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3.1 Introduction
ird-harmonic generation (THG) and second-harmonic generation (SHG) are both coherent nonlinear
processes that can be used for microscopy, and that can be produced and detected simultaneously using
a single laser. However, they are sensitive to dierent electronic symmetries and obey dierent phase-
matching conditions, so that THG and SHG images are usually both distinct and complementary. Since
the principles and applications of SHG microscopy are discussed at large in other chapters, the aim of
this chapter is to discuss the parameters that govern THG eciency, and to present a few examples where
combined THG−SHG−2PEF imag ...