Research On The Optical Trapping Of A Rayleigh Dielectric Sphere

Optical trapping technology or optical tweezers, which make use of light’s mechanical effect to manipulate micro-particles effectively, is one of the great invitations in laser technology areas at the end of the20th century. And in recent years, optical trapping technology has become a powerful tool, particularly in biophysical science, for trapping living cells and organelles, and in atomic physics for manipulating the neutral atoms. Focused on the interaction between the trapping beam and the Rayleigh particle, this thesis completed the following work.1. The analytical expression for the propagation of elegant Hermite-cosine-Gaussian (EHcosG) beams through a paraxial ABCD optical system has been derived and used to study the radiation forces produced by highly focused this kind of beams in the Rayleigh scattering regime. Owing to the characteristics of focused EHcosG beams, which have a dark-centered configuration at the focal point and a double-peak configuration nearby the focal point, it is expected to simultaneously trap and manipulate particles with low-refractive index at the focal point and particles with high-refractive index nearby the focal point. Finally, the conditions for effective trapping and manipulating the particle have been analyzed.2. The analytical expression for the propagation of elegant Hermite-cosh-Gaussian (EHCHG) beams through a paraxial ABCD optical system is derived and used to study the radiation forces produced by highly focused EHCHG beams acting on a Rayleigh dielectric particle. Owing to the characteristics of this kind of beams our analysis shows that it can be expected to simultaneously trap and manipulate dielectric spheres with low-refractive index at the focal point, and particles with high-refractive index nearby the focal point. Finally, the stability conditions for effective trapping and manipulating particles by this kind of focused beams are discussed.3. Radiation forces acting upon a dielectric sphere produced by focused controllable dark-hollow beams (CDHBs) in the Rayleigh regime of light scattering are analyzed theoretically and calculated numerically. Results show that focused CDHBs can be used to trap and manipulate particles with high-refractive index at the focal point. Furthermore, the trapping effect and optical trap stiffness can be improved by increasing either the beams order or central dark size controlled parameter, which indicates that the trapping stability can be improved by optimizing CDHBs parameters. Finally, the trapping stability conditions are analyzed.4. The focusing properties of anomalous hollow beams (AHBs) are theoretically and numerically investigated, and the radiation forces acting upon a Rayleigh dielectric sphere produced by focused AHBs are also studied. Results show that focused AHBs can be used to trap and manipulate micro-sized dielectric spheres with high-refractive index at the focal point. Finally, the stability conditions for effective trapping particles are analyzed in detail.5. The analytical propagation expression of partially coherent flat-topped vortex beams through a paraxial optical ABCD system is derived, and then is used to investigate the coherence vortex properties of partially coherent flat-topped vortex beams in the Fourier transform and fractional Fourier transform systems. It is shown that in the Fourier transform system the coherence vortex depends on the flat-topped beam order N, spatial coherence parameter a and position (x1, y1) of the reference point, whereas in the fractional Fourier transform system the flat-topped beam order N does not affect the spectral degree of coherence. Furthermore, in both transform systems, depending on the choice of the reference point, the zero value point of the spectral degree of coherence may be present or absent. In particular, if x1=y1=0is selected, the phase at the zero value point of the spectral degree of coherence may be determinate, thus the coherence vortex does not exist.6. The analytical expression for the propagation of Airy-related beams generated from flat-topped Gaussian beams through an ABCD optical system is derived, and used to study the propagation of this type of beams through a chiral slab. The influence of several factors, such as the optical beam order N, and the chirality’s parameter y of the chiral slab, on the beam propagation prosperities both in near-and far-zones have been discussed in detail. Results show that the Airy tails of high order beams decay more quickly than those of low order beams in the chiral slab; the constructive interference effect between the LCP and RCP beams in the chiral medium is more significant as the chirality’s parameter γ increases; the LCP and RCP beams are not separated at the near-zone, while the two beams are obviously separated in the far-zone, and accordingly the interference peaks decrease as the propagation distance increases in the chiral slab.