Abstract
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.
Highlights
Enantiomer sorting has attracted tremendous attention owing to its significant applications in both material science and the drug industry[1,2,3,4,5]
The chiral particles used in reported experiments are tens of micrometers in size, in the geometrical-optics regime, where the mechanism and methodology are quite different from the dipole approximation and Mie theories
Unlike the optical lateral force on dipole chiral particles (R ≤ 100 nm)[23], whose sign depends only on the chirality of the particle, our results show that the sign of the optical lateral force on the micro-chiral particles (R ≥ 300 nm) could directly depend on the size (Fig. 1e, f) and chirality (Fig. 1g, h)
Summary
Enantiomer sorting has attracted tremendous attention owing to its significant applications in both material science and the drug industry[1,2,3,4,5]. Polarized (CP) beams can induce spin-dependent lateral forces on achiral spherical particles when they are placed near an interface[15,16]. 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]. The chiral particles used in reported experiments are tens of micrometers in size, in the geometrical-optics regime, where the mechanism and methodology are quite different from the dipole approximation and Mie theories
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