Physical Limits on Raman Scattering: The Critical Role of Pump and Signal Co-design

We present a rigorous method for deriving limits on Raman scattering in structured media. We exploit this framework to constrain the maximum Raman signal resulting from a planewave incident on two experimentally relevant systems, consisting of either a single Raman molecule in the vicinity of a structured medium or a designable Raman medium (a distribution of Raman molecules). Results pertaining to metallic and dielectric structures illustrate the efficacy of structural optimization and the importance of accounting and co-designing for the nonlinear interplay between pump and signal fields. In particular, we show that treating the pump-focusing and signal-extraction processes separately, as has been done in prior works, leads to highly unrealistic predictions of achievable enhancements. The formulation could readily find applications in guiding further improvements on surface-enhanced Raman scattering (SERS) sensitivity and Raman-assisted lasing.