ABSTRACT
T he advent of the tunable laser has made possible the study of processes in which more than one photon is absorbed so that the atomic sys-
tem gains several ~ω of energy-this chapter deals with such multiphoton processes. Highly excited states may be populated by these mechanisms creating population inversions with respect to intermediate states so that lasing or superradiant emission (a form of coherent spontaneous emission with short lifetime) occurs. It is even possible to observe lasing action from virtual states (dressed atoms) as in coherent anti-Stokes Raman scattering (CARS).
