===Focused Gaussian Beams===
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[[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>'''The beam radius has to be at least λ<sub>0</sub>/π; otherwise, strong fields appear outside the excitation box.'''</u></font>
* The direction of the Gaussian Beam is determined by the incident '''Theta''' and '''Phi''' angles in degrees. You can also set the '''Polarization''' of the Gaussian Beam and choose from the three options: '''TM<sub>z</sub>''', '''TE<sub>z</sub>''' and '''User Defined'''.
* Unlike plane waves, a Gaussian beam is a localized field. Therefore, you need to specify the '''Beam Properties'''. This includes the coordinates of the beam's '''Focus''', which is the beam's waist center in the world coordinate system as well as the beam's '''Radius''' in project units.
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[[Image:FDTD47.png]]
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Figure 1: [[FDTD Module]]'s Gaussian Beam dialog.
A Gaussian beam box placed around a horizontal PEC plate. The trident at the corner of the box shows the propagation vector as well as the E-field and H-field polarization vectors. The titled transparent green circle shows the footprint of Gaussian beam at its focal (waist) point.