Calculated Static limit of the SHG coefficients is zero?

[Dean]

Dear all,
I am doing calculations about the SHG in 2D materials.
But my results obtained from GW-BSE approximation show that the Static limit of the SHG coefficients is zero? Is this possible and how to analyze this feature?
Thanks in advance.

[myrta gruning]

Just a couple of questions to understand better the problem:
- The static limit is found by extrapolation. Are you finding zero SHG at finite laser frequency or it is the extrapolation that gives zero?
- Are you sure that you are looking at a SHG component that should be different from zero?
Best
Myrta

[Dean]

Dear Myrta,
Thanks for your reply.

1.png

From this figure, you can see the SHG coefficients is very close to zero when the photon energy is zero.
Furthermore, I am not sure that the SHG component should be zero or not? I am a new learner about the calculation of SHG.
Thanks in advance.

[myrta gruning]

Thanks. So that SHG is not zero in general. I forgot to ask if you tried simple independent particle calculations and got the same result.
Anyway, the SHG is a 3D tensor corresponding to 3 spatial directions. The first two correspond to the applied field. These are determined by

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% ExtF_Dir

while the third direction is the emitted signal, that you read from

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o.YPP-X_probe_order_2

.
Which component is relevant it depends on the system and how the unit cell is built. If you know the symmetry of the system, there are tables, e.g. on the Boyd of the non-zero SHG components.