Billion-Fold Increase in Tip-Enhanced Raman Signal

Abstract

A billion-fold increase in the Raman signal over conventional tip-enhanced Raman spectroscopy/microscopy (TERS) is reported. It is achieved by introducing a stimulating beam confocal with the pump beam into a conventional TERS setup. A stimulated TERS spectrum, closely corresponding to its spontaneous TERS counterpart, is obtained by plotting the signal intensity of the strongest Raman peak of an azobenzene thiol self-assembled monolayer versus the stimulating laser frequency. The stimulated TERS image of azobenzene thiol molecules grafted onto Au ⟨111⟩ clearly shows the surface distribution of the molecules, whereas, when compared to the simultaneously recorded surface topography, it presents an image contrast of different nature. The experimentally obtained stimulated gain is estimated at 1.0 × 10(9), which is in reasonable agreement with the theoretically predicted value. In addition to the signal increase, the signal-to-noise ratio was 3 orders of magnitude higher than in conventional spontaneous TERS. The proposed stimulated TERS technique offers the possibility for a substantially faster imaging of the surface with respect to normal TERS.

Nonlinear spectroscopy of plasmonic nanoparticles

“…The latter leads to homodyning with the incoming probe wave. Wickramasinghe et al [109] used the field enhancement at the sharp gold tip of a scanning tunnelling microscope to locally increase the stimulated Raman scattering efficiency.…”

Tip-enhanced Raman spectroscopy: From concepts to practical applications

“…[31] Wickramasinghe et al demonstrated high sensitivity in stimulated TERS, reporting billion-fold enhancement of an azobenzene thiol band when compared to spontaneous TERS. [32] While significant advancements have been made in the field, some issues remain unresolved. For example, high peak powers within individual laser pulses cause intense electromagnetic fields and likely heat at the tip-sample junction, potentially affecting the stability of molecules in the enhancing region.…”

The optical near-field: super-resolution imaging with structural and phase correlation

“…First tunneling to a gold surface has been shown to be an effective method to enhance Raman of molecular species placed on gold [66]. Second tunneling is a known method to investigate plasmonic optical effects [67]. Third tunneling is a good method to induce carriers in semiconductors.…”