It depends very much on the symmetry of the material. In general, there is always a sort of mixture contributions either from surface dipoles, bulk dipoles (non centrosymmetric), bulk quadrupoles, spatial dispersion, and even magnetic contributions for magnetic materials. The separation of these effects e.g. determining how much comes from the surface and bulk can be challenging and requires careful analysis of the symmetry e.g. group theory by looking at the tensor components and nonlinear modelling especially using the so called SBHM. I recommend the paper:
http://iopscience.iop.org/article/10.1088/2040-8978/18/3/035501/pdf
where it is shown that SHG in Si111 is dominated by interface dipole and also bulkquadrupole with relatively the same strength if you measure SHG in reflection. Meanwhile, for SHG from zincblende which are non centrosymmetric such as GaAs the main contribution is clearly bulk dipoles as is evident from SBHM measurements which must include Fresnel coefficients. This is along with group theory because the second order nonlinear susceptibility from dipolar contributions does not vanish in such system contrary to Si. I recommend you to read more about this in:
https://www.osapublishing.org/josab/abstract.cfm?uri=josab-33-2-195
I hope my answer can help you further
Thank you very much for your information Hendradi Hardhienata
I will find and carefully read the paper you recommended.
You are welcome also this may help good luck on your research..
http://link.springer.com/article/10.1007/BF01538233
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