Weak-pulse transparency enhancement in an optically dense three-level medium induced by a 2π pulse in a neighboring transition (V-scheme)

The coherent V-configuration interaction between an optically dense resonantly-absorbing three-level medium (neon) and two ultrashort superradiance pulses with converging wave fronts is investigated experimentally and theoretically. Both separate and combined propagation of pulses with wavelengths λ1=614.3 nm (strong field, θ1⩾π) and λ3=594.5 nm (weak field, θ3≈/20) are studied. For propagation of a separate strong-field pulse, supertransparency of the absorbing medium was observed, which is associated with the generation of a soliton-like pulse at the difference frequency (Δν≈1700 MHz) and the dispersion-diffraction stabilization effect. Under these conditions a weak-field pulse is completely absorbed. Combined propagation of the pulses leads to novel effects. A below-threshold pulse (weak field) was observed to pass through the absorber while interacting coherently with a strong-field pulse at a neighboring transition. It is shown theoretically that absorption of the weak pulse is reduced for two reasons: first, as a result of incoherent transparency of the resonance transition caused by emptying of the lower level by the field of the strong pulse, and second, as a result of coherent transfer of polarization between the upper levels via the two-photon processes. When the conditions for combined propagation are met, the latter mechanism ensures inversionless amplification of a weak pulse over a wide band of frequencies. In this case, the gain can even exceed the linear absorption coefficient in absolute value. A difference in propagation velocities of the weak and strong pulses was recorded experimentally, along with a shift in the carrier frequency of the weak field towards the red (≈600 MHz). A mechanism for transfer of phase modulation from a strong pulse to a weak pulse via the common lower level is discussed theoretically.