In SRS microscopy, like CARS microscopy, both the pump and Stokes photons are incident on the sample. If the frequency difference ωSRS = ω pump – ω Stokes matches a molecular vibration (ωvib) 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.
Learn more about stimulated Raman scattering:
Vibrational imaging based on stimulated Raman scattering microscopy
New Journal of Physics