Experimental demonstration of phase control of dispersion effects for an ultrashort pulse train propagating in a resonant medium

Influence of propagation and external phase in sequential two-photon absorption of femtosecond pulses

We study the influence of shaping the phase of the optical field when time-delayed femtosecond pulse pairs excite a sequential two-photon transition in rubidium atoms. Propagation through this optically dense medium modifies the pulse profile. When an external phase is introduced into one of the pulses, partial cancellation of the two-photon absorption signal is observed for temporal delays much greater than the pulse duration. This behavior is qualitatively explained in terms of a negative group delay.