Research On The Focus Shaping Of Vector-vortex Beams And The Force Distribution In Optical Tweezers

With the deepening cognition of optics,how to manipulate the light fields in a more flexible and effective way is gradually becoming a new research hotspot.As two essential attributes of light,amplitude and phase have been fully explored and widely utilized to control the optical fields in space domain.However,polarization state,as another crucial parameter to describe the vectorial properties of light,did not receive enough attention until the concept of vector beams was proposed.The so-called vector beams are actually a kind of structured light field with inhomogeneous distribution of states of polarization(So Ps)in its cross-section and thus exhibit numerous unique features compared with the conventional scalar beams(i.e.,the homogeneously polarized beams including linearly-,circularly-,and elliptically-polarized light fields).Especially under the tight focusing conditions,the incident polarization states play a dominant role in determining the spatial distributions of focused fields.The vector beams with such rich geometric configurations of So Ps will provide more flexible degrees of freedom for customizing the highly confined focused electromagnetic fields and consequently produce more novel phenomena with great potential for new applications.As another kind of structured light field,optical vortex beams have also attracted widespread attention of researchers in recent years due to their spatially-variant phase distributions and a certain orbital angular momentum carried by each of photons.By combining the structural characteristics of the two novel light fields,the focus shaping of vector-vortex beams and its application in optical tweezers were systematically investigated in this paper.The specific contents are listed as follows:1.An overview of the Richards-Wolf vectorial diffraction theory,the research foundation of this paper,is made by deriving the expressions of the focused electromagnetic fields for the case of a linearly-polarized incident plane beam.On the basis of that,the generalized integrated formulae for calculating the three-dimensional(3D)focal field distributions of locally linearly polarized vector-vortex beams and hybridly polarized vector-vortex beams are further built.2.Using the established analytical models,the focusing properties of four types of incident vector-vortex beams are discussed in detail,including azimuthally-variant locally linearly polarized vector-vortex beams,radially-variant locally linearly polarized vector-vortex beams,spatially-variant locally linearly polarized vectorvortex beams,and hybridly polarized vector-vortex beams.By tailoring the input polarization and phase,a series of focused light fields with special intensity distributions are theoretically obtained.For instance,3D hollow beams,bottle-hollow beams,and the focal spots with highly suppressed sidelobes,etc.3.According to the dipole approximation method,the optical force distributions exerted on a spherical Rayleigh particle under the illumination of different focused electromagnetic fields are calculated.The simulated results reveal that the particle's longitudinal trapping position can be adjusted by introducing a tunable focal shift along the optical axis.In addition,the optical cage created by amplitude and polarization modulations enables a stable capture of the particle with lower refractive index than ambient.More importantly,a new method to realize the identification and separation of a pair of nanoparticles with opposite chirality in the transverse plane is demonstrated in theory.