How can I calculate the far-field radiation pattern of a second-harmonic signal generated inside a 3D periodic structure in COMSOL?

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I am modeling second-harmonic generation (SHG) in a 3D GaAs metasurface/grating using the Wave Optics Module. The structure is illuminated at the fundamental frequency through a Port boundary condition, and the second-harmonic field is generated inside the nonlinear material through a nonlinear polarization source.

The excitation is a plane wave incident at 45° with respect to the surface normal, so the generated SH field is expected to be radiated into multiple diffraction orders and potentially over a wide angular range.

My goal is to obtain the far-field radiation pattern of the generated second-harmonic field.

The challenge is that the SH field is not excited by a Port; it is generated internally by the nonlinear polarization. Therefore, I am unsure whether COMSOL's standard Far-Field Calculation feature can be applied directly in this situation.

The model is fully 3D. The SH field is generated by a nonlinear polarization. The structure is periodic and diffraction orders are present. The SH field contains all vector components ((E_x), (E_y), and (E_z)), so a simple scalar FFT may not be sufficient. Is there a way to compute the far-field pattern of the generated second-harmonic field directly within COMSOL? Any guidance or examples of similar SHG simulations in COMSOL would be greatly appreciated.



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