Splitting of the peak of electromagnetically induced transparency by the higher-order spatial harmonics of the atomic coherence

The interaction of a two-level system with a strong standing wave is examined by the probe field resonant to the adjacent transition in the Λ configuration of Ar II. The known peak of electromagnetically induced transparency is observed in the absorption spectrum at the detuning of the probe field proportional to the detuning of the strong field. Along with it, the different structure has been found at the line center, independently of the strong-field frequency. The calculations clarify that the central structure is induced by the higher-order spatial harmonics of the atomic coherence induced at the probe transition. The effect results in steep frequency dependence of the refractive index at the line center.