2D photon echo

Proc. Natl Acad. Sci. USA 113, 11800–11805 (2016)

Nonlinear 2D rotational spectroscopy of gas-phase molecules has been performed in the terahertz (THz) frequency range by researchers from the USA and Israel. Two time-delayed collinear THz pulses were generated from a LiNbO₃ crystal by optical rectification and focused into a gas cell containing CH₃CN at room temperature at 70 torr. The field strength of each pulse was 400 kV cm⁻¹ inside the sample gas cell. The transmitted THz pulses were detected by electro-optical sampling with a ZnTe crystal. The 2D time-domain nonlinear signal was recorded as a function of the time difference between the THz pulses and the read-out delay time. The team observed four types of third-order signal that arose from three THz field interactions with the molecule dipoles: rephasing (photon echo), non-rephasing, two-quantum, and pump–probe signals. The different phenomena could be distinguished from the time-domain traces of the nonlinear signal. The 2D rotational spectroscopy permitted direct measurement of rotational dephasing, population relaxation dynamics and spectral correlations.