Coherent Raman scattering (CRS) is a popular technique for ultrafast spectroscopy and microscopy studies. CRS is based on a third-order nonlinear light-matter interaction, characterized by a nonlinear susceptibility with nonzero elements for bulk samples. It is challenging to perform CRS measurements at interfaces, as bulk contributions can often overwhelm. We have developed a surface-sensitive approach for CRS spectroscopy and microscopy, which enhances the sensitivity to interfacial processes by at least tenfold.
We have previously demonstrated a total internal reflection, wide-field CARS microscope, where the signal is enhanced with the aid of a thin gold layer that supports surface plasmon polariton resonances. This surface-enhanced CARS microscope is capable of generating images of lipid structures in close proximity (<100 nm) to the glass substrate at excitation densities that are 4 orders of magnitude lower than in point-scanning CARS microscopy. In this contribution, we demonstrate its application to visualizing lipids in aqueous media, including imaging of cells, with a unique surface-sensitive contrast that cannot be obtained with conventional CARS microscopy.