Terahertz beats of vibrational modes in methanol and ethanol selectively excited by tr-CARS technique

Abstract

A recently developed time-resolved coherent anti-Stokes Raman scattering (tr-CARS) technique allows the measurement of vibrational coherences with high frequency differences with the ambient environment. The method is based on the short spatial extension of femtosecond pulses with a broadband tunable nonlinear optical parametric amplifier (NOPA) and an internal time delay between the probe and pump/Stokes pulse pair in the CARS process. The different beat frequencies between Raman modes can be selectively detected as oscillations in the tr-CARS transient signal with the broadband tunable NOPA. In this work, we aim at the Raman C—H stretching vibrations from 2800 cm−1 to 3000 cm−1, within which the different vibrational modes in both ethanol and methanol are selectively excited and simultaneously detected. The high time resolution of the experimental set-up allows one to monitor the vibrational coherence dynamics and to observe the quantum beat phenomena on a terahertz scale. This investigation indicates that the femtosecond tr-CARS technique is a powerful tool for the real-time monitoring and detection of molecular and biological agents, including airborne contaminants such as bacterial spores, viruses and their toxins.