We describe experimentally accessible diagnostics for the excitation of optically dense frequency-selective media by linear frequency-chirped pulses using a sensitive pump–probe technique on the to transitions of 1.0% Distinct features within a transmitted cw probe pulse are used to identify the combination of linear chirp rate and optical power needed to produce an average Bloch-vector rotation of 90°. The resulting superposition state is thus an equal mixture of the ground and excited states on average. We find experimentally a linear relationship between the applied chirp-pulse intensity and chirp rate required to produce this half-inversion, a conclusion supported by both analytical calculations made using a Landau–Zener approach, and detailed computer simulations using the Maxwell–Bloch model. The numerical simulations predict experimentally observed phenomena such as the reshaping of probe pulses by stimulated emission or absorption. Finally, we quantify the relationship between chirp rate and optical power for half-inversion as a function of the optical density of the medium. The pump–probe experimental techniques and simulation analysis techniques developed here can be extended to produce an arbitrary mixture of ground and excited states, on average, in media spanning a wide range of optical density. Preparation of such media by chirped pulses for applications in quantum computing, photon-echo-based time-domain storage, and signal processing will be aided by these techniques.
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