Research On Propagation And Scattering Characteristics Of Terahertz Structured Beams

Because of its special bandwidth range and extensive application prospect,terahertz wave has become a research hotspot in wireless communication,biophysics and medicine,target detection,super resolution imaging and radar.As a kind of electromagnetic wave whose amplitude,phase and polarization vary with spatial position,the structured beam has excellent characteristics,such as orbital angular momentum,tight focusing beyond diffraction limit,self-splitting,self-shaping and self-bending propagation.The interaction between the structured beam and matter exhibits novel effects and phenomena,such as information and energy transfer,momentum and angular momentum transfer,lateral displacement and spin.On the other hand,the experimental implementation of metamaterials has opened up a new research field,and new beam manipulation techniques and phenomena are waiting to be reexamined.The beam manipulation technology is also widely used in wireless communication and imaging systems,for example,the study of the propagation of structured beams in plasma media is expected to solve the blackout problem.In addition,the most materials mainly existed in the state of particles,and the analysis of the scattering characteristics of particles may help to improve the manipulation of small particles and retrieve the physical properties of objective particles.In this dissertation,the structured beam is introduced into the terahertz band,and it is studied that several aspects,such as the propagation characteristics of terahertz structured beams,including Laguerre-Gaussian vortex beams,non-diffracting Bessel beams and self-accelerating Airy beams,in dispersive left-handed and gradient-index meta-materials;Vector wave analysis and dynamical characteristics of these structured beams upon reflection and refraction;Reflection,transmission,and absorption of terahertz structured beams propagation in an inhomogeneous magnetized plasma slab;Scattering of terahertz structured beams by complex particles.The primary work is summarized as follows:1.In this chapter,we intend to derive the explicit expressions for the electromagnetic field components of typical structured beams,including Laguerre-Gaussian beams,Bessel beams,and Airy beams,based on the vector angular spectrum representation.Also,distributions of the electromagnetic fields and the phase are displayed for these typical structured beams.2.Based on the ABCD transfer matrix and Collins formula,the analytical expressions of the complex amplitude of the non-diffracting Bessel beam in a gradient-index medium are derived.Through numerical calculations,the propagation of Bessel beams in a gradient-index medium is examined.Based on the vector angular spectrum representation method,the analytical expressions for the electric and magnetic field components of Laguerre-Gaussian beams in dispersion left-handed material are derived,and used to calculate the momentum,spin and orbital angular momentum.Further,the propagation dynamics of Laguerre-Gaussian beams in the left-handed material are examined.3.According to the matrix transformation theory model,a hybrid method based on the angular spectrum representation and vector potential in the Lorenz gauge is introduced to describe the full vector theoretical analysis model and dynamical quantities of structured beams upon reflection and refraction.The explicit analytical expressions for the electric and magnetic field components of Laguerre Gaussian beams,Bessel beams and Airy beams reflected and refracted at an air-medium interface are derived in detail.Local-field patterns and magnitude profiles,as well as dynamical quantities with different parameters,are displayed.4.From the governing equation in the high temperature plasma,and using the continuous boundary conditions of electric and magnetic fields,the reflection and transmission coefficients of terahertz wave are obtained.Additionally,the transmission characteristics,including reflectivity,transmissivity and absorptivity,of terahertz waves propagating in high temperature magnetized plasma are studied numerically and theoretically.Based on the scattering matrix method algorithm,the reflection and transmission coefficients are derived,and the effects of the magnetization direction,external magnetic field strength,collision frequency on left-hand and right-hand polarized terahertz wave in the uniformly magnetized plasma are examined.Based on the angular spectrum representation and the 4×4 transfer matrix method,the reflected and transmitted electric fields of Laguerre-Gaussian beams are expressed by the inverse Fourier transform of the product of the angular spectrum amplitude of the incident beam,and the propagation properties of Laguerre-Gaussian beams in an inhomogeneous magnetized plasma slab are analyzed.5.By making use of the vector angular spectrum decomposition method,the beam shape coefficients of Gaussian beams and Laguerre-Gaussian beams are obtained and expressed by a vector angular spectrum,and the two structured beams are expressed in terms of vector spherical wave functions and beam shape coefficients.Expressions for the internal and near-surface fields,as well as the differential scattering cross section are derived and computed.The correctness of the beam shape coefficients is verified.Also,the effects of beam parameters and size parameters of the sphere on the distributions of internal and near-surface field intensity,as well as the differential scattering cross sections are analyzed;The electromagnetic field components of internal and near-surface fields of the spherical particle illuminated by Laguerre-Gaussian beams are calculated,and some numerical results are included to show the dynamical characteristics of the internal and near-surface fields of a sphere when it is illuminated by Laguerre-Gaussian beams with different beam parameters.6.The implementation of typical structured beams,including the Hermite-Gaussian beams,Laguerre-Gaussian beams,Bessel beams,and Airy beams,in the discrete dipole approximation is presented.The detailed processes for the implementation of these beams in discrete dipole approximation are given.The calculated result for our codes of Gaussian beams is compared with that from the Amsterdam discrete dipole approximation codes,and very good agreement is observed,and the scattering of these structured beams by spherical particles is illustrated and discussed;The electromagnetic field components of internal and near-surface fields of composite particles illuminated by above four structured beams are calculated,and some numerical results are included to show the electric field intensity of the internal and near-surface fields of composite particles when it is illuminated by these typical structured beams with different beam parameters.