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Based on our experimental observations of the resonant stimulated Raman gain and loss spectra in Rb atomic vapor, we propose an alternative interpretation of electromagnetically induced transparency (EIT). We find that, in the presence of a coupling field, a probe beam can exhibit both gain and loss depending on the frequencies of the incident beams. The gain and loss can coexist in a Doppler-broadened system, leading to polarization interference between atoms of different velocities, with a sharp transition from gain to loss as the probe-field frequency is scanned. We use the concept of Raman gain, instead of the commonly accepted quantum Fano interference, to explain the phenomenon of EIT and its transparency window as being due to the suppression of linear absorption by the Raman gain. Using this concept, the phenomena of EIT and Autler-Townes splitting may be explained within the same framework.

The effects of Doppler broadening on Autler-Townes (AT) splitting in six-wave mixing (SWM) are investigated by the dressed-state model. We analyze the velocities at which the atoms are in resonance with the dressed states through Doppler frequency shifting and find that, depending on the wave-number ratio, there may be two resonant velocities which can originate from resonance with one of the dressed states or from resonance with two different dressed states. Based on this model, we discuss a novel type of AT doublet in the SWM spectrum, where macroscopic effects play an important role. Specifically, the existence of resonant peaks requires polarization interference between atoms of different velocities in addition to a change in the number of resonant atoms involved. Our model can also be employed to analyze electromagnetically induced transparency resonance and other types of Doppler-free high-resolution AT spectroscopy.