Ultranarrow Dispersive Profiles due to Optical Pumping Effect

An unexpected profile has been found in the coherent two-field excitation spectrum of a two-level system. This profile is dispersive and its width can be as small as 0.02Γ, where Γ is the natural linewidth of the excited state. Our study shows that the profile is due to the ultranarrow resonance in the spontaneously transferred population convoluting the Mollow dispersion.