  The propagation of two-color laser fields through optically thick atomic
ensembles is studied. We demonstrate how the interaction between these two
fields spawns the formation of co-propagating, two-color soliton-like pulses
akin to the simultons found by Konopnicki and Eberly [Phys. Rev. A 24, 2567
(1981)]. For the particular case of thermal Rb atoms, exposed to a combination
of a weak cw laser field resonant on the D1 transition and a strong, sub-ns
laser pulse resonant on the D2 transition, simulton formation is initiated by
an interplay between the 5s_{1/2} -- 5p_{1/2} and 5s_{1/2} -- 5p_{3/2}
coherences which amplifies the D1 field at the arrival of the D2 pulse
producing sech-squared pulse with a length of less than 10 microns. This
amplification is demonstrated in a time-resolved measurement of the light
transmitted through a thin thermal cell. We find good agreement between
experiment and a model that includes the hyperfine structure of the relevant
levels. With the addition of Rydberg dressing, quasi-simultons offer
interesting prospects for strong photon-photon interactions in a robust
environment.

Atomic Physics