Investigation of novel optical scattering properties of micron-scale dielectric particles

This dissertation presents an examination of novel optical scattering properties of micron-scale dielectric particles with applications to light scattering spectroscopy and ultrasensitive nanoparticle detection.; Using the finite-difference time-domain (FDTD) method and Wentzel-Kramers-Brillouin (WKB) approximation, the spectral dependence of scattering cross sections of inhomogeneous and nonspherical dielectric particles is investigated. The equivalent volume-averaged scattering behavior is identified in light scattering by randomly inhomogeneous dielectric spheres, and the concept of the equiphase sphere is proposed to model light scattering by nonspherical dielectric particles. These approaches are useful for analyzing the spectral dependence of light scattering by inhomogeneous and nonspherical particles such as biological cells and nuclei. Applications of these studies include light scattering spectroscopy for probing cellular structures and morphology, and noninvasive detection of precancerous and early cancerous changes in the human epithelium.; In addition, the near-field scattering properties of dielectric microcylinders and microspheres are investigated using the FDTD method and rigorous analytical theory. In particular, physical phenomena involving localized photonic "nanojets" are explored. First, it is shown that photonic nanojets have waists as narrow as about 100 nm (smaller than the diffraction limit), and propagate over several optical wavelengths without significant diffraction. Second and even more remarkably, it is shown that a photonic nanojet can increase the backscattering of visible light by a nanometer-scale particle located within the nanojet by several orders of magnitude. Both analytical and perturbation analyses are provided which show that the resulting backscattering by the nanoparticle is proportional to the third power of its size parameter. This is a significant dimensional increase relative to the classical Rayleigh backscattering intensity, which is proportional to the sixth power of the size parameter. Potential applications of this "super-enhanced" backscattering phenomenon include visible-light detection and characterization of nanoparticles as small as clusters of a few hundred atoms. Other potential applications include manipulation and modification of similarly sized nanoparticles.