On The Three-dimensional Light Scattering By A Large Nonspherical Particle Based On Vectorial Complex Ray Model

The study of the light scattering by particles is attracting more and more researchers because it concerns an increasing number of optical technologies.Various light scattering theories and calculation methods have been proposed and developed,while different models and methods have different ranges of applicability.For a particle of complex shape and of size much larger than the wavelength of incident light,the existing analytical theories and numerical methods are no longer applicable,while the precision by geometrical optics approximation(GOA)method is always limited because it is hard for GOA to account for the divergence of wavefront and the phase shift due to focal lines.Ren et al.have developed the vectorial complex ray model(VCRM),which integrates the curvature of wavefront as a new intrinsic property of light ray,thus providing a feasible way to solve the light scattering by a large and smooth particle of any shape.However,the current numerical implementation of VCRM addresses only the scattering when light rays propagate within a two-dimensional(2D)plane.This thesis is devoted to the systematic research on the calculation method based on VCRM and the corresponding experimental verification for the scattered intensity of plane wave and shaped beam by large nonspherical particles in three-dimensional(3D)space.The main work and achievements are summarized as follows:1.As the first step,the calculation method based on VCRM for the 2D scattered intensity of plane wave by a cylinder of any smooth cross section is proposed.The calculation method for solving the coordinates of each interaction point with the cylinder is provided.The surface curvature at each point on the cylinder,the reflected and refracted wavefronts at each interaction,the phase and the amplitude for each ray scattered by a cylinder with arbitrarily smooth cross section are derived under the framework of VCRM.The proposed method is applied to solving the scattered intensity of plane wave by a composite elliptical cylinder(CEC),whose cross section can take various shapes ranging from circular,elliptical to highly-deformed.The scattering patterns of the CECs whose cross sections take the shapes of natural raindrops are calculated.The effects of shape deformation,refractive index and the direction of incident wave on the scattering fields,especially on the rainbows,are quantitatively analyzed.2.This thesis reports the first realization of a calculation method in the framework of VCRM for the 3D scattered intensity by a large particle of any smooth surface.Based on VCRM,the ray tracing,the phase shifts due to focal lines and optical path,the divergence and convergence of wavefront,and the cross polarization effects in 3D scattering are addressed.Besides,by developing a triangulation-based interpolation algorithm,the calculation for the inference intensity of scattered rays in 3D space has been achieved,thus breaking through the bottle-neck problem for VCRM in the extension to 3D scattering.The proposed algorithm is very flexible and only requires the particle to have a smooth surface(differentiable to the second order).Besides,the result of programming shows that this algorithm also has significant advantage in computational efficiency,allowing one to calculate the 3D scattered intensity by a large nonspherical particle even on a personal computer.3.The calculation method proposed for the 3D scattering by large nonspherical particles is applied to solving the scattering of plane wave by a real liquid jet.Having a non-zero curvature along the jet axis,the geometric profile of a real liquid jet is much more complex than that of an infinite cylinder.For an incident plane wave which propagates perpendicularly to the jet axis,the intensity distribution of the 3D scattering field is successfully simulated.Furthermore,taking advantage of the ability of VCRM for interpreting the scattering mechanism,a systematic analysis is made for the scattered light of different orders,in regard to their separation or interference in 3D space.To verify the proposed method for 3D scattering and to examine the simulated result,an experimental setup has been established for measuring the 3D scattered intensity of plane wave by a liquid jet.An incident wave of limited width 1 mm which has approximately uniform intensity and planar wavefront is achieved by using 4 f optical system and a spatial filter,ensuring the experimental realization of the light beam in simulation.The 3D scattering field measured by experiment agrees well with that by simulation.4.In the framework of VCRM,a ray description method for incident elliptical Gaussian beam is proposed,thus making it possible to calculate the 3D scattered intensity of laser beam by a large particle of any smooth surface.At each point of the incident beam,not only the propagation direction,amplitude and phase of the beam but also the principal curvatures and the principal directions of the local wavefront are characterized.Then based on the calculation method for the 3D scattering of plane wave,the calculation for the 3D far-field scattered intensity of elliptical Gaussian beam by a real liquid jet is successfully achieved.The 3D scattering fields near the first-and second-order rainbows for incident elliptical Gaussian beams of different divergence angles are investigated.A quantitative analysis is also made for the spatial characteristics of the scattering field when a tightly focused beam illuminates the jet.The calculation for the 3D scattered intensity of elliptical Gaussian beam by a large nonspherical particle/target not only verifies the flexibility of VCRM,but also provides important methodology for solving the 3D scattering of other shaped beams under the framework of VCRM.