Two-color excitation fluorescence microscopy through highly scattering media

We study the performance of two-color excitation (2CE) fluorescence microscopy [Opt. Lett. 24, 1505 (1999)] in turbid media of different densities and anisotropy. Excitation is achieved with two confocal excitation beams of wavelengths lambda(1) and lambda(2), which are separated by an angular displacement theta, where lambda(1) not equal lambda(2), 1/lambda(e) = 1/lambda(1) + 1/lambda(2), and lambda(e) is the single-photon excitation wavelength of the sample. 2CE fluorescence is generated only in regions of the sample where the two excitation beams overlap. The 2CE fluorescence intensity is proportional to the product of the two excitation intensities and could be detected with a large-area photodetector. The requirement of spatiotemporal simultaneity for the two excitation beams makes 2CE fluorescence imaging a promising tool for observing microscopic objects in a highly scattering medium. Optical scattering asymmetrically broadens the excitation point-spread function and toward the side of the focusing lens that leads to the contrast deterioration of the fluorescence image in single- or two-photon (lambda(1) = lambda(2)) excitation. Image degradation is caused by the decrease in the excitation energy density at the geometrical focus and by the increase in background fluorescence from the out-of-focus planes. In a beam configuration with theta not equal 0, 2CE fluorescence imaging is robust against the deleterious effects of scattering on the excitation-beam distribution. Scattering only decreases the available energy density at the geometrical focus and does not increase the background noise. For both isotropic and anisotropic scattering media the performance of 2CE imaging is studied with a Monte Carlo simulation for theta = 0, pi/2, and pi, and at different h/d(s) values where h is the scattering depth and d(s) is the mean-free path of the scattering medium.     

Scattering in twin-cavity narrow-band interference filters illuminated by a monochromatic beam

The scattering behavior of the all-dielectric twin-cavity narrow-band interference filter is studied both in theory and in experiment in two cases, l(1) = l(2) and l (1) not equal l(2), where l(1) and l(2) are the optical thicknesses of the two cavities. It has been shown that the scattering properties are determined mainly by the spacers in which the electric-field intensities are large because of the presence of large standing-wave fields. The scattered light cones are found on both sides of the filter illuminated by a monochromatic light of which the wavelength (lambda(L)) is shorter than the peak wavelength (lambda(0)?) of the filter. The scattering angle of each cone is equal to the tilted angle of the filter when the peak wavelength of the filter shifts to the illumination wavelength. For the case l(1) not equal l(2), the distributions of the scattered light on both sides of the filter are quite different. The analytical calculations are in good agreement with experimental results. The possible applications of scattering in the twin-cavity filter in determining the bandwidth of the peak transmittance and the optical thicknesses of two spacers are addressed.     

Use of the delta-P1 approximation for recovery of optical absorption, scattering, and asymmetry coefficients in turbid media

We introduce a robust method to recover optical absorption, reduced scattering, and single-scattering asymmetry coefficients (microa, micro's, g1) of infinite turbid media over a range of (micro's/microa) spanning 3 orders of magnitude. This is accomplished through the spatially resolved measurement of irradiance at source-detector separations spanning 0.25-8 transport mean free paths (l*). These measurements are rapidly processed by a multistaged nonlinear optimization algorithm in which the measured irradiances are compared with predictions given by the delta-P1 variant of the diffusion approximation to the Boltzmann transport equation. The ability of the delta-P1 model to accurately describe radiative transport within media of arbitrary albedo and on spatial scales comparable to l* is the key element enabling the separation of g1 from micro's.     

Modeling low-coherence enhanced backscattering using Monte Carlo simulation

Constructive interference between coherent waves traveling time-reversed paths in a random medium gives rise to the enhancement of light scattering observed in directions close to backscattering. This phenomenon is known as enhanced backscattering (EBS). According to diffusion theory, the angular width of an EBS cone is proportional to the ratio of the wavelength of light lambda to the transport mean-free-path length l(s)* of a random medium. In biological media a large l(s)* approximately 0.5-2 mm > lambda results in an extremely small (approximately 0.001 degrees ) angular width of the EBS cone, making the experimental observation of such narrow peaks difficult. Recently, the feasibility of observing EBS under low spatial coherence illumination (spatial coherence length Lsc < l(s)*) was demonstrated. Low spatial coherence behaves as a spatial filter rejecting longer path lengths and thus resulting in an increase of more than 100 times in the angular width of low coherence EBS (LEBS) cones. However, a conventional diffusion approximation-based model of EBS has not been able to explain such a dramatic increase in LEBS width. We present a photon random walk model of LEBS by using Monte Carlo simulation to elucidate the mechanism accounting for the unprecedented broadening of the LEBS peaks. Typically, the exit angles of the scattered photons are not considered in modeling EBS in the diffusion regime. We show that small exit angles are highly sensitive to low-order scattering, which is crucial for accurate modeling of LEBS. Our results show that the predictions of the model are in excellent agreement with the experimental data.     

Effects of the Experimental Geometry on the Vortex Dynamics in High-T c, Superconductors

A brief overview is given on the macroscopic electrodynamics of type-II superconductors with and without vortex pinning, of various shapes and in an applied magnetic field, but also with applied transport current and with finite London penetration depth . The extension to >0 is important at low inductions B to describe the transition to the Meissner state, and for films with thickness not much larger than . The finite width of the surface layer with screening currents and the correct dc and ac responses in various geometries follow from an equation of motion for the current density, with a dependent integral kernel.     

Influence of size parameter and refractive index of the scatterer on polarization-gated optical imaging through turbid media

The influence of incident polarized light, refractive index, and size parameter of the scatterer on achievable resolution and contrast (image quality) of polarization-gated transillumination imaging in turbid media is reported here. Differential polarization detection led to significant improvement of image quality of an object embedded in a medium of small-sized scatterers (diameter Dor=lambda,g>or=0.7), the improvement in image quality was less pronounced using either linear or circular polarization gating when the refractive index of the scatterer was high (ns=1.59), but for a lower value of refractive index (ns=1.37), image quality improved with the differential circular polarization gating. We offer a plausible explanation for these observations.     

Comparison of the ocean inherent optical properties obtained from measurements and inverse modeling

A model developed recently by Loisel and Stramski [Appl. Opt. 39, 3001-3011 (2000)] for estimating the spectral absorption a(lambda), scattering b(lambda), and backscattering b(b)(lambda) coefficients in the upper ocean from the irradiance reflectance just beneath the sea surface R(lambda, z = 0(-)) and the diffuse attenuation of downwelling irradiance within the surface layer ?K(d)(lambda)?(1) is compared with measurements. Field data for this comparison were collected in different areas including off-shore and near-shore waters off southern California and around Europe. The a(lambda) and b(b)(lambda) values predicted by the model in the blue-green spectral region show generally good agreement with measurements that covered a broad range of conditions from clear oligotrophic waters to turbid coastal waters affected by river discharge. The agreement is still good if the model estimates of a(lambda) and b(b)(lambda) are based on R(lambda, z = 0(-)) used as the only input to the model available from measurements [as opposed to both R(lambda, z = 0(-)) and ?K(d)(lambda)?(1) being measured]. This particular mode of operation of the model is relevant to ocean-color remote-sensing applications. In contrast to a(lambda) and b(b)(lambda) the comparison between the modeled and the measured b(lambda) shows large discrepancies. These discrepancies are most likely attributable to significant variations in the scattering phase function of suspended particulate matter, which were not included in the development of the model.     

Calculation of radiative fluxes in flame power units

An engineering procedure is suggested for calculation of radiation properties of gas-dust media. It is based on using factors (attenuation, scattering factors, etc.) tabulated for discrete spectral ranges and averaged on particle size fractions. Application of the procedure is demonstrated using the example of a coal dust flame and the furnace working volume with account of ash and triatomic gases.Notation C_i mass fraction of the i-th component - _j mass fraction of the j-th particle fraction - n(r) calculated function of the particle size distribution - r particle radius - radiation wave length - diffraction parameter - m complex refraction index of the particle material - k_s attenuation, scattering, and scattering anisotropy factors for individual particles - K_s attenuation, scattering, and scattering anisotropy factors for the whole dust - averaged scattering anisotropy cosine - a_a, b_n Mie coefficients - , , volumetric attenuation, scattering, absorption factors for a dust flow - single-scattering albedo - emissivity All-Union Research and Design Institute of Metallurgical Heat Power Engineering, Nonferrous Metallurgy, and Refractory Materials, Ekaterinburg, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 3, pp. 287–291, March, 1993.     

Photon diffusion coefficient in scattering and absorbing media

We present a unified derivation of the photon diffusion coefficient for both steady-state and time-dependent transport in disordered absorbing media. The derivation is based on a modal analysis of the time-dependent radiative transfer equation. This approach confirms that the dynamic diffusion coefficient is given by the random-walk result D = cl(*)/3, where l(*) is the transport mean free path and c is the energy velocity, independent of the level of absorption. It also shows that the diffusion coefficient for steady-state transport, often used in biomedical optics, depends on absorption, in agreement with recent theoretical and experimental works. These two results resolve a recurrent controversy in light propagation and imaging in scattering media.     

Scattering delay time of Mie scatterers determined from steady-state and time-resolved optical spectroscopy

We have determined the scattering delay time of Mie scatterers (r = 255 nm quartz spheres in polyester resin) from a combination of steady-state (integrating-sphere) and time-resolved (frequency-domain) measurements performed in the multiple-scattering regime. The effective transport velocity of light was derived from intensity and phase measurements at four different wavelengths by using the time-integrated microscopic Beer-Lambert law. We could demonstrate a systematic underestimation of the effective transport velocity compared with the phase velocity in the medium. Assuming that this discrepancy was caused entirely by the transient nature of a single-scattering process, the data presented resulted in time delays of between 18 fs (lambda = 678 nm) and 177 fs (lambda = 1,064 nm) per scattering event. For three out of four wavelengths investigated, the measured values are in excellent agreement with values predicted by a theoretical model for the scattering delay time based on Mie theory.     

Coupled electron/photon Monte Carlo calculations of X-ray scattering with application to industrial radiography

Monte Carlo transport methods are used to simulate the scattering of X-rays in polystyrene and iron slabs. The calculations are made with monoenergetic X-ray sources in the energy region from 30 keV (100 keV for iron) to 20 MeV. This energy range includes the energy regions for diagnostic radiology (0.03–0.15 MeV), nuclear medicine (0.1–2.0 MeV) and industrial radiography (0.2–20 MeV). slab thicknesses for polystyrene were 53, 100, and 210 mm and for iron 7 and 14 mm. The present calculations include the effects of secondary electron/positron radiation which become quite important at high energies. As a function of the incident photon energy, the ratio of the scattered to the total radiation (scatter fraction) was found to have a characteristic ‘N’ shape. Increasing the atomic number of the scattering media has the effect of ‘squeezing the N’.     

Location of the effective diffusing-photon source in a strongly scattering medium

When a narrow laser beam illuminates a strongly scattering medium, the effective pointlike source of diffusing photons appears inside the medium. By the method worked out, which is based on measurements of the diffusive intensity of light emerging from a turbid spherical sample, the depth of this source site (the penetration depth) is determined relatively to the sample diameter, which is known accurately. By using this method of locating the effective source, we have discovered that its position inside the medium is unexpectedly deep. We obtained the penetration depth D(0) = 4.6 l* +/- 0.7 l* instead of one transport mean free path, where l* is the value of D(0) in the standard diffusion theory. Information about this source dipping is useful in diffusing-photon correlation spectroscopy because of its influence on the geometric factor calculated from the diffusion equation.     

An electron-phonon contribution to the stoner enhancement in GaMo4X8 compounds

Using the Stoner-Wohlfarth model of itinerant ferromagnetism for GaMo₄X₈ compounds, we determine the electron-phonon enhancement from their electronic specific heats. These compounds have unstable structures due to strong electron-phonon coupling. The room-temperature structure can be stabilized by doping with nonmagnetic isoelectronic impurities. The Curie temperature is decreased by a factor of three as a consequence of itinerant-electron parallel-spin scattering on nonmagnetic impurities. This property demonstrates that a long-range ferromagnetic interaction contributes to the Stoner enhancement. We speculate that the amplitude of this extra contribution is governed by the electron-phonon coupling, which depends on the mass. We are able to predict the experimental values of and the amplitude of the possible isotope effect.     

A new method for the evaluation of thermal conductivity and thermal diffusivity from transient hot strip measurements

The standard straight-line fit to data of a transient hot strip (THS) experiment to determine the thermal conductivity and thermal diffusivitya suffers from two major drawbacks: First, due to the statistical nature of the estimation procedure, there is no relation between the uncertainty of the measured value on one hand and the transport properties obtained on the other. Second, in order to account for he heat capacity of the strip and outer boundary conditions, two intervals of the plot must he rejected before analyzing it. So far, these intervals are selected arbitrarily. We now treat the THS working equation as a function of the four parameters concerned. a.U ₀ (initial voltage), andt ₀ (time delay). Chi-square fittings. following the Levenberg-Marquardt algorithm. are performed separately for several overlapping time intervals of the entire plot to find and a with minimal standard deviation. In the course of subsequent iterations an individual weighting factor is applied to each point to account for systematic errors. This procedure yields the "best" values of anda along with their individual errors. comprising the systematic and the statistical errors. Experimental results on Pyrex glass 7740 were taken to verify the new procedure.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder. Colorado, U.S.A.     

Scattering of light by a periodic structure in the presence of randomness. III. Limit of the extended matched filtering method for the detection of weak periodicities

It is known that, for scattering of light by a rough grating, the periodically occurring intensity maxima that are characteristic of the periodic part of the scattering medium get smeared off and hence become undetectable if coherence length r0 of the randomness is much smaller than wavelength A of the periodic part of the grating. It is also known that by conventional intensity measurements the detectability region is r0/lambda > or = 0.33, which can be extended to lower regions by intensity interferometry. It is established here that an extended matched filtering method enables one to detect the periodic part of the medium, even in the presence of higher roughness (i.e., lower r0), and the limit is placed at r0/lambda approximately 0.11 by our numerical experiments.     

The influence of structural properties on the dye diffusion and dyeabilty of PA 6 fibres

The relationships were investigated between the structural and dyeing properties of different structurally modified PA 6 fibers. PA 6 monofilament yarn samples were applied having different crystalline degrees and different content of alpha, respectively gamma crystalline modification. The diffusion coefficients of two different acid dyes were determined together with the content of dyestuff absorbed (under specific conditions) by fiber samples. In order to estimate colorimetry method from the viewpoint of polymer structural change detection, the colors of the dyed PA 6 samples were determined (L*, a*, b*, C*, h coordinates) and the color differences (L*, a*, b*, C*, H*, E*) were calculated between untreated standard and structurally modified samples. The dyeability of PA 6 fibers depended mainly on the way in which crystallinity had been achieved, i.e. on the material's history. Different crystalline modifications caused different sorption properties and fiber dyeability. Smaller increases (app 20%) of crystallinity degree caused a decrease in diffusion coefficients. Contrary to expectations, the significant increase of crystallinity degree (app 50%) had an influence on the increase of dyeability, presumably owing to the formation of larger empty spaces in the structure. The results achieved by colorimetry were in good correlation with dye absorption measurements. The colorimetry of dyed fiber samples was sensitive enough to detect even small differences in the quantities of absorbed dyestuff.     

Cube-like mixed-phases TiO2 mesocrystalline hollow boxes from in situ topotactic transformation for highly efficient dye-sensitized solar cells

The cube-like TiO₂ mesocrystalline hollow boxes (TiO₂-MHBs) are fabricated by a topotactic transformation method. TiO₂-MHBs consist of one-dimensional nanorod-like rutile TiO₂ combined with anatase TiO₂. The ordering of TiO₂-MHBs is in favor of efficient electron transport. TiO₂-MHBs with mesoporous structure show excellent light scattering performance. Dye-sensitized solar cell with TiO₂-MHBs light scattering overlayer exhibits an 18.3% increment of cell efficiency (9.51%) compared with the TiO₂ nanoparticles film cell (8.04%). The improved photovoltaic performance is attributed to its unique crystallographic property and highly porous structure, which can enhance light scattering capability, quicken electron transport and electrolyte diffusion, and reduce charge recombination.     

Study of rotons in superfluid4He by Raman scattering

There is a long standing disagreement between neutron and Raman scattering from rotons in superfluid⁴He near T. In neutron scattering the linewidth becomes very large and the roton signal seems to disappear at T. A substantially smaller linewidth is observed with Raman scattering and the roton signal is present even at T. We have interpreted this difference (J. Low Temp. Phys. 93, 879 (1993)) as due to a modulation of the roton energy by a fluctuating local superfluid velocity due to proliferation of vorticity as T is approached. This gives rise to an extra contribution to the roton linewidth in neutron scattering but not in Raman scattering in which two rotons with almost opposite momenta are excited. We propose a test of this explanation with evanescent field Raman scattering. It has been suggested (A. Kuklov, A. Bulatov and J.L. Birman, Phys. Rev. Lett. 72, 3855 (1994)) that in such a measurement the scattering by a single roton should be measurable. In this case the presence of a fluctuating local superfluid velocity should show up as in neutron scattering and corroborate our proposal.     

Irradiance inversion algorithm for estimating the absorption and backscattering coefficients of natural waters: Raman-scattering effects

We modify an algorithm for retrieving the absorption (a) and backscattering (b(b)) coefficient profiles in natural waters by inverting profiles of downwelling and upwelling irradiance so as to include the presence of Raman scattering. For a given wavelength of interest, lambda, the light field at the appropriate Raman excitation wavelength lambda(e) is first inverted to obtain the Raman source function at lambda. Starting from estimates of the inherent optical properties at lambda, the contribution to the irradiances at lambda from Raman scattering is then estimated and subtracted from the total irradiances to obtain the elastically scattered irradiances. We then inverted the elastically scattered irradiances to find new estimates of a and b(b) using our original method [Appl. Opt. 37, 3886 (1998)]. The algorithm then operates iteratively: The new estimates are used with the Raman source function to derive a new estimate of the Raman contribution, etc. Sample results are provided that demonstrate the working of the algorithm and show that the absorption and scattering coefficients can be retrieved with accuracies similar to those in the absence of Raman scattering down to depths at which the light field is significantly perturbed by it, e.g., with ~90% of the upwelling light field originating from Raman scattering.