Coherent control of excitation pathways in optical two-dimensional coherent spectroscopy of rubidium atoms

We demonstrate that controlled initial-state preparation in rubidium (Rb) vapor fundamentally alters its nonlinear optical response, as probed by two-dimensional coherent spectroscopy (2DCS). By varying the intensity of a prepulse, we tune its pulse area, thereby manipulating the initial-state population dynamics of the D_2 transition (5^2S_{1/2} → 5^2P_{3/2}). This selectively activates different quantum pathways, revealed in the 2DCS spectra by distinct one- and two-atom state amplitudes that oscillate with prepulse power, a clear signature of Rabi-type population cycling. A time-dependent two-level Rabi model confirms this behavior, showing that the \pi-pulse condition coincides with the maximum amplitude of the two-atom state. Fits based on this model connect the measured 2DCS peak amplitudes directly to the population dynamics induced by the prepulse, enabling quantitative extraction of the population-transfer efficiency. This establishes 2DCS as a powerful probe of population-prepared coherence dynamics, enabling direct mapping of how initial-state populations govern nonlinear optical responses in atomic vapors.