Wrong interference pattern (Double-Slit experiment)

I want to simulate the double slit experiment. The width of each slit is 0.1 and the distance between them 10. My try at the code is the following (using Split Step Fourier Function)

clc; clear all; format long;
N=1024; L=100; x=(L/N)*(-N/2:N/2-1); dx=x(2)-x(1);
k=(2*pi/L)*[0:N/2-1 -N/2:-1]; 
z=linspace(-50,50,1);
w=10; uin=exp(-(x/w).^2).*exp(1i*k.*z); % initial condition
%figure(1); plot(x,abs(uin),'r');
a=1/10; d=10;
if (x>=0) & (x<=a)
    po=zeros(1,N);
elseif (x>=10+a) & (x<=10+2*a)
    po=zeros(1,N);
elseif (x>a) & (x<10+a)
    po=10*ones(1,N);
else
    po=ones(1,N);
end;

S=1000; zmax=20; h=zmax/S; z=0; count=0; dim=0; savestep=10; u0=uin; 
for m=1:S
u1=ifft(fft(u0).*exp(-1i*0.5*h*k.^2));   % First step of SSF
u2=exp(1i*h*po).*u1;                               % Second step of SSF
u3=ifft(fft(u2).*exp(-1i*0.5*h*k.^2));   % Second step of SSF
u0=u3; z=z+h; count=count+1;
if (count==savestep)
    dim=dim+1; field(dim,:)=u0; pot(dim,:)=po; z1(dim)=z; count=0;
end;
end;
[X,Z]=meshgrid(x,z1);
figure(2); surf(X,Z,abs(field)); shading interp; axis square; axis tight; view([0 90]);

As you can see, this is not what I should be getting. I think that po should be somewhat different, but I don't know how. Any help would be appreciated.