Research On The Interaction Between Shaped Beam And Complex Particles Based On High-Order Conformal FDTD Method

In recent years,with the rapid development of laser technology and light field control technology,various electromagnetic and structured light beams with novel characteristics have received extensive attention from researchers.Based on the high-order time domain finite difference method FDTD(2,4),this thesis contributes to the reconstruction of shaped beams in the FDTD method.Based on the traditional method,using the fourth-order center difference approximate the first-order partial derivatives of the space in the rotation operator,an improved performance of numerical dispersion and anisotropy properties is obtained.The reconstruction of fundamental Gaussian beam and high-order Bessel beam in FDTD have been realized.On the basis of shaped beams in FDTD,the the far-field scattering characteristics of chiral and anisotropic particles interaction with Bessel beam and near-field characteristics of Non-spherical particles with shaped beams are studied.The reconstruction of the shaped beams in FDTD method opens a new way for the study of the interaction between beams and complex shaped particles.The main work of this thesis are as follows:1.Research on high-order FDTD method.Theoretical deriving the high-order FDTD iterative formula,analyzing the numerical dispersion and mesh anisotropy of the high-order FDTD method when using fourth-order central difference.Deriving the high-order conformal formula for dielectric medium,thus compensates the error caused by the cubic grid approximation.The modified formula of the total field scattering field under high-order conditions is given,allowing the incident field can be injected in a high-order form.The near-field transformed technique based on the equivalent principle is discussed.2.Reconstructing fundamental mode Gaussian beam in FDTD method.Based on the fifth-order approximation expression,the Gaussian beam is reconstructed by applying a single plane as the excitation in beam space.There are several steps for this.Firstly,selecting a Gaussian beam cross section plane on which the tangential component electromagnetic field used as the excitation source.Secondly,Calculating the amplitude and phase of the field component on this plane by using the fifth-order approximation of the Gaussian beam,and correcting the phase discontinuity.Then,converting the phase to time delay using Fourier transform.Finally,the modulated Gaussian pulse plane wave is corrected and applied to the excitation surface source by the amplitude and time delay of the beam at each grid position.What's more,an equivalent source of Gaussian beam can be formed by creating another target space,copying the tangential component on a closed surface from the beam space to the target space at each time steps.3.Reconstructing Bessel beam source in FDTD method.Based on the angular spectrum representation(ASR),the incident Bessel beam is described as a superposition of plane waves whose wavevectors covering a conical surface.This decomposition of Bessel beam is then approximated by a finite collection of plane waves,which are injected into FDTD simulation domain using the total-field/scattered-field(TF/ST)method.By changing the polarization of the plane wave spectrum,arbitrary polarization Bessel beam including angular and radial polarization can be generated.By tuning the phase of the plane wave spectrum,arbitrary high order Bessel beam can be generated.Combined with the rotation of the Euler angle,oblique incident beam can be formed.By calculating the time of each plane spectrum reaches the off-axis point,different retarded time are given and reconstruction of the off-axis beams are realized.The conditions for high-order beam calculating in time domain are also discussed.This method can be extended to the numerical reconstruction of other structural beams in FDTD,which provides assistance for the analysis of interaction between structural light field and complex target.4.Scattering of aspherical chiral and anisotropic particles by Bessel beam is studied.Based on the reconstruction of Bessel beam in FDTD method,the scattering of non-spherical particles by Bessel beam is studied.The iterative formula in FDTD for chiral media and anisotropic media is derived.Numerical calculates the influence of the order of the Bessel beam,the size of the half cone,the different chiral parameters and the anisotropic parameters on the far-field scattering characteristics of the particles5.Focus properties of shaped beam by semi-ellipsoid particles is analyzed.The nearfield focus properties of semi-ellipsoid particles is studied by applying the reconstruction of Gaussian beam and Bessel beam in FDTD.Firstly,the near-field focus of Gaussian beam by particles with different geometrical dimensions and different refractive indices is studied.The influence of different locations of particles inside Gaussian beams on the convergence is analyzed.Then the focus of graded-index semi-ellipsoids is discussed.An extremely narrow focus spot size can be obtained by choosing the right refractive index and geometry of the particle.Finally,the convergence of Bessel beams by semi-ellipsoidal particles is studied.The influence of Bessel beams with different orders,different polarizations and different incident directions on focus properties of particle are discussed.