Kerker‐Type Intensity‐Gradient Force of Light

Abstract

The intensity gradient of light represents the most important property for optical tweezers to manipulate small particles, which is known to produce a conservative optical force that is either attractive or repulsive. Here, it is shown that Kerker interference, the interplay between electric and magnetic dipoles induced in nanoparticles, permits the intensity gradient to exert a nonconservative optical force, in the case of a standard optical trap created with linearly or elliptically polarized Gaussian beams. The Kerker‐type intensity‐gradient force has an “anisotropic” directionality, and it tends to repel particles away from the beam axis. Such repulsive effects can greatly sensitize the particle trapping behavior of optical tweezers to the particle size. Utilizing these peculiar properties, all‐optical sorting of Si nanoparticles is theoretically demonstrated, with tunable size‐selection criterion and accuracy.

Unveiling dipolar spectral regimes of large dielectric Mie spheres from helicity conservation

“…that electric and magnetic dipolar optical forces [33][34][35][36], radiation pressure [37,38], light transport phenomena [39,40], and the novel concept of anapole modes [41], originally derived for small nanoparticles, can be extended to larger HRI dipolar spheres. Moreover, the novel dipolar regions where the EM helicity is preserved can be used to enhance the sensitivity of circular dichroism spectroscopy of chiral particles [42,43].…”

Kerker Conditions upon Lossless, Absorption, and Optical Gain Regimes

“…This phenomenon arises since the asymmetry parameter (g), which encodes the particle's optical response, is equivalent to the EM helicity at the direction perpendicular to the incoming wave when the object is excited by a beam with well-defined helicity (σ ¼ AE1), namely, Λ π=2 ¼ 2σg [33]. This relation straightforwardly links the EM helicity with the g parameter, which appears in multiple branches of physics such as optical forces [34][35][36], light transport phenomena [37][38][39], or wavelength-scale errors in optical localization [40]. Remarkably, this wavelength's error limit can be drastically surpassed at the first Kerker condition for dipolar particles, where an optical vortex arises in the backscattering region [41].…”

Multipole Engineering of Attractive−Repulsive and Bending Optical Forces

“…As a result of this rather dramatic scattering diagram reshaping (recall that a Gaussian beam can be decomposed into a sum of plane waves), new transverse forces, orthogonal to both gradient contribution and the radiation pressure, can emerge. [ 54 ]…”

“…In a Gaussian beam, the antitrapping effect was previously obtained only for interfering electric and magnetic dipoles. [ 54 ] Obtaining an antitrapping regime on the dipole−quadrupole interference of the same nature moments is inherent for high‐index particles only.…”

“…[ 49–51 ] However, high‐index dielectric particles supporting a variety of Mie resonances [ 52,53 ] introduce new interaction channels beyond simple dipolar polarizability terms. [ 54,55 ] For example, coherent interaction between the electric and magnetic responses of silicon particles was shown to provide either pulling or pushing forces, depending on system parameters. [ 56 ] Sorting of silicon particles with laser beams was shown in the study by Shilkin et al [ 57 ] Core−shell geometries allow designing multipole resonances within a structure.…”