Balancing the Effects of Extinction and Enhancement for Optimal Signal in Surface-Enhanced Femtosecond Stimulated Raman Spectroscopy

The field of ultrafast surface-enhanced Raman spectroscopy (SERS) is rapidly expanding; however, few applications for these new techniques have been demonstrated. One obstacle for the widespread application of ultrafast SERS is the addition of highly enhancing and scattering plasmonic substrates to already complex nonlinear spectroscopies. The competition between extinction and enhancement in ultrafast SERS techniques complicates the optimization of a number of experimental parameters. Here we study the concentration and path length dependences of signal quality in surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS). We find that in contrast to previous studies of spontaneous SERS which use signal magnitudes to define optimal experimental parameters, signal-to-noise ratios (SNRs) are the best measure of ideal experimental parameters in SE-FSRS. We report ideal concentrations and path lengths to use in transmissive geometry SE-FSRS experiments with colloidal nanoparticle substrates. Our results indicate that despite competing effects from SERS and FSRS mechanisms, similarly performed SE-FSRS and SERS experiments yield maximum SNRs using the same concentration and path length due to the overwhelming effects of extinction. By understanding how to optimize SE-FSRS experimental parameters, ultrafast SERS, and SE-FSRS in particular, can be more readily applied to future plasmonically enhanced spectroscopic studies