Quantum theory of SRS microscopy

Stimulated Raman scattering (SRS) is a fundamental optical process, discovered more than 60 years ago. While the early SRS spectroscopy studies have provided valuable insights to materials systems, the advent of SRS microscopy in 2008 has launched a rapidly growing field in chemical imaging. However, fundamental understanding of the molecular response under SRS spectroscopy and microscopy are still lacking. Herein we present a new framework to introduce molecule-intrinsic stimulated Raman scattering cross sections, σSRS, in the unit of Göppert-Mayer (GM). The absolute SRS cross sections determined for real molecular systems challenge the conventional wisdom that Raman spectroscopy is always a weak process. The enormous rate acceleration of SRS, captured by an apparent SRS cross section, stems from a synergistic effect between the field and the molecule. Our new framework goes beyond the conventional optics-centric view and presents a molecule-inclusive perspective, thus offering a comprehensive foundation for the future growth of SRS spectroscopy and microscopy.