Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase

TL;DR

This paper demonstrates ultrafast two-dimensional terahertz spectroscopy of gas-phase molecules, revealing detailed rotational dynamics and couplings through nonlinear signals and photon echoes.

Contribution

It introduces a novel experimental method for 2D terahertz spectroscopy of gas-phase molecules at room temperature, enabling direct observation of rotational transitions.

Findings

Observation of photon echoes in gas-phase molecules

Mapping of nonlinear rotational spectra in the frequency domain

Detection of J-state-resolved rotational dynamics

Abstract

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.