Terahertz pulse shaping through propagation in a gas of symmetric top molecules

Symmetric top molecules of methyl iodide are irradiated with a terahertz pulse generated by a two-color plasma and shaped by a short propagation in air. Free-induction decay is emitted by the excited molecular sample and then propagates in air before detection. The experimental data show that the input terahertz (THz) pulse undergoes strong reshaping through absorption and dispersion. This leads to narrow wave packets at revival times due to the excitation of high rotational energy levels. Typically, a THz burst of duration $\ensuremath{\simeq}15\ensuremath{-}20$ ps is produced periodically, with a central frequency of $\ensuremath{\simeq}1$ THz and a width that can be as narrow as 60--80 GHz. Pulse shaping based on propagation can be useful for quantum control in molecules. We provide a theoretical description of this wave propagation based on the Maxwell-Bloch equation. The observed experimental signal is in good agreement with the numerical simulations.