Thanks for your reply . I try to simulate the 3d model of a 2 slit . and get the far field from that. I want to plot the far. field distribution and my plot axis be wavelength vs angle. I did it in 2d FDTD and could get the result but for 3d I couldnt have it. here is how I simulate the structure and the script would be as follow like the picture. in my script the only change I made is the , I change the farfieldexact2d to 3d. my monitor is 2D monitor z normal .