
Second Harmonic Generation Imaging56
3.2.5 Size Effects
We consider the case of a sphere of diameter d with a nonlinear susceptibility
χ
(3)
= 1 centered at the
focus, and embedded in a medium with the same index of refraction, but a null nonlinear susceptibility.
We note that, because there is no far-eld THG signal from an isotropic normally dispersive homoge-
neous medium [4], and because the propagation of the harmonic eld is a linear process, it is equivalent
to consider an object with a nonlinear susceptibility
χ
1
3( )
embedded in a medium with a nonlinear
susceptibility
χ
2
3( )
, or to consider an object with a nonlinear suscept