Michael F. G. Wood

Angular measurements of light scattered by turbid chiral media using linear Stokes polarimeter

The effects of turbid chiral media on light polarization are studied in different directions around the scattering samples using a refined linear Stokes polarimeter, which simplifies the signal analysis, and allows for the detailed investigations of scattered light. Because no moving parts are involved in a measurement at a specific detection direction, the determination accuracy of polarization states is increased. The results show that light depolarization increases with both turbidity and detection angle for low and moderately turbid samples; however, the angular dependence decreases with increasing turbidity. When the turbidity is increased to approximately 100 cm(-1), the depolarization becomes higher in the forward than in the backward direction. Polarization sensitive Monte Carlo simulations are used to verify some experimental observations. The results also demonstrate that surviving linear polarization fraction and overall intensity are more sensitive to the increase of glucose concentration in backward than in the forward direction in highly turbid media, indicating that backward geometry may be preferable for potential glucose detection in a biomedical context. Comparison measurements with optically inactive glycerol suggest that the refractive index matching effect, and not the chiral nature of the solute, dominates the observed optical rotation engendered by glucose in highly turbid media.

Monte Carlo study of pathlength distribution of polarized light in turbid media

Photon pathlength distributions as a function of the number of scattering events in cylindrical turbid samples are studied using a polarization-sensitive Monte Carlo model with linearly polarized light input. Sample scattering causes extensive depolarization, yielding a photon field comprised of polarized and depolarized sub-populations. It is found that the pathlength of polarization-preserving photons is distributed within a defined spatial range with strong angular dependence. This pathlength, averaged over the range, is 2-3X smaller than the one averaged over the widely-spread range of all (polarized + depolarized) collected photons. It is also demonstrated that changes in optical properties of the media affect the pathlength distributions.

Depolarization of light in turbid media: a scattering event resolved Monte Carlo study

Details of light depolarization in turbid media were investigated using polarization-sensitive Monte Carlo simulations. The surviving linear and circular polarization fractions of photons undergoing a particular number of scattering events were studied for different optical properties of the turbid media. It was found that the threshold number of photon scattering interactions that fully randomize the incident polarization (defined here as <1% surviving polarization fraction) is not a constant, but varies with the photon detection angle. Larger detection angles, close to backscattering direction, show lower full depolarization threshold number for a given set of samples optical properties. The Monte Carlo simulations also confirm that depolarization is not only controlled by the number of scattering events and detection geometry, but is also strongly influenced by other factors such as anisotropy g, medium linear birefringence, and the polarization state of the incident light.

Effects of formalin fixation on tissue optical polarization properties

Formalin fixation is a preparation method widely used in handling tissue specimens, such as biopsies, specifically in optical studies such as microscopy. In this note, we examine how formalin fixation affects the polarization properties of porcine myocardium and liver as assessed by optical polarimetry. Spatial maps of linear retardance and depolarization were derived from four myocardial and four liver samples before and after formalin fixation. Overall, linear retardance and depolarization increased after fixation for both myocardium (15% and 23% increase, respectively) and liver (38% and 51%, respectively). The relative increase in retardance was greater in liver compared to myocardium, although the absolute increase in retardance was comparable for both. The effect of fixation on bulk optical properties was also investigated for myocardium where the scattering coefficient increased from 92 to 132 cm(-1) and the absorption coefficient remained constant at 1.1 cm(-1).

Stokes polarimetry in multiply scattering chiral media: effects of experimental geometry

The spatial distribution of optical rotation alpha and surviving linear polarization fraction beta(L) of light scattered from cylindrical turbid chiral (glucose-containing) and achiral samples is studied using a linear Stokes polarimeter. alpha and beta(L) are measured in and off the incident plane as the detection angle changes from the forward to the backward direction. The experimental results exhibit a complex dependence on the detection geometry: alpha is more sensitive to glucose presence off the incident plane, whereas beta(L) exhibits larger effects in-plane, as validated by polarization sensitive Monte Carlo simulations. A rigorous methodology is presented for optimizing the experimental geometry in the polarimetric examinations of complex random systems.

Polarized light based birefringence measurements for monitoring myocardial regeneration

Myocardial infarction leads to remodeling of the myocardium, resulting in a deterioration of cardiac function. This remodeling involves changes in the extracellular matrix, particularly an increase in collagen. Recently developed stem cell based regenerative treatments have been shown to reduce myocardial remodeling and collagen formation after infarction leading to an improvement in overall cardiac function. However, this emerging field is in dire need of biomarkers to monitor the progress and success of these treatments. Collagen is a fibrous protein and exhibits birefringence due to different refractive indices parallel and perpendicular to the direction of the fibers. As a result, changes in the collagen content and organization in the myocardium should lead to changes in birefringence. Birefringence measurements were made through ex vivo myocardial tissues from rats with induced myocardial infarctions including a number that had undergone regenerative treatment with mesenchymal stem cells. Results show a decrease in birefringence from normal to infracted myocardium, indicating a decrease in tissue organization associated with scar formation, however, an increase in birefringence was seen in those myocardial tissues that had undergone regenerative treatment indicating reorganization of tissue structure. These results demonstrate promise for this technique and are motivating further work towards performing measurements in vivo.

Turbid polarimetry for tissue characterization

We have developed a novel turbid polarimetry platform for characterization of biological tissues. Currently, we are exploring the use of this platform for characterization of the extracellular matrix particularly for use in monitoring regenerative treatments of myocardial infarctions. Collagen is a fibrous protein and exhibits birefringence due to different refractive indices parallel and perpendicular to the direction of the fibers. As a result, changes in the collagen content and organization in the tissue lead to changes in birefringence. We demonstrate our ability to measure these extracellular changes in vivo using a mouse dorsal window chamber model. Collagenase was injected into a region of the chamber to denature the extracellular matrix. Birefringence measurements show a large decrease in birefringence associated with the destruction of collagen fibers. Birefringence measurements were also made through ex vivo myocardial tissues from rats with induced myocardial infarctions including a number that had undergone regenerative treatment with mesenchymal stem cells. Results show a decrease in birefringence from normal to infracted myocardium, indicating a decrease in tissue organization associated with scar formation, however, an increase in birefringence was seen in those myocardial tissues that had undergone regenerative treatment indicating reorganization of tissue structure.

Submicrometer Epsilon-Near-Zero Electroabsorption Modulators Enabled by High-Mobility Cadmium Oxide

Epsilon-near-zero materials provide a new path for tailoring light–matter interactions at the nanoscale. In this paper, we analyze a compact electroabsorption modulator based on epsilon-near-zero confinement in transparent conducting oxide films. The nonresonant modulator operates through field-effect carrier density tuning. We compare the performance of modulators composed of two different conducting oxides, namely, indium oxide (In2O3 ) and cadmium oxide (CdO), and show that better modulation performance is achieved when using high-mobility (i.e., low loss) epsilon-near-zero materials such as CdO. In particular, we show that nonresonant electroabsorption modulators with submicron lengths and greater than 5 dB extinction ratios may be achieved through the proper selection of high-mobility transparent conducting oxides, opening a path for device miniaturization and increased modulation depth.

Effects of detection geometry on polarimetric measurements of scattered light from turbid media containing optically active glucose molecules

The effects of optically turbid medium on polarization states of incident light are studied using a novel linear Stokes polarimeter. The optical rotation and surviving linear polarization fraction of light scattered from highly turbid media (scattering coefficient μs = 100 cm-1) are measured both in and off the incident plane while the detection angle changes from forward direction (0°) to backward directions (135°, 145° and 155°). The response of the optical rotation and surviving linear polarization to the presence of glucose molecules (0.06M - 0.9M) is also studied. The results show that in the absence of glucose, the scattering-induced optical rotation is zero in the incident plane for all detection angles, and increases with detection angle when measured off the incident plane. Conversely, the surviving linear polarization fraction increases with detection angle in the incident plane, and decreases when off the incident plane. Thus, when measured in the incident plane, optical rotation is least sensitive to glucose in the turbid medium, whereas the surviving linear polarization is most sensitive. For the above turbidity and glucose concentration ranges, the optimal glucose detection sensitivity using optical rotation is at 135° detection angle, 2 mm off the incident plane, while it is at 135° detection angle in the incident plane if surviving linear polarization is used as a glucose probe. This work demonstrates the complexity of polarimetry in turbid chiral media and underscores the importance of detection geometry in making and interpreting turbid polarimetry measurements.

Multivariate analysis methods for spectroscopic blood analysis

Blood tests are an essential tool in clinical medicine with the ability diagnosis or monitor various diseases and conditions; however, the complexities of these measurements currently restrict them to a laboratory setting. P&P Optica has developed and currently produces patented high performance spectrometers and is developing a spectrometer-based system for rapid reagent-free blood analysis. An important aspect of this analysis is the need to extract the analyte specific information from the measured signal such that the analyte concentrations can be determined. To this end, advanced chemometric methods are currently being investigated and have been tested using simulated spectra. A blood plasma model was used to generate Raman, near infrared, and optical rotatory dispersion spectra with glucose as the target analyte. The potential of combined chemometric techniques, where multiple spectroscopy modalities are used in a single regression model to improve the prediction ability was investigated using unfold partial least squares and multiblock partial least squares. Results show improvement in the predictions of glucose levels using the combined methods and demonstrate potential for multiblock chemometrics in spectroscopic blood analysis.