Low Frequency Collinear Pre-Resonant Impulsive Stimulated Raman Microspectroscopy

In this work, we extend low frequency impulsive stimulated Raman microspectroscopy to the pre-electronic resonance regime, using a broadband two-color collinear pump probe scheme which can be readily extended to imaging. We discuss the difficulties unique to this type of measurements in the form of competing resonant two-photon absorption processes and the means to overcome them, and successfully reduce the noise which arises due to those competing processes by eliminating the detected spectral components which do not contribute to the vibrational signature of the sample yet introduce most of the noise. Finally, we demonstrate low frequency spectroscopy of crystalline samples under near-resonant pumping showing both enhancement and spectral modification due to coupling with the electronic degree of freedom.