Adiabatic Following and Slow Optical Pulse Propagation in Rubidium Vapor

Short pulses of narrow-line low-intensity dye-laser light nearly resonant with the Zeeman-split ^2P₁₂ resonance line (7948) of rubidium were observed to propagate through dilute rubidium vapor as slowly as (114) c. These slow pulse velocities showed that most of the energy in the propagating wave was contained by the vapor as coherent atomic excitation. The observed pulse velocities v₏ are in good agreement with the equation v₏=ddk for the group velocity obtained from linear-dispersion theory. Also, the experimental results are quantitatively explained by adiabatic following, in which the pseudomoments of the atoms remain aligned along the effective field of the laser light. The adiabatic-following model allows for a direct comparison of our results with the work on self-induced transparency. For high-intensity light, adiabatic following predicts a nonlinear pulse velocity and the possibility of observing self-steepening.