A higher-order Poincaré sphere and Stokes parameter representation of the higher-order states of polarization of vector vortex beams that includes radial and azimuthal polarized cylindrical vector beams is presented. The higher-order Poincaré sphere is constructed by naturally extending the Jones vector basis of plane wave polarization in terms of optical spin angular momentum to the total optical angular momentum that includes higher dimensional orbital angular momentum. The salient properties of this representation are illustrated by its ability to describe the higher-order modes of optical fiber waveguides, more exotic vector beams, and a higher-order Pancharatnam-Berry geometric phase.
Light at wavelengths in the near-infrared (NIR) region allows for deep penetration and minimal absorption through high scattering tissue media. NIR light has been conventionally used through the first NIR optical tissue window with wavelengths from 650 to 950 nm. Longer NIR wavelengths had been overlooked due to major water absorption peaks and a lack of NIR-CCD detectors. The second NIR spectral window from 1100 to 1350 nm and a new spectral window from 1600 to 1870 nm, known as the third NIR optical window, were investigated. Optical attenuation measurements from thin tissue slices of normal and malignant breast and prostate tissues, pig brain, and chicken tissue were obtained in the spectral range from 400 to 2500 nm. Optical images of chicken tissue overlying three black wires were also obtained using the second and third spectral windows. Due to a reduction in scattering and minimal absorption, longer attenuation lengths and clearer optical images could be seen in the second and third NIR optical windows compared to the conventional first NIR optical window. A possible fourth optical window centered at 2200 nm was noted.
Near-infrared (NIR) radiation has been employed using one- and two-photon excitation of fluorescence imaging at wavelengths 650–950 nm (optical window I) for deep brain imaging; however, longer wavelengths in NIR have been overlooked due to a lack of suitable NIR-low band gap semiconductor imaging detectors and/or femtosecond laser sources. This research introduces three new optical windows in NIR and demonstrates their potential for deep brain tissue imaging. The transmittances are measured in rat brain tissue in the second (II, 1,100–1,350 nm), third (III, 1,600–1,870 nm), and fourth (IV, centered at 2,200 nm) NIR optical tissue windows. The relationship between transmission and tissue thickness is measured and compared with the theory. Due to a reduction in scattering and minimal absorption, window III is shown to be the best for deep brain imaging, and windows II and IV show similar but better potential for deep imaging than window I.
This review highlights time-resolved fluorescence kinetics and photon transport in tissues and other biomedical media with a special emphasis on ultrafast measurements of key optical parameters. Measurements of fluorescence decay lifetimes from human breast and atherosclerotic artery tissues in the uv and visible region are described after a brief description of fundamentals of fluorescence kinetics. A time-dependent diffusion model for photon migration and various ultrafast methods for time-resolved light scattering measurements to obtain key optical parameters of tissues and other model turbid media are presented. The usefulness of optical parameters as markers in optical diagnostics and imaging is considered. Time-gated measurements of ballistic and snake photons to obtain shadowgrams and an inverse numerical reconstruction of the interior map of a turbid medium from time-resolved data in the context of optical tomography are presented.
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