
Second Harmonic Generation (SHG) 173
correlation. Owing to the characteristics of ultrashort pulses, a number of new
features unknown in the conversion of cw light have to be considered [12–16].
We will examine first the relatively simple case of type I SHG, in which the
fundamental wave propagates as an ordinary (o) or extraordinary (e) wave, pro-
ducing an extraordinary or ordinary second harmonic (SH) wave, respectively.
We will briefly discuss at the end of this section the more complex case of type
II SHG, in which the nonlinear polarization, responsible for the generation of a
second harmonic propagating as an e wave, is proportional to the product ...