Exploring Spin-Orbit Interaction Dynamics through the Lens of Structured Light

Kumar, Ram Nandan (2025) Exploring Spin-Orbit Interaction Dynamics through the Lens of Structured Light. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Ram Nandan Kumar (17IP025)) 17IP025.pdf - Submitted Version Restricted to Repository staff only Download (14MB)

Abstract

In this thesis, we explore the spin-orbit interaction (SOI) dynamics of light in both paraxial and non-paraxial regimes, with a particular focus on its applications in interferometry and optical tweezers. Through a combination of theoretical analysis and experimental investigations, this work advances our understanding of SOI effects and provides a foundation for potential applications in classical spin quantization, non-separability of vector-vortex beams, the optical Hall effect, optical trapping, particle manipulation, beam shaping, and the development of light-driven micromotors. In the paraxial regime, this thesis reports the development of a novel rectangular loop interferometer (RLI) that confines light within a rectangular path geometry and enables the generation of a wide variety of vector and vortex beams. By exploiting the reflection and transmission properties of optical elements (beam splitters, a combination of a half-wave plate (HWP) and a polarizing beam splitter (PBS)), the RLI computes the sum of numerous converging mathematical geometric series with values between zero and one at the speed of light, limited only by the detector bandwidth, achieving an accuracy of 90-98%. Additionally, by introducing a combination of a q-plate and HWP inside the RLI, the interferometer generates vortex beams for an input circularly polarized Gaussian beam, carrying orbital angular momentum (OAM), and its value depends on the number of times the beam is circulated into the RLI. For an input linearly polarized beam, the RLI produces higher-order vector beams with complex polarization distributions. These results demonstrate the potential of the interferometer for controlling and manipulating the angular momentum of light and the polarization of the vector beam, offering a unique approach to studying SOI in the paraxial regime. In the non-paraxial regime, we explore the tight focusing of structured scalar and vector beams, revealing remarkable SOI effects. In particular, experiments conducted using optical tweezers uncovered several optical phenomena, such as the “rotational spin-Hall effect,” allowing us to visualize the SOI effect. By employing first-order radially and azimuthally polarized vector beams, we have probed the effect of pure transverse spin angular momentum (TSAM), which is generated due to the longitudinal component of the field arising from tight focusing. Our configuration is unique in the sense that the LSAM for such first-order vector beams is zero by symmetrical construction. In addition, our system allows us to separately probe the effects of electric and magnetic TSAM, which we demonstrated using input first-order radially and azimuthally polarized vector beams, respectively. Furthermore, for the same input vector beam, we have presented the inhomogeneous spin momentum-induced orbital motion of birefringent particles around the beam’s propagation axis, referred to as origin-dependent OAM. We also show that the linear combination of spinless radially and azimuthally polarized vector beams, i.e., spirally polarized vector (SPV) beam, can generate spatially resolved longitudinal SAM (LSAM) of light, mediated by the SOI effect. Finally, the composite effects of tight focusing and the interaction of spin-polarized light with the anisotropic medium of an LC particle result in helicity flipping (from RCP to LCP, and vice versa), enabling planetary-like motion at the microscale. We term this phenomenon “micromotors driven by the SOI of light”. These effects are thoroughly quantified using the complete vector diffraction theory described by Debye and Wolf, through numerical simulations of the focused electric field and electromagnetic dynamical quantities such as spin and orbital AM of light. The effect of SOI is further enhanced by introducing a refractive index (RI) stratified medium in the path of the focused beam. Additionally, we have developed a Müller-matrix-based model of the composite effects of tight focusing and the interaction of spin-polarized light with the anisotropic medium of an LC particle, revealing various optical phenomena, including the spin-Hall effect, vortex generation from geometric phase gradients, spin flipping (opposite helicity generation), spin-to-orbit conversion, and many more. All these optical phenomena have been experimentally observed using various beam configurations, including fundamental Gaussian beams, Hermite-Gaussian beams, and first-order vector beams with diverse polarization distributions in optical tweezers. The rotational dynamics induced by SOI in these configurations allowed for the precise manipulation of trapped particles. Overall, this thesis provides a comprehensive understanding of SOI effects in both paraxial and non-paraxial regimes. Our findings demonstrate how SOI can be harnessed for precise control of angular momentum in light-matter interactions, leading to practical applications in optical trapping, the optical Hall effect, spin-momentum locking, classical entanglement, optical communications, and the development of photonic devices. This work also establishes a foundation for future research into more complex optical systems and dynamic light-matter interactions, offering a pathway toward innovative technologies based on structured light and SOI.

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