Surface-enhanced coherent nonlinear optical signals can dramatically improve detection sensitivity of spectroscopic imaging techniques. Large enhancement factors (EFs) of many orders of magnitude are expected for coherent Raman scattering of molecules in local fields of plasmonic nanostructures. However, only small EFs, several orders of magnitude less than the predicted values, were experimentally observed. To understand this discrepancy we measured the spatial variation of the shape of surface-enhanced coherent anti-Stokes Raman scattering (SECARS) spectra of pyridazine on randomly aggregated gold nanoparticles. We developed a model to simulate the dependence of SECARS spectra on the position and linewidth of the surface plasmon resonance, and attribute small (and even negative) EFs to local destructive interference. We report measurements of nanoscale phase effects in SECARS, and propose strategies to increase experimental EFs towards theoretical predictions.

• Received 31 July 2013

DOI:https://doi.org/10.1103/PhysRevA.89.043841