Research Of Optical Trapping And Optical Manipulation With Archimedes Spiral Structure

In the past few decades,because light waves can carry angular momentum,this provides a new dimension for optical communication and optical manipulation,and its potential application has attracted great attention from researchers.In free space,light waves carrying OAM exhibit a helical phase wavefront and are therefore commonly referred to as optical Vortex.To rotate a Mie particle with a circularly polarized plane wave,it is often necessary that the particle itself is anisotropic,absorptive,or birefringent,such that the momentum transfer imbalance across the particle causes the particle to spin.However,isotropic uniform Mie particles are subj ected to linear momentum in the direction of the optical axis under the action of a circularly polarized plane wave,resulting only straight motion with no rotation.If a phase rotation factor is added to the plane wave,the wavefront is no longer a planar structure,but a spiral phase structure,that is,a vortex light field,and the uniform particle receives the orbital angular momentum transferred by the photon,and rotation can occur.It is one of the key issues to develop new multifunctional nano-optical tweezers that must be resolved to generate controllable vortex light with OAM and realize optical capture and optical micromanipulation of isotropic particles.Archimedes spiral metasurface can provides an additional geometrical phase that can generate light vortex which carrying OAM.Moreover,when the chirality of the incident light(left-handed or right-handed circular polarization)is the same or opposite to the helical structure,the excited surface plasmonic field will also exhibit two completely different forms,which has opened up broad prospects in biomedical and optical manipulation field.The two-dimensional Archimedes spiral metasurface,which can generate a focus/optical vortex at the incidence of left-handed/right-handed circularly polarized light,capable of capturing or rotating isotropic particles.Compared with the traditional laser tweezers,plasmonic optical tweezer has many advantages such as lower input power,stronger light gradient force.However,the plasmonic field formed by the metasurface of the two-dimensional Archimedes spiral structure exists only on the surface of the gold film,and the corresponding capture region is only on the surface,which has a large limitation for manipulating irregularly particles.It is also not conducive to real three-dimensional micro-manipulation.In this paper,a 3D Plasmonic Archimedes Spiral Lens(3D PASL)is proposed for the first time,and the theoretical analysis and numerical simulation of the 3D structure are carried out.By controlling the angular momentum of the near-field plasmons,a focus or plasmonic vortex field can be obtained at the center of the structure.We studied in detail the effects of several structural parameters on the focus or plasmonic vortex position and the intensity of the focus.It was found that the focal point/plasmonic vortex position can be adjusted by changing the height of the structure,thus overcoming the defects of the 2D Archimedes spiral that the particles can only be manipulated on the surface of the gold film.Because the cup-shaped of the 3D PASL,it can directly hold liquid biological samples,and perform local agitation stratification or capture specific biological living cells.This natural advantage is beneficial for practical applications.Theoretical analysis found that the adj acent spiral spacing in 2D PASL must be equal to the surface plasm on wavelength to form the focus or optical vortex,and the 3D PASL in this paper has no strict parameter constraints on the adjacent spiral spacing,which makes the structure more flexible than 2D PASL.The structure finds many potential applications in micromechanical systems,optofluidic handling devices,microbiology and biophotonics research,as well as in the analysis of conformational change of DNA or protein by providing a controllable trapping or local vortex turbulence.