Progress in Optics (Enhanced Edition‪)‬

Nonlinear optical phenomena can generally be ascribed to nonlinear
contributions to the polarization of a suitable medium, i.e., contributions
which are quadratic, cubic, etc. in the electric field strength of the applied
electromagnetic field. Since these nonlinear polarization terms become
large enough to produce observable effects only for fields comparable in
their amplitude with the interatomic field acting upon the electron (about
3 x lo8 V/cm), it was only after the advent of the laser, as a new and
powerful light source, that nonlinear optics became accessible to experimental
study, whereas nonlinear effects due to static electric fields - the
Pockels and Kerr effect - were already well established for some time. It
was just one year after the first laser had been successfully operated
(MAIMAN[ 1960]), that two pioneering experiments demonstrating the
nonlinear response of matter to an intense optical wave, were performed,
name!y, the observation of a two-photon absorption process (KAISERa nd
GARRETT[1 961]), and the generation of the second harmonic (FRANKEN,
HILL, PETERS and WEINREIC[H1 961]). Since then rapid developments in
this field, both experimental and theoretical, have taken place.