Browsing by Subject "optical scattering"
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Item Analysis of Photonic Nanojets Formed by a Focused Incident Beam Using Vector Spherical Harmonics Expansion(2015-05-13) Dong, AotuoThe analysis of photonic nanojets formed by dielectric spheres generally assumes that the incident field is a plane wave. In this work, using vector spherical harmonics representations, we analyze the case of a more realistic incident field consisting of a focused beam formed by microscope objectives with low as well as high numerical aperture. The nanojet solution is exact with practical number of spherical harmonic terms. In contrast with a plane wave analysis, we are able to include in the analysis cases in which the sphere is at the focal plane of the focus beam and away from the focal plane. We found that the nanojet beam waist dimension is less sensitive with respect to the azimuthal angle when compare with the plane wave case. Also, by shifting the particle away from the focal plane, the nanojet beam waist can be positioned outside the particle which otherwise would be inside or at the particle surface. Different relative positions of the particle and the focused beam along the propagating axis are discussed as well as different particle size. The bwam waist can achieve 130 nm when the diameter of the particle is 2 um. The results are compared with the plane wave case.Item Ultrasound-modulated optical tomography in soft biological tissues(Texas A&M University, 2007-09-17) Sakadzic, SavaOptical imaging of soft biological tissues is highly desirable since it is nonionizing and provides sensitive contrast information which enables detection of physiological functions and abnormalities, including potentially early cancer detection. However, due to the diffusion of light, it is dificult to achieve simultaneously both good spatial resolution and good imaging depth with the pure optical imaging modalities. This work focuses on the ultrasound-modulated optical tomography - a hybrid technique which combines advantages of ultrasonic resolution and optical contrast. In this technique, focused ultrasound and optical radiation of high temporal co-herence are simultaneously applied to soft biological tissue, and the intensity of the ultrasound-modulated light is measured. This provides information about the optical properties of the tissue, spatially localized at the interaction region of the ultrasonic and electromagnetic waves. In experimental part of this work we present a novel implementation of high-resolution ultrasound-modulated optical tomography that, based on optical contrast, can image several millimeters deep into soft biological tissues. A long-cavity confocal Fabry-Perot interferometer was used to detect the ultrasound-modulated coherent light that traversed the scattering biological tissue. Using 15-MHz ultrasound, we imaged with high contrast light absorbing structures placed 3 mm below the surface of chicken breast tissue. The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120????m, respectively. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has potential for broad biomedical applications. In the theoretical part we present various methods to model interaction be-tween the ultrasonic and electromagnetic waves in optically scattering media. We first extend the existing theoretical model based on the diffusing-wave spectroscopy approach to account for anisotropic optical scattering, Brownian motion, pulsed ul-trasound, and strong correlations between the ultrasound-induced optical phase in-crements. Based on the Bethe-Salpeter equation, we further develop a more general correlation transfer equation, and subsequently a correlation diffusion equation, for ultrasound-modulated multiply scattered light. We expect these equations to be applicable to a wide spectrum of conditions in the ultrasound-modulated optical tomography of soft biological tissues.