In SRS microscopy, like CARS microscopy, both the pump and Stokes photons are incident on the sample. If the frequency difference SRS = pump - Stokes matches a molecular vibration (vib) stimulated excitation of the vibrational transition occurs. Unlike CARS, in SRS there is no signal at a wavelength that is different from the laser excitation wavelengths. Instead, the intensity of the scattered light at the pump wavelength experiences a stimulated Raman loss (SRL), with the intensity of the scattered light at the Stokes wavelength experiencing a stimulated Raman gain (SRG). The key advantage of SRS microscopy over CARS microscopy is that it provides background-free chemical imaging with improved image contrast, both of which are important for biomedical imaging applications where water represents the predominant source of nonresonant background signal in the sample.
Figure 1: Stimulated Raman scattering (SRS) energy diagram for the SRS four-wave mixing process (right). Label-free stimulated Raman gain imaging of lipids in human melanocytes. Image courtesy of Andreas Volkmer (University of Stuttgart, Germany).