Low frequency coherent Raman imaging with optical scattering robustness (Invited Paper)

Raman microscopy is a valuable approach to label-free chemical imaging. Raman spectra provide a rich and powerful label-free probe of biological systems. While the narrow spectral features make Raman spectroscopy extremely attractive, the weak strength of Raman scattering makes it difficult to image deeply in many situations. In the case of spontaneous scattering, where inelastically scattered light is detected from a sample illuminated by a narrow-linewidth laser, the persistent challenge with Raman spectroscopic methods is low signals that are difficult to separate from background autofluorescence. In addition, low frequency vibrational modes are very difficult to detect with spontaneous Raman scattering. We will discuss imaging with impulsive stimulated Raman scattering (ISRS). In ISRS, the pump pulse produces a vibrational coherence that leads to a time variation of the effective linear refractive index that drives spectral scattering in the time-delayed probe pulse. ISRS microscopy allows for high quality hyperspectral imaging of low and fingerprint Raman vibrational frequencies when using optical interferometry. We will present methods that allow for high-quality low-frequenc