Surface-enhanced Raman spectroscopy (SERS) has attracted a lot of attention in molecular sensing because of the remarkable ability of plasmonic metal nanostructures to enhance the weak Raman scattering process.

On the other hand, coherent vibrational spectroscopy triggered by impulsive excitation using ultrafast laser pulses provides complete information about the temporal evolution of molecular vibrations, allowing dynamical processes in molecular systems to be followed in“real time”.

Recently, researchers at Shanghai Institute of Optics and Fines Mechanics (SIOM) of Chinese Academy of Sciences have combined these two concepts and demonstrate surface-enhanced impulsive vibrational spectroscopy. The study was published in Scientific Reports.

They combined the advantages of surface enhancement and impulsive laser spectroscopy to simultaneously address both the ground- and excited-state vibrations in the time scale of molecular motions. This technique fully satisfies the requirements for nanoscale real-time molecular sensing, which include small background, rapid signal acquisition, high enhancement, good reproducibility, as well as high temporal and spectral resolution.

Then, this possibility was demonstrated by using thin films of MEH-PPV, one of the most important semiconducting polymers, spin-coated on a substrate covered with monodisperse silver (Ag) NPs.

Real-time molecular vibrations contributed from both the electronic ground and excited states are triggered by the impulsive excitation with sub-10 fs laser pulses, which give rise to coherent wave packet motions in the molecules.

The local-field enhancement supported by the Ag NPs is shown to couple to the dynamics of molecular vibrations associated with the electronic transition in MEH-PPV. The time-resolved enhancement factors of the vibrational modes due to both excited- and ground-state wave packet motions are recorded as a function of the probe wavelength.

They found that the time-dependent spectroscopic signals close to the NP surface could be enhanced by a factor of 470, while at the front surface of MEH-PPV the signal can be enhanced by a factor of 4.6 compared to a similar reference sample with no NPs. Importantly, the temporal shape of the signals remains essentially unchanged in spite of the enhanced signal levels.

Figure: Results with/without NPs.
Show in (a,b,c) are experimental results for MEH-PPV, and shown in (d,e,f) are those for NP/MEH-PPV. (a,d) Two-dimensional (probe wavelength versus probe delay time) difference absorption spectra; (b,e) molecular vibrations extracted from (a,d), respectively; (c,f) molecular vibrational   spectra obtained by taking Fourier transforms of (a,d), respectively. (Image by Du Juan)