Towards Fully Loss-Corrected SRS Imaging

Stimulated Raman scattering (SRS) is a powerful microscopy technique, enabling label-free and chemically-selective investigation of biochemical and medical samples. The contrast in SRS is based on probing the imaginary part of the third order nonlinear susceptibility which is related to specific molecular vibrational resonances. However, as SRS is typically detected as an intensity modulation on either the pump or Stokes beam with a lock-in amplifier, variations in SRS signal strength resulting from spatial changes in chemical composition cannot be distinguished from spatially varying optical losses across heterogeneous and thick samples [1], complicating, e.g., quantitative pharmacokinetic investigations of drugs in tissue [2]. When detecting SRS as stimulated Raman gain (SRG) on the Stokes beam with both the pump and Stokes beam experiencing optical loss, then the SRG signal strength can be expressed as follows: