  Ultrafast two-dimensional spectroscopy utilizes correlated multiple
light-matter interactions for retrieving dynamic features that may otherwise be
hidden under the linear spectrum. Its extension to the terahertz regime of the
electromagnetic spectrum, where a rich variety of material degrees of freedom
reside, remains an experimental challenge. Here we report ultrafast
two-dimensional terahertz spectroscopy of gas-phase molecular rotors at room
temperature. Using time-delayed terahertz pulse pairs, we observe photon echoes
and other nonlinear signals resulting from molecular dipole orientation induced
by three terahertz field-dipole interactions. The nonlinear time-domain
orientation signals are mapped into the frequency domain in two-dimensional
rotational spectra which reveal J-state-resolved nonlinear rotational dynamics.
The approach enables direct observation of correlated rotational transitions
and may reveal rotational coupling and relaxation pathways in the ground
electronic and vibrational state.
