Abstract
By setting laser beams in two-photon Doppler-free configurations for both cascade and Λ-type subsystems in a four-level inverted-Y system, a four-wave mixing (FWM) process is studied and optimized within the Doppler profile. The two-photon ac-Stark effect in the FWM signal is observed, which can be used to determine the energy level shift of the atom due to dressing fields.