Propagation Properties Of Partially Coherent Vortex Beams And Their Radiation Forces

The vortex beam has become a research hotspot due to its special wave-front structure and important application prospect in optical communication,biomedicine,quantum information,and many other fields.Under the same environmental conditions,decreasing the coherence has advantages in space optical communication,optical trapping,and atomic cooling.Therefore,partially coherent vortex beams(PCVBs)are more attractive in overlapping applications.At present,the practical application of PCVBs is in the primary stage.The spatial coherence structure has an essential influence on the beam's propagation characteristics,topological charge measurement,and optical trapping characteristics.Based on the partially coherent modified Bessel Gaussian(PCMBG)beams,partially coherent elegant Laguerre-Gaussian(PCe LG)beams,partially coherent anomalous vortex(PCAV)beam as an example,respectively,the vortex structure by gradient-index(GRIN)medium,the measurement method of vortex structure(topological charge),and the radiation forces on different Rayleigh particles are studied systematically.The main content and innovation points of this thesis are as follows:1.The propagation characteristics of the PCMBG beam in the GRIN medium are studied.Based on the generalized Huygens-Fresnel diffraction integral formula and ABCD matrix of the GRIN medium,the cross-spectral density(CSD)function expression of the PCMBG beam in the GRIN medium is derived.The evolution characteristics and influencing factors of vortex structure in the GRIN medium are also calculated and simulated.The results show that periodic changes characterize the beam propagation trajectory – first,focusing and then divergence.The change period is only related to the gradient index factor.When passing through half-period integer times,the rotation direction of the vortex phase is reversed.The vortex phase structure is the same as that of the initial vortex when the vortex is located at half periodic integer times.The gradient index factor only affects the propagation period,and the coherence parameter only affects the beamwidth.The change of topological charge,however,does not affect the propagation period.The higher-order optical vortices do not split when the higher-order topological charges are applied.The results show that the propagation characteristics of the PCVBs in the GRIN medium are different from those in free space.Choosing the appropriate GRIN medium can be used to carry out the optical field transformation effectively.The propagation evolution law of the vortex phase in the GRIN medium provides a theoretical basis for long-distance optical communication.2.A new method for measuring the topological charge of a PCe LG beam is proposed.Based on the generalized Huygens-Fresnel diffraction integral formula,the far-field CSD of a PCe LG beam is derived.The special structural distribution and influencing factors of the complex degree of coherence(CDOC)of the beam are studied.The results show that the far-field CDOC of the PCe LG beam exhibits the ring dislocation,and the number of dark rings in the CDOC is corresponding to the topological charge number |l| but is not related to the radial index p.A mathematical explanation is given for this phenomenon.The finding of N=|l| is verified experimentally.Therefore,a method of measuring the topological charge of a PCe LG beam is demonstrated.At low coherence,this method has the advantages of clear results and simple observation.The results reveal a one-to-one correspondence between the structural distribution of CDOC and the number of topological charges: N=|l|.This confirms that the correlation structure of a PCe LG beam is utterly different from that of a partially coherent standard Laguerre-Gaussian beam.The finding provides a theoretical basis for optical communication and optical trapping.3.The focusing properties of a PCAV beam and its radiation force on dielectric Rayleigh particles are investigated.The study relied on the generalized Huygens-Fresnel diffraction integral formula to calculate and simulate the CSD function of a PCAV beam near the focal plane,the evolution characteristics and influencing factors of the optical field near the focal plane.From the Rayleigh scattering theory,the radiation force and influencing factors of PCAV beams on dielectric Rayleigh particles are deduced and simulated.The results show that the PCAV beams have strong beam shaping ability in the focal plane and its vicinity,forming the distribution of hollow,flat-top,and solid light intensity.In the focal plane,the high refractive index dielectric Rayleigh particles can be trapped in three dimensions by using solid light intensity distribution.At the point of deviation from the focal plane,low refractive index dielectric Rayleigh particles can be transversely trapped.By adjusting beam order,topological charge,and coherence length,the magnitude of radiation force and trapping range can be adjusted.The results also show that the radiation force can be finely regulated by using many parameters of PCVBs.Using the special evolution characteristics and shaping ability of the PCVB,a special operation such as the "particle screening" function to the particles may be carried out,which may provide a theoretical basis for novel optical tweezers technology.4.Tightly focusing properties of a partially coherent circularly polarized anomalous vortex(PCCPAV)beam and its radiation forces on metallic Rayleigh particles are studied.The CSD function of a tightly focused PCCPAV beam are deduced from the vector diffraction theory and partial coherence theory,and the intensity distribution and the influencing factors calculated and simulated.From Rayleigh scattering theory,the radiation force of a tightly focused PCCPAV beam on metallic Rayleigh particles and its influencing factors are equally calculated and simulated.The results show that spin direction,topological charge,and numerical aperture have effects on the intensity distribution after tightly focusing.The coherence length,beam order,and beamwidth only change the intensity value,but not the intensity distribution.When the beam is tightly focused in the axial direction,the spin angular momentum is converted to OAM.Tightly focused PCCPAV beams can trap the metallic Rayleigh particles if the appropriate parameters are selected.The radiation force can be changed by adjusting the topological charge and beam order.The results may provide a theoretical basis for the application of novel optical tweezers technology.