Study On The Optical Tweezers Technique By Using Optical Focal Field With Tailored Intensity Distribution

The photodynamic effect is derived from the momentum exchange in the interaction between light and matter.With this mechanical effect,optical tweezers,capable of stable capture and flexible manipulation of nanometer to micron scale particles under non-contact and non-destructive living conditions,has been developed.The optical tweezers has made people's research on small objects from passive observation to active observation,and has been widely used in biology,physics,chemistry and other fields.However,the capture sources used in conventional optical trapping systems are scalar optical fields,limiting the capture efficiency of the specific types of particles in terms of capture range and well depth.With the rapid development of laser technology,people have been able to effectively control the vector distribution of the optical field in and out of the cavity.This vector optical field with a special polarization distribution has spawned a large number of novel optical trapping techniques,which are a good complement and enhancement of the optical trapping technology.It is well known that the mechanical effects of particles in the optical field are inextricably linked to the intensity distribution of the optical field,so their stability and dynamic behavior will be controlled by the intensity modulation of the focal field.By means of the unique field distribution characteristics of the vector optical field under tightly focused conditions,we have developed a series of focal field intensity control techniques based on the modulation of the incident field polarization state and explored the application of such optical field in the optical trapping technology.In the experimental aspect,we built a continuous laserbased optical trapping system,which uses a variety of lasers with different wavelengths to achieve stable capture of micron-scale dielectric particles.Theoretically,we have focused on Richards-Wolf vector diffraction theory.The tightly focusing characteristics of the cylindrical vector vortex beams are analyzed in detail.The influence of the numerical aperture of the objective lens,the polarization state of the incident optical field and the topological charge on the Gouy phase and the wavefront spacing of the focal field are given.The relationship between the Gouy phase and the wavefront spacing distribution are explained.Based on the characteristics of the tightly focused vector beam,we generate a spherical hollow focal spot using two pairs of radially polarized beam with first-order Laguerre-Gaussian distribution and use the dipole approximation theory to study the mechanical properties of the particles near the focus.Analysis confirmed that the focal field is suitable for three-dimensional stable capture and manipulation of high refractive index and low refractive index particles.In addition,we propose a method for generating a two-dimensional flattop focal field using a second-order full Poincare sphere beam and verify the feasibility of the flat-top light field capturing nanoparticles in three-dimensional space.The research work in this paper has opened up new ideas for the modulation of vector optical field and its application in the optical field.It may provide new solutions in the field of focal field phase research,surveying,optical capture and micro-manipulation.