
96 Second Harmonic Generation Imaging
the target to be in a bulk noncentrosymmetric microenvironment, THG does not have this constraint.
However, THG is not highly versatile as a change in refractive index is required when focused beams
are used. is is because the signal arises from optical heterogeneities in the specimen where there are
changes in the third-order nonlinear susceptibility,
χ
(3)
. One point of comparison comes from imag-
ing biological membranes with both SHG and THG. SHG images the two-dimensional membrane
itself where dye molecules have assembled parallel to each other [16,21,57]. In contrast, THG does so
by probing the volu ...