The Study On Optical Manipulations Of Chiral Nanoparticles In Plane Waves

Photons carry momentum and interact with particles to exchange momentum,the optical force and optical torque induced on the objects.Optical manipulation is based on the optical force and optical torque frame to manipulate and study the physical characteristics of microscopic particles by using the laser beam.This article theoretical study on optical force and optical torque and trace their physical origins,which is conducive to accurately controlling the direction and magnitude of optical force and optical torque,and provides a theoretical reference for actual optical manipulation applications.In recent years,most of the researches on optical manipulation have focused on two parts: On the one hand,using different light fields to manipulate particles,it can induce some novel phenomenon such as pushing,pulling,binding,and rotating,which have attracted the attention of the majority of researchers.On the other hand,according to the different characteristics of particles manipulate objects,such as colloidal spheres,dielectric spheres,and metal nanospheres.However,the chiral materials with geometrically asymmetric properties exist widely in nature.Because of the complex calculations,it is also worth that further research to solve this problem.With the development of chiral drugs,chiral molecules and chiral materials,the new challenge have been put forward for the identification and separation of chiral particles.For this reason,this article is mainly devoted to the theoretical calculation of the optical force and optical torque on chiral particles in the plane waves,which mainly includes the following two aspects:(1)The optical binding and lateral optical force on chiral particles are investigated in the linear polarization plane waves.By the method based on full-wave simulation and the theory of dipole expansion approximation,the optical binding force and lateral optical force induced on the chiral particles with different chiral combinations in four pairs of particles are calculated.The optical binding force presents the characteristics,such as equal magnitude and opposite direction,and it is not sensitive to the chirality parameters.In the x-and y-polarized plane waves,the nearest dimer is formed with a separation of 1.25? and1? respectively.And analytical analysis within the dipole approximation indicated that the different combinations have the same physical origin: the gradient force induced by multiple scattering between particles.This discovery can be used to simultaneously bonded two chiral particles in the plane waves.At the same time,the lateral optical force presents the characteristic that as long as one particle has chirality,it can be induced.It depends on the chirality of the particle and has multiple origins: in addition to the gradient force,there are radiation force,curl force,and the spin force make the contributions.By tailoring the polarization of the incident plane wave and chirality parameters of the particles,the lateral optical forces of different behaviors can be adjusted.Furthermore,the chiral particles can be pushed to one side or rotated around the center of mass according to the characteristics of the lateral optical force induced on the chiral particles in the plane waves.These findings provide valuable theoretical references for the separation and sorting of chiral particles in industries.(2)The optical force and optical torque are studied on chiral particle in the plane waves.Using methods based on the generalized Lorentz-Mie theory and the multipole expansion theory,it is found that in the plane waves with the electric field along the xpolarization and left-circular polarization,the chiral particles will induce multipole coupling,which the contributes of quadrupole item keep the dominant state on the optical force and optical torque when the incident wavelength is compared with the particle size.Through further investigation,it is found that the contribution of the chiral term in the optical force and the optical torque is dominant,and has exceeded the contribution of the electric multipole in value in the case of multipole coupling.It is worth noting that the extinction cross-section maintains the same trend as the optical force and optical torque,reaching the upper bound at a wavelength of 0.3 microns,and the cross-section corresponds to the peak of the optical force and optical torque one-to-one.These findings provide new ideas for manipulating the optical force and optical torque on chiral particles in plane waves.