Studies On Dynamic Characteristics Of Vortex Vector Beams Interacting With Microparticles

With the rapid development of laser technology and associated interdisciplinary research fields,the optical field modulation technique has been paid much attention.A series of special mode optical fields can be produced through the modulation of the fundamental parameters such as the frequency,amplitude,phase and polarization state of the optical field,which are widely used in optical communications,optical micromachining,optical microscopy,optical micromanipulation and so on.Light beams carry energy and optical momentum.In light-matter interactions,both the energy and momentum are transferred from light to matter,certainly resulting in mechanics effects.Since the pioneering work demonstrated by Ashkin of American Bell Lab in 1970 that the laser can trap and move suspended micro-particle,optical micromanipulation technology — a technology that using the mechanical effect produced by tightly focused laser beams to manipulate particles of the order of several nanometers to several hundred microns,has been widely used in modern scientific researchs especially in biology,physics,chemistry and medicine,due to its unique advantages of noncontact,noninvasive and high precision operation capabilities.The interactions of special mode optical fields with particles show some special phenomena and effects,which are new hotspots in the research field of optical micromanipulation in recent years.By modulating the phase and polarization state of light beams,the vortex vector beams' focusing properties and the particles' dynamics characteristics were investigated in detail in this thesis in the following aspects.1.The computational method and program for focusing special mode optical fields,the optical force and the optical torque on particles is established.The Richards-Wolf vectorial diffraction theory under nonparaxial approximation is used to calculate the distribution of the focused field to compensate for the deficiencies of scalar diffraction theory under tightly focusing conditions.A rigorous electromagnetic scattering model is used to calculate the scattering field around the particle,greatly reducing the calculating load and being suitable forsolving various complex shaped particles.The conservation of optical momentum and angular momentum as well as the integration of time-averaged Maxwell stress tensor are used to calculate the optical force and torque on particles,ensuring the accuracy of numerical calculations.2.A method of spinning particles in the transverse direction(vertical to optical axis)— transverse spin is proposed.The traditional circularly polarized beam has the longitudinal spin angular momentum(SAM)that causes the particle to spin axially.While the focused field of the cylindrical vector beam carries a considerable transverse SAM that can induce a transverse(between the radial and azimuthal direction)spin of the particle,which makes it possible to rotate particles along a non-axial direction and allows for additional rotation degrees of freedom in optical manipulation.Further,a vectorial beam with azimuthally varied polarization is proposed to manipulate the particle.Tightly focusing of such beam exhibits petal-shaped intensity distributions and carries purely transverse SAM,which can be used to trap multiple particles simultaneously and spin particles along the azimuthal direction.3.The orbiting motion of particles in various polarized vortex beams is analyzed.Optical vortex(OV)beams carry optical orbital angular momentum(OAM)and can induce an orbiting motion of trapped particles in optical manipulation.The state of polarization(SOP)of vortex beams will affect the motion state of this optically induced rotation to some extent.Focusing the OV beams with circular,radial or azimuthal polarizations can induce a uniform orbiting motion on the trapped particle,while in the focused field of OV beam with linear polarization the particle experiences a non-uniform orbiting motion.The direction of orbiting motion is determined by the sign of the topological charge of the OV beam.And each OV beam has an optimal topological charge corresponding to the maximum orbital torque.4.The spinning motion of particles in different polarized vortex beams is studied.The focused fields of OV beams also carry the optical SAM due to focusing that can drive a spinning motion of the trapped particles simultaneously.In the focusedOV beam with circular or radial polarization,the particle orbits around the optical axis while also experiences a non-axial(between the azimuthal and axial direction)spinning motion.The direction of spinning motion is not only deterimined by the sign of topological charge of the OV beam,but also by the polarization state.In addition,the motion of particles in a tightly focused double-vortex(DV)beam obtained by superposing two OV beams with equal but opposite topological charges is also studied.It is intuitively presumed that the OAM of such a mixed field will be cancelled out and not rotate the trapped particles.However,the results show that the focused field of such beam carries SAM,which can induce the particles spin along the beam axis or transversely to the beam axis.5.The stable form of non-spherical particles in various focused polarized beams is revealed.A spheroidal particle represents a typical non-spherical particle.Although the geometry seems to be relatively simple,the spheroid shape can actually model many irregular particles in optical micromanipulation.Through the investigations of the three-dimensional spatial orientation of spheroidal particles and the polarization properties of light beams,it is found that the focused fields of various polarized beams can trap not only the spheroidal particle stably,but also control the orientation of the particle as well.