WEAK-PULSE TRANSPARENCY ENHANCEMENT IN AN OPTICALLY DENSE 3-LEVEL MEDIUM INDUCED BY A 2-PI PULSE IN A NEIGHBORING TRANSITION (V-SCHEME)

The coherent V-configuration interaction between an optically dense re sonantly-absorbing three-level medium (neon) and two ultrashort superr adiance pulses with converging wave fronts is investigated experimenta lly and theoretically. Both separate and combined propagation of pulse s with wavelengths lambda(1) = 614.3 nm (strong field, theta(1) greate r than or equal to pi) and lambda(3) = 594.5 nm (weak field, theta(3) approximate to pi/20) are studied. For propagation of a separate stron g-field pulse, supertransparency of the absorbing medium was observed, which is associated with the generation of a soliton-like pulse at th e difference frequency (Delta upsilon approximate to 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) wa s observed to pass through the absorber while interacting coherently w ith a strong-field pulse at a neighboring transition. It is shown theo retically that absorption of the weak pulse is reduced for two reasons : first, as a result of incoherent transparency of the resonance trans ition caused by emptying of the lower level by the field of the strong pulse, and second, as a result of coherent transfer of polarization b etween the upper levels via the two-photon processes. When the conditi ons for combined propagation are met, the latter mechanism ensures inv ersionless amplification of a weak pulse over a wide band of frequenci es. In this case, the gain can even exceed the linear absorption coeff icient in absolute value. A difference in propagation velocities of th e weak and strong pulses was recorded experimentally, along with a shi ft in the carrier frequency of the weak field towards the red (approxi mate to 600 MHz). A mechanism for transfer of phase modulation from a strong pulse to a weak pulse via the common lower level is discussed t heoretically. (C) 1998 American Institute of Physics.