Photon Diffusion Research Articles

Noninvasive Estimation of Glycated Hemoglobin In-Vivo Based on Photon Diffusion Theory and Genetic Symbolic Regression Models.

The diagnosis and management of diabetes require frequent monitoring of blood sugar levels. Prolonged exposure of most of the monosaccharides in the bloodstream results in the glycation of hemoglobin. This glycated hemoglobin (HbA1c) based test plays an important role to avoid diabetic complications. However, noninvasive estimation of HbA1c is a very new, promising, and challenging topic in modern bioengineering scopes. The purpose of this study is to develop and verify mathematical models in order to quantify the glycated hemoglobin in-vivo percentage non-invasively. This research utilized photon diffusion theory to develop the finger models and genetic symbolic regression methods to solve the models to estimate the level of glycated hemoglobin in the blood. The validation of these models with human participants indicated a high degree of correlation (0.887 and 0.907 Pearson's r value), and high precision (2.56% and 2.96% coefficient of variation (%CV)) for transmission and reflection type noninvasive digital volume pulse-based signals. This research will be a breakthrough for the application of noninvasive HbA1c estimation.

Ultrafast carrier and phonon dynamics in thin films of bismuth telluride on a flexible substrate

Thin films of topological insulators (TIs) possess exotic nonlinear optical properties such as strong light-matter interaction, broadband spectral sensitivity, thickness-dependent tunable bandgap, higher harmonic generation, etc. Due to the presence of metallic surface states with Dirac fermions and an insulating bulk band in TI, they are projected to be a viable material for studying novel physics, resulting in exciting new properties and technologies. The peculiar electron-phonon interactions at the surface have been linked to various unexpected physical features of topological insulators. Although electron behaviour in topological insulators has been extensively investigated on non-flexible substrates, electron-phonon interactions at TI bulk and surfaces states are less well known on a flexible substrate. Because of its potential uses in wearable devices, communications, sensors, and other fields, there is a significant need for the manufacture of high performance flexible optoelectronic responses employing novel exotic materials. In this paper, we preformed ultrafast pump-probe method to explore TI (Bi2Te3) thin films on a flexible PET (polyethylene terephthalate) substrate. We studied the dynamics of Bi2Te3 thin films' hot carrier relaxation progression and coherent phonon behaviour using transient absorbance measurements. Thickness-dependent low-frequency coherent acoustical phonon oscillations are observed in 10 nm thick films, and the changes vanish for 25 nm thick films, and high-frequency optical phonon oscillations are absent in our work. Subpicosecond range of time constant for the photon excitation, diffusion, carrier thermalization, and relaxation are reported. Longer characteristics time was observed for 25 nm film as compared to 10 nm film, and its variation has been discussed here. The thickness-dependent coherent acoustic phonon oscillating in the terahertz frequency range has been experimentally calculated. The film's terahertz frequency response varies with its thickness, allowing it to be employed in future terahertz applications based on flexible topological insulator thin films.

Numerical Simulations of Convective Three-dimensional Red Supergiant Envelopes

We explore the three-dimensional properties of convective, luminous (L ≈ 104.5–105 L ⊙), hydrogen-rich envelopes of red supergiants (RSGs) based on radiation hydrodynamic simulations in spherical geometry using Athena++. These computations comprise ≈30% of the stellar volume, include gas and radiation pressure, and self-consistently track the gravitational potential for the outer ≈3M ⊙ of the simulated M ≈ 15M ⊙ stars. This work reveals a radius, R corr, around which the nature of the convection changes. For r > R corr, though still optically thick, diffusion of photons dominates the energy transport. Such a regime is well studied in less luminous stars, but in RSGs, the near- (or above-)Eddington luminosity (due to opacity enhancements at ionization transitions) leads to the unusual outcome of denser regions moving outward rather than inward. This region of the star also has a large amount of turbulent pressure, yielding a density structure much more extended than 1D stellar evolution predicts. This “halo” of material will impact predictions for both shock breakout and early lightcurves of Type IIP supernovae. Inside of R corr, we find a nearly flat entropy profile as expected in the efficient regime of mixing-length theory (MLT). Radiation pressure provides ≈1/3 of the support against gravity in this region. Our comparisons to MLT suggest a mixing length of α = 3–4, consistent with the sizes of convective plumes seen in the simulations. The temporal variability of these 3D models is mostly on the timescale of the convective plume lifetimes (≈300 days), with amplitudes consistent with those observed photometrically.

Photon Walk in Transparent Wood: Scattering and Absorption in Hierarchically Structured Materials

AbstractThe optical response of hierarchical materials is convoluted, which hinders their direct study and property control. Transparent wood (TW) is an emerging biocomposite in this category, which adds optical function to the structural properties of wood. Nano‐ and microscale inhomogeneities in composition, structure, and at interfaces strongly affect light transmission and haze. While interface manipulation can tailor TW properties, the realization of optically clear wood requires detailed understanding of light–TW interaction mechanisms. Herein is shown how material scattering and absorption coefficients can be extracted from a combination of experimental spectroscopic measurements and a photon diffusion model. Contributions from different length scales can thus be deciphered and quantified. It is shown that forward scattering dominates haze in TW, primarily caused by refractive index (RI) mismatch between the wood substrate and the polymer phase. Rayleigh scattering from the wood cell wall and absorption from residual lignin have minor effects on transmittance, but the former affects haze. Results provide guidance for material design of transparent hierarchical composites toward desired optical functionality; it is demonstrated experimentally how transmittance and haze of TW can be controlled over a broad range.

Numerical Study of Near-Infrared Light Propagation in Aqueous Alumina Suspensions Using the Steady-State Radiative Transfer Equation and Dependent Scattering Theory

Understanding light propagation in liquid phantoms, such as colloidal suspensions, involves fundamental research of near-infrared optical imaging and spectroscopy for biological tissues. Our objective is to numerically investigate light propagation in the alumina colloidal suspensions with the mean alumina particle diameter of 55 nm at the volume fraction range 1–20%. We calculated the light scattering properties using the dependent scattering theory (DST) on a length scale comparable to the optical wavelength. We calculated the steady-state radiative transfer and photon diffusion equations (RTE and PDE) using the DST results based on the finite difference method in a length scale of the mean free path. The DST calculations showed that the scattering and reduced scattering coefficients become more prominent at a higher volume fraction. The anisotropy factor is almost zero at all the volume fractions, meaning the scattering is isotropic. The comparative study of the RTE with the PDE showed that the diffusion approximation holds at the internal region with all the volume fractions and the boundary region with the volume fraction higher than 1%. Our findings suggest the usefulness of the PDE as a light propagation model for the alumina suspensions rather than the RTE, which provides accurate but complicated computation.

Repressing high‐temperature radiative heat transfer in thermal barrier coatings

AbstractPhoton diffusion in thermal barrier coatings (TBCs) significantly deteriorates the overall performance of gas turbines operating at high temperatures. This study presents the strategy of high‐temperature photon suppression, based on a ceramic composite consisting of the second component with a smaller refractive index and controlled particle size. Using the Mie theory, it is theoretically demonstrated that controlling the second component particle size closer/equal to the infrared radiation wavelength region (1–5 μm) could reduce photon diffusion. Ceramic composites comprised of 8 wt.% yttria‐stabilized zirconia (8YSZ, matrix) and corundum (second component) with different particle sizes were prepared. The total and the photon thermal conductivity of the 8YSZ/corundum composites are lower than pure 8YSZ by ∼48.9% and ∼96.4% at 1200°C, respectively. With the addition of corundum into 8YSZ, the thermal radiation transport of 8YSZ is significantly suppressed due to the photon scattering produced by the lower refractive index and proper particle size of the corundum. Besides, the fracture toughness and hardness of composites increased by ∼20% and ∼13%, respectively, compared to the 8YSZ. Composite with the corundum particles size of 1 μm displays the lowest values of total and photon thermal conductivity at high temperature.

Comparison of functional activation responses from the auditory cortex derived using multi-distance frequency domain and continuous wave near-infrared spectroscopy.

.Significance: Quantitative measurements of cerebral hemodynamic changes due to functional activation are widely accomplished with commercial continuous wave (CW-NIRS) instruments despite the availability of the more rigorous multi-distance frequency domain (FD-NIRS) approach. A direct comparison of the two approaches to functional near-infrared spectroscopy can help in the interpretation of optical data and guide implementations of diffuse optical instruments for measuring functional activation.Aim: We explore the differences between CW-NIRS and multi-distance FD-NIRS by comparing measurements of functional activation in the human auditory cortex.Approach: Functional activation of the human auditory cortex was measured using a commercial frequency domain near-infrared spectroscopy instrument for 70 dB sound pressure level broadband noise and pure tone (1000 Hz) stimuli. Changes in tissue oxygenation were calculated using the modified Beer–Lambert law (CW-NIRS approach) and the photon diffusion equation (FD-NIRS approach).Results: Changes in oxygenated hemoglobin measured with the multi-distance FD-NIRS approach were about twice as large as those measured with the CW-NIRS approach. A finite-element simulation of the functional activation problem was performed to demonstrate that tissue oxygenation changes measured with the CW-NIRS approach is more accurate than that with multi-distance FD-NIRS.Conclusions: Multi-distance FD-NIRS approaches tend to overestimate functional activation effects, in part due to partial volume effects.

Sample size effects for Lévy flight of photons in atomic vapors.

Lévy flight superdiffusion consists of random walks characterized by very long jumps that dominate the transport. However, the finite size of real samples introduces truncation of long jumps and modifies the transport properties. We measure typical Levy flight parameters for photon diffusion in atomic vapor characterized by a Voigt absorption profile. We observe the change of Lévy parameter as a function of truncation length. We associate this variation with size-dependent contributions from different spectral regions of the emission profile with the Doppler core dominating the transport for thin samples and Lorentz wings for thick samples. Monte Carlo simulations are implemented to support the interpretation of results.

Features of photon diffusion in a dispersed medium

Energy transfer by thermal radiation in a dispersed medium with a variable refractive index is discussed. This transfer can be described by a surprisingly simple diffusion equation. The process is naturally to interpret as the photon diffusion. The diffusion equation is free from strict conditions of applicability of the radiation transfer equation, which are usually not satisfied in disperse media with densely packed inhomogeneities. Quantum constraints on the value of the photon diffusion coefficient are derived. These restrictions turn out to be similar to the conditions for the applicability of geometric optics. The lower limit of the thermal conductivity coefficient is obtained, which is easier to verify in the experiment. An independent derivation of this limitation is given from considerations of symmetry and dimension.

Photon Recycling in CsPbBr3 All-Inorganic Perovskite Nanocrystals.

Photon recycling, the iterative process of re-absorption and re-emission of photons in an absorbing medium, can play an important role in the power-conversion efficiency of photovoltaic cells. To date, several studies have proposed that this process may occur in bulk or thin films of inorganic lead-halide perovskites, but conclusive proof of the occurrence and magnitude of this effect is missing. Here, we provide clear evidence and quantitative estimation of photon recycling in CsPbBr3 nanocrystal suspensions by combining measurements of steady-state and time-resolved photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. The steady-state PL shows clear spectral modifications including red shifts and quantum yield decrease, while the time-resolved measurements show prolonged PL decay and rise times. These effects grow as the nanocrystal concentration and distance traveled through the suspension increase. Monte Carlo simulations of photons diffusing through the medium and exhibiting absorption and re-emission account quantitatively for the observed trends and show that up to five re-emission cycles are involved. We thus identify 4 quantifiable measures, PL red shift, PL QY, PL decay time, and PL rise time that together all point toward repeated, energy-directed radiative transfer between nanocrystals. These results highlight the importance of photon recycling for both optical properties and photovoltaic applications of inorganic perovskite nanocrystals.

Simplified derivation of the Kompaneets equation

An isotropic electromagnetic field in a plasma of thermalized electrons undergoes changes in energy as a result of Compton scattering and an Einstein–Hopf drag force on the electrons, eventually approaching a Bose–Einstein photon distribution at the electron temperature. The rate of change of field energy due to the combined effects of Compton scattering and the drag force is shown to be described by the Kompaneets equation for photon diffusion in frequency space. A similarity is noted between this approach and Einstein's derivation of the Planck spectrum based on the recoil of atoms as they absorb and emit radiation.

Comparison of Different Wavelengths for Estimating HbA1c and SpO₂ Noninvasively Using Beer-Lambert Law and Photon Diffusion Theory Derived Models

Photoplethysmography (PPG) is a widely used method for detecting the blood volume changes in the limbs of human body using different wavelengths of light. Many physiological parameters are estimated through the use of the PPG signals in recent studies including glycated hemoglobin (HbA1c) and blood oxygenation (SpO2). However, the processes of estimating the physiological parameters are dependent on the wavelengths of light used to acquire the PPG signal. In this study, we compare wavelengths from 300nm to 1100nm for four models to estimate HbA1c and SpO2 levels with higher accuracy. These four models were constructed from Beer-Lambert and photon diffusion theories. From the analyses of 300nm to 1100nm, it is seen that the best sets of wavelengths are found to be {426nm, 721nm, 1100nm} and {426nm, 678nm, 1100nm} for two Beer-Lambert based models, respectively. On the other hand, for both of the photon diffusion-based models, the {426nm, 384nm, 1100nm} set is found to be the best performing wavelengths. Moreover, from the analysis of the six widely used wavelengths in different PPG systems, the {465nm, 525nm, 660nm} is found to be the best performing set of wavelengths that results in higher accuracy for HbA1c and SpO2 estimation using all four models.

Phenomenological Interpretations of Some Somatic Temporal and Spatial Patterns of Biophoton Emission in Humans

Biophoton emission remains controversial. The photo-genic origin of biophoton has been attributed to the oxidative stress or free radical production. However, there are considerable gaps in quantitative understanding of biophoton emission. I propose an analytical hypothesis for interpreting a few patterns of steady-state biophoton emission of human, including the dependency on age, the diurnal variation, and the geometric asymmetry associated with serious asymmetrical pathological conditions. The hypothesis is based on an alternative form of energy state, termed vivo-nergy, which is associated with only metabolically active organisms that are also under neuronal control. The hypothesis projects a decrease of the vivo-nergy in human during growth beyond puberty. The hypothesis also proposes a modification of the vivo-nergy by the phases of systematic or homeostatic physiology. The hypothesis further postulates that the deviation of the physiology-modified vivo-nergy from the pre-puberty level is deteriorated by acquired organ-specific pathological conditions. A temporal differential change of vivo-nergy is hypothesized to proportionally modulate oxidative stress that functions as the physical source of biophoton emission. The resulted steady-state diffusion of the photon emitted from a photo-genic source in a human geometry simplified as a homogeneous spherical domain is modeled by photon diffusion principles incorporating an extrapolated zero-boundary condition. The age and systematic physiology combined determines the intensity of the centered physiological steady-state photo-genic source. An acquired pathology sets both the intensity and the off-center position of the pathological steady-state photo-genic source. When the age-commemorated, physiology-commanded, and pathology-controlled modifications of the steady-state photo-genetic sources are implemented in the photon diffusion model, the photon fluence rate at the surface of the human-representing spherical domain reveals the patterns on age, the temporal variation corresponding to systematic physiology, and the geometric asymmetry associated with significant asymmetric pathological condition as reported for spontaneous biophoton emission. The hypothesis, as it provides conveniences for quantitative estimation of biophoton emission patterns, will be extended in future works towards interpreting the temporal characteristics of biophoton emission under stimulation.

Interpretation of anatomic correlates of outer retinal bands in optical coherence tomography.

By providing the sectioning capability to differentiate individual retinal layers, optical coherence tomography (OCT) is revolutionizing eye disease diagnosis and treatment evaluation. A better understanding of the hyper- and hypo-reflective bands in retinal OCT is essential for accurate interpretation of clinical outcomes. In this article, we summarize the interpretations of clinical OCT and adaptive optics (AO) OCT (AO-OCT) of the outer retina in the human eye, and briefly review OCT investigation of the outer retina in animal models. Quantitative analysis of outer retinal OCT bands is compared to established parameters of retinal histology. The literature review and comparative analysis support that both inner/outer segment (IS/OS) junction and IS ellipsoid zone nonexclusively contribute to the second band; and OS, OS tips, and retinal pigment epithelium apical processes contribute to the third band in conventional OCT. In contrast, AO-OCT might predominantly detect the IS/OS junction and OS tip signals at the second and third bands due to its improved sectioning capability and possible AO effect on the sensitivities for recording ballistic and diffusive photons from different regions of the outer retina.

A Low-mass Binary Neutron Star: Long-term Ejecta Evolution and Kilonovae with Weak Blue Emission

Abstract We study the long-term evolution of ejecta formed in a binary neutron star (NS) merger that results in a long-lived remnant NS by performing a hydrodynamics simulation with the outflow data of a numerical relativity simulation as the initial condition. At the homologously expanding phase, the total ejecta mass reaches ≈ 0.1 M ⊙ with an average velocity of ≈ 0.1 c and lanthanide fraction of ≈ 0.005. We further perform the radiative transfer simulation employing the obtained ejecta profile. We find that, contrary to a naive expectation from the large ejecta mass and low lanthanide fraction, the optical emission is not as bright as that in GW170817/AT2017gfo, while the infrared emission can be brighter. This light-curve property is attributed to preferential diffusion of photons toward the equatorial direction due to the prolate ejecta morphology; large opacity contribution of Zr, Y, and lanthanides; and low specific heating rate of the ejecta. Our results suggest that these light-curve features could be used as an indicator for the presence of a long-lived remnant NS. We also found that the bright optical emission broadly consistent with GW170817/AT2017gfo is realized for the case in which the high-velocity ejecta components in the polar region are suppressed. These results suggest that the remnant in GW170817/AT2017gfo is unlikely to be a long-lived NS but might have collapsed to a black hole within s.

Diffuse optical localization imaging for noninvasive deep brain microangiography in the NIR-II window

Fluorescence microscopy is a powerful enabling tool for biological discovery, albeit its effective penetration depth and resolving capacity are limited due to intense light scattering in living tissues. The recently introduced short-wave infrared cameras and contrast agents featuring fluorescence emission in the second near-infrared (NIR-II) window have extended the achievable penetration to about 2 mm. However, the effective spatial resolution progressively deteriorates with depth due to photon diffusion. Here we introduce diffuse optical localization imaging (DOLI) to enable super-resolution deep-tissue fluorescence microscopy beyond the limits imposed by light diffusion. The method is based on localization of flowing microdroplets encapsulating lead sulfide (PbS)-based quantum dots in a sequence of epi-fluorescence images acquired in the NIR-II spectral window. Experiments performed in tissue mimicking phantoms indicate that high-resolution detection of fluorescent particles can be preserved over 4 mm depth range, while in vivo microangiography of murine cerebral vasculature can be accomplished through intact scalp and skull. The method further enables retrieving depth information from planar fluorescence image recordings by exploiting the localized spot size. DOLI operates in a resolution-depth regime previously inaccessible with optical methods, thus massively enhancing the applicability of fluorescence-based imaging techniques.

Effective field theory of stochastic diffusion from gravity

Planar black holes in AdS have long-lived quasinormal modes which capture the physics of charge and momentum diffusion in the dual field theory. How should we characterize the effective dynamics of a probe system coupled to the conserved currents of the dual field theory? Specifically, how would such a probe record the long-lived memory of the black hole and its Hawking fluctuations? We address this question by exhibiting a universal gauge invariant framework which captures the physics of stochastic diffusion in holography: a designer scalar with a gravitational coupling governed by a single parameter, the Markovianity index. We argue that the physics of gauge and gravitational perturbations of a planar Schwarzschild-AdS black hole can be efficiently captured by such designer scalars. We demonstrate that this framework allows one to decouple, at the quadratic order, the long-lived quasinormal and Hawking modes from the short-lived ones. It furthermore provides a template for analyzing fluctuating open quantum field theories with memory. In particular, we use this set-up to analyze the diffusive Hawking photons and gravitons about a planar Schwarzschild-AdS black hole and derive the quadratic effective action that governs fluctuating hydrodynamics of the dual CFT. Along the way we also derive results relevant for probes of hyperscaling violating backgrounds at finite temperature.

Static laser speckle suppression using liquid light guides.

Static laser speckle suppression using multimode fibers has practical limitations as the technique requires an extremely long fiber to achieve an acceptable speckle contrast. An effective method based on liquid light guides was developed in this study to suppress laser speckle. In this study, a speckle simulation model of the liquid light guide was established for numerically calculating the speckle contrast without solving the analytical solution of the photon diffusion equation. The obtained simulation results were compared with the experimental results for the dependence of speckle contrast on the required length and numerical aperture with different liquid core types of liquid light guides. A speckle contrast of 12% and a speckle suppression efficiency of 5 was achieved at the end of a 2.4 m long liquid light guide. For the same fiber length, liquid light guides were found to suppress speckle more efficiently when compared to multimode fibers.

Accuracy of homogeneous models for photon diffusion in estimating neonatal cerebral hemodynamics by TD-NIRS.

We assessed the accuracy of homogenous (semi-infinite, spherical) photon diffusion models in estimating absolute hemodynamic parameters of the neonatal brain in realistic scenarios (ischemia, hyperoxygenation, and hypoventilation) from 1.5 cm interfiber distance TD NIRS measurements. Time-point-spread-functions in 29- and 44-weeks postmenstrual age head meshes were simulated by the Monte Carlo method, convoluted with a real instrument response function, and then fitted with photon diffusion models. The results show good accuracy in retrieving brain oxygen saturation, and severe underestimation of total cerebral hemoglobin, suggesting the need for more complex models of analysis or of larger interfiber distances to precisely monitor all hemodynamic parameters.

Diffuse photon remission from thick opaque media of the high absorption/scattering ratio beyond what is accountable by the Kubelka-Munk function.

The Kubelka-Munk (KM) theory of diffuse photon remission from opaque media is widely applied to quality-control processes. Recent works based on radiative transfer revealed that the KM function as the backbone parameter of the method may saturate at strong absorption to cause the KM approach to be unfit to predict the change of diffuse reflectance from the medium at strong absorption. We demonstrate by empirical means based on Monte Carlo results that diffuse photon remission from a strong-absorbing medium depends simply upon the absorption/scattering ratio when evaluated over a large area centered at the point of illumination differing in geometry from those convenient for the KM approach. Our empirical prediction gives ∼11% mean errors of the diffuse photon remission from thick opaque medium having an absorption coefficient ranging 0.001 to up to 1000 times stronger than the reduced-scattering coefficient. A slight modification to the KM function in terms of the absorption weighting and absorption-scattering coupling for use within the KM approach also noticeably improves the prediction of diffuse photon remission from thick opaque medium of strong absorption. Our empirical model and the KM approach using the modified KM function were compared against measurements from a thick opaque medium, of which the absorption coefficient was changed over four orders of magnitude.