We study phase-conjugate six-wave mixing spectroscopy based on electromagnetically-induced-transparency in a Doppler-broadened folded four-level system.It is found that the six-wave mixing spectrum can be either Doppler-free or very broad,depending on whether the interference between the polarisations of atoms with different velocities is constructive or destructive.To obtain the Doppler-free six-wave mixing spectrum in the folded four-level system,the conditions are more stringent in comparison with those in the cascade and N-type four-level systems.This polarisation interference can be controlled in the presence of a strong coupling field.
We study the electromagnetically-induced transparency(EIT)in a Doppler-broadened cascaded three-level system.We decompose the susceptibility responsible for the EIT resonance into a linear and a nonlinear part,and the EIT resonance reflects mainly the characteristics of the nonlinear susceptibility.It is found that the macroscopic polarization interference effect plays a crucial role in determining the EIT resonance spectrum.To obtain a Doppler-free spectrum there must be polarization interference between atoms of different velocities.A dressed-state model,which analyzes the velocities at which the atoms are in resonance with the dressed states through Doppler frequency shifting,is employed to explain the results.
