Nanooptics

The sensitivity of molecular spectroscopy based on surface-enhanced coherent anti-Stokes Raman scattering (SECARS) is limited by the spectrally overlapping background from nonresonant four-wave mixing (FWM). While the SECARS signal is mediated by the molecular vibrational eigenstates and exhibits long lifetime (a few picoseconds), the FWM background stems from the instantaneous electronic polarization and decays rapidly with the vanishment of excitation. Therefore, CARS and FWM can be separated by time-resolved CARS (trCARS) using ultrashort pulsed lasers. The broad spectral bandwidth also enables broadband CARS (BCARS) for simultaneous identification of multiple vibrational signatures. This work combines trCARS and BCARS with an optimized plasmonic system to demonstrate time-resolved surface-enhanced BCARS) and show its capability in obtaining background-suppressed broadband vibrational spectra from a monolayer of 4-Aminothiophenol (4-ATP) molecules self-assembled on a gold grating. The vibrational dephasing time of the ring-breathing mode for the 4-ATP monolayer on the gold grating (≈1.00 ± 0.17 ps) is significantly shortened compared to that for 4-ATP powder on a glass substrate (2.10 ± 0.05 ps). This work presents an effective method to suppress FWM background and investigate the vibrational dynamics of molecules in the vicinity of plasmonic nanostructures.