Negative optical torque on a microsphere in optical tweezers

Diniz, K.; Dutra, R. S.; Pires, Luís Barbosa; Viana, Nathan B.; Nussenzveig, H. M.; Neto, Paulo A. Maia

doi:10.1364/oe.27.005905

Cited by 35 publications

(37 citation statements)

References 46 publications

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“…Only a few experimental observations of spin-dependent lateral forces have hitherto been reported. These lateral forces, associated with optical spin-orbit interactions, differ from the "chirality-dependent" lateral forces induced by linearly polarized beams, which deflect dipolar chiral particles with opposite handedness towards opposite lateral directions [20][21][22][23][24] . Most examples of optical lateral forces induced by chirality are only theoretical predictions based on dipole (radius ≤ 50 nm) or geometricaloptics (e.g., radius > 10 µm) particles under the illumination of beams with intensity gradients 25,26 .…”

Section: Introductionmentioning

confidence: 92%

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

et al. 2020

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Lateral optical forces induced by linearly polarized laser beams have been predicted to deflect dipolar particles with opposite chiralities toward opposite transversal directions. These "chirality-dependent" forces can offer new possibilities for passive all-optical enantioselective sorting of chiral particles, which is essential to the nanoscience and drug industries. However, previous chiral sorting experiments focused on large particles with diameters in the geometrical-optics regime. Here, we demonstrate, for the first time, the robust sorting of Mie (size~wavelength) chiral particles with different handedness at an air-water interface using optical lateral forces induced by a single linearly polarized laser beam. The nontrivial physical interactions underlying these chirality-dependent forces distinctly differ from those predicted for dipolar or geometrical-optics particles. The lateral forces emerge from a complex interplay between the light polarization, lateral momentum enhancement, and out-of-plane light refraction at the particle-water interface. The sign of the lateral force could be reversed by changing the particle size, incident angle, and polarization of the obliquely incident light.

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Section: Introductionmentioning

confidence: 92%

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

et al. 2020

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“…In the case of very thin shells, t < ∼ 50 nm, the curves corresponding to different values of κ approach each other and recover the result for a homogeneous achiral sphere as t → 0, as expected. For homogenous achiral spheres, the rotation angle α is usually opposite to the polariza-tion handedness of the trapping beam [36]. Hence we find a positive α in the limit t → 0 when taking right-handed circular polarization (negative spin angular momentum) as in the example of Fig.…”

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confidence: 61%

Enantioselective manipulation of single chiral nanoparticles using optical tweezers

et al. 2020

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We put forward a twofold enantioselective method for chiral nanoparticles using optical tweezers. First, we demonstrate that the optical trapping force in a typical, realistic optical tweezing setup with circularly-polarized trapping beams are sensitive to the chirality of core-shell nanoparticles, allowing for efficient enantioselection. Second, we propose another enantioselective method based on the rotation of core-shell chiral nanoparticles' equilibrium position under the effect of a transverse Stokes drag force. In this case, the chirality of the particle shell gives rise to an additional twist, thus leading to a strong enhancement of the optical torque driving the rotation. Both chiral resolution methods are shown to be robust against the variation of the size of the system and material parameters, suggesting it could be applied to a wide range of experimental situations, particularly those of biological interest for which optical tweezing is widely used. Our results provide alternative enantioselective mechanisms and pave the way for all-optical manipulation and enantiopure synthesis of chiral nanoparticles. In addition, they can also be applied in the characterization of chirality for testing existing methods of enantioselection.

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“…For dielectric Mie particles, their orbital motions in the field are, in many cases, 'negative', that is, in the opposite direction of the spin momentum flow. This negative orbital motion is recently observed in a circularly-polarized (Gaussian-like) focused field in an indirect way [16]. The difficulty of observing directly such orbital motions (positive or negative) lies in the fact that the radial gradient force always tends to restrict the probe particle on the ring of intensity maximum where the spin momentum flow vanishes.…”

Section: Introductionmentioning

confidence: 81%

Spin momentum-dependent orbital motion

Yan

Liang

et al. 2020

New J. Phys.

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We present a theoretic analysis on (azimuthal) spin momentum-dependent orbital motion experienced by particles in a circularly-polarized annular focused field. Unlike vortex phase-relevant (azimuthal) orbital momentum flow whose direction is specified by the sign of topological charge, the direction of (azimuthal) spin momentum flow is determined by the product of the field's polarization ellipticity and radial derivative of field intensity. For an annular focused field with a definite polarization ellipticity, the intensity's radial derivative has opposite signs on two sides of the central ring (intensity maximum), causing the spin momentum flow to reverse its direction when crossing the central ring. When placed in such a spin momentum flow, a probe particle is expected to response to this flow configuration by changing the direction of orbital motion as it traversing from one side to the other. The reversal of the particle's orbital motion is a clear sign that spin momentum flow can affect particles' orbital motion alone even without orbital momentum flow. More interestingly, for dielectric particles the spin momentum-dependent orbital motion tends to be 'negative', i.e., in the opposite direction of the spin momentum flow. This arises mainly because of spin-orbit interaction during the scattering process. For the purpose of experimental observation, we suggest the introduction of an auxiliary radially-polarized illumination to adjust the particle's radial equilibrium position, for the radial gradient force of the circularly-polarized annular focused field tends to constrain the particle at the ring of intensity maximum.

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Negative optical torque on a microsphere in optical tweezers

Cited by 35 publications

References 46 publications

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

Enantioselective manipulation of single chiral nanoparticles using optical tweezers

Spin momentum-dependent orbital motion

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