Description: | [Pre-Submission Seminar] Ramnandan Kumar (IISER-K) -- "Exploring Spin-Orbit Interaction Dynamics through the Lens of Structured Light" |

Date: | Monday, Jul 29, 2024 |

Time: | 3 p.m. - 4:30 p.m. |

Venue: | Asima Chatterjee Lecture Theatre |

Details: | Spin-to-orbit conversion of light is a dynamic optical phenomenon in nonparaxial fields leading to various manifestations of the optical Hall effect, such as the spin Hall effect, orbital Hall effect, and spin-orbit Hall effect [1-3]. However, the effects of the spin-orbit interaction (SOI) have not been explored extensively for structured Gaussian and structured vector beams carrying no intrinsic spin or orbital angular momentum. Our thesis is concerned in studying SOI in such beams, and generating varied applications of the effects of SOI, using the experimental setup demonstrated in Figure 1. First, we show that the SOI effects on such structured beams can be directly visualized due to the azimuthal rotation of their transverse intensity profiles, a phenomenon we call the rotational spin-Hall effect. Thus, for an input circularly polarized (right or left) Hermite-Gaussian (HG10) mode, the SOI leads to a significant azimuthal rotation of the transverse intensity distribution of both the orthogonal circularly polarized (left or right) component and the longitudinal field intensity with respect to the input intensity profile. We validate our theoretical and numerically simulated results experimentally by tightly focusing a circularly polarized HG10 beam in an optical tweezers configuration and projecting out the opposite circular polarization component and the transverse distribution of the longitudinal field intensity at the output of the tweezers. [4] In the non-paraxial regime, the longitudinal component of the electromagnetic field plays an important role in the generation of pure transverse spin angular momentum (TSAM). Our next study provides an effective and optimal strategy for generating TSAM in optical tweezers by tightly focusing first-order radially and azimuthally polarized vector beams with no intrinsic angular momentum (AM) into a refractive index-stratified medium. The choice of such input fields ensures that the longitudinal spin angular momentum (LSAM) arising from the electric (magnetic) field for the radial (azimuthal) polarization is zero. As a result, the effects of the electric and magnetic TSAM are exclusively observed separately in the case of input first-order radially and azimuthally polarized vector beams on single optically trapped birefringent particles [5]. In addition, we have also observed inhomogeneous spin and origin-dependent orbital momentum-induced orbital motion of birefringent particles in tight focusing of these vector beams [6]. Besides this, using the linear combination of radially and azimuthally polarized vector beams, we can generate spatially resolved positive and negative helicity of light [7]. In another work, we have designed inhomogeneous structured vector beams to probe simultaneous spin angular momentum (ϭ+ and ϭ-) of light [8]. Another manifestation of the SOI effect of light is engineering the spin dynamics to design the micromachines of birefringent liquid crystal particles. Next in the thesis, we report, both theoretically and experimentally, the breaking of helicity RCP/LCP into LCP/RCP as a fundamental consequence of SOI of light in a non-paraxial regime, and the manifestation of both helicity components in the rotational motion of particles [9]. Finally, we have also developed a rectangular loop interferometer (RLI) that is capable of confining light in a rectangular path geometry. Using this, we computed the sum of numerous mathematical geometric series converging to different values between zero and one, employing intensity and polarization-dependent optical elements. Furthermore, we also physically created complex vector and vortex beams that carry orbital angular momentum (OAM) of all orders of topological charge using circularly polarized input beams with a combination of a half-wave plate and q-plate inserted into the interferometer path [10]. Figure 1: Experimental setup corresponding to works mentioned in the abstract. References: [1] Bliokh, K. Y et al., Phys. Rev. Lett. 96, 073903 (2006). [2] Bliokh, K. Y. et al., Optica 3, 1039–1047 (2016). [3] A. Kavokin et al., Phys. Rev. Lett. 95, 136601 (2005) Sample chamberNA 1.4ObjectiveImmersion OilCover slipGlass slide1.8141.5161.331.5165 μm160 μm35 μm1.5 mmZ-axisYX1234CCDPolarizerBeam expander671 nmBeamsplitterMirrorNA 1.4Objectiveq-plateMicroscopeHWP(λ/2)Laser |

Calendar: | Seminar Calendar (entered by sangita.jr) |

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