Multipole Engineering of Attractive−Repulsive and Bending Optical Forces

Abstract

Focused laser beams allow controlling the mechanical motion of objects and can serve as a tool for assembling micro and nanostructures in space. While small particles mainly experience attractive gradient forces and repulsive radiation pressure, introducing additional flexibility suggests approaching new capabilities. Herein, optical forces acting on a high refractive index sphere in a focused Gaussian beam are analyzed and new regimes are revealed. Multipolar analysis allows separating an optical force into interception and recoil components, resulting in different mechanical actions. In particular, interplaying interception radial forces and multipolar resonances within a particle can lead to either trapping or antitrapping, depending on the system parameters. At the same time, the recoil force generates a significant azimuthal component along with an angular‐dependent radial force. Those contributions enable enhancing either trapping or antitrapping and also introduce bending reactions. These effects are linked to the far‐field multipole interference and, specifically, to asymmetric scattering patterns. The latter approach is extremely useful, as it allows assessing the nature of optomechanical motion by observing far‐fields. Multipolar engineering of optical forces, being quite a general approach, is not necessarily linked to simple spherical shapes and paves a way to new possibilities in microfluidic applications, including sorting and microassembly.

Polarization‐Dependent Optical Forces Arising from Fano Interference

“…As single Si nanospheres (SiNSs) support both electric and magnetic multipoles, [4][5][6][7] we use SiNSs of different sizes as the Fano-resonant media, with a permittivity of Si adapted from Ref. [22].…”

“…[4] Magnetic multipoles are generally present in "artificial magnetic" structures or high-index dielectric nanomaterials such as Si nanospheres. [5][6][7] The interaction between the magnetic multipoles leads to a magnetic Fano resonance (MFR). Significant MFR or EFR can produce unique scattering properties, rendering Fano-resonant media particularly attractive for the development of chemical or biological sensors and optical antenna or switching.…”

“…Recently, several studies have been performed on the Fano resonance force for a variety of Mie nanostructures. [6,7,10,[15][16][17] The force has also been explored for pulsed light, by considering the nonlinear effects induced in the materials. [18][19][20] It shows that the total optical force is sensitive to the light wavelength and the size of the nanostructures.…”

Superhybrid Mode-Enhanced Optical Torques on Mie-Resonant Particles

“…The scattering cross-section can be obtained via the normalization of the scattering power to the incident energy flux, , where ε m is the relative permittivity of the media, amd ε 0 and μ 0 are the permittivity and permeability of vacuum, respectively. The optical scattering force (ruling out the optical gradient force and spin density force) on small particles can be approximately written in the relation of the scattering cross-section as , − where k represents the light vector ( k = | k |). Typically, the optical force on an electric dipole in SI units can be expressed as − where α e is the electric polarizability of the particle, σ e ext is the electric extinction cross-section, ⟨ S ( i ) ⟩ = Re­( E i × H i * ) is time-averaged Poynting vector of the incident field, ⟨ L Se ⟩ = i E i × E i * is time-averaged electric spin density.…”

Optical forces in photonic Weyl system

“…So far, optical tweezers have been widely used in multiple disciplines such as nanophotonics [11,12], near field optics [13], plasmomics [14][15][16], optical cooling [17] and biological science [18,19]. Optical force, which transfers the momentum from electromagnetic (EM) radiation to the particle, plays a vital role in these important optical manipulating applications [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34].…”