Evidence for a Local Field Effect in Surface Plasmon-Enhanced Sum Frequency Generation Vibrational Spectra

Abstract

Surface plasmon-enhanced vibrational spectroscopy has been demonstrated to be an important highly sensitive diagnostic technique, but its enhanced mechanism is yet to be explored. In this study, we couple femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) with surface plasmon generated by the excitation of localized gold nanorods/ nanoparticles and investigate the plasmonically enhanced factors (EFs) of SFG signals from poly(methyl methacrylate) films. Through monitoring the SFG intensity of carbonyl and ester methyl groups, we have established a correlation between EFs and the coupling of localized surface plasmon resonance with SFG and visible beams. It is found that the total enhanced factor is approximately proportional to the square of an enhanced factor of the SFG electromagnetic field and the fourth power of the enhanced factor of the visible electromagnetic field. The local field effect is roughly expressed to be the square of an enhanced factor of the visible electromagnetic field. This finding will help to guide the experimental design of plasmonenhanced SFG to drastically improve the detection sensitivity and thus provide greater insight into the ultrafast dynamics near plasmonic surfaces.

Enhanced Sum Frequency Generation for Monolayers on Au Relative to Silica: Local Field Factors and SPR Effect

“…Surface plasmon resonance (SPR) is reported to play a significant role in SFG signal enhancement. − ,,, Recently, Tan et al provided evidence that the SFG enhancement factor is proportional to the enhanced factors of the sum and visible electromagnetic fields due to the coupling of the localized surface plasmon generated by the excitation of the gold nanorods/nanoparticles with the sum and visible beams . The remaining enhancement factors of 8.1 and 11.5 for the ss and as modes in our study most likely result from the SPR contribution.…”

“…Distinct influencing factors, especially the SPR effect and the interplay between F ijk and χ ijk terms, should be carefully considered when investigating the SFG signal enhancement. It is worth mentioning that some particular molecular groups polarized by Au surface plasmon (for example, phenyl and pyridine) and a χ (3) term (geometrically thick ones) might contribute to the observed SFG enhancements. ,,, These possible influencing factors are beyond the scope of this study; further theoretical and experimental studies are necessary.…”

“…It is worth mentioning that some particular molecular groups polarized by Au surface plasmon (for example, phenyl and pyridine) and a χ (3) term (geometrically thick ones) might contribute to the observed SFG enhancements. 34,82,93,94 These possible influencing factors are beyond the scope of this study; further theoretical and experimental studies are necessary.…”

“…[28][29][30][31]34,89,90 Recently, Tan et al provided evidence that the SFG enhancement factor is proportional to the enhanced factors of the sum and visible electromagnetic fields due to the coupling of the localized surface plasmon generated by the excitation of the gold nanorods/nanoparticles with the sum and visible beams. 34 The remaining enhancement factors of 8.1 and 11.5 for the ss and as modes in our study most likely result from the SPR contribution. Enhancement factors from the SPR effect and the interplay between the F ijk and χ ijk terms are listed in Table 3.…”

“…As a second-order nonlinear optical technique, sum frequency generation (SFG) vibrational spectroscopy, with intrinsically molecular-level selectivity and sensitivity at surfaces and interfaces, has been developed into a powerful tool to specifically probe surfaces and interfaces where the inversion symmetry is broken under the electrical dipole approximation. − Meanwhile, surface-enhanced SFG has been utilized to detect molecules on metals or nanoparticle surfaces. − Nonetheless, analogous to nonlinear coherent vibrational spectroscopy, one of the biggest issues regarding SFG is the strong interference between nonresonant (NR) signals and molecular resonance signals in terms of the peak position, peak width, amplitude, and phase, − leading to the peak shift and distortion and even the creation of ghost peaks . This problem is frequently a barrier to the SFG spectral analysis, especially when interpreting the SFG spectra of surface-anchored molecules on metals where the NR signals drastically overwhelm the molecular resonant signals.…”