Nonlinear excitation by quantum interference in a Doppler-broadened rubidium atomic system

We analyze nonlinear excitation in a four-level atomic system that exhibits electromagnetically induced transparency induced by a strong coupling laser. We show that, at the line center of the atomic transition, the nondegenerate two-photon excitation in the dressed states can be enhanced by constructive quantum interference in two excitation paths while the linear absorption is inhibited by destructive quantum interference. We report an experimental study of the interference-enhanced two-photon absorption in a multilevel Λ-type rubidium atomic system and compare the measurements with the theoretical calculations.