[1d fdtd yee algorithm using fortran 90] unexpected graph- what is the problem?

!d fdtd Simulation in free space 
subroutine fd1d01(f0,miu,delta,S,E0)
implicit none

real                               :: f0 
real                               :: miu
real                               :: delta 
real                               :: S 
real                               :: E0
integer                            :: iinit 
integer                            :: ilast 
integer                            :: i,j
integer                            :: t,t0
double precision                   :: di
integer                            :: n
double precision                   :: Ca
double precision                   :: Da
double precision                   :: tdelta 
double precision                   :: c
double precision,dimension(100)    :: Ez
double precision,dimension(100)    :: Hy
real, parameter                    :: pi = 3.14159265
double precision                   :: Cb
double precision                   :: Db
double precision                   :: lambda
double precision                   :: alpha
character(len=20)                  :: filename

f0 = 1.0
miu = 1.0
delta = 1.0 
S = 1.0
E0 = 1.0
iinit = 0 
ilast = 100
 
c = 3.e8
lambda = c/f0
alpha = 0.04*lambda
tdelta = 1.0*alpha/(S*c)

!initialization
Ez = 0
Hy = 0

Ca = 1.0
Cb = tdelta/(delta*alpha)
Da = 1.0
Db = tdelta/(miu*alpha)

do n = 0,500
 write (filename, "('ez',I3.3,'.dat')") n
 open (unit=130,file=filename)
  do j = 0,2*ilast
     di = 0.5*j 
     if (Mod(j,2) == 0 ) then  !j is even; hence di is whole number
        Ez(Int(di-0.5)) = Ca*(Ez(Int(di-0.5)))+Cb*(Hy(Int(di))-Hy(Int(di-1)))
     else  !j is odd; hence di is 1/2 of whole number
        Hy(Int(di)) = Da*(Hy(Int(di)))+Db*(Ez(Int(di+0.5))-Ez(Int(di-0.5)))
     end if
  write (130,*) Ez(Int(di))
  end do

  !plane wave source
  Ez(50) = E0*sin (2*pi*f0*n*tdelta)

 close (unit=130)

end do !n

end SUBROUTINE fd1d01