This PDF file contains the front matter associated with SPIE Proceedings Volume 12845, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Studying the depolarization rate of light emerging from a turbid medium holds promise for the non-invasive characterization of its single-scattering properties, with relevant application in the quality analysis of different specimens or for diagnostic purposes in the biomedical field, to name a few. However, irrespective of sample geometry, the dynamics of light depolarization takes place on a time scale of few ps, which is too fast for traditional detection methods. Here, we present experimental results on the time-domain evolution of the depolarization ratio of light that is diffusely reflected from a scattering medium, using linearly polarized fs pulses in an all-optical gating scheme. Time-resolved reflectance curves are recorded in the parallel and perpendicular polarization channels relative to the illumination beam, granting direct access to the depolarization rate. We demonstrate our experimental approach on a lipid emulsion, fitting the data with a polarized Monte Carlo simulation to retrieve the average particle size and scattering asymmetry factor using just two time-domain reflectance measurements in a semi-infinite geometry.

Mueller matrix microscopy can provide a comprehensive description of sample’s polarization properties non-invasively, and has shown attractive applications in many different fields such as biomedicine, marine particle monitoring, and material identification. Mueller matrix measurements are based on multiple light intensity measurements under illuminations of different polarization states, therefore have more error sources compared to other imaging methods, which reduce the image quality and hinder efforts for quantitatively characterizing the polarization features of weak polarization biological samples such as living cells. Measurement errors of a Mueller matrix microscope are mainly classified into three categories including systematical error, random noise, and mismatch error. In this paper, based on the physical realizability of Mueller matrix and image frequency analysis, different categories and sources of measurement errors in Mueller matrix microscopy are analyzed in detail, a no-reference Mueller matrix image quality evaluator (MIQE) is also proposed to estimate the error level of biomedical sample’s Mueller matrix.

One common approach to separatingmelanin and hemoglobin distribution from a color image is Independent Component Analysis (ICA). In this study, we propose a method based on deep learning to automatically detect suitable areas for successful facial pigmentation analysis. To do that, three deep learning models are utilized for segmentation and localization to offer a candidate region for ICA. The experiment was conducted using cross-polarized facial images selected from 200 subjects, and results showed that the deep learning-guided ICA can effectively identify regions of hyperpigmentation and successfully separate melanin and hemoglobin maps for evaluation.

Polarized light microscopy (PLM) is an established technique for the inspection of thin tooth sections in dental research. However, conventional PLM is mainly based on the qualitative evaluation of color-dependent birefringence, depolarization and transmittance but lacks a quantitative interpretation and, thus, the comparison with other polarimetric imaging methods. Here, we describe an easy to implement extension of PLM that enables measuring the degree of polarization (DOP). By replacing the analyzer and RGB camera in PLM with a monochrome polarization camera, linear polarization states can be directly determined from the transmitted light. Additionally rotating the polarizer in the illumination path and, by that, the state incident into the sample facilitates reconstructing a DOP image from multiple linear measurements. The resulting depolarization measurements are compared with conventional PLM images as well as X-ray micro-computed tomography (μCT) data of the intact teeth. Our results show that caries and demineralization appear to be directly related to depolarization in enamel. However, the interpretation is more complicated for dentin, which shows a reduced DOP also in sound tissue. We assume that these insights support the development and analysis of future dental polarimetry techniques in vivo, such as intra-oral polarization-sensitive optical coherence tomography (PS-OCT)

We have developed a new method to detect the optical rotation of glucose in the aqueous humor with high accuracy, separating the influence of the cornea with a phase difference thousands of times larger than that of glucose. Using this method and a simple experimental system simulating the eye (cornea and aqueous humor), we confirmed the detection of optical rotation of glucose at a normal glucose concentration equivalent (approximately 100 (mg/dl)) to aqueous glucose solution. We believe that this result will significantly advance the development of painless noninvasive glucose sensors.

We present the optimization and calibration of a Mueller matrix imaging polarimeter. The polarimeter is intended to be used as a compact tool for biomedical diagnosis, in particular for skin examination. The device uses a pixelated-polarization camera along with a fixed variable retarder for the polarization state analyzer, minimizing the number of elements used in the device for Full-Mueller matrix measurements. For the polarization state generator the device uses an LED as light source, and a polarizer followed by a pair of LCVRs to generate any polarization state over the Poincare sphere. With this system the polarization properties of a sample can be obtained with a total of 8 measurements to extract the 16 elements of the Mueller matrix. We study the optimization strategies by minimizing the condition number of the instrument matrix, to maximize the signal to noise ratio, and reducing the effect of experimental errors on the optimization. As a compact and transportable tool, the polarimeter will be used in different environments, so an auto-calibration method is also required. In this work we explore the necessary conditions to successfully use the Eigenvalue calibration method in a polarized-camera and liquid crystals based polarimeter. The study presented will help to measure the Mueller matrix of a sample with high accuracy and precision levels, needed for the study of biological samples.

We present the development of a subpicosecond spectropolarimeter enabling high sensitivity balanced detection of time-resolved circular dichroism (TRCD) signals from chiral sample in solution. The signals are measured with a conventional femtosecond pump-probe set-up using the combination of a quarter-waveplate and a Wollaston prism. This simple and robust method allows access to TRCD signals with improved signal-to-noise ratio and very short acquisition times. We provide a theoretical analysis of the artifacts of such detection geometry and the strategy to eliminate them. We illustrate the potential of this new detection with the study of the [Ru(phen)₃]·2PF₆ complexes in acetonitrile.

We present a method to determine the Mueller matrix of a sample using polarization-entangled photon pairs. One of the photons of a pair goes through a sample and is then subject to a polarization projection measurement. The other photon, which does not go through the sample, is also subject to a polarization projection. The measured quantum correlations are equivalent to polarimetry measurements, where the initial state of the photon going through the sample is determined by the polarization projection on the entangled partner that does not go through the sample. The correspondence with the classical system is acausal because quantum measurements apply to distinct Hilbert spaces. We tested this method with standard optical elements finding excellent agreement with the expectations. Thus it can be used as an alternative to classical Mueller polarimetry for conditions that would be challenging to do otherwise.

Polarization-sensitive optical coherence tomography (PS-OCT) has been developed to measure the depth-resolved polarization properties of biological tissues. As the polarization states transmit through tissue layers in a round-trip manner, the optic axis is affected by overlying layers of tissue in PS-OCT. In this paper, we mathematically derived the optic axis of biological tissue for PS-OCT measurements and proposed a computationally effective algorithm for the reconstruction of the optic axis. The derivation is based on the fact that the Mueller matrix of an elliptical retarder is known to be a rotation matrix that rotates at an angle of phase retardation around an axis determined by the optic axis orientation and ellipticity angle. Assuming the fibers of PS-OCT and the tissue as elliptical and cascaded linear retarders, respectively, we showed that the Stokes vectors measured by PS-OCT rotate around an axis at the angles of double-pass phase retardation for each layer of tissue. The rotation axis is the optic axis cumulatively rotated around the optic axes at the angles of single-pass phase retardations of overlaying layers. Then, an algorithm to reconstruct the optic axis from the rotation axes and angles of Stokes vectors measured from each tissue layer was proposed. The algorithm was validated by a simulation and a PSOCT measurement of 3D printer filaments stacked as a triangle. The inner angles of the triangle were measured from the en-face OCT structural image and a cross-section of the relative optic axis orientation. The difference between both measurement methods is less than 21°.

Polarimetric techniques have widely demonstrated their potential in biophotonics due to its capability to obtain relevant information from biological samples in a noninvasive and nondestructive way. Different polarimetric observables, obtained from the Mueller matrix of a sample, are used to explore the potential of these techniques in pathology detection or different biological structures classification. The physical properties of a sample related to polarization can be divided in three main groups: retardance, dichroism and depolarization. In this work, we propose the study of the polarimetric observables related to these physical properties for the identification of different structures in a biological sample by means of different pseudo-coloration methods. In particular, we study pseudo-coloration functions based on the Gaussian and Cauchy probabilistic functions. These probabilistic functions allow us to compute the probability of a given part of a sample to belong to a particular class (i.e. healthy or pathological or different structures in the same sample) where, this probability depends on the polarimetric observables obtained from the studied sample. We present a study of the different observables and methods to find the best approach for brain tissue identification (identification of gray and white matter in ex-vivo cow brain) and, which may be of interest in multiple biomedical scenarios such as early pathology detection and diagnosis or enhanced visualization of different structures for clinical applications.

Hematoxylin-eosin staining is the gold standard for anatomic pathology as the general stain for histological examination of human tissue. Birefringent property is a valuable and commonly used metric for characterizing the anisotropic structure of tissue. When assessing the birefringent property of tissue slices, unstained slice is often preferred to avoid the potential effects of staining dyes. However, obtaining high-quality adjacent tissue samples is often difficult due to possible distortion, rotation, and uneven thickness of the tissue slices during sample preparation. In this study, we measure the Mueller matrix images of the unstained, hematoxylin stained, eosin stained, and hematoxylin-eosin stained tissue slices using the transmission Mueller matrix microscope developed in our previous study. Then the Mueller matrix polar decomposition parameter of linear phase retardance is calculated to describe the linear birefringent property of tissue. It is shown that the linear birefringent property is enhanced by the combination of dye molecules and birefringent structures, which may be due to the parallel attachment of the dye molecules to the fibers. To evaluate in detail the contrast consistency of tissue linear birefringence imaging from differently stained slices, we perform a similarity analysis using the Bhattacharyya coefficient, which demonstrated that hematoxylin, eosin, and hematoxylin-eosin staining do not generate linear birefringent property on region of tissue samples without birefringent structures. Therefore, it can be acceptable to prepare hematoxylin-eosin stained slice to obtain both bright field image and linear phase retardance image simultaneously for digital pathologic diagnosis.

Polarization imaging techniques have been extensively employed in biomedical studies and clinical applications. As a comprehensive polarization representation, the Mueller matrix (MM) can provide abundant polarization-related information of biomedical samples. For quantitative information acquisition, groups of polarization basic parameters (PBPs) have been obtained through MM analysis and demonstrated as effective structural characterization tools of tissue samples. Recently, MM polarimetric endoscopy has shown great clinical diagnostic potential. For in-vivo polarimetry such as gastrointestinal MM endoscopy of human internal organ cavities, the complicated undulating tissue surface geometry delivers an inescapable occurrence of oblique incidence, which induces a prominent aberration to backscattering tissue polarimetry. Thus, quantitatively analyzing and reducing the polarimetric aberration induced by oblique incidence are crucial for in-vivo precision MM endoscopy. In this study, we quantitatively analyze the polarimetric aberration induced by tissue surface geometry on PBPs. A correlation heatmap is obtained as applicable criteria to select appropriate incident angle for different PBPs. Based on the analyzing results, we propose two aberration correction strategies of parameter selection and azimuth rotation, which are suitable for tissue samples with randomly and well-aligned fiber textures, respectively. Both strategies are demonstrated to be effective in the ex-vivo human gastric muscularis tissue experiment. The findings presented in this study can be useful to provide accurate polarization imaging results, widely applied on in-vivo polarimetric endoscopy for tissues with complicated surface topography.