Mechanisms of epi-detected stimulated raman scattering microscopy

Epi-detection is indispensible for stimulated Raman scattering (SRS) imaging of opaque samples. We present an analysis of the mechanisms underlying the generation of epi-detected SRS signals. By study of forward- and epi-detected SRS signals from pharmaceutical films of controlled thickness and scatterer density, we show that the epi-detected SRS signal arises from the backscattering of the forward-propagating probe photons. Furthermore, we show that both forward- and epi-detected SRS signal intensity linearly depends on the local oscillator power at the detector at the same slope. Under the same excitation conditions, stimulated Raman loss produces a larger epi-detected signal than stimulated Raman gain due to the larger scattering cross section of light with a shorter wavelength.