Retrieving the Complex Vibrational Susceptibility with Interferometric Stimulated Raman Scattering

Coherent Raman Scattering (CRS) is a third-order optical process, which drives a nonlinear polarization in the sample, proportional to the third order nonlinear vibrational susceptibility χ
⁽³⁾
(ω) = χR
⁽³⁾
+χNR
⁽³⁾
. This is a mixture of a complex resonant term, responsible for the different vibrational transitions, and a purely real, non-resonant and frequency-independent term, due to electronic transitions of the target molecules and the surrounding medium. The two most common CRS techniques, in their standard implementation, do not allow the retrieval of the full complex χ
⁽³⁾
signal: the CARS signal is a mixture of its real and imaginary parts, while SRS only retrieves the imaginary part of the nonlinear vibrational susceptibility. Measurement of the complex χ
⁽³⁾
signal would offer instead a richer spectroscopic information to be used, e.g., to distinguish molecules with similar amplitude response but different spectral phase. Numerical methods can be used to reconstruct the full vibrational χ
⁽³⁾
, but their main limitation is the requirement of the entire vibrational spectrum, while most CRS-based microscopies can reach very high imaging speed working with narrowband configurations. Interferometric techniques can overcome this limitation and have been demonstrated for CARS.