[[Image:FDTD47.png|thumb|250px|[[FDTD Module]]'s Gaussian Beam dialog]]
[[EM.Cube]] gives you an option to illuminate objects with a focused beam instead of a uniform plane wave. The focused beam is a Gaussian beam, which is a solution of the paraxial approximation to the Helmholtz equation. The fundamental Gaussian beam is rotationally-symmetric about its propagation axis, and its transverse field distribution follows a Gaussian function profile. The critical parameter is the beam radius w<sub>0</sub>; it is the point where the field drops by 1/e from its value at the center. The beam opens up into a cone along the propagation direction, with a cone angle of tan θ = λ<sub>0</sub>/(π.ω<sub>0</sub>) (λ<sub>0</sub> is the free-space wavelength). <font color="#a52a2a"><u>'''Â {{note|The beam radius has to be at least λ<sub>0</sub>/π; otherwise, strong fields appear outside the excitation box.'''</u></font>}}
The Gaussian beam box is displayed in the project workspace as a green wireframe box enclosing the structure. To define a new Gaussian Beam source, follow these steps: