Origin of Dispersive Line Shapes in Plasmonically Enhanced Femtosecond Stimulated Raman Spectra

• Published in: Journal of physical chemistry. C Vol. 120; no. 37; pp. 20998 - 21006
• Main Authors: Mandal, Aritra, Erramilli, Shyamsunder, Ziegler, L. D
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22.09.2016
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b03303

A theoretical model is described to explain the observed dispersive-like vibrational line shapes reported in previous studies of plasmonically enhanced (PE) femtosecond stimulated Raman spectroscopy (FSRS). These Raman line shapes are rationalized in terms of interference or heterodyne terms between the PE stimulated Raman response from the molecules located in the plasmonically enhancing regions of Au nanoparticles and the nonlinear emission/scattering due to the resonant surface plasmon resonance (SPR) of these nanoparticles. The treatment of the FSRS signal response follows the standard perturbative polarization approach apart from the familiar g 4 field enhancement factor found in spontaneous SERS description, where g represents the local electromagnetic field enhancement due to proximity with the plasmonic nanoparticles. The SPR contribution is modeled as a PE resonant two-level system. The heterodyne cross-term between these two complex third-order responses is large because both of the components are plasmonically enhanced and results in the observed dispersive-like line shape and plasmon resonance detuning dependence. The corresponding calculated spontaneous SERS spectrum for this system only exhibits a normal Lorentzian vibrational resonance, apart from a very weak broad baseline, in agreement with experimental observations.