Polarized light scattering from complex micro and nano scale optical systems

Saha, Sudipta (2019) Polarized light scattering from complex micro and nano scale optical systems. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sudipta Saha (12IP020)) 12IP020.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

In this thesis, we have studied some of the intriguing aspects of polarized light scattering from a diverse class of micro and nano scale optical systems. We start with the spin-orbit interaction (SOI) of light which is usually associated with inter-conversion between the longitudinal spin and orbital angular momenta (SAM and OAM) of light. Recently, it has been discovered that in addition to the conventional longitudinal angular momenta, structured (inhomogeneous) optical fields can exhibit an unusual transverse SAM, which is independent of the helicity. Moreover, such inhomogeneous fields also demonstrate helicity-dependent transverse component of Belinfante’s spin momentum. Such highly nontrivial structure of the momentum and spin densities in the structured optical fields has led to a number of fundamentally interesting and intricate phenomena, e.g., the quantum spin Hall Effect of light and the optical spin-momentum locking in surface optical modes. For structured fields, finite longitudinal components of the electric and the magnetic fields are the sources of the transverse spin momentum. Moreover, the transverse SAM appears when the longitudinal field component is shifted in phase with respect to the transverse field components. Despite this relatively candid origin of the transverse SAM and transverse spin momentum in structured optical fields, until recently these unusual angular momentum entities have not been recognized or experimentally observed. The general solution of scattering of plane waves from spherical particles also yields such phase-shifted longitudinal component of the scattered electric and magnetic fields. The question therefore arises – does scattering of plane waves produce such highly nontrivial structure of the momentum and spin densities? In order to answer this question, we have studied the SAM density and Poynting vector components for the scattered field in diverse micro and nano optical systems. We have shown that indeed scattering leads to the generation of helicity-independent transverse SAM and helicitydependent transverse momentum components. We have also shown that these fundamentally interesting quantities can be enhanced (both in magnitude and spatial extent) using the interference of different scattering modes in case of a single nano-sized plasmonic sphere and non-resonant dielectric microsphere. We have further demonstrated that scattering of plane waves from a hybrid structure (such as a metal-dielectric-metal sphere) whose dispersion curve shows a well-defined avoided crossing phenomenon may lead to significant enhancement of the transverse SAM resulting from the highly structured field at the avoided crossing. We have then studied the scattering properties of a truncated spatial Kramers-Kronig (KK) optical medium to reveal the effect of ‘non-reciprocity in reflection’ and its wave-guiding properties. We have shown that a finite slab of KK medium can support one-sided null scattering under bidirectional identical illumination. This is shown to be a direct consequence of ‘nonreciprocity in reflection’ in an asymmetric structure with loss or gain. We have then investigated the possibility of surface modes with expected local field enhancement in such a medium. We have then explored the experimental quantitative polarimetry studies and its applications in fluorescence scattering via Mueller matrix formalism. The Mueller matrix is conventionally defined for processes like reflection, refraction and elastic scattering. However for inelastic scattering (e.g. fluorescence), the Mueller matrix formalism is not explored much. We have shown that fluorescence spectroscopic Mueller matrix measurements and its inverse analysis can probe and quantify exclusive information on the molecular orientation and organization of complex fluorescent samples. The signatures of the preferential orientation and organization of the molecules manifesting as differential excitation and emission of fluorescence for orthogonal polarizations (both linear and circular polarizations) are shown to be characteristically encoded in the various elements of the fluorescence spectroscopic Mueller matrix. The corresponding information is shown to be successfully gleaned through Mueller matrix inverse analysis and quantified via a set of newly defined fluorescence anisotropy metrics, namely, fluorescence diattenuation (differential excitation of fluorescence by orthogonal polarizations) and fluorescence polarizance (differential emission of fluorescence by orthogonal polarizations). The experimental measurements and its inverse analysis are performed on various systems and information of the ‘true’ anisotropies related to the morphological and structural organizations are extracted from the recorded fluorescence Mueller matrix. These studies show considerable promise of using Mueller matrix of fluorescence scattering for characterization of different complex fluorescent samples including biological tissues. The research outputs presented in this thesis thus bring new insight on the scattering properties of light and indicate considerable promise for developing polarization (spin) based photonic devices for potential applications. It is hoped that the research will stimulate further research in the exciting fields of spin-controlled nanophotonics and polarimetry in biophotonics which is still at its infancy.

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