%**************************************************************************
%     2-D FDTD TE code with PML absorbing boundary conditions
%**************************************************************************
%
%     Program author: Susan C. Hagness
%                     Department of Electrical and Computer Engineering
%                     University of Wisconsin-Madison
%                     1415 Engineering Drive
%                     Madison, WI 53706-1691
%                     608-265-5739
%                     hagness@engr.wisc.edu
%
%     Date of this version:  February 2000
%
%     This MATLAB M-file implements the finite-difference time-domain
%     solution of Maxwell's curl equations over a two-dimensional
%     Cartesian space lattice comprised of uniform square grid cells.
%
%     To illustrate the algorithm, a 6-cm-diameter metal cylindrical 
%     scatterer in free space is modeled. The source excitation is 
%     a Gaussian pulse with a carrier frequency of 5 GHz.
%
%     The grid resolution (dx = 3 mm) was chosen to provide 20 samples
%     per wavelength at the center frequency of the pulse (which in turn
%     provides approximately 10 samples per wavelength at the high end
%     of the excitation spectrum, around 10 GHz).
%
%     The computational domain is truncated using the perfectly matched
%     layer (PML) absorbing boundary conditions.  The formulation used 
%     in this code is based on the original split-field Berenger PML. The
%     PML regions are labeled as shown in the following diagram: 
%
%            ----------------------------------------------
%           |  |                BACK PML                |  |
%            ----------------------------------------------
%           |L |                                       /| R|
%           |E |                                (ib,jb) | I|
%           |F |                                        | G|
%           |T |                                        | H|
%           |  |                MAIN GRID               | T|
%           |P |                                        |  |
%           |M |                                        | P|
%           |L | (1,1)                                  | M|
%           |  |/                                       | L|
%            ----------------------------------------------
%           |  |                FRONT PML               |  |
%            ----------------------------------------------
%
%     To execute this M-file, type "fdtd2D" at the MATLAB prompt.
%     This M-file displays the FDTD-computed Ex, Ey, and Hz fields at 
%     every 4th time step, and records those frames in a movie matrix, 
%     M, which is played at the end of the simulation using the "movie" 
%     command.
%
%**************************************************************************
clc;
clear;
tic;
%**************************************************************************
%     Fundamental constants
%**************************************************************************
cc=2.99792458e8;            %speed of light in free space
muz=4.0*pi*1.0e-7;          %permeability of free space
epsz=1.0/(cc*cc*muz);       %permittivity of free space

freq=5.0e+9;                %center frequency of source excitation
lambda=cc/freq;             %center wavelength of source excitation
omega=2.0*pi*freq;

%**************************************************************************
%     Grid parameters
%**************************************************************************
ie=100;           %number of grid cells in x-direction
je=50;            %number of grid cells in y-direction
ib=ie+1;
jb=je+1;
is=15;            %location of z-directed hard source
js=je/2;          %location of z-directed hard source

dx=3.0e-3;        %space increment of square lattice
dt=dx/(2.0*cc);   %time step

nmax=300;         %total number of time steps

iebc=8;           %thickness of left and right PML region
jebc=8;           %thickness of front and back PML region
rmax=0.00001;     %反射系數(shù)。
orderbc=2;        %多項式指數(shù)。
ibbc=iebc+1;
jbbc=jebc+1;
iefbc=ie+2*iebc;
jefbc=je+2*jebc;
ibfbc=iefbc+1;
jbfbc=jefbc+1;
%**************************************************************************
%     Material parameters
%**************************************************************************
media=2;

eps=[1.0 1.0];      %
sigma=[0.0 1.0e+7]; %電導(dǎo)率。
mur=[1.0 1.0];      %
sigmaM=[0.0 0.0];   %磁導(dǎo)率。
%**************************************************************************
%     Wave excitation
%**************************************************************************
rtau=160.0e-12;
tau=rtau/dt;
delay=3*tau;

source=zeros(1,nmax);
for n=1:7.0*tau
  source(n)=sin(omega*(n-delay)*dt)*exp(-((n-delay)^2/tau^2));
end

%**************************************************************************
%     Field arrays
%**************************************************************************

ex=zeros(ie,jb);           %fields in main grid 
ey=zeros(ib,je);
hz=zeros(ie,je);

exbcf=zeros(iefbc,jebc);   %fields in front PML region
eybcf=zeros(ibfbc,jebc);
hzxbcf=zeros(iefbc,jebc);
hzybcf=zeros(iefbc,jebc);

exbcb=zeros(iefbc,jbbc);   %fields in back PML region
eybcb=zeros(ibfbc,jebc);
hzxbcb=zeros(iefbc,jebc);
hzybcb=zeros(iefbc,jebc);

exbcl=zeros(iebc,jb);      %fields in left PML region
eybcl=zeros(iebc,je);
hzxbcl=zeros(iebc,je);
hzybcl=zeros(iebc,je);

exbcr=zeros(iebc,jb);      %fields in right PML region
eybcr=zeros(ibbc,je);
hzxbcr=zeros(iebc,je);
hzybcr=zeros(iebc,je);

%**************************************************************************
%     Updating coefficients
%**************************************************************************

for i=1:media
  eaf  =dt*sigma(i)/(2.0*epsz*eps(i));
  ca(i)=(1.0-eaf)/(1.0+eaf);
  cb(i)=dt/epsz/eps(i)/dx/(1.0+eaf);
  haf  =dt*sigmaM(i)/(2.0*muz*mur(i));
  da(i)=(1.0-haf)/(1.0+haf);
  db(i)=dt/muz/mur(i)/dx/(1.0+haf);
end

%**************************************************************************
%     Geometry specification (main grid)
%**************************************************************************

%     Initialize entire main grid to free space
caex(1:ie,1:jb)=ca(1);     
cbex(1:ie,1:jb)=cb(1);

caey(1:ib,1:je)=ca(1);
cbey(1:ib,1:je)=cb(1);

dahz(1:ie,1:je)=da(1);
dbhz(1:ie,1:je)=db(1);

%     Add metal cylinder
diam=20;          % diameter of cylinder: 6 cm
rad=diam/2.0;     % radius of cylinder: 3 cm
icenter=4*ie/5;   % i-coordinate of cylinder's center
jcenter=je/2;     % j-coordinate of cylinder's center

for i=1:ie
    for j=1:je
        dist2=(i+0.5-icenter)^2 + (j-jcenter)^2;
        if dist2 <= rad^2 
            caex(i,j)=ca(2);
            cbex(i,j)=cb(2);
        end
        dist2=(i-icenter)^2 + (j+0.5-jcenter)^2;
        if dist2 <= rad^2 
            caey(i,j)=ca(2);
            cbey(i,j)=cb(2);
        end
    end
end

%**************************************************************************
%     Fill the PML regions
%**************************************************************************

delbc=iebc*dx;
sigmamam=-log(rmax/100.0)*epsz*cc*(orderbc+1)/(2*delbc);
bcfactor=eps(1)*sigmamam/(dx*(delbc^orderbc)*(orderbc+1));

%     FRONT region 

caexbcf(1:iefbc,1)=1.0;
cbexbcf(1:iefbc,1)=0.0;
for j=2:jebc
  y1=(jebc-j+1.5)*dx;
  y2=(jebc-j+0.5)*dx;
  sigmamay=bcfactor*(y1^(orderbc+1)-y2^(orderbc+1));
  ca1=exp(-sigmamay*dt/(epsz*eps(1)));
  cb1=(1.0-ca1)/(sigmamay*dx);
  caexbcf(1:iefbc,j)=ca1;
  cbexbcf(1:iefbc,j)=cb1;
end
sigmamay = bcfactor*(0.5*dx)^(orderbc+1);
ca1=exp(-sigmamay*dt/(epsz*eps(1)));
cb1=(1-ca1)/(sigmamay*dx);
caex(1:ie,1)=ca1;
cbex(1:ie,1)=cb1;
caexbcl(1:iebc,1)=ca1;
cbexbcl(1:iebc,1)=cb1;
caexbcr(1:iebc,1)=ca1;
cbexbcr(1:iebc,1)=cb1;

for j=1:jebc
  y1=(jebc-j+1)*dx;
  y2=(jebc-j)*dx;
  sigmamay=bcfactor*(y1^(orderbc+1)-y2^(orderbc+1));
  sigmamays=sigmamay*(muz/(epsz*eps(1)));
  da1=exp(-sigmamays*dt/muz);
  db1=(1-da1)/(sigmamays*dx);
  dahzybcf(1:iefbc,j)=da1;
  dbhzybcf(1:iefbc,j)=db1;
  caeybcf(1:ibfbc,j)=ca(1);
  cbeybcf(1:ibfbc,j)=cb(1);
  dahzxbcf(1:iefbc,j)=da(1);
  dbhzxbcf(1:iefbc,j)=db(1);
end

%     BACK region 

caexbcb(1:iefbc,jbbc)=1.0;
cbexbcb(1:iefbc,jbbc)=0.0;
for j=2:jebc
  y1=(j-0.5)*dx;
  y2=(j-1.5)*dx;
  sigmamay=bcfactor*(y1^(orderbc+1)-y2^(orderbc+1));
  ca1=exp(-sigmamay*dt/(epsz*eps(1)));
  cb1=(1-ca1)/(sigmamay*dx);
  caexbcb(1:iefbc,j)=ca1;
  cbexbcb(1:iefbc,j)=cb1;
end
sigmamay = bcfactor*(0.5*dx)^(orderbc+1);
ca1=exp(-sigmamay*dt/(epsz*eps(1)));
cb1=(1-ca1)/(sigmamay*dx);
caex(1:ie,jb)=ca1;
cbex(1:ie,jb)=cb1;
caexbcl(1:iebc,jb)=ca1;
cbexbcl(1:iebc,jb)=cb1;
caexbcr(1:iebc,jb)=ca1;
cbexbcr(1:iebc,jb)=cb1;

for j=1:jebc
  y1=j*dx;
  y2=(j-1)*dx;
  sigmamay=bcfactor*(y1^(orderbc+1)-y2^(orderbc+1));
  sigmamays=sigmamay*(muz/(epsz*eps(1)));
  da1=exp(-sigmamays*dt/muz);
  db1=(1-da1)/(sigmamays*dx);
  dahzybcb(1:iefbc,j)=da1;
  dbhzybcb(1:iefbc,j)=db1;
  caeybcb(1:ibfbc,j)=ca(1);
  cbeybcb(1:ibfbc,j)=cb(1);
  dahzxbcb(1:iefbc,j)=da(1);
  dbhzxbcb(1:iefbc,j)=db(1);
end

%     LEFT region 

caeybcl(1,1:je)=1.0;
cbeybcl(1,1:je)=0.0;
for i=2:iebc
  x1=(iebc-i+1.5)*dx;
  x2=(iebc-i+0.5)*dx;
  sigmamax=bcfactor*(x1^(orderbc+1)-x2^(orderbc+1));
  ca1=exp(-sigmamax*dt/(epsz*eps(1)));
  cb1=(1-ca1)/(sigmamax*dx);
  caeybcl(i,1:je)=ca1;
  cbeybcl(i,1:je)=cb1;
  caeybcf(i,1:jebc)=ca1;
  cbeybcf(i,1:jebc)=cb1;
  caeybcb(i,1:jebc)=ca1;
  cbeybcb(i,1:jebc)=cb1;
end
sigmamax=bcfactor*(0.5*dx)^(orderbc+1);
ca1=exp(-sigmamax*dt/(epsz*eps(1)));