%***********************************************************************
% 3-D FDTD code with PEC boundaries
%***********************************************************************
%
% 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 three-dimensional
% Cartesian space lattice comprised of uniform cubic grid cells.
% 
% To illustrate the algorithm, an air-filled rectangular cavity 
% resonator is modeled. The length, width, and height of the 
% cavity are 10.0 cm (x-direction), 4.8 cm (y-direction), and 
% 2.0 cm (z-direction), respectively.
%
% The computational domain is truncated using PEC boundary 
% conditions:
% ex(i,j,k)=0 on the j=1, j=jb, k=1, and k=kb planes
% ey(i,j,k)=0 on the i=1, i=ib, k=1, and k=kb planes
% ez(i,j,k)=0 on the i=1, i=ib, j=1, and j=jb planes
% These PEC boundaries form the outer lossless walls of the cavity.
%
% The cavity is excited by an additive current source oriented
% along the z-direction. The source waveform is a differentiated 
% Gaussian pulse given by 
% J(t)=-J0*(t-t0)*exp(-(t-t0)^2/tau^2), 
% where tau=50 ps. The FWHM spectral bandwidth of this zero-dc-
% content pulse is approximately 7 GHz. The grid resolution 
% (dx = 2 mm) was chosen to provide at least 10 samples per 
% wavelength up through 15 GHz.
%
% To execute this M-file, type "fdtd3D" at the MATLAB prompt.
% This M-file displays the FDTD-computed Ez fields at every other
% time step, and records those frames in a movie matrix, M, which 
% is played at the end of the simulation using the "movie" command.
%
%***********************************************************************

clear

%***********************************************************************
% 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

%***********************************************************************
% Grid parameters
%***********************************************************************

ie=50; %number of grid cells in x-direction
je=24; %number of grid cells in y-direction
ke=10; %number of grid cells in z-direction

ib=ie+1; 
jb=je+1; 
kb=ke+1; 

is=26; %location of z-directed current source
js=13; %location of z-directed current source

kobs=5;

dx=0.002; %space increment of cubic lattice
dt=dx/(2.0*cc); %time step

nmax=500; %total number of time steps

%***********************************************************************
% Differentiated Gaussian pulse excitation
%***********************************************************************

rtau=50.0e-12;
tau=rtau/dt;
ndelay=3*tau;
srcconst=-dt*3.0e+11;

%***********************************************************************
% Material parameters
%***********************************************************************

eps=1.0;
sig=0.0; 

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

ca=(1.0-(dt*sig)/(2.0*epsz*eps))/(1.0+(dt*sig)/(2.0*epsz*eps));
cb=(dt/epsz/eps/dx)/(1.0+(dt*sig)/(2.0*epsz*eps));
da=1.0;
db=dt/muz/dx;

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

ex=zeros(ie,jb,kb);
ey=zeros(ib,je,kb);
ez=zeros(ib,jb,ke);
hx=zeros(ib,je,ke);
hy=zeros(ie,jb,ke);
hz=zeros(ie,je,kb);

%***********************************************************************
% Movie initialization
%***********************************************************************

tview(:,:)=ez(:,:,kobs);
sview(:,:)=ez(:,js,:);

subplot('position',[0.15 0.45 0.7 0.45]),pcolor(tview');
shading flat;
caxis([-1.0 1.0]);
colorbar;
axis image;
title(['Ez(i,j,k=5), time step = 0']);
xlabel('i coordinate');
ylabel('j coordinate');

subplot('position',[0.15 0.10 0.7 0.25]),pcolor(sview');
shading flat;
caxis([-1.0 1.0]);
colorbar;
axis image;
title(['Ez(i,j=13,k), time step = 0']);
xlabel('i coordinate');
ylabel('k coordinate');

rect=get(gcf,'Position');
rect(1:2)=[0 0];

M=moviein(nmax/2,gcf,rect);

%***********************************************************************
% BEGIN TIME-STEPPING LOOP
%***********************************************************************

for n=1:nmax

%***********************************************************************
% Update electric fields
%***********************************************************************

ex(1:ie,2:je,2:ke)=ca*ex(1:ie,2:je,2:ke)+...
cb*(hz(1:ie,2:je,2:ke)-hz(1:ie,1:je-1,2:ke)+...
hy(1:ie,2:je,1:ke-1)-hy(1:ie,2:je,2:ke));

ey(2:ie,1:je,2:ke)=ca*ey(2:ie,1:je,2:ke)+...
cb*(hx(2:ie,1:je,2:ke)-hx(2:ie,1:je,1:ke-1)+...
hz(1:ie-1,1:je,2:ke)-hz(2:ie,1:je,2:ke));

ez(2:ie,2:je,1:ke)=ca*ez(2:ie,2:je,1:ke)+...
cb*(hx(2:ie,1:je-1,1:ke)-hx(2:ie,2:je,1:ke)+...
hy(2:ie,2:je,1:ke)-hy(1:ie-1,2:je,1:ke));

ez(is,js,1:ke)=ez(is,js,1:ke)+...
srcconst*(n-ndelay)*exp(-((n-ndelay)^2/tau^2));

%***********************************************************************
% Update magnetic fields
%***********************************************************************

hx(2:ie,1:je,1:ke)=hx(2:ie,1:je,1:ke)+...
db*(ey(2:ie,1:je,2:kb)-ey(2:ie,1:je,1:ke)+...
ez(2:ie,1:je,1:ke)-ez(2:ie,2:jb,1:ke));

hy(1:ie,2:je,1:ke)=hy(1:ie,2:je,1:ke)+...
db*(ex(1:ie,2:je,1:ke)-ex(1:ie,2:je,2:kb)+...
ez(2:ib,2:je,1:ke)-ez(1:ie,2:je,1:ke));

hz(1:ie,1:je,2:ke)=hz(1:ie,1:je,2:ke)+...
db*(ex(1:ie,2:jb,2:ke)-ex(1:ie,1:je,2:ke)+...
ey(1:ie,1:je,2:ke)-ey(2:ib,1:je,2:ke));

%***********************************************************************
% Visualize fields
%***********************************************************************

if mod(n,2)==0;

timestep=int2str(n);
tview(:,:)=ez(:,:,kobs);
sview(:,:)=ez(:,js,:);

subplot('position',[0.15 0.45 0.7 0.45]),pcolor(tview');
shading flat;
caxis([-1.0 1.0]);
colorbar;
axis image;
title(['Ez(i,j,k=5), time step = ',timestep]);
xlabel('i coordinate');
ylabel('j coordinate');

subplot('position',[0.15 0.10 0.7 0.25]),pcolor(sview');
shading flat;
caxis([-1.0 1.0]);
colorbar;
axis image;
title(['Ez(i,j=13,k), time step = ',timestep]);
xlabel('i coordinate');
ylabel('k coordinate');

nn=n/2;
M(:,nn)=getframe(gcf,rect);

end;

%***********************************************************************
% END TIME-STEPPING LOOP
%***********************************************************************

end

movie(gcf,M,0,10,rect);