Studies on spin dependent momentum and intensity distribution of light in a stratified medium using optical tweezers

Pal, Debapriya (2019) Studies on spin dependent momentum and intensity distribution of light in a stratified medium using optical tweezers. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Debapriya Pal (14MS029)) 14MS029.pdf - Submitted Version Restricted to Repository staff only Download (6MB)

Abstract

Spin-orbit interaction of light couples the spin and trajectory of motion in spinning particles. The e↵ect of SOI - and thus the accompanying Berry phases - is to bring about an interdependence of trajectory and polarization (spin). We study the interesting e↵ects of the spin orbit interaction (SOI) of light in tightly focused light beams in optical tweezers designed with a stratified medium in the path of the trapping light beam. The stratified medium is designed by constructing sample chambers with microscope cover slips that are refractive index (RI) mismatched with the immersion oil of the tweezers microscope objective lens. While focusing a Gaussian beam tightly, we study the occurrences of diverse phenomena dealing with the formation of lobes in the radial and axial direction with respect to beam propagation with input linear polarized light. We experimentally verify our predictions for trapping at di↵erent locations along the longitudinal or axial direction of beam propagation. Furthermore we then proceed to demonstrate by a simple calculation that tight focusing of a circularly polarized Gaussian beam in optical tweezers leads to spin-momentum locking, where the transverse spin angular momentum density is rendered independent of helicity, while the transverse Poynting vector becomes helicity dependent. Our simulations also reveal that the presence of a stratified medium in the path of the trapping beam can significantly enhance the magnitude of transverse spin angular momentum and transverse Poynting vector. We finally verify our predictions by observing interesting rotational motion resembling nutation of single trapped elongated dielectric particles due to interaction with a tightly focused circularly polarized light propagating through a stratified medium. Our experiment is possibly the first experimental demonstration of the e↵ects of the elusive Belinfante spin in propagating light beams manifested in the spin-dependent rotation of a single particle trapped under optical tweezers.

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