2-PHOTON GAIN AND LASING IN STRONGLY DRIVEN 2-LEVEL ATOMS

Driven two-level atoms are shown to exhibit strong, resonantly enhanced, two-photon gain. This gain, unlike that observed in nonlinear mixing processes, is not constrained by a phase-matching condition involving the pump beam. Physically, the gain can be associated with inverted two-photon transitions between dressed states or multiphoton stimulated Raman scattering, and it may be useful in the realization of a two-photon laser. Comparison with the well-known single-photon gain found in the same system is made. © 1990 The American Physical Society.