Analysis of trapping force exerted on multi-layered chiral sphere induced by laser sheet

Theoretical study on optical trapping of multi-layered chiral sphere has attracted more and more attentionfor its important applications in many frontier scientific fields such as chemical engineering, biomedicine, opticaltweezers, micro/nano lithography etc. In order to trap and manipulate chiral multi-layered particles efficiently,the present paper aims at developing the theoretical research of trapping force (TF) exerted on a multi-layeredchiral sphere induced by laser sheet which might have great potential to improve the light performance inoptical trapping as well as capture, suspension, and high-precision delivery of chiral cells. Here, based on theGeneralized Lorenz Mie theory and the completeness of spherical vector wave functions (SVWFs), theelectromagnetic field of incident laser sheet are expanded in terms of SVWFs. Accordingly, by introducing thebeam scattering theory and the conservation law of electromagnetic momentum (EM), the analysis of TFexerted on multi-layered chiral sphere can be analytically expressed in terms of the incident and scatteringcoefficients. Taking the chiral cell as an example, the TF induced by laser sheet is simulated numerically.Numerical effects of the varying chirality, polarization states, beam waist width, inner material loss andoutmost size on the TF induced by laser sheet are analyzed and compared with those by circular Gaussianbeam incidence in detail. It is found that the introduction of chirality parameter may reduce the axial TFexerted on chiral multi-layered cell. Thus, it is more difficult to trap and manipulate stratified chiral cells thanto trap general isotropic cells. Also it is shown that the TF of chiral cells can be significantly discriminatory innature, depending upon both the handedness of the interacting particles and the polarization of the incidentlight. Thus, an appropriately polarized beam should be considered in trapping chiral cells. For chiral multi-layered cells with small loss in the inner layer, when the inner refractive indices are less than the outmostrefractive index, the TF of multi-layered chiral cell becomes stronger with the outmost radius decreasing.Conversely, for the inner refractive indices are greater than the outer refractive index, TF becomes weaker asthe outmost radius decreases. Besides, compared with the traditional circular Gaussian beam, the strongconvergence of elliptical Gaussian beam can be easier to achieve three-dimensional capture of stratified chiralcells, which may provide a recipe to understand the light interaction with more complex chiral cells with the aidof the analytical approach and could be a promising avenue for the design of optical trapping systems.