Selective Plasmonic Platforms Based on Nanopillars to Enhance Vibrational Sum-Frequency Generation Spectroscopy

The enhancement of the vibrational sum-frequency generation (SFG) signal from molecules adsorbed on metallic nanopillars excited at a resonance frequency matching their localized surface plasmons is reported. The nanopillars stand vertically on a metal surface and possess two plasmon modes that can be selectively excited by either the incident visible laser beam, or the generated SFG beam itself. This nanostructured platform increases the molecular SFG signal of a monolayer by two orders of magnitude. The localization and the geometry of the two plasmon modes enables to probe the molecules adsorbed onto the vertical nanopillar wall, or on the top of it, or on the flat surface between the pillars, selectively. In practice, this spatial selectivity is set by switching the polarization of the visible and SFG beams at resonance. Owing to the largely improved sensitivity combined with a specific spatial selectivity, plasmon-enhanced SFG boosts the versatility of second-order vibrational SFG spectroscopy or microscopy. This makes them promising platforms in the development of analytical molecular devices. Molecules adsorbed on metallic nanopillars give an enhanced vibrational sum-frequency generation (SFG) signal when a localized surface plasmon resonance is excited. The nanopillars possess two plasmon modes that can be selectively excited by either the incident visible or the generated SFG polarized laser beams. Depending on which mode is excited, one can probe molecules adsorbed at different regions of the nanostructured surface selectively, i.e., the vertical pillar sidewall, the pillar top, or the flat surface between the pillars. Owing to the largely improved sensitivity combined with a specific spatial selectivity, a plasmon-enhanced SFG boosts the versatility of SFG spectroscopy with regard to specific chemical sensing.