I am not certain what the problem is, however even with a relatively high mesh density, there is no detail that would suggest something similar to the posted image.
syms x y
lambda = 1060E-9;
M =1;
z=100;
k=2*pi/lambda;
b=0.1;
wo=0.02;
delta=(1i*k)/(2*z);
A = ((k.^2)-(4.*z.^2.*delta));
B = ((1i.*k.*b)./(2.*z.*wo.*delta));
C = ((b.^2)./(4.*wo.^2.*delta));
f1=(k/(2*z)).^2;
f2=(1/(2.*1i.*sqrt(delta))).^M;
f3=exp((A.*x.^2)+(B.*x)+C);
f4=exp((A.*y.^2)+(B.*y)+C);
I(x,y)=f1.*f2.*f3.*f4 % Create 'I' As A Funciton Of '(x,y)'
figure
fcontour(real(I), [-1 1 -1 1]*5E-6, 'Fill','on', 'MeshDensity',150)
colormap(turbo)
axis('equal')
figure
fsurf(real(I), [-1 1 -1 1]*5E-6, 'MeshDensity',150)
colormap(turbo)
% axis('equal')
.
