• Advanced Photonics Nexus
  • Vol. 4, Issue 2, 026006 (2025)
Ram Nandan Kumar1,*, Sauvik Roy1, Subhasish Dutta Gupta1,2,3, Nirmalya Ghosh1,*, and Ayan Banerjee1,*
Author Affiliations
  • 1Indian Institute of Science Education and Research Kolkata, Department of Physical Sciences, Mohanpur, India
  • 2Tata Institute of Fundamental Research, Hyderabad, India
  • 3Indian Institute of Technology, Department of Physics, Jodhpur, India
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    DOI: 10.1117/1.APN.4.2.026006 Cite this Article Set citation alerts
    Ram Nandan Kumar, Sauvik Roy, Subhasish Dutta Gupta, Nirmalya Ghosh, Ayan Banerjee, "Spatially resolved spin angular momentum mediated by spin–orbit interaction in tightly focused spinless vector beams in optical tweezers," Adv. Photon. Nexus 4, 026006 (2025) Copy Citation Text show less

    Abstract

    We demonstrate an effective and optimal strategy for generating spatially resolved longitudinal spin angular momentum (LSAM) in optical tweezers by tightly focusing the first-order spirally polarized vector (SPV) beams with zero intrinsic angular momentum into a refractive index stratified medium. The stratified medium gives rise to a spherically aberrated intensity profile near the focal region of the optical tweezers, with off-axis intensity lobes in the radial direction possessing opposite LSAM (helicities corresponding to σ = + 1 and -1) compared to the beam center. We trap mesoscopic birefringent particles in an off-axis intensity lobe as well as at the beam center by modifying the trapping plane and observe particles spinning in opposite directions depending on their location. The direction of rotation depends on the particle size with larger particles spinning either clockwise or anticlockwise depending on the direction of spirality of the polarization of the SPV beam after tight focusing, while smaller particles spin in both directions depending on their spatial locations. Numerical simulations support our experimental observations. Our results introduce new avenues in spin–orbit optomechanics to facilitate novel yet straightforward avenues for exotic and complex particle manipulation in optical tweezers.
    Erad/azi(ρ,z)=(E0/w0)ρexp(ρ2w2)ei,i=ρ,ϕ.

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    ESPVACW/CW(ρ,ϕ,z)=(E0/w0)ρexp(ρ2w2)(cosϕ±sinϕsinϕcosϕ).

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    E(ρ,ψ,z)=0θmax02πE(θ,ϕ)eikzcosθeikρsinθcos(ϕψ)sinθdθdϕ.

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    [ExEyEz]SPVACW/CW=[i(I1cosψ±I2sinψ)i(I1sinψI2cosψ)I0],

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    [HxHyHz]SPVACW/CW=[±i(I1cosψI2sinψ)±i(I1sinψ±I2cosψ)I0],

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    [ExEyEz]SPV(ACW/CW)=i(I1iI2)2eiψ[1i0]+i(I1±iI2)2eiψ[1+i0]I0[001].

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    S=14ωn2Im[ϵ(E*×E)+μ(H*×H)].

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    Sxe=i(I1sinψI2  cos  ψ),Sye=i(I1  cos  ψ±I2sinψ),Sze=(I1*I2I1I2*),

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    Ram Nandan Kumar, Sauvik Roy, Subhasish Dutta Gupta, Nirmalya Ghosh, Ayan Banerjee, "Spatially resolved spin angular momentum mediated by spin–orbit interaction in tightly focused spinless vector beams in optical tweezers," Adv. Photon. Nexus 4, 026006 (2025)
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