Near-Field Infrared Vibrational Dynamics and Tip-Enhanced Decoherence

By: Xiaoji G. Xu and Markus B. Raschke

Nano Letters, 13, 1588 (2013)

Ultrafast infrared spectroscopy can reveal the dynamics of vibrational excitations in matter. In its conventional far-field implementation, however, it provides only limited insight into nanoscale sample volumes due to insufficient spatial resolution and sensitivity. Here, we combine scattering-scanning near-field optical microscopy (s-SNOM) with femtosecond infrared vibrational spectroscopy to characterize the coherent vibrational dynamics of a nanoscopic ensemble of C-F vibrational oscillators of polytetrafluoroethylene (PTFE). The near-field mode transfer between the induced vibrational molecular coherence and the metallic scanning probe tip gives rise to a tip-mediated radiative IR emission of the vibrational free-induction decay (FID). By increasing the tip-sample coupling, we can enhance the vibrational dephasing of the induced coherent vibrational polarization and associated IR emission, with dephasing times up to T2NF ~ 370 fs in competition against the intrinsic far-field lifetime of T2FF ~ 680 fs as dominated by nonradiative damping. Near-field antenna-coupling thus provides for a new way to modify vibrational decoherence. This approach of ultrafast s-SNOM enables the investigation of spatiotemporal dynamics and correlations with nanometer spatial and femtosecond temporal resolution.