Monte Carlo modeling of polarized light propagation: Stokes vs. Jones. Part I.

This bipartite comparative study aims at inspecting the similarities and differences between the Jones and Stokes-Mueller formalisms when modeling polarized light propagation with numerical simulations of the Monte Carlo type. In this first part, we review the theoretical concepts that concern light propagation and detection with both pure and partially/totally unpolarized states. The latter case involving fluctuations, or "depolarizing effects," is of special interest here: Jones and Stokes-Mueller are equally apt to model such effects and are expected to yield identical results. In a second, ensuing paper, empirical evidence is provided by means of numerical experiments, using both formalisms.

Monte Carlo modeling of polarized light propagation: Stokes vs. Jones. Part II.

In this second part of our comparative study inspecting the (dis)similarities between "Stokes" and "Jones," we present simulation results yielded by two independent Monte Carlo programs: (i) one developed in Bern with the Jones formalism and (ii) the other implemented in Ulm with the Stokes notation. The simulated polarimetric experiments involve suspensions of polystyrene spheres with varying size. Reflection and refraction at the sample/air interfaces are also considered. Both programs yield identical results when propagating pure polarization states, yet, with unpolarized illumination, second order statistical differences appear, thereby highlighting the pre-averaged nature of the Stokes parameters. This study serves as a validation for both programs and clarifies the misleading belief according to which "Jones cannot treat depolarizing effects."

Absolute measurement of molecular two-photon absorption cross-sections using a fluorescence saturation technique.

We have developed a fluorescence saturation technique for accurate measurements of the absolute molecular two-photon absorption (TPA) cross-section of fluorescent dyes. We determine the TPA crosssection both from measurements at excitation intensities well below saturation onset (in the square power-law regime) and from data obtained near the onset of saturation. The two estimates have different sensitivities to potential sources of errors. Using the square power-law regime requires calibration of the overall collection efficiency of the detection channel, including the quantum yield of the dye. In the saturation regime, the two key requirements are a good knowledge of the excitation profile and an adequate model of the two-photon excitation transition. To fulfill the former requirement, we developed diagnostic tools to characterize the tightly focussed excitation beam. To satisfy the latter requirement, we included the correct polarization dependent averaging over molecular orientations in our model. We measured the TPA cross-section of Rhodamine B (RhB) and Rhodamine 6g (Rh6g) in methanol at 798 nm for linear and circular polarization. For RhB we observed excellent agreement between the TPA cross-section estimate < sigma2 > obtained from the square power-law regime and that obtained from the saturation regime, < sigma2 >(sat). For the case of linear polarization we found: < sigma2 > = 12 +/- 2 GM and < sigma2 >(sat) = 10.5 +/- 2 GM. For the case of circular polarization we obtained: < sigma2 > = 8.4+/-2 GM and < sigma2 >(sat) = 7.5+/-2 GM. The results obtained with linear polarization are in good agreement with previously published non-linear transmission data (delta= 2sigma = 20.4 GM at 800nm). For Rh6g the difference between < sigma2 > and < sigma2 >(sat) is larger, but still considerably smaller than the variance of sigma2 values found in the literature.

Multiple irradiation sensing of the optical effective attenuation coefficient for spectral correction in handheld OA imaging.

Spectral optoacoustic (OA) imaging enables spatially-resolved measurement of blood oxygenation levels, based on the distinct optical absorption spectra of oxygenated and de-oxygenated blood. Wavelength-dependent optical attenuation in the bulk tissue, however, distorts the acquired OA spectrum and thus makes quantitative oxygenation measurements challenging. We demonstrate a correction for this spectral distortion without requiring a priori knowledge of the tissue optical properties, using the concept of multiple irradiation sensing: recording the OA signal amplitude of an absorbing structure (e.g. blood vessel), which serves as an intrinsic fluence detector, as function of irradiation position. This permits the reconstruction of the bulk effective optical attenuation coefficient µeff,λ . If performed at various irradiation wavelengths, a correction for the wavelength-dependent fluence attenuation is achieved, revealing accurate spectral information on the absorbing structures. Phantom studies were performed to show the potential of this technique for handheld clinical combined OA and ultrasound imaging.

In situ fiber-optical monitoring of cytosolic calcium in tissue explant cultures.

We present a fluorescence-lifetime based method for monitoring cell and tissue activity in situ, during cell culturing and in the presence of a strong autofluorescence background. The miniature fiber-optic probes are easily incorporated in the tight space of a cell culture chamber or in an endoscope. As a first application we monitored the cytosolic calcium levels in porcine tracheal explant cultures using the Calcium Green-5N (CG5N) indicator. Despite the simplicity of the optical setup we are able to detect changes of calcium concentration as small as 2.5 nM, with a monitoring time resolution of less than 1 s.

Determining the optical properties of a gelatin­TiO(2) phantom at 780 nm.

Tissue phantoms play a central role in validating biomedical imaging techniques. Here we employ a series of methods that aim to fully determine the optical properties, i.e., the refractive index n, absorption coefficient µ(a), transport mean free path [Formula: see text], and scattering coefficient µ(s) of a TiO(2) in gelatin phantom intended for use in optoacoustic imaging. For the determination of the key parameters µ(a) and [Formula: see text], we employ a variant of time of flight measurements, where fiber optodes are immersed into the phantom to minimize the influence of boundaries. The robustness of the method was verified with Monte Carlo simulations, where the experimentally obtained values served as input parameters for the simulations. The excellent agreement between simulations and experiments confirmed the reliability of the results. The parameters determined at 780 nm are [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]The asymmetry parameter g obtained from the parameters [Formula: see text] and [Formula: see text] is 0.93, which indicates that the scattering entities are not bare TiO(2) particles but large sparse clusters. The interaction between the scattering particles and the gelatin matrix should be taken into account when developing such phantoms.

Characterization of optical properties of ZnO nanoparticles for quantitative imaging of transdermal transport.

Widespread applications of ZnO nanoparticles (NP) in sun-blocking cosmetic products have raised safety concerns related to their potential transdermal penetration and resultant cytotoxicity. Nonlinear optical microscopy provides means for high-contrast imaging of ZnO NPs lending in vitro and in vivo assessment of the nanoparticle uptake in skin, provided their nonlinear optical properties are characterized. We report on this characterization using ZnO NP commercial product, Zinclear, mean-sized 21 nm. Two-photon action cross-section of this bandgap material (E(bg) = 3.37 eV, λ(bg) = 370 nm) measured by two techniques yielded consistent results of [Formula: see text] = 6.2 ± 0.8 µGM at 795 nm, and 32 ± 6 µGM at 770 nm per unit ZnO crystal cell, with the quantum efficiency of [Formula: see text] = (0.9 ± 0.2) %. In order to demonstrate the quantitative imaging, nonlinear optical microscopy images of the excised human skin topically treated with Zinclear were acquired and processed using [Formula: see text] and [Formula: see text]values yielding nanoparticle concentration map in skin. Accumulations of Zinclear ZnO nanoparticles were detected only on the skin surface and in skin folds reaching concentrations of 800 NPs per µm(3).