Researches On Algorithm Of Implicit Finite-Difference Time-Domain And Model For Achieving Infrared Perfect Absorption In Monolayer Black Phosphorous

Currently,to make the finite-difference time-domain method(FDTD)overcome the limitation of Courant-Friedrichs-Lewy(CFL)in temporal increment,the implicit FDTD methods have been developed so that CPU time and computational efficiency can be improved.In general,alternate-direction-implicit FDTD(ADI-FDTD)and Crank-Nicolson FDTD(CN-FDTD)methods are the most popular implicit FDTD methods.Numerical dispersion and computational errors of the ADI-FDTD method are higher than those of the CN-FDTD method,when temporal increment is enlarged.Therefore,the CN-FDTD method can be considered to resolve the airborne transient electromagnetics(ATEM)system which adopts the techniques of 'transmit in air/ground,receive in air'.This EM system belongs to the extremely low frequency and electrical small size problem.Since 2014,black phosphorous(BP),as one of the emerging 2D materials,has unique in-plane anisotropic property,which is different from the in-plane isotropy of graphene.Besides,BP shows its good optoelectronic properties.However,weak light-matter absorption in BP for infrared frequency is observed,and BP is easily oxidized.However,the BP-light interaction can be enhanced by manipulating the angle of incident light and polarization,electronic doping concentration and dielectric layer thickness so that the infrared absorption in BP monolayer can be improved.The significance of this dissertation is mainly to study and develop the algorithm for proposing the implicit CN-FDTD based on CFS-PML and unsplit-field and its application in extremely low frequency subsurface sensing problems;at the same time,it proposes a method for achieving near unity infrared absorption in designed structures with a monolayer BP.However,the designed 3D structures with a monolayer BP belong to the multiscale problems.Therefore,the proposed CFS-PML-based CNCSU-FDTD method can not be enough efficient in computing multiscale problems.Finally,to reduce the CPU time and memory very much,the FEM simulation with the surface current density is adopted.The originality and achievements in this dessertation are mainly listed as follows:1.An unsplit-field and accurate Crank-Nicolson-cycle-sweep-uniform FDTD(CNCSU-FDTD)method based upon the CFS-PML formulations and the ADE technique is proposed,which is characterized with high numerical accuracy and implicit time integration to circumvent the CFL stability condition.Therefore,the computational efficiency can be greatly increased by enlarging the temporal increment.The proposed CFS-PML-based CNCSU-FDTD method can be applied to solving three-dimensional(3D)extremely low frequency subsurface electromagnetic sensing problems(namely for 0.1-100 Hz),such as the airborne transient electromagnetics(ATEM)problems,and overcome low computational efficiency of the conventional explicit FDTD method.For example,four hundred thousand time steps are needed in one period for regular FDTD methods when the low frequency is 25 Hz with the maximum frequency of 191 Hz and spatial cell size of 50 m,leading to impracticalness and inefficiency.However,the temporal increment in CNCSU-FDTD can be 1300 times larger than that in the regular FDTD for the low frequency sensing problems,resulting in achieving higher computational efficiency.2.As an emerging anisotropic 2D material,few-atomic-layer black phosphorus has shown some the promising potentials for infrared optoelectronics.Engineering and enhancing its light-matter interaction is significant for many novel photonic devices.In view of this,we aim to achieve perfect infrared absorption in monolayer BP with/without microstructural patterning.By optimizing the polarization and angle of incident light,the dielectric thickness,and the electron doping concentration,respectively,infrared radiation can be sufficiently coupled to optical absorption of monolayer BP in a multiscale structure.Moreover,monolayer BP with defects can also reach near unity infrared absorption by the proposed method.This research provides a simple and efficient scheme to trap infrared light for developing promising optoelectronic devices based on monolayer BP and potentially other anisotropic 2D materials.