Non-invasively focusing light into strongly scattering media, such as biological tissue, is highly desirable but challenging. Recently, ultrasonically guided wavefront shaping technologies have been developed to address this limitation. So far, the focusing resolution of most implementations has been limited by acoustic diffraction. Here, we introduce nonlinear photoacoustically guided wavefront shaping (PAWS), which achieves optical diffraction-limited focusing in scattering media. We develop an efficient dual-pulse excitation approach to generate strong nonlinear photoacoustic (PA) signals based on the Grueneisen relaxation effect. These nonlinear PA signals are used as feedback to guide iterative wavefront optimization. As a result, light is effectively focused to a single optical speckle grain on the scale of 5-7 µm, which is ~10 times smaller than the acoustic focus with an enhancement factor of ~6,000 in peak fluence. This technology has the potential to benefit many applications that desire highly confined strong optical focus in tissue.

/pdf/photoacoustically-guided-wavefront-shaping-for-enhanced-2ejc1yqvyy.pdf

Photoacoustically guided wavefront shaping for enhanced optical focusing in scattering media

An objective analysis is carried out of the matricial models representing the polarimetric properties of light and material media leading to the identification and definition of their corresponding physical quantities, using the concept of the coherency matrix. For light, cases of homogeneous and inhomogeneous wavefront are analyzed, and a model for 3D polarimetric purity is constructed. For linear passive material media, a general model is developed on the basis that any physically realizable linear transformation of Stokes vectors is equivalent to an ensemble average of passive, deterministic nondepolarizing transformations. Through this framework, the relevant physical quantities, including indices of polarimetric purity, are identified and decoupled. Some decompositions of the whole system into a set of well-defined components are considered, as well as techniques for isolating the unknown components by means of new procedures for subtracting coherency matrices. These results and methods constitute a powerful tool for analyzing and exploiting experimental and industrial polarimetry. Some particular application examples are indicated.

/pdf/polarimetric-characterization-of-light-and-media-physical-foqb2ja25u.pdf

Polarimetric characterization of light and media - Physical quantities involved in polarimetric phenomena

This tutorial-review introduces the fundamentals of polarized light interaction with biological tissues and presents some of the recent key polarization optical methods that have made possible the quantitative studies essential for biomedical diagnostics. Tissue structures and the corresponding models showing linear and circular birefringence, dichroism, and chirality are analyzed. As the basis for a quantitative description of the interaction of polarized light with tissues, the theory of polarization transfer in a random medium is used. This theory employs the modified transfer equation for Stokes parameters to predict the polarization properties of single- and multiple-scattered optical fields. The near-order of scatterers in tissues is accounted for to provide an adequate description of tissue polarization properties. Biomedical diagnostic techniques based on polarized light detection, including polarization imaging and spectroscopy, amplitude and intensity light scattering matrix measurements, and polarization-sensitive optical coherence tomography are described. Examples of biomedical applications of these techniques for early diagnostics of cataracts, detection of precancer, and prediction of skin disease are presented. The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.

https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-21/issue-7/071114/Polarized-light-interaction-with-tissues/10.1117/1.JBO.21.7.071114.pdf

Polarized light interaction with tissues

Utilization of Mueller matrix formalism to obtain optical targets depolarization and polarization properties

A product decomposition of a depolarizing Mueller matrix consisting in the sequence of five factors—a diagonal depolarizer stacked between two retarder and diattenuator pairs—is proposed. This novel “symmetric” decomposition allows for a straightforward interpretation and parameterization of an experimentally determined Mueller matrix in terms of an arrangement of polarization devices and their characteristic parameters: diattenuations, retardances, and axis azimuths. Its application is illustrated on experimentally determined Mueller matrices.

Analysis of depolarizing Mueller matrices through a symmetric decomposition.

We report on measurements in transmission of the speckle produced by scattering liquid media: diluted milk and water solutio ns of polystyrene-microspheres of different diameters. The speckle size is affected not only by scattering parameters such as the optical thickness, but also by the dimensions of the scatters. From the speckle measurement, we propose a method to differentiate media. Moreover, a calculation of the transmitted light profile by Monte Carlo simulation allowed us to get a better insight on the speckle size evolution versus scattering.

Scattering through fluids: speckle size measurement and Monte Carlo simulations close to and into the multiple scattering

This report about backscattering measure-ments of the speckle produced by strongly-scattering liquid media shows that the size of the backscattered speckle depends on scattering and anisotropy coefficients. These measurements were aimed at assessing the effects of polarization characteristics of the incident laser beam and of the scattered light on speckle size. The samples under study consisted of monodisperse polystyrene microspheres in solutions, mixtures of different sized-microspheres, milk, blood and pig skin. Such measurements of speckle size in polarization give information on strongly scattering media, allow their discrimination and enable one to characterize the undergone changes.

Backscattered speckle size as a function of polarization: influence of particle-size and -concentration

Further to the irradiation of pig with cobalt, the polarimetric properties of skin samples were experimentally investigated, and the Mueller matrices were measured at different wavelengths. Whatever the studied sample, both increase of irradiation rate and reduction of wavelengths decreased the depolarization. Finally, we demonstrate that, 70 days after irradiation, polarization measurements allow one to distinguish a healthy pig skin sample from a 15 Gy- or 20 Gy-irradiated one.

G. Le Brun

Papers

Scattering through fluids: speckle size measurement and Monte Carlo simulations close to and into the multiple scattering

Backscattered speckle size as a function of polarization: influence of particle-size and -concentration

Analysis of the depolarizing properties of irradiated pig skin

Decomposition algorithm of an experimental Mueller matrix

Mueller matrix decomposition for biological tissue analysis