lived charges which cause high photoconductivity and photocatalytic activity. However, TMC have no absorption in the region of visible and near-Infrared (NIR) light. The optical resonance of Au nanorods (NRs) depending mainly on their length and width (aspect ratio) can be used to design panchromatic absorbers, covering most of the useful solar spectrum. Therefore, in this article, Au NRs and TMC composites were prepared by the ligand exchange method to show highly efficient H2 production (924µmolh−1g−1) under visible-NIR light irradiation. The efficient H2 production is explained by surface plasmon resonance (SPR) of Au NRs, electron injection to TMC, efficient charge transport in TMC due to the superstructure of TMC and water reduction on TMC. Single-particle confocal fluorescence microscopy and time-resolved diffuse reflectance measurements confirmed the efficient hot electron injection and charge separation from Au NRs with strong SPR to the superstructure of TMC.

Retrieval of the optical properties of a semiinfinite compartment in a layered scattering medium by single-distance, time-resolved diffuse reflectance measurements

Functional analysis of the human brain requires methods that take the layered structure of the head into account. In this work we introduce an improved theoretical model that describes light propagation in multilayered, turbid cylinders with a infinitely thick bottom layer, which simplifies calculations and reduces computation times. Our approach was validated with Monte Carlo simulations and single distance, time-resolved experiments on a three-layered phantom, where the absorption of the deepest layer was gradually modified. We were able to retrieve both, the scattering and absorption coefficient of this layer within reasonable errors. Hereby, changes in scattering were found to have less effect on the experimental data than absorption changes, making the reliable estimation of the reduced scattering coefficient more difficult in comparison to absorption. Stability of the implemented fitting routine was thoroughly analyzed, revealing that special care is needed to obtain accurate values for the reduced scattering coefficient.

Experimental estimation of the photons visiting probability profiles in time-resolved diffuse reflectance measurement

A time-gated intensified CCD camera was applied for time-resolved imaging of light penetrating in an optically turbid medium. Spatial distributions of light penetration probability in the plane perpendicular to the axes of the source and the detector were determined at different source positions. Furthermore, visiting probability profiles of diffuse reflectance measurement were obtained by the convolution of the light penetration distributions recorded at different source positions. Experiments were carried out on homogeneous phantoms, more realistic two-layered tissue phantoms based on the human skull filled with Intralipid–ink solution and on cadavers. It was noted that the photons visiting probability profiles depend strongly on the source–detector separation, the delay between the laser pulse and the photons collection window and the complex tissue composition of the human head.

Time-Resolved Diffuse Reflectance Using Small Source-Detector Separation and Fast Single-Photon Gating

We demonstrate the feasibility of time-resolved diffuse reflectance measurements at small source-detector separations using a single-photon avalanche diode operated in time-gated mode. Photon time distributions at an interfiber distance of 2 mm were obtained on a homogeneous tissue phantom with a dynamic range of 10(6) and collecting photons at arrival times up to 4 ns. Moreover, we were able to detect a local inhomogeneity deeply buried within a diffusive medium with better spatial resolution, higher signal intensity, and same contrast of a larger (20 mm) interfiber distance. Finally, the proposed approach proved valuable to detect in vivo a task-related brain activation.

Effect of Excitation Wavelength on Ultrafast Electron–Hole Recombination in Titanium(IV) Oxide Powders Irradiated by Femtosecond Laser Pulses

Abstract Femtosecond time-resolved diffuse reflectance measurement was carried out for anatase and rutile TiO2 powder systems in air. Anomalous rapid decay in the first few picoseconds after a rise in absorption was observed in both anatase and rutile TiO2 powders when TiO2 powder was excited by light pulses of near-band-gap energy, while it was not observed in the case of over-band-gap irradiation. These results suggest that the recombination dynamics does not depend on the crystal structure, if the wavelength dependence is normalized using energy relative to their absorption edges.

A hierarchical local weighted calibration and classification approach to depth-resolved quantification in scattering media using photon time-of-flight measurements

Chemometric methods for quantitative analysis in layered scattering/absorbing paper samples are described. A radial array of time-resolved diffuse reflectance measurements have been made to obtain multi-perspective information sensitive to changing sample composition. For analysis, the stepwise multilinear regression (SMLR) method was used a basis of a hierarchical locally weighted sample calibration. Estimates of the absorption coefficient in each sample region were made either by linear regression following by binning of the result (HLRB) or by K-nearest neighbour classification analysis (HCKNN). With these approaches, the tomographic reconstruction problem may be linearized by utilizing a priori information about sample composition in upper regions to direct subsequent calibrations for lower regions. A comparison of different reconstruction methodologies is made along with their efficacy in the presence of added noise. Results demonstrate that estimations of the absorption coefficient deep within a highly scattering/absorbing sample are obtainable at greater than 50% accuracy, using the HCKNN approach. This represents a 20% improvement at all sample depths over the SMLR approach. The use of locally weighted calibrations and sample classification to constrain the solution of a tomographic reconstruction is shown as a powerful new tool for quantification in layered scattering/absorbing media.

The Limiting Role of Iodide Oxidation in cis-Os(dcb)2(CN)2/TiO2 Photoelectrochemical Cells

Porous nanostructured photoanodes prepared from colloidal solutions of DeGussa or sol−gel processed TiO2 material have been derivatized with cis-M(dcb)2(CN)2, where M = Ru(II) or Os(II) and dcb is 4,4‘-(COOH)2-2,2‘-bipyridine. When employed in a regenerative solar cell with iodide donors, cis-Ru(dcb)2(CN)2/TiO2 converts absorbed photons to electrons nearly quantitatively while cis-Os(dcb)2(CN)2/TiO2 does not. In-situ time-resolved diffuse reflectance measurements reveal a sluggish iodide oxidation rate is responsible for the low photocurrent efficiency of cis-Os(dcb)2(CN)2/TiO2. Transient absorption measurements support this conclusion and allow full wavelength characterization of the charge-transfer processes.

Experimental tests of a simple diffusion model for the estimation of scattering and absorption coefficients of turbid media from time-resolved diffuse reflectance measurements

When a picosecond light pulse is incident upon a turbid medium such as tissue, the temporal distribution of diffusely reflected and transmitted photons depends on the optical absorption and scattering properties of the medium. From diffusion theory it is possible to derive analytic expressions for the pulse shape in terms of the optical interaction coefficients of a homogeneous semi-infinite medium. Experimental tests of this simple model in tissue-simulating liquid phantoms of different geometries are presented here. The results of these tests show that, in a semi-infinite phantom, the application of the diffusion model provides estimates of the absorption and transport-scattering coefficients that are accurate to better than 10%. Comparable accuracy was also obtained with this simple model for finite slab, cylindrical, and spherical volumes as long as the objects were of sufficient size. For smaller volumes the absorption coefficient was overestimated because of the significant loss of photons at the bounda ries of the object.

Quantitative evaluation of diffuse reflectance spectra

A survey is given of methods for determining molar absorption coefficients and kinetic parameters from diffuse reflectance spectra and R ∞- t-plots respectively. The description of photoreactions of molecules adsorbed on light-scattering media and the procedures for calculating the quantum yields of these reactions both from transmission spectra of immersed samples and diffuse reflectance data of powdered samples, are considered in more detail. Furthermore, various methods are discussed enabling rate constants of deactivation processes to be determined from time-resolved diffuse reflectance measurements.