Optical Trapping Of Multiple Particlesand Optical Rotation Of Non-sphericalparticles In Tightly Focused Gaussian Beams

Optical tweezers have been an important tool of micromanipulation and microfabircation since optical trapping is non-contacting and scathless to particles. Optical trapping and manipulation have been focused by scientests and researchers and have been widely used in many fields of biology, physics and chemistry. The tightly focused Gaussian beam focused by a high numerical objective is very common in optical tweezers. Based on the scattering of tightly focused beams by microparticles, we propose study the influence factors of optical tweezers and develop some useful models to simulate optical trapping and manipulation. The investigation in this dissertation will supply certain physical foundation to optical trapping and manipulation technology.The optical force and torque exerted on microparticles in tightly focused Gaussian beams were calculated using T-matrix method. The effects of the wavelength and polarization of beams, numerical aperture of objectives, size and refractive index of spheres were investigated for the optical force exerted on the spherical particle. In addition, the optical torque landscapes on prolate and oblate ellipsoids located on the beam axes were presented. Based on the optical torque landscapes, the rotation directions of these two ellipsoids were analyzed. The parameters of the beams and these two ellipsoids were studied as well. We found that these parameters have different effects on the optical torque.In addition, the model of optical trapping of a multiple-particle system was developed, which was used to to simulate the optical trapping and disordered motion of multiple-particle systems in optical traps. The maximum number of spheres, which can be trapped in a single tightly focused Gaussian beam, were calculated with different parameters of spheres. Some factors which will affect the multiple trapping were found due to our results. Meanwhile, the multiple-block model was presented for the cylinders with a large aspect ratio. The accuracy of T-matrix was presented as well. We analyzed the application range of this model and the reason of unsuccessful usage.The optical binding forces on spheres in two tightly focused Gaussian beams was studied based on the multiple scattering theory. we discussed the impact factors of the axial optical binding force in two counter-propagating beams and lateral optical binding force in two co-propagating beams, such as the beam wavelength, the distance between two beam focuses, numerical aperture of objectives, the polarization of beams, the size and refractive index of the spheres. These factors have different effects on optical binding force in our results. The investigation gives theory foundation to optical tweezers with multiple beams.We developed the movement equations of non-spherical particles in tightly focused beams. The model of optical trapping and rotation of any shaped non-spherical particles in tightly focused beams was developed as well, which can be used to find the movement trajectaries, equillibrim positions and stable orientations of any-shaped particles. T-matrices from DDA were used to calculate the optical force and torque on the particles. The equillibrim positions and orientations of micro-cylinders and nanowires with aspect ratios larger than1were investigated in the tightly focused Gaussian beams. According to the orientaitons of cylinders, the orientation landscapes were divided into four regions: untrapped region, vertical region, horizontal region and the region between the vertical and horizontal regions (0o <θ <90o). The effects of the beam wavelength, numerical aperture of objectives and the refractive index of cylinders on the orientation landscape were discussed. We finally found that the circularly-polarized beam, high numerical aperture of the objective and lower relative refractive index (>1) of the particle are of great help to make the cylinder stand up in a single tightly focused beam.