Doppelspalt

Input Parameter;
Clear["Global\.b4*"]
a    =  
        2;
(Subscript[Δ, 1])  =  
        1;
(Subscript[Δ, 2])  =  
        10;
λ    =   .1;
zs   =  100;

iferenz[xs_, ys_]  :=  
        Block[{}, 
          k = ((2 π )/λ) {xs, ys}/(Sqrt[(xs^2) + (ys^2) + (zs^2)]);  
NIntegrate[(E^(I k . {x, y})) , {x, -(Subscript[Δ, 1])/
                  2, +(Subscript[Δ, 1])/2}, {y, -(Subscript[Δ, 2])/
                  2, (Subscript[Δ, 2])/
                  2}] ((E^(I k[[1]] a))  + (E^(-I k[[1]] a)))];

t = 
        Table[{x, Abs [iferenz[x, 0]]^2}, {x, -20, 
            20,  .1}];
ListPlot[t, PlotJoined -> True, 
      PlotRange -> All]