Origin of dispersive line shapes in plasmon-enhanced stimulated Raman scattering microscopy

Plasmon-enhanced stimulated Raman scattering (PESRS) microscopy has been recently developed to reach single-molecule detection limit. Unlike conventional stimulated Raman spectra, dispersive-like vibrational line shapes were observed in PESRS. Here, we propose a theoretical model together with a phasor diagram to explain the observed dispersive-like line shapes reported in our previous study. We show that the local enhanced electromagnetic field induced by the plasmonic nanostructure interferes with the molecular dipole-induced field, resulting in the dispersive profiles of PESRS. The exact shape of the profile depends on the phase difference between the plasmonic field and the molecular dipole field. We compared plasmon-enhanced stimulated Raman loss (PESRL) and plasmon-enhanced stimulated Raman gain (PESRG) signals under the same pump and Stokes laser wavelength. The PESRL and PESRG signals exhibit similar signal magnitudes, whereas their spectral line shapes show reversed dispersive profiles, which is in an excellent agreement with our theoretical prediction. Meanwhile, we verify that the nonresonant background in PESRS mainly originates from the photothermal effect. These new insights help the proper use of PESRS for nanoscale bioimaging and ultrasensitive detection.

Automated summary

Plasmon-enhanced stimulated Raman scattering (PESRS) microscopy has been developed to achieve single-molecule detection limit. The dispersive-like vibrational line shapes observed in PESRS can be explained by the interference between the local enhanced electromagnetic field induced by the plasmonic nanostructure and the molecular dipole-induced field. The PESRL and PESRG signals exhibit similar signal magnitudes but show reversed dispersive profiles under the same laser wavelength.