Invited
Two-dimensional infrared femtosecond spectroscopy of ultrafast quantum dynamics with local fields of plasmonic antenna

Studies of molecular structure and its environment using localized vibrational modes as sensitive probes is frequently done with vibrational spectroscopy by observing the shifts of the vibrational frequency and change in the transition lineshape. However, vibrational excitations are often dominated by inhomogeneous broadening, making the corresponding studies extremely challenging. Third-order two dimensional femtosecond vibrational spectroscopy (2DIR) meets these challenges. Here, the spectrum is spread in two dimensions, revealing the correlation between the excitation and detection frequencies. It separates between the inhomogeneous and homogeneous lineshape components, reveals intra- and inter-molecular coupling, and greatly simplifies interpretation of the congested spectra. The waiting time dependence of the vibrational frequency correlations provides direct access to the observables associated with ultrafast dynamics.

Another difficulty in vibrational spectroscopy arises from the low transition dipole strengths of vibrational excitations and poor detectivity in the mid-infrared, which limits the applications to those cases, where large amounts of analyte are available. To address this problem, surface enhancement of the signal by the near-fields of noble metal nanostructures supporting localized plasmon resonances in the infrared is used. Here, the nanostructures’ material, size, and shape can be engineered to manipulate the near-fields on the length scale several orders of magnitude below the wavelength of the excitation light.

Surface-enhanced 2DIR allows studies of ultrafast structural dynamics in minute amounts of molecules located within the reach of the enhanced near-fields. 2DIR signal enhancement of up to five orders of magnitude have been observed, whereas the molecular quantum dynamics have not been affected by the interaction with the plasmonic excitation. However, among the consequences of signal amplification are dispersive lineshapes of the vibrational transitions, competition between the near-field coupling and the radiation damping enhancement mechanisms, and lack of polarization selectivity imposed by the surface boundary conditions.