Optical Micromanipulation Based On Spatial Modulation Of Optical Fields

Because of their non-intrusive way operation,wide range of manipulated objects and high precision in measuring tiny force and displacement,optical tweezers have been widely applied to life science and physics.By modulating the amplitude,phase and/or polarization of light,it is possible to create optical fields with complex configuration,such as point-traps array,optical vortex,and self-accelerating light beams(travel along a curved trajectory).Such an ability to handle optical fields advances optical micro-manipulation from conventional single-laser-beam tweezers into three-dimensional dynamic manipulations including optical rotation and optical guiding.Furthermore,due to the capability of manipulating particles with high precision,optical micro-manipulation has become a powerful tool in the study of colloidal interactions,optical sorting,crystallographic texture fabrication,and micro-chemical reaction,etc.It is therefore,from both application and development points of view,of considerable importance to develop an effective technique of generating novel optical fields,thus extending the functionality of optical micro-manipulation.Focused on spatial-light-modulation-based optical micro-manipulation,the research in this thesis covers the following aspects:1.A compact,sophisticated and multifunctional optical micro-manipulation apparatus is designed and built.Owing to a modular design,the apparatus has a volume as small as~400×400×500 mm~3 and a power utilization as high as 40%.A LabVIEW program with a user-interface is developed for real-time dynamic micro-manipulation,with which tens of microparticles can be simultaneously trapped and manipulated with a refresh rate of~13Hz.Using this apparatus,we can trap and manipulate particles with diameter ranging from subwavelength to tens of micrometers,including cells,colloidal particles,etc.The system aberration is corrected by using a high-order vortex phase.Such a scheme is simple,without requiring any modification to the system,but efficient accompanied with an improvement on the beam modulation efficiency by a factor of64%as well as an increase in trap stiffness in the(x,y)direction from(94.61,133.02)pN/?m to(222.06,192.17)pN/?m,corresponding to a factor of ratio of(2.35,1.44).2.Simultaneous optical trapping and imaging of microparticles in the axial plane is realized using a 45°-microreflector-based axial-plane imaging technique and an axial-plane Gerchberg-Saxton(GS)algorithm(powered by the axial-plane fast Fourier transform).Axial-plane imaging allows for a direct visualization and thus a more sophisticated optical micromanipulation in the axial plane,as opposed to the field of view of conventional optical trapping systems being confined to the lateral plane.With the proposed axial-plane imaging technique,we demonstrate an axial-plane holographic optical trapping with field-of-view up to 300?m.Besides,the proposed axial-plane GS algorithm permits direct generation of novel beams in the axial plane,showing much higher speed and efficiency than three dimensional iterative algorithms.By combining the axial-plane imaging technique and the axial-plane GS algorithm,we demonstrate optical trapping with 1×3 traps array in the axial direction and calibrate the stiffness of all the traps.3.The particle dynamics in various optical vortex fields are studied.Transfer of angular momentum occurring in light-matter interaction,induces torque on the object and makes it rotate.Knowledge on the rotation and spinning dynamics of particles gains us insight into the underlying physic and promotes the applications of optical induced rotation.We investigate the particle dynamics in Laguerre-Gaussian(LG)beams and analyze theoretically and experimentally the transfer of optical orbital angular momentum.We also study the optical rotating of low-index particles in quasi-perfect optical vortices and propose a method to enhance the rotation speed of low-index particles.To investigate transfer of optical spin angular momentum,we analyze the particle dynamics in circularly polarized optical vortex fields.We propose a method to improve the spinning speed of birefringent microparticles by using circularly polarized low-order vortex and demonstrate the observation of simultaneous orbital rotation and spinning of birefringent microparticles in circularly polarized high-order vortex beams.4.Optical guiding along straight paths with laser beams like Bessel beams,high-order radial index LG beams and tilted Bessel beams,and along curved paths with self-accelerating laser beams like Airy beams and snake-like beams are demonstrated with the axial-plane imaging technique.Optical guiding is emerging as an increasingly important tool in optical sorting,in which non-diffracting beams are ideal candidates for their long working distance.The proposed axial plane imaging and trapping in one technique is highly suitable for studying optical guiding with non-diffracting beams.Optical guiding over a long distance up to~300?m is realized with zero-order Bessel beams and high-order radial index LG beams,respectively.By using tilted Bessel beams,we demonstrate the optical guiding along a tilted path with tilted angle of 18.7°.Furthermore,we investigate the optical guiding along curve trajectories with Airy beams and snake-like beams.Study on optical guiding with varieties of non-diffracting beams will promote the application of optical sorting in complex environment.