
86 Second Harmonic Generation Imaging
SHG nonlinear process. All third-order nonlinear processes can be viewed as four-wave mixing pro-
cesses, and the material wave in the case of TPA is induced by two optical waves of the same frequency.
TPA in semiconductors is among the most thoroughly studied in nonlinear optics [22]. e resonant
condition (a stable upper state) is characterized by an imaginary
χ
(3)
, while a real
χ
(3)
is attributed to the
third-order nonlinear refraction, much in the same way that the real and imaginary components of the
linear susceptibility
χ
(1)
are associated for linear refraction and absorption.
4.2.3 THG Photophysics ...