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

Abstract

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 bio-imaging and ultrasensitive detection.

A unified framework for the description of plasmonically enhanced ultrafast and cw Raman spectroscopies

“…The enhancement of stimulated Raman spectroscopies, coherent anti-Stokes Raman scattering (CARS), [6][7][8][9][10][11] stimulated Raman gain/loss (SRG/L), [12][13][14][15] femtosecond stimulated Raman scattering (FSRS), [16][17][18][19] impulsive stimulated Raman scattering (ISRS) 20 due to plasmonic effects has been experimentally demonstrated as well, albeit with varying degrees of enhancement efficiencies, and have shown some spectroscopic complications. Although dispersive vibrational line shapes are often observed in homodyne detected spectroscopies with Raman resonances, such as CARS and sum frequency generation (SFG), dispersive Raman line shapes have more recently been observed in heterodyne detected PE-FSRS and PE-SRG/L spectra.…”